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  • Lithology, stratigraphy, chemostratigraphy, and depositional environment of the Mississippian Sycamore rock in the SCOOP and STACK area, Oklahoma, USA: Field, lab, and machine learning studies on outcrops and subsurface wells
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-12
    Benmadi Milad; Roger Slatt; Zou Fuge

    This paper presents an interpreted depositional environment, developed 2nd order sequence stratigraphy framework, and detailed lithofacies identification from two Mississippian Sycamore outcrops (I-35 Sycamore and Speake Ranch) and subsurface wells in the SCOOP (South Central Oklahoma Oil Province). Then these rocks calibrate the rock properties with wireline log responses to identify the best landing zones. Qualitative and quantitative techniques of field, laboratory, and machine learning studies were conducted. For the field studies, we measured the complete 450 ft of the outcrop stratigraphic section, another separate 50 ft outcrop, examined the underlying Woodford Shale and overlying Caney Shale boundary contacts, documented sedimentary structures, constructed an outcrop gamma-ray profile, and developed a sequence stratigraphic framework. Laboratory studies included petrographic analyses, detailed X-ray Fluorescence (XRF), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD). For machine learning studies, a principal component analysis (PCA), elbow method, and self-organizing map (SOM) were used to analyze the electrofacies and chemofacies from the outcrop and a subsurface uncored well. The outcrop hand-held gamma ray profile was obtained and correlated with subsurface wells. Five major outcrop lithofacies and chemofacies, within six stratigraphic units of alternating siltstone and shale strata, were identified from the wireline logs. A Maximum Flooding Surface (MFS), and two major 2nd order Sequence Boundaries (SB) were recognized at the outcrop and a nearby subsurface well. Bouma sequences and repetitive cycles of sedimentary structures indicated sediment gravity flow deposition on a marine slope setting. This study provides geologic insights to better understanding the depositional environment and the lithology of the Sycamore rocks. The bioturbated siliceous shale and/or the sandy siltstone can be potential target zones due to their reservoir quality, lithology, bed continuity, and brittleness. This information can be of direct benefit to the exploration and development programs of many companies in the SCOOP area, particularly in the Anadarko and Ardmore basins in Oklahoma.

    更新日期:2020-02-12
  • Mudstone/sandstone ratio control on carbonate cementation and reservoir quality in Upper Permian Rotliegend sandstones, offshore the Netherlands
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-11
    Johannes M. Miocic; Jean-Pierre Girard; Robert Schöner; Reinhard Gaupp

    The eolian-fluvial sandstones of the Upper Permian Rotliegend formation, which were deposited in the Southern Permian Basin, are today deeply buried (∼3–4 km) and constitute important gas reservoirs in the Netherlands and the southern North Sea. The reservoir properties of the sandstones have been documented to be strongly affected by diagenesis, but the primary diagenetic factors impairing reservoir quality and their cause remain variably interpreted in the literature. Here, we present the results of a detailed investigation on the diagenetic processes controlling reservoir quality in the Lower Slochteren formation in the L and K blocks offshore the Netherlands, where fluvial and aeolian sandstones intercalate with playa lake muds in a delta setting. Quantitative analysis of the diagenetic mineral phases occurring in all main depositional facies (eolian, fluvial, playa-lake) was carried out on more than 200 samples from 21 wells, with the authigenic mineral composition of an additional 500 samples being evaluated qualitatively. The integration of petrographical observations with log data and core descriptions reveals that cement distribution/abundance is not dominantly driven by depositional facies, nor are reservoir properties. Early pore-filling dolomite cement can be as high as 40% and represents the main control on reservoir properties. Detailed analysis of its spatial distribution shows it to be distinctly related to mudstone proximity and mudstone/sandstone (M/S) ratio. Sandstones occurring as thin beds in mudstone-rich depositional sequences (high M/S ratio) typically exhibit strong pervasive carbonate cement regardless of sedimentary facies. In contrast, sandstones forming thick beds in mudstone-poor sequences (low M/S ratio) are commonly free/low in dolomite cement. Our results demonstrate, for the first time, that reservoir quality in the Rotliegend sandstones in the delta setting of the Netherlands is primarily controlled by early dolomite cement. The latter is most developed in areas with high (>70%) vertical M/S ratio, where it may be a devastating factor for reservoir quality. Best reservoir sandstones should be expected where the depositional stacking pattern is poor in shaly deposits (playa-lake and distal sheet-flood sediments), such as in the southern/south-eastern part of the studied area.

    更新日期:2020-02-11
  • Source rock characterization and oil-to-source rock correlation of a Cambrian -Ordovician fold-and-thrust belt petroleum system, western Newfoundland
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-11
    Martin Schwangler; Nicholas B. Harris; John W.F. Waldron

    In this study, we use biomarker and isotope data to address the complexities of petroleum system analysis in fold-thrust systems, specifically related to hydrocarbon generation and migration, such as variable thermal maturities of source rocks, complex migration pathways, and mixing. Source rocks, extracts, and oil samples are taken from Paleozoic rocks along the Appalachian structural front in western Newfoundland. Oil seeps along this fold-and-thrust belt have motivated episodic exploration efforts over the last 150 years. However, economic development has been unsuccessful to date, in part because of the complex nature of oil-to-source relationships. Pyrolysis analyses identify promising source intervals in Lower Ordovician (Floian) formations, with an excellent source potential containing type I/II organic matter (TOC up to 9.35%, HI up to 840). A second good source interval, identified within the late Cambrian (Furongian) continental slope and rise sediments, contains type II/III organic matter (TOC up to 2.34%, HI = 380). Source rock samples from outcrops are marginally mature to mature with Tmax values between 436 and 447. Geochemical analyses of source rock extract imply a clastic shale-dominated source rock with a minor contribution from carbonate source rocks. Chemometric analyses yield 3 extract groups. Groups 1 and 2 contain lower Ordovician samples with organic matter derived from oxidized micro-plankton with varying amounts of Gleocapsomorpha prisca. Extract group 3 originates from Cambrian samples and shows evidence of a bacterial-derived organic matter. Similar genetic relationships among 10 oil samples from natural oil seeps and abandoned well sites are indicated by high-resolution biomarker analyses. Seven oil samples originated from Ordovician source rocks that produced high API oils with low bisnorhopane/hopane, tricyclic/hopane ratios, and high sterane concentrations, supporting interpretation of a source containing algal-derived organic matter. Three oils originated from Cambrian shales. These high API oils show high bisnorhopane/hopane and tricyclic/hopane ratios, low sterane concentrations, and low carbon isotope values, all indicative of organic matter derived from bacteria, generated over a wide range of maturities (0.98–1.26 %Ro). Thermal maturity-sensitive biomarkers and naphthalene ratios show that source rocks generated oils with a wide range of maturities related to their dipping character in the imbricated thrust stack. The Cambrian source generated oils that locally experienced secondary cracking during Acadian inversion, whereas the Ordovician source generated oil with low maturities after the Acadian inversion. Oil operaters exploring fold-and-thrust belt petroleum systems may expect extreme changes of oil properties (for example, API gravity) in spacially restricted areas.

    更新日期:2020-02-11
  • Depositional and organic carbon-controlled regimes during the Coniacian-Santonian event: First results from the southern Tethys (Egypt)
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-11
    Ahmed Mansour; Michael Wagreich; Thomas Gentzis; Seare Ocubalidet; Sameh S. Tahoun; Ashraf M.T. Elewa

    Geochemical and palynological proxies were determined for 31 samples representing the upper Coniacian-lowermost Campanian Abu Roash A Member, collected from the BED2-3 well. This was done to assess the prevailing paleoredox conditions, productivity levels, and water-column settings to: (1) understand controls and mechanisms related to accumulation of organic carbon within the sedimentary record; (2) assess principal environmental processes that triggered the preservation of organic matter-poor, calcareous shale and limestone facies of the Abo Roash A Member; and (3) to interpret the paleoceanographic settings and confirm the occurrence of either Oceanic Anoxic Event 3 (OAE3) or Cretaceous Oceanic Red Beds (CORBs) within the Abu Gharadig Basin in the southern part of the Tethyan Ocean, Egypt. Sedimentation rates were reconstructed based on the biostratigraphic-age constraints of marine dinoflagellate cysts (dinocysts). Trace elements-based ratios and relationships were used in order to study the role of clastic terrestrial input. Three groups of dinocysts and freshwater Pediastrum algae were used to study the water-column conditions. Results showed that the Abu Roash A Member was deposited under oxic to intermittently dysoxic conditions in a distal inner to middle neritic environment, with varying sedimentation rates from 32 to 119 mm kyr−1. Productivity appeared to be a significant factor for determining magnitudes of the organic carbon accumulation; however, low productivity was prevalent during deposition. Carbonate content was high, which resulted in dilution of organic matter during varied rates of sediment input. Such results indicate that the paleoceanographic conditions in the southern Tethys witnessed the absence of prevalent oceanic anoxia (i.e., the Coniacian-Santonian OAE 3) and deposition of the CORBs as in several parts of the Tethys, the Pacific, and the Indian oceans.

    更新日期:2020-02-11
  • Sr and Nd isotopes of cold seep carbonates from the northern South China sea as proxies for fluid sources
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-10
    Lu Ge; Wei Chen; Bi Zhu; Meitong Fan; Tao Yang; Shaoyong Jiang

    In this study, Sr isotopes combined with Nd isotopes of modern cold seep carbonates were studied to provide reliable proxies for sources and pathways of seep fluids. We presented Sr and Nd isotopes together with stable carbon and oxygen isotope data of cold seep carbonates recovered from the northern continental slope of the South China Sea (SCS). Based on the evidence for negligible detrital contamination and diagenetic alteration, carbonate samples are considered dependable archives which record Sr and Nd isotope signals of the original pore waters from which carbonates have precipitated. Excellent correlation between 87Sr/86Sr ratios and 1/Sr observed in carbonate samples indicates two end-members with distinct Sr isotopic signatures, including modern seawater as a major source and a lesser contribution from methane-rich seep fluid driven from a deep source. Besides these two end-members, release of Nd from organic matter and Fe–Mn oxyhydroxide is an additional contribution to pore water Nd. Furthermore, Sr and Nd isotopic compositions suggest water-rock interactions between upward seep fluid and detrital component in sediments, which resulted in the isotopic shift towards pronounced 143Nd depleted and slightly 87Sr enriched in the fluid phase. This study provides insight into the knowledge for tracing seepage fluid circulation by Sr–Nd isotope system at modern seep systems.

    更新日期:2020-02-10
  • Characterisation of submarine depression trails driven by upslope migrating cyclic steps: Insights from the Ceará Basin (Brazil)
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-10
    Daniele Maestrelli; Vittorio Maselli; Benjamin Kneller; Domenico Chiarella; Nicola Scarselli; Paola Vannucchi; Luigi Jovane; David Iacopini

    Circular to elliptical topographic depressions, isolated or organized in trails, have been observed on the modern seabed in different contexts and water depths. Such features have been alternatively interpreted as pockmarks generated by fluid flow, as sediment waves generated by turbidity currents, or as a combination of both processes. In the latter case, the dip of the slope has been hypothesized to control the formation of trails of downslope migrating pockmarks. In this study, we use high-quality 3D seismic data from the offshore Ceará Basin (Equatorial Brazil) to examine vertically stacked and upslope-migrating trails of depressions visible at the seabed and in the subsurface. Seismic reflection terminations and stratal architecture indicate that these features are formed by cyclic steps generated by turbidity currents, while internal amplitude anomalies point to the presence of fluid migration. Amplitude Versus Offset analysis (AVO) performed on partial stacks shows that the investigated anomalies do not represent hydrocarbon indicators. Previous studies have suggested that the accumulation of permeable and porous sediments in the troughs of vertically stacked cyclic steps may create vertical pathways for fluid migration, and we propose that this may have facilitated the upward migration of saline pore water due to fluid buoyancy. The results of this study highlight the importance of gravity-driven processes in shaping the morphology of the Ceará Basin slope and show how non-hydrocarbon fluids may interact with vertically stacked cyclic steps.

    更新日期:2020-02-10
  • Vertical effective stress and temperature as controls of quartz cementation in sandstones: Evidence from North Sea fulmar and Gulf of Mexico Wilcox sandstones
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-08
    Olakunle J. Oye; Andrew C. Aplin; Stuart J. Jones; Jon G. Gluyas; Leon Bowen; Joseph Harwood; Ian J. Orland; John W. Valley

    We present quantitative petrographic data, high spatial resolution oxygen isotope analyses of quartz cement, basin modelling and a kinetic model for quartz precipitation for two Paleocene-Eocene Wilcox Group sandstones from Texas and two Jurassic Fulmar Formation sandstones from the Central North Sea. At each location, one sandstone has been buried to ca. 145 °C and one to ca. 185 °C. A key difference between the Wilcox and Fulmar burial histories is that the Wilcox sandstones are currently at higher vertical effective stresses and, from basin modelling studies, have been subjected to generally higher vertical effective stresses through their burial history. The amounts of quartz cement in the Wilcox sandstones are between 12 and 18%, and between 2 and 6% in the Fulmar sandstones. High-spatial-resolution oxygen isotope data obtained from the quartz cements suggest temperature ranges for quartz precipitation from 60 to 80 °C to values approaching maximum burial temperature. Factors such as grain coatings or the timing of petroleum emplacement cannot explain the differences in the amounts of quartz cement. Petrographic data show that most of the silica for quartz cement can be derived from intergranular pressure dissolution. Although the sample set is small, we interpret the results to suggest that the differences in quartz cementation in Fulmar and Wilcox sandstones can be explained better by differences in their vertical effective stress history than their temperature history; in this case, the supply of silica rather than the precipitation of quartz becomes an important control on the rate and extent of cementation.

    更新日期:2020-02-10
  • Diagenetic and environmental control of the clay mineralogy, organic matter and stable isotopes (C, O) of Jurassic (Pliensbachian-lowermost Toarcian) sediments of the Rodiles section (Asturian Basin, Northern Spain)
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-08
    Jean-François Deconinck; Juan Jose Gómez; François Baudin; Hugo Biscay; Ludovic Bruneau; Theophile Cocquerez; Olivier Mathieu; Pierre Pellenard; Anne-Lise Santoni

    Clay mineralogical and geochemical analyses (δ18Ocarb, and paired carbonate and organic matter carbon isotope), completed by the characterization of organic matter, have been performed on the Pliensbachian sediments exposed on the Rodiles section (Asturian Basin, Northern Spain). The objectives were to precise the Pliensbachian climate evolution at a relatively low paleolatitude of the Tethyan domain by comparison with previously published high-resolution clay mineralogical and geochemical data from the NW Tethyan domain (e.g. Paris Basin and Cardigan Bay Basin, West Wales). The clay mineral assemblages are dominantly composed of illite and illite-smectite mixed-layers (I–S) associated with minor proportions of chlorite and kaolinite. Smectites are not identified what may result either to their absence at the time of sedimentation or to their progressive illitization due to the burial diagenesis. Rock-Eval data show that organic matter reaching more than 5% in black-shale horizons is thermally mature with Tmax comprised between 444 and 454 °C indicating that the section is in the oil window and that the burial temperature reached or exceeded 80–90 °C. The low δ18Obulk values comprised between −3 and −6‰ clearly results from a diagenetic influence (thermal effect due to burial) but their fluctuations compared with δ18O measured on well-preserved belemnites guards show that the overall trend is however preserved. By comparison with higher latitude sedimentary basins, the most striking feature is the very small proportions of kaolinite (generally less than 5% while the proportions of this mineral reach 40% at higher paleolatitudes), except at the Pliensbachian/Toarcian transition, where the proportion of kaolinite increases significantly. Clay minerals likely originated from the erosion of the Iberian Massif located to the South of the Asturian Basin in a semi-arid climatic belt. This may explain the scarcity of kaolinite by comparison with sedimentary basins located at higher latitude influenced by more humid climate. The increasing proportions of kaolinite by the end of the Pliensbachian suggest southward expansion of the humid belt as a consequence of cooler temperatures and the establishment of steeper latitudinal gradients of temperature likely resulting from the formation of restricted polar ice cap. The main δ13C (on both 13Cinorg and 13Corg) Pliensbachian excursions including the negative excursion of the Sinemurian/Pliensbachian Boundary Event (SPBE) and the Late Pliensbachian positive excursion are well recognized.

    更新日期:2020-02-10
  • Creeping microstructure and fractal permeability model of natural gas hydrate reservoir
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-07
    Jianchao Cai; Yuxuan Xia; Cheng Lu; Hang Bian; Shuangmei Zou

    The successful pilot testing of depressurization production for natural gas hydrate reservoir was conducted at Shenhu Area in the South China Sea. However, pressure changes due to depressurization production cause shrinkage of creeping pore and throat space for fluid flow, ultimately resulting in permeability damage in this reservoir. To optimize gas recovery for natural gas hydrate reservoir, it is important to understand the substantial variations of the pore structure and physical properties under different pressure conditions. In this study, by integrating computed tomography imaging with water flow experiments on clayey silt sediment samples, we analyze how pore scale structural parameters (such as average pore and throat radius, pore and throat median radius, maximum pore and throat radius, and fractal dimension) change under different axial strains by using fractal geometry approach. It is found that there is a negative relationship between axial stress and structural parameters. Meanwhile, with the axial stress increases, the range of pore and throat radius distribution located on the right of center distribution decreases. On the basis of fractal geometry, a fractal model is then proposed to explain the effects of axial strains and creeping microstructure on permeability for natural gas hydrate reservoir. Results show that the model provides good match with experimental data when the axial stress is larger, while poor agreements with experimental results at low pressure. This study helps us understand fundamental mechanism for permeability changes during the depressurization of gas hydrate reservoir.

    更新日期:2020-02-07
  • Clay mineral content, type, and their effects on pore throat structure and reservoir properties: Insight from the Permian tight sandstones in the Hangjinqi area, north Ordos Basin, China
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-07
    Ren Wang; Wanzhong Shi; Xiangyang Xie; Wei Zhang; Shuo Qin; Kai Liu; Arthur B. Busbey

    Clay mineral content and pore throat structure, including type, morphology, size distribution, and porosity and permeability, are very important in factors that controll gas storage in tight sandstone reservoirs. In this study, integrated thin section and field emission scanning electron microscope (FE-SEM) observation, X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and rate-controlled mercury porosimetry (RCP) analysis were used to investigate pore throat structure and the effects of clay minerals on pore throat connectivity, size distribution and reservoir properties of tight sandstones in the middle Permian successions of the Hangjinqi area, north Ordos Basin, China. Results show that the tight sandstones have very low porosity and permeability (averaged 9.76% and 0.968 mD) with diverse clay mineral compositions and grain sizes. The main pore types are intergranular and intragranular dissolution pores, intercrystalline and interstitial micropores. Most pores and throats are infilled with authigenic clay minerals, such as acicular chlorite, booklet kaolin, and scaly and filamentous illite with minor silica and calcareous cements. The pore and throat sizes in tight sandstones exhibit large variation, with 147.648 μm and 1.051 μm in sandy conglomerates, 160.998 μm and 1.285 μm in pebbly coarse-grained sandstones, 155.016 μm and 0.868 μm in coarse-grained sandstones, 158.134 μm and 1.178 μm in medium-grained sandstones, and 154 μm and 0.379 μm in fine-grained sandstones, respectively. The results indicate that clay minerals affected the pore throat connectivity, size distribution and reservoir properties of different lithofacies. The properties of tight sandstone reservoirs are heavily influenced by illite content, and throat size played a more important role influencing porosity and permeability. This study helps to clarify the relationship of clay minerals, pore throat structure and reservoir properties of the Permian tight sandstones in the Hangjinqi area and can guide associated work on tight sandstone reservoirs in the Ordos Basin and other basins worldwide.

    更新日期:2020-02-07
  • Facies analysis relationships depositional environments of the subsurface stratigraphy of the Snake Cave Interval in the Bancannia Trough, western Darling Basin, New South Wales, SE Australia
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-06
    M.K.H. Khalifa; K.J. Mills

    This work presents the first detailed research on the subsurface stratigraphy of the sedimentary Snake Cave Interval (late Early to late Middle Devonian) in the Bancannia Trough, the westernmost component of the Darling Basin, in western New South Wales, Australia. The study is based on data from wireline-log responses (especially gamma-ray and sonic logs), and core and cuttings descriptions in three exploration wells that are used to develop a model for depositional environments in the Bancannia Trough. This study provides re-evaluation and re-interpretation of the positions of the top and bottom boundaries of the Snake Cave Interval using distinct patterns changes in gamma-ray and sonic log signatures in relation to lithological characteristics in the Bancannia South-1, Bancannia North-1 and Jupiter-1 wells. The log facies interpretation of the Snake Cave Interval section enabled the detailed facies description of four lithostratigraphic units (A, B, C and D, in ascending order) in the study area. The Snake Cave Interval displays units with ten distinct sedimentary facies identified from core and then tied to the gamma ray log characteristics, forming facies associations that are visible in both vertical and lateral sections. Five facies associations have been identified: FA-I, braided fluvial multistory channel belt complex deposits; FA-II, meandering fluvial distributary channel sandstone complex deposits; FA-III, sandy braided fluvial distributary channels deposits; FA-IV, meandering fluvial deposits; and FA-V, fluvial channel-fill association with fluvio-tidal transition zone complex deposits. Detailed comparison of lithologic logs with wireline-log suite signatures (mainly gamma-ray logs) that are dominantly braided fluvial and meandering fluvial deposits, with minor fluvio-tidal transition zone complex deposits in this trough. This article presents a new depositional model that will promote exploration and discovery of potential petroleum resources and stratigraphic-trap configurations in this trough and adjacent regions of the Darling Basin.

    更新日期:2020-02-07
  • Regional structure and polyphased Cretaceous-Paleocene rift and basin development of the mid-Norwegian volcanic passive margin
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-05
    Dmitry Zastrozhnov; Laurent Gernigon; Iakov Gogin; Sverre Planke; Mansour M. Abdelmalak; Stéphane Polteau; Jan Inge Faleide; Ben Manton; Reidun Myklebust

    The Møre and Vøring basins of the mid-Norwegian margin are characterized by thick accumulations of Cretaceous to Paleocene sedimentary strata. They were formed during a series of Late Mesozoic-Early Cenozoic extensional events and represent vast underexplored areas with a limited number of wells. Recently, a new generation of long-offset 2D seismic reflection lines and 3D seismic data, together with new well data, has permitted a significant improvement in the regional understanding of the Møre and Vøring basins. This has enabled much better imaging of the deep Cretaceous subbasins and sub-basalt structures. In light of this significant data improvement, we performed a regional tectonostratigraphic synthesis of the pre-breakup development of the Møre and Vøring basins. We have interpreted eight regional Cretaceous and Paleocene horizons and constructed a series of structural and thickness maps. The new interpretations allow us to examine the sequential evolution of the Cretaceous to Paleocene sedimentary infill and to discuss its relationship to the deep crustal structures and regional tectonic events. We conclude that the long and polyphased development of the Møre and Vøring basins is partly controlled by deep-seated structural highs. We show that active deposition in the Early Cretaceous was mainly focused in the Møre Basin, while the main Cenomanian and subsequent Late Cretaceous-Paleocene depocentres developed principally in the Vøring Basin and migrated sequentially west towards the present continent-ocean boundary. We argue that the outer Møre and Vøring basins are likely underlain by a relatively thick continental crust compared to the inner part of the regional sag basin. In this setting our observations do not support evidence for a large zone of exhumed upper mantle, which has previously been proposed to have formed before magmatism and breakup.

    更新日期:2020-02-06
  • Transition metal catalysis in natural gas generation: Evidence from nonhydrous pyrolysis experiment
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-05
    Jinliang Gao; Caineng Zou; Wei Li; Yunyan Ni; Yilin Yuan

    Transition metal catalysis has been advocated as a critical mechanism for natural gas generation. But its presence and significance in geological settings have long been debated due to conflicting results of laboratory experiments and the lack of reliable field indicator. In this study, a low rank coal sample from the Shahezi Formation of the Songliao Basin was subjected to nonhydrous pyrolysis with and without transition metal. The chemical and isotopic compositions of gaseous products in experiments were analyzed to investigate the mechanism and potential indicator of transition metal catalysis in sedimentary basins. The chemical and carbon isotopic compositions of hydrocarbon gases generated in pyrolysis experiments revealed that transition metal could catalyze both decomposition of organic matters and abiogenic hydrocarbon synthesis such as Fischer-Tropsch Type reactions during maturation of source rocks under experimental conditions resembling geological settings. Catalytic methane has uniform δD values unrelated to source rock maturity, which is helpful in identification of catalytic natural gases in sedimentary basins. Transition metal catalysis is probably not a common mechanism for natural gas generation in sedimentary basins due to the positive correlation between δDCH4 of natural gases and maturities of source rocks observed in most natural cases. However, transition metal catalysis could well interpret the restricted range of δDCH4 and carbon isotopic reversal phenomenon of natural gases in the Yingcheng Formation of the Xujiaweizi fault-depression whose origin has long been debated. So we inferred that the transition metal catalysis may have occurred in deep strata of the Xujiaweizi fault-depression and played significant role in natural gas generation. Our study revealed that restricted range of δDCH4 of natural gases with different maturities may serve as an effective indicator of transition metal catalysis in sedimentary basins. It provides a new perspective to investigate the significance and universality of transition metal catalysis in different geological settings.

    更新日期:2020-02-06
  • Origin and timing of palaeovalleys in the Carpathian Foredeep basement (Sędziszów Małopolski-Rzeszów area; SE Poland) in the light of palynological studies
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-04
    Przemysław Gedl; Elżbieta Worobiec

    The northern margin of the Carpathian basins and their closest foreland are not preserved nowadays being partly destroyed by the subduction processes, and partly hidden under several kilometres thick cover of overthrusted Carpathian nappes and autochthonous Miocene deposits. Seismic data show along the whole arch of the Carpathian orogene lowered areas that form elongated structures in the basement, approximately perpendicular to the front of the overthrust. These structures were interpreted as palaeovalleys incised in an uplifted basin margin prior to the overthrust of folded Carpathian nappes. Studies of their sedimentary sequences, for these profiles located at the depths accessible by the boreholes, have yielded ambiguous results so far. One such a palaeovalley filled with initial stage deposits was detected in the vicinity of Rzeszów, SE Poland. Organic-rich, fine-grained deposits from the middle part of this sequence yielded palynomorphs allowing their dating. We present timing of the Rzeszów palaeovalley formation based on palynological interpretations and comparison with regional geology of the Carpathian domain.

    更新日期:2020-02-04
  • Origin of enigmatic sand injectite outcrops associated with non-tectonic forced-folding structure in Bahrah area, northern Kuwait Bay
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-04
    Mubarak M. Al-Hajeri; Aimen Amer; Deddy Djawair; David Green; Mohammad Al-Naqi

    This is a quantitative and qualitative study for the first time linking surface with subsurface geology to understand the occurrence and origin of enigmatic sand injectite outcrops in two sites within the Bahrah coastal plain area, northern Kuwait Bay. Field work observations and aerial photography were used to describe and characterize the surface geology and stratigraphy of the study area. Thirty-one rock samples were obtained for petrographical, fluid inclusions and strontium isotopes analyses. 3D seismic data were also used to assess the subsurface geology of the area. The petrography and microthermometry of fluid inclusions were used to understand the origin of fluids and gases associated with the formation of these sand injectites. We found that these surface geological features represent surface manifestation of subsurface geological and biological activities/processes. Therefore, we proposed a near-surface complex focused fluid injection mechanism/model responsible for the formation of the sand pipes. The model consists of five episodes: Episode-1 represents the development of near-surface geobody (? Paleoshoreline) sealed by a shale layer; Episode-2 represents the entrapment of hydrocarbons that migrated laterally up-dip within geobody, interacting with mixed seawater intrusion and fresh groundwater; Episode-3 represents the effect of biodegradation biological processes of the entrapped hydrocarbons generating CH4, CO2 and H2S gases; Episode-4 represents overpressure build-up due to gas generation exceeding pore volume of geobody and forming non-tectonic forced-folds and polygonal to semi-circular joints/fractures network; and Episode-5 represents overpressure release triggered by external factor(s) (seismic activities?) facilitating gases, water and sand remobilisation through weakened zones of natural fractures.

    更新日期:2020-02-04
  • How far can hydraulic fractures go: A comparative analysis of water flowback, tracer, and microseismic data from the Horn River Basin
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-02-01
    Yingkun Fu; Hassan Dehghanpour

    The potential environmental impacts of hydraulic fracturing is a significant public concern. However, a key question is: How far can fractures go beyond the well? We analyze microseismic, pressure interaction (frac-hit and interference) and tracer data recorded during fracturing of and production from 52 wells in the Horn River Basin. The results show that the wells communicate through fracture-like pathways which allow water, gas, and proppant flow. The lateral and vertical well communications occur at distances above 1 km and up to 130 m, respectively. Well communication may last for over 1.7 years possibly due to proppant flow which prevents complete fracture closure during flowback and post-flowback processes.

    更新日期:2020-02-03
  • Reducing the risk of finding a working petroleum system using SAR imaging, sea surface slick sampling, and geophysical seafloor characterization: An example from the eastern Black Sea basin, offshore Georgia
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-31
    Harry Dembicki

    As new plays emerge in deepwater settings, sea surface slicks are often overlooked as a source of geochemical information to help define potential petroleum systems. To demonstrate how sea surface hydrocarbon slicks can be used, a case study is presented from the eastern Black Sea in the Rioni Basin, offshore Republic of Georgia. There source rocks of Oligo-Miocene age (Maikop) are suspected of charging upper Miocene deepwater channel-levee sands in fold and thrust system traps. To reduce exploration risk, direct evidence of hydrocarbon generation and migration was sought using sea surface slicks observed there. To provide confirmation of charge, a collection of diverse data was used. First, synthetic aperture radar satellite images were utilized to verify the presence of large recurring sea surface slicks over prospective structures. These slicks were then sampled for later analysis during the acquisition of 3-D seismic data. To verify that the slicks were related to the subsurface expulsion, seafloor bathymetric and amplitude data were extracted from the 3-D seismic volume and used to identify potential seafloor features consistent with hydrocarbon seepage. Some of these seafloor features were found to be coincident with the apparent sea surface origins of the slicks. Seismic imaging was also used to demonstrate potential migration pathways which may link suspected deep subsurface traps and the seafloor. And finally, geochemical analysis showed that the compositional characteristics of the slicks’ hydrocarbons were similar to known Maikop sourced oils in the region. This combination of data provided a high level of confidence that the seismically imaged traps in offshore Republic of Georgia are charged, suggesting a working petroleum system was present. What it cannot tell us is how much petroleum may be in these structures. That question can only be answered by the drill bit.

    更新日期:2020-01-31
  • Nitrogen-rich gas shale logging evaluation and differential gas-bearing characterization of lower Cambrian formation in northern Guizhou, south China
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-30
    Shuangbiao Han; Songtao Bai; Zhiyuan Tang; Yurun Rui; Dajian Gong; Jinchuan Zhang

    There are significant differences in the gas content between the two sets of lower Paleozoic shale in south China. The nitrogen content of the lower Silurian shale gas that has been commercially developed is very low, whereas that of the lower Cambrian shale (LCS) is relatively high. Shale has complex gas-bearing mechanistic characteristics. The genesis of nitrogen is complex and sources are diverse. A gas containing high nitrogen and low hydrocarbons increases the risk and difficulty of developing a shale gas resource. Based on the key geological parameters of the LCS in the northern Guizhou area (NGA), this study investigates the characteristics of nitrogen in the highly mature shale and discusses the logging evaluation method for nitrogen-bearing shale gas reservoirs in this complex tectonic area. Changes in the gas-bearing characteristics of the LCS in the study area are complex. The shale gas composition was found to vary widely due to differential enrichment; the methane content was between 1.2% and 82.4%, and the nitrogen content was between 0.03% and 97.4%. The nitrogen-rich shale formations exhibited a high compensation density (2.46–2.58 g/cm3), high compensation neutron (10.2–17.9%), and low acoustic time difference (192–230 μs/m). There were two different types of resistivity responses in nitrogen-bearing shale gas reservoirs, which had distribution characteristics of 21–56 Ω m and 100–1000 Ω m. With these response characteristics, the logging evaluation of nitrogen-bearing shale gas reservoirs was interpreted by selecting differential gas-sensitive parameters, and different gas-bearing types were divided into dry, nitrogen, mixed, and methane gas layers. Furthermore, the identification and computational evaluation of the gas composition (methane/nitrogen) and content were carried out quantitatively. Differential gas-bearing property was characterized and that is beneficial to the exploration practice in the study area.

    更新日期:2020-01-31
  • Laboratory evaluation of flow properties of Niutitang shale at reservoir conditions
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-30
    Decheng Zhang; P.G. Ranjith; M.S.A. Perera; Guowei Ma

    Absolute and water saturation dependent gas flow under reservoir conditions is important for the exploration of deep energy resources. Steady-state and various unsteady-state argon permeability tests were conducted on dry Niutitang shale during the confinement loading and unloading cycles, then different gases were used for steady-state permeability tests at the final confinement of 20 MPa. Permeability decreased by 36% with the confinement increasing from 5 MPa to 60 MPa, and around 66% of the permeability reductions was recovered after the unloading cycles. The apparent steady-state permeabilities were 38% higher than those derived by unsteady-state methods due to the enhanced slip flow. Two cycles of water imbibition and gas drainage tests were conducted with the long water evaporation process completed in the second gas drainage process. Argon to carbon dioxide permeability ratio ranged from 1.2 to 2.1, while the water permeability was 30–45% of the gas intrinsic permeability. The argon breakthrough pressure of water-saturated Niutitang shale sample was 8 MPa, and the permeability increased by 10 times after the long-term evaporation process. Argon permeability decreased by 90% and increased by 21.6%–29.6% respectively after the first and second gas drainage processes compared with that under dry condition with the residual water saturation being 29.7% and 11.3% respectively, which were caused by the water blockage and cracking respectively. Influences of carbon dioxide adsorption and induced swelling reached a minimum at residual water saturation.

    更新日期:2020-01-31
  • Dolomitisation, cementation and reservoir quality in three Jurassic and Cretaceous carbonate reservoirs in north-western Iraq
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-28
    Omar K. Mohammed Sajed; Paul W.J. Glover

    Dolomitisation is a key diagenetic process, commonly improving reservoir quality in carbonate rocks. It is important in the Jurassic and Cretaceous carbonate reservoirs of north and north-western Iraq, creating primary and secondary pays in many oilfields (Kirkuk, Ain Zalah and Butmah). This paper addresses the impact of dolomitisation and cementation on the reservoir quality of three dolomitised carbonate formations (Butmah, Mauddud and Wajnah). Three different dolomitisation types were recognised: reflux dolomitisation, mixing zone, and burial dolomitisation. Extensive petrophysical measurements were carried out and have shown that differences in dolomite crystal size and cementation control the complexity of porosity, pore and pore throat size distributions, pore network connectivity, and permeability. Reflux dolomitisation exhibits widely spread porosities and permeabilities compared to the other two characterised dolomitisation models, indicating better reservoir properties wherever anhydride cement is either absent or subsequently dissolved (as in the Butmah Formation). Mixing zone dolomitisation was encountered in the Mauddud and Wajnah formations, providing moderate to good reservoir properties, whereas burial dolomitisation generally provides poor to moderate reservoir properties in all three studied formations. It was noted that dolomite cementation provides poor reservoir properties in both the Butmah and Mauddud formations. Both the Mauddud and Wajnah formations show poroperm relationships with the degree of scatter dependent on both differences in crystal size and pore connectivity, indicating the degree of formation heterogeneity. Combined, both quantitative and qualitative observations indicate that dolomitisation cannot be considered in isolation, but is co-active and co-dependent upon dissolution, cementation and fracturing. All of these processes can both cause and amplify dolomitisation, but can themselves be caused or amplified by dolomitisation. We have combined all of our observations to propose a new model which relates different reservoir quality outcomes to the occurrence, intensity and history of different diagenetic processes as a first step in the petrophysical quantification of the effect of diagenetic processes on reservoir quality.

    更新日期:2020-01-29
  • Dynamic in-situ imaging of methane hydrate formation and self-preservation in porous media
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-28
    Viktor V. Nikitin; Geser A. Dugarov; Anton A. Duchkov; Mikhail I. Fokin; Arkady N. Drobchik; Pavel D. Shevchenko; Francesco De Carlo; Rajmund Mokso

    We present the results of dynamic in-situ 3D X-ray imaging of methane hydrates microstructure during methane hydrate formation and dissociation in sand samples. Short scanning times and high resolution provided by synchrotron X-rays allowed for better understanding of water movement and different types of gas-hydrate formation. Complementing previous observations, we conclude that the process of gas-hydrate formation is accompanied by the water movements caused by cryogenic water suction that happens in sequences of short fast movements with longer equilibrium states in between (when the water is immobile). Based on the 3D microstructure we identified two types of gas-hydrate formation: (i) into the gas pockets and (ii) inside water volumes. For both mechanisms we do not see problems in gas or water supply to support the gas-hydrate formation. The rate of dissociation in the self-preservation mode (pressure drop at negative temperatures) appears to be different for these two types of gas hydrates. This means that the history of the gas-hydrate formation may influence its behaviour at the dissociation stage (e.g. gas-hydrate production).

    更新日期:2020-01-29
  • ‘Halo-kinematic’ sequence stratigraphic analysis adjacent to salt diapirs in the deepwater contractional province, Liguro-Provencal Basin, western mediterranean
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-28
    V. Mianaekere; J. Adam

    This study investigates the coupled halokinetic and depositional processes of the contractional diapiric-minibasin province in the distal deepwater Liguro-Provencal basin. The Liguro-Provencal basin is situated above young oceanic crust developed during the NW-SE Miocene rifting and back-arc extension. Messinian evaporites succession was deposited contemporaneously on extended continental crust and young oceanic crust. Post-Messinian postrift isostatic adjustments of the continental shelves influenced prograding sedimentary wedges, gravitational failure and gravity-driven deformation above salt, hence extensive salt diapirism is situated in a contractional domain in the deep basin overlying oceanic crust. The kinematic evolution of salt structures in the contractional province is analysed from preserved depositional patterns in successive genetic depocenters adjacent to salt diapirs. Within minibasin successions, four dominant depositional patterns have been classified as a layered, truncated-onlap wedge, onlap-wedge and truncated-hook. The truncated-onlap wedge sequences are early syn-kinematic depositional packages contemporaneous to salt growth and may converge towards the rising salt structure prior to diapirism. They are therefore categorised as the pre-diapiric sequence. The onlap wedge and truncated hook styles form later syn-kinematic packages in diapiric growth phase and are labelled as halo-kinematic sequences. The layered sequence, maybe pre-kinematic and show parallel beds all truncating on the diapir flank. This onlap wedge consists of on-lapping beds at the basal sequence boundary, stratal thinning or convergence towards the diapir flank and angular truncations on an erosional surface on the top depositional boundary proximal to the diapir flank. The truncated-hook sequence reflects an absence of beds on-lapping on the diapir roof within the sequence, beds truncate at diapir flank and on an erosional/slip surface on top depositional boundary proximal to diapir flank. Truncated-onlap wedge sequence reflects on-lapping beds in the middle of the sequence and older and younger beds that truncate on the top depositional boundary.

    更新日期:2020-01-29
  • Soft-sediment deformation structures as indicators of tectono-volcanic activity during evolution of a lacustrine basin: A case study from the Upper Triassic Ordos Basin, China
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-27
    Yi Gao; Zaixing Jiang; James L. Best; Jingong Zhang

    The Ordos Basin, central China, is a large-scale Late Triassic intracratonic lacustrine basin, and in which the Upper Triassic Yanchang Formation comprises interbedded sandstones, mudstones and tuffs, representing deposits of a complete lacustrine life cycle. Based on observation of extensive well cores, various types of soft-sediment deformation structures have been identified in the lacustrine deposits of the southwest basin, including deformed lamination/bedding, load casts and flame structures, pseudonodules, microfaults, autoclastic brecciation, sand dykes, dyke-sill complexes, and sandy and muddy slumps. Liquefaction and fluidization of unconsolidated permeable sediments, together with brittle fracturing of semi-consolidated sediments, are the major deformation mechanisms that generated these structures. Both the scale and prevalence of the deformed sediments, as well as the geological setting of the lacustrine basin, suggest that seismic events (synsedimentary tectonic activity) were the main trigger to initiate deformation processes, although sedimentary overloading could also have been a minor localized trigger. Many volcanic tuffs preserved in both the primary undisturbed deposits and deformed sediments serve as support for synsedimentary tectono-volcanic activities that induced deformation in these lacustrine sediments. The deformation structures usually form vertical clusters of several different types, representing active syndepositional tectonic periods. Stratigraphic correlation shows three such deformation clusters to be widespread in the area detailed herein, and likely recorded three major tectonic periods during deposition. A thick extensive oil shale, with various deformation structures, also reveals another active tectonic period. These four intervals coincide well with the transition of the sedimentary architecture in the area, indicating four phases of most intense tectonic periods and their influence on lacustrine evolution.

    更新日期:2020-01-27
  • Redox effects on relative permeability in Fe-rich clay bearing sandstones
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-25
    Evren Unsal; Hilbert van der Linde; Ove B. Wilson

    The presence of iron (Fe)-rich clay minerals makes the hydrocarbon-bearing sandstone reservoirs prone to Fe oxidation-reduction (redox) reactions. At depths relevant to hydrocarbon recovery applications, oxygen is scarce and Fe2+ is the most common oxidation state of Fe on the grain surface and in the free liquid (reduced state). At surface conditions, where oxygen is abundant, Fe2+ in clays can oxidize to Fe3+ which is referred to as the oxidized state. The change in the oxidation state of structural Fe can alter the surface wettability of the clays as well as the arrangement of the clay layers and the pore morphology. Consequently, the distribution of fluid phases and their flow properties in the pore space may be influenced. In this study, the effects of redox conditions on rock-fluid interactions in sandstone samples with Fe-rich clays were investigated by a series of stead-state coreflooding experiments. Two types of sandstone core materials were tested; each contained different types of Fe-rich clays, such as smectite and chlorite. For corefloods at reduced conditions, a reducing agent (erythorbic acid) was added in the brine. For corefloods at oxidized conditions, the brine that was prepared at room conditions and contained oxygen. Corefloods started with an oxidized-brine/oil co-injection sequence followed by reduced-brine/oil co-injection. Effects of the redox conditions were analyzed by comparing brine and oil relative permeability curves obtained at oxidized and reduced conditions. Both core types were responsive to the changes in the redox conditions; the core samples with chlorites was relatively more sensitive than the samples with other Fe-rich clays, such as smectites. This suggested that the initial condition of the core is important, and measurements taken in oxidized core samples may not be representative of reservoir conditions where the rock is in reduced state. Using a reducing agent should always be considered for coreflooding studies, especially when the Fe-rich clays are present in the core samples.

    更新日期:2020-01-26
  • Influence of paleoenvironmental conditions on distribution and relative abundance of saturated and aromatic hydrocarbons in sediments from the NW part of the Toplica basin, Serbia
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-24
    Nikola Burazer; Aleksandra Šajnović; Nebojša Vasić; Milica Kašanin-Grubin; Dragana Životić; João Graciano Mendonça Filho; Predrag Vulić; Branimir Jovančićević

    The investigation of the relationship between paleoenvironmental conditions and distribution and relative abundance of specific saturated and aromatic hydrocarbons was the main objective of this study, thus marking the parameters, which were most sensitive to environmental changes. Insights on the type, generative potential, and maturity of organic matter (OM), as well as paleoclimate conditions, along with the reconstruction of depositional settings of the northwest part of the Toplica basin (Serbia), were provided. Organic petrographic, palynofacies, organic geochemical, mineralogical, and XRF analyses were carried out to investigate 40 sediment samples of the Prebreza and Čučale sedimentary units. Investigated samples were deposited in the saline and anoxic environment under semi-arid to semi-humid/humid climate conditions, with the constant inflow of volcanoclastic material. The predominance of δ-methyltrimethyltridecil chroman (δ-MTTC) within euxinic portions of the stratified water column was associated with the increase in salinity, which was noticed for the sediments of the Prebreza unit. The sediments from this stratigraphic unit showed a higher contribution of algae precursor, whereas the sediments of the Čučale unit showed higher portions of microbiologically reworked OM. Most of the samples contained oil-prone kerogen type II. Maturity of OM for the sediments of the Prebreza unit ranged from immature to early-mature, while for samples of the Čučale unit varied from early-mature to mature stages. Distribution of hopane biomarkers typical for crude oil indicated that depth of 1 km was a boundary for the genesis of thermodynamic, more stable compounds. A significant portion of semifusinite was correlated with the paleofire event, which affected the distribution of n-alkanes. The high production of hydrocarbons was related to volcanic activity. Parameters, which proved to be highly susceptible at the stratigraphic boundary between the Prebreza and Čučale units were C-value, S/H, α-MTTC, δ-MTTC, β-/γ-MTTC, and (1,3- + 1,6-)/(1,4 + 1,5-DMC), respectively.

    更新日期:2020-01-24
  • A novel hybrid method for gas hydrate filling modes identification via digital rock
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-23
    Huaimin Dong; Jianmeng Sun; Muhammad Arif; Naser Golsanami; Weichao Yan; Yihuai Zhang

    Gas hydrates are one of the most abundant clean energy resources and can effectively solve the energy crisis with a lower carbon footprint. Gas hydrates typically exhibit complex microscopic filling modes, including adhesive mode, cemented mode and scattered mode. Furthermore, the scientific understanding of such filling modes and the associated petrophysical parameters is essential for successful exploitation and development of hydrate formations. However, a precise identification of such filling modes remains a big challenge. Thus, here we proposed a novel hybrid gas hydrate filling mode identification method via digital rock technique. The hybrid method uses a combination of multiple methodologies. Firstly, the dry core samples were scanned by the high-resolution micro-scale 3D X-ray computed tomography (μCT) and the digital rock models were built. Then the resistivity was simulated as a function of gas hydrate saturation with three idealized gas hydrate-filling modes. The differences between the resistivity of three idealized gas hydrate-filling modes were analyzed, and based on the correlations obtained, the idealized gas hydrate saturation calculation models were formulated. We then considered well log data from a real hydrate reservoir in order to evaluate the application of proposed hydrate saturation model and identify the filling mode. Essentially, the gas hydrate saturation results from digital rock models and field well logging data are compared, and the filling modes in the research reservoir were successfully identified.

    更新日期:2020-01-24
  • Control of structural style by large, Paleogene, mass transport deposits in the Mexican ridges and Salina del Bravo, western Gulf of Mexico
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-23
    Jack J. Kenning; Paul Mann

    The Mexican Ridges fold-belt (MR) of the western Gulf of Mexico (GOM) is a 600 km long, 120–220 km wide, deep-water, passive margin, fold-belt trending parallel to the eastern continental margin of Mexico. Shortening within the deep-water MR is related to a primary sub-horizontal, shale detachment linked to an up-dip, 50 km wide zone of listric, normal faults along the continental shelf. A 20,000 line-km grid of 2D depth-converted industry seismic data tied to one, deep well was used to map the frequency, location, lateral extent, and thickness distributions of twelve, individual, mass transport deposits (MTDs) of Paleogene age that together form a <3.5 km thick, stacked zone extending above the basal, shale detachment. Deposition of numerous lobate Paleogene MTDs are related to south-to-north post-Laramide deformation and exhumation of orogenic belts along the coastal area of eastern Mexico. The thickness, fine-grained dominance, and mechanical ductility of the stacked MTDs collectively acted as a control on the structural evolution of the overlying MR and the adjacent salt-rich Salina del Bravo. The <3.5 km thick MTD section and their overlying units in the northern MR control the distribution of the largest amplitude fold structures. Regional folds often possess shorter wavelengths, higher amplitudes, and tighter interlimb angles compared to those formed above the thinner, < 2 km thick MTD section in the southern part of the study area. Other important relationships between MTDs, the regional structure, and hydrocarbon prospectivity of the MR include: 1) MTDs are likely over-pressured, and that over-pressure distribution is a control on fold detachment, 2) stacked, inter-bedded, and over-pressured clay-rich MTDs are inferred to form regional seals for hydrocarbons generated in the underlying Mesozoic section, 3) despite the presence of thicker detachment units in the northern MR, there is potential hydrocarbon prospectivity within folded Miocene reservoirs, where thrust faults could provide possible hydrocarbon migration pathways through the underlying MTDs.

    更新日期:2020-01-23
  • Karst dissolution along fracture corridors in an anticline hinge, Jandaíra Formation, Brazil: Implications for reservoir quality
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-23
    Fabio Luiz Bagni; Francisco H. Bezerra; Fabrizio Balsamo; Rubson P. Maia; Marcello Dall'Aglio

    Folded, fractured, and karstified structures have been recognized in several carbonate reservoirs. However, they are rarely fully identified from seismic or well data and have been poorly described. The present study investigates the geological and structural controls that regional anticlines may exert on solution-enlarged fracture corridors and epigenic karst concentrations in folded-fractured carbonate units. We performed regional mapping based on 106 2D seismic lines, 51 well logs, unmanned aerial vehicle (UAV) imagery of four major outcrop sites (I, II, III and IV) and field investigations to parameterize fracture and karstic features (density, spacing, length, and aperture). The study area is Potiguar Basin, Brazil, where we identified the gentle NE-trending Apodi fold, ∼10 km wide and ∼20 km long. The fold formed along an inverted extensional fault during the latest Miocene-Quaternary stress field, and the envelope of the meandering trend of the Apodi River coincides with the NE-trending fold axis. The fracture pattern in the fold hinge zone consists of two orthogonal, syn-folding fracture sets: a NE-SW-striking hinge-parallel set and an orthogonal NW-SE-striking set. These fractures and superposed karstification are mostly concentrated in the hinge zone along fracture corridors in an area 1.5 km wide and 20 km long. We conclude that fracture corridors in anticline hinges are likely sites to have high fracture densities and wide apertures. These corridors provide reservoir spaces that are favorable for the formation of karst cavities, with significant increases in permeability and porosity along the fracture sets. These features act as important fluid pathways and storage areas where matrix porosity is low. The findings can be used for first-order prediction in karstified carbonate reservoirs.

    更新日期:2020-01-23
  • Tectonic and structural controls on Neogene fluid release in the Patagonian Continental Margin
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-22
    J.I. Isola; J.P. Ormazabal; G. Flores; S. Arismendi; M. Druet; A. Muñoz; J.L. del Río; S.D.A. Etienot; M.P. Gomez Ballesteros; S. Principi; N.D. Bolatti; A.A. Tassone

    Analysis of high-resolution multi-beam bathymetry, 2D multi-channel seismic, and high-resolution seismic sub-bottom profiles revealed the presence of widespread fluid escape features in the middle slope of the Patagonian Continental Margin. On the sea-bottom, these features correspond to pockmarks and mud volcanoes, whereas in the sub-surface they are represented in the seismic records by several acoustic anomalies such as chimneys, acoustic blanking, enhanced reflectors, and reverse-phase enhanced reflectors among others. Some of these acoustic signatures can be traced to syn-rift deposits that fill a deep re-activated and inverted graben. Analysis of pockmarks elongation and pockmarks alignments show a good correlation with inverted normal faults, suggesting that faults and fractures have influenced the pockmark's shape and might have acted as pathways for upward fluid migration. Some of the acoustic anomalies associated with pockmarks are interpreted as evidence of gas. The gas observed in the seismic data seems to be thermogenic, and the seismic data suggests a deep origin associated with over-pressured syn-rift deposits. Two morphometrically different sets of pockmarks were identified: an older set hosted by Miocene aged rocks, and a younger set hosted by Quaternary deposits. Different potential triggers are discussed for the genesis of the Miocene pockmarks in relation to the seismostratigraphy, structural geology and regional tectonics. It is concluded that, tectonic activity associated with the Neogene inversion of the graben faults, due to Andean compression, is the most likely cause for the formation these pockmarks. The presence of gas charged sediment and young pockmarks also suggest that after the Middle Miocene tectonic climax, a more recent pulse of release of fluids occurred.

    更新日期:2020-01-23
  • Regional variation of permeability in Thamama-B reservoirs of Abu Dhabi
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-22
    S.N. Ehrenberg; J. Zhang; J.S. Gomes

    To better understand reservoir quality in a major oil-producing limestone unit, the Thamama-B zone of the Lower Cretaceous (Barremian) Kharaib Formation, this study compiles and analyzes average values of porosity and permeability measured on 330 well cores in both onshore and offshore areas of the Abu Dhabi Emirate. Most average and individual values plot along a single porosity-permeability trend, which is unaffected by porosity-preserving early oil emplacement and only moderately influenced by texture and clay content. The main trend corresponds with the established field for mud-dominated, microporous limestones, but a subordinate proportion, termed “hi-K” plug samples, have significantly higher permeability-for-given-porosity. The hi-K samples are more abundant in the southwestern part of the study area and also show spatial clustering within individual oilfields. The present macro-scale view of Thamama-B reservoir quality is a new resource that can be useful for both exploration and production planning.

    更新日期:2020-01-22
  • Postaccumulation sandstone porosity evolution by mechanical compaction and the effect on gas saturation: Case study of the Lower Shihezi Formation in the Bayan'aobao area, Ordos Basin, China
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-22
    Lu Xia; Zhen Liu; Yingchang Cao; Wei Zhang; Junbang Liu; Chunlan Yu; Yingjie Hou

    Porosity evolution and reservoir quality are strongly affected by mechanical compaction. However, the compaction-induced porosity reduction process in sandstone reservoirs during the postaccumulation period and its effect on gas saturation remain poorly understood. By studying the sandstones in the Bayan'aobao area, we were able to quantify the porosity evolution and gas saturation variations caused by mechanical compaction during the postaccumulation period through fluid inclusion analysis and mathematical modeling. Moreover, we verified our findings by compaction experiments and geological statistics. Fluid inclusion microthermometry was employed to determine the hydrocarbon accumulation periods, and the highest homogenization temperature of the two-phase aqueous fluid inclusions coexisting with natural gas fluid inclusions, i.e., ∼115 °C (with a corresponding geological age of ∼140 Ma), was regarded as the beginning of the postaccumulation stages. With these data, we constructed an inversion evolution model (based on the geologic statistical model of sandstone mechanical compaction) for the postaccumulation period and determined the gas saturation changes (based on compaction porosity evolution). The modeling results indicate that the sandstone porosity decreased greatly during postaccumulation subsidence and increased slightly during late-stage uplift, while the gas saturation increased with both the porosity decrease during the postaccumulation subsidence and the porosity increases during the later uplift. The modeling results of subsidence are concordant with the compaction experiments, which also show that the larger the initial porosity, the greater the porosity reduction. The uplift modeling results demonstrate that an erosion-induced decrease in burial depth allowed the sandstone porosity to increase, which is supported by some scholars’ findings (e.g., Neuzil and Pollock, 1983; Jiang et al., 2004; Zhang, 2013) and by the positive correlation between sandstone porosities and restored geologic unit thicknesses in the study area. However, the inversion modeling indicates that the reason for the porosity rebound in litharenites is that the increase in elastic porosity slightly exceeds the decrease in viscoplastic porosity, which would have been different if the uplift-induced erosion rate had been distinctly lower. Gas production test data and restored geologic unit thicknesses indicate that uplift and erosion may be able to enrich sandstone gas reservoirs to a certain extent, consistent with the calculated gas saturation variation results, but a full understanding of gas saturation evolution awaits further studies. The inversion model can provide a powerful tool for quantitative analysis of the sandstone compaction process during the postaccumulation period, which could be used for reference in other similar regions.

    更新日期:2020-01-22
  • Structural geological interpretations from unrolled images of drill cores
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-21
    Megha Chakraborty; Soumyajit Mukherjee

    Structures hidden at depth can be assessed through drill cores recovered from them. A crucial part of petroleum geology study is to interpret structures from these cores collected from vertical and inclined drilling operations reaching different depths. We deduce analytically ideal structures in unrolled/unwrapped images of cylindrical drill cores. These structures are: (i) orthogonal and conjugate fractures, (ii) single generation periodic folds and type-1 and type-2 superposed folds, (iii) listric faults and (iv) different types of angular unconformities. The sinuous curves as found in unrolled images are not always characteristic of the structures. Therefore accurate identification of structures would require additional information, such as the drill core itself.

    更新日期:2020-01-22
  • Orogen-transverse structures in the eastern Himalaya: Dextral Reidal shear along the Main Boundary Thrust in the Garu-Gensi area (Arunachal Pradesh, India), implication in hydrocarbon geoscience
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-18
    T.K. Goswami; B.N. Mahanta; S. Mukherjee; B.R. Syngai; R.K. Sarmah

    Structural mapping and analyses reveal deformation in the rocks of the Lesser Himalayan Miri and Gondwana, and Lower Siwalik Dafla successions across the Miri Thrust (MT) and the Main Boundary Thrust (MBT) in the Garu-Gensi area (Arunachal Himalaya, India). The-north-dipping Gondwana sequence is sandwiched between the Miri Group of rocks in the north and the Lower Siwalik Dafla sandstones in the south. The Gondwana rocks folded four times (D1-D4). Detail outcrop mapping and structural analyses substantiated by satellite images elucidate that the Miri, the Gondwana and the Lower Siwalik rock sequences in the area are traversed by two sets of NW and NE trending cross-fault zones with∼70–100 m thickness mappable for ∼70 km. The cross-faults trend NW around the locality Daring, while in the western part around Gensi, they trend NE. The westernmost fault slipped dextrally and the easternmost one at Daring sinistrally. These transverse faults offset the MBT: 0.52–0.59 km for the western- and 0.58–1.41 km for the eastern faults. The deformation in the Gondwana sequence may be constrained as syn-Himalayan (i.e., India-Asia collisional event). This was followed by a compression from E/ESE due to the post-Eocene Indo-Burmese collision. The culminating phase of combined India-Asia and Indo-Burmese compression is manifested in field with a strong NE-directed compression with dextral Reidal shear developing in the transverse conjugate fault zones in the area with maximum displacement towards SE. Cluster of the earthquake data indicates record of the events (M ≥ 6) within 30–80 km depth-both N and S of the MBT in the study area. Further, the southerly continuation of the transverse faults below the alluvium can provide vital clue in studying the potential hydrocarbon trap in the northern fringe of the Brahmaputra river.

    更新日期:2020-01-21
  • Composite rock types as part of a workflow for the integration of mm-to cm-scale lithological heterogeneity in static reservoir models
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-18
    Achyut Mishra; Kuncho D. Kurtev; Ralf R. Haese

    Lithological heterogeneity in the form of sedimentary structures such as cross and planar and as interfaces between massive beddings is known to impact fluid flow and fluid-rock reactions in the subsurface. Such heterogeneity exists at mm-to cm-scale and is typically not accounted for in conventional geological models due to technical restrictions posed by the wireline log resolution and the inefficiency of reservoir modelling softwares to handle such fine scale information. Hence, multiphase flow and reactive transport simulations based on conventional models fail to appropriately account for the impact of sedimentary heterogeneity. We present a new workflow which allows building reservoir models where such fine-scale lithological heterogeneity can be integrated, thereby overcoming the limitations associated with log resolution and modelling softwares. First, the rock types are coupled as composites in a systematic manner at a discretization level that can be handled by the modelling softwares. The mm-scale rock properties quantified from core samples are accounted for in the composite rock type characterization. Lithotype logs of the composites are derived by modifying the color-coded depth record based on a combination of different wireline logs. The lithotype logs form the basis for property population in the reservoir models. The coupling of core and wireline data allows to account for the mm-to cm-scale heterogeneity in coarsely discretised reservoir models. The application of the new workflow is demonstrated for the development of a 2-D reservoir model at the CO2CRC's Otway Research site with a vertical resolution of 5 cm while incorporating sedimentary heterogeneity at 5 mm scale. The composite rock type method would allow for the impact of fine scale heterogeneity to be directly accounted for in static reservoir models.

    更新日期:2020-01-21
  • Regional porosity variation in Thamama-B reservoirs of Abu Dhabi
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-16
    S.N. Ehrenberg; J. Zhang; J.S. Gomes

    We present a large dataset of average porosity, depth, thickness and well locations for one of the Earth's major oil-bearing limestone reservoirs and discuss possible controlling factors. Petrologic data are not included in this study, but regional petrology data from earlier studies and detailed results from one representative oilfield provide a useful basis for interpretation of the present compilation. Porosity decreases with depth similar to porosity-depth trends from other limestone strata, but the main data cluster, representing the largest fields in the depth range 7000–9500 ft (2134–2896 m), follows a much steeper trend approximately parallel with the crest-flank trends of individual fields where crestal porosity has apparently been preserved by early arrival of oil. Maps of depth, porosity, thickness, and departure of porosity from the overall trend illustrate the geographic variation of these parameters. Solidity (thickness with porosity removed) is constant from crest to flanks in the largest fields, indicating that crest-flank porosity differentiation took place by closed-system compaction and cementation. The principal value created by this work is the framework established for focusing continuing studies that can further constrain the controlling processes and thus develop predictive tools for this particular reservoir and similar micropore-dominated limestones worldwide.

    更新日期:2020-01-17
  • The impact of grain-coating chlorite on the effective porosity of sandstones
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-16
    Haoran Xia; Elizabeth Hernandez Perez; Thomas Dunn

    Chlorite grain coatings in sandstones can inhibit quartz overgrowths and preserve porosity in hot reservoirs, but coatings may also introduce high irreducible water saturation and therefore reduce effective porosity. In order to derive a quantitative model to predict microporosity and effective porosity, five sets of sandstone samples with well-developed chlorite grain coatings were studied. Scanning electron microscopy and X-ray diffraction results on each set indicate various types of chlorite morphology and a broad range of chlorite abundance, though the ratio of the average size to the average thickness of chlorite platelets is constant in all samples. Microporosity, defined in this study as the product of total porosity and irreducible water saturation, is correlated to chlorite by volume content and chemical composition: ϕmicro = 7.08 × Vchl/(0.39 × Vchl + 2.26) + 0.927 for iron-rich chlorite, and ϕmicro = 7.08 × Vchl/(1.88 × Vchl + 7.83) + 0.927 for magnesium-rich chlorite, where ϕmicro is microporosity in porosity units (p.u.) and Vchl is volume percentage (%) of chlorite in the bulk sample. When the total porosity for a sample is known, its effective porosity is the difference between the total porosity and the microporosity.

    更新日期:2020-01-16
  • Organofacies composition of the upper Jurassic – Lowermost Cretaceous source rocks, Danish central graben, and insight into the correlation to oils in the Valdemar field
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-15
    L. Ponsaing; A. Mathiesen; H.I. Petersen; J.A. Bojesen-Koefoed; N.H. Schovsbo; H.P. Nytoft; L. Stemmerik

    The organic-rich marine shales of the Upper Jurassic - lowermost Cretaceous Farsund Formation are the main hydrocarbon source rocks for the Cretaceous chalk reservoir of the Valdemar Field (Danish North Sea). Geochemical analyses of oil extracts from core samples across the greater Valdemar Field show substantial heterogeneity of the reservoir oils, suggesting contribution from kitchen areas with source rocks of variable organofacies and thermal maturity. In order to better understand the source rock facies that charged the field, a refined organofacies characterization of the Upper Jurassic - lowermost Cretaceous marine shale source rock succession (Farsund Formation) in the Danish Central Graben was established. This organofacies characterization is based on integration of petrographic and geochemical data and demonstrates large variations in both the regional and local distribution of the organofacies at the sequence level in the shales within the Danish Central Graben. In addition, the organofacies characterization shows that the respective sequences in the succession may be represented by several organofacies at the same time. Part of the explanation to the heterogeneity in the oil types accumulated within the Valdemar Field may lie in the presence of different organofacies within the uppermost sequences (Vol-3 to Ryaz-1) in the three kitchen areas. Each organofacies presumable generates different types of oils and the variation in the relative proportions of the different organofacies likely determines the composition of the generated oils. One dimensional (1-D) maturity modelling at three pseudo-well locations, each representing a potential Farsund Formation kitchen, shows that all kitchens are in the main oil window and that mainly oil was expelled from the Volg-3 to Ryaz-1 sequences. Modelling indicates that (i) average values of TOC, HI, thickness and the kinetic model are the main controls on generation and expulsion of hydrocarbons; and (ii) by increasing these parameters, expulsion could also occur from the lower part of the succession.

    更新日期:2020-01-15
  • 更新日期:2020-01-15
  • Cenozoic structural history of the Gippsland Basin: Early Oligocene onset for compressional tectonics in SE Australia
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-15
    Elizabeth M. Mahon; Malcolm W. Wallace

    The Gippsland Basin contains some of the largest hydrocarbon accumulations in Australia, and has been in production since the 1920's. These hydrocarbons are trapped by large growth anticlines offshore, in reservoirs of the Cretaceous to Eocene-aged Latrobe Group. Despite the obvious importance of these growth anticlines, the timing of their formation, and the overall Cenozoic tectonic history of the basin is not well understood. Here, we present a detailed growth strata analysis of the faults and anticlines within the Cenozoic sediments of the Gippsland Basin. This indicates two major phases of tectonism in the basin: 1. Late Cretaceous to Eocene extension, and 2. Oligocene to Holocene compression. Detailed analysis of the extensional phase indicates the development of numerous normal growth faults, which display an overall reduction in the magnitude of extension from the Late Cretaceous to the Eocene, commonly terminating at the top of the Latrobe Group. The shift to compressional tectonism occurred at approximately the Eocene-Oligocene boundary (∼34 Ma). A major and widespread episode of compression then occurred, with evidence of growth on anticlines and reverse faults beginning in the early Oligocene. This previously unrecognized early Oligocene event produced significant growth (20–50% total growth) of the major anticlines which host hydrocarbon accumulations. The early Oligocene event represents the first phase of the compressional tectonic regime that continues to the present day in SE Australia. It appears likely that this Oligocene event affected other basins in SE Australia and probably contributed to uplift of the Eastern Highlands. The underlying tectonic cause for this Oligocene compressional regime is enigmatic and may be related to far field tectonic processes. A second pulse of compressional tectonism and anticline growth occurred during the mid-Miocene and is generally more significant in the onshore regions of the basin. The youngest phase of compressional tectonism beginning in the Late Miocene (∼10 Ma) is also more intense in onshore regions and is marked by an unconformity that is widespread in SE Australia.

    更新日期:2020-01-15
  • Sequence stratigraphy of deep-water systems
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-15
    Octavian Catuneanu

    Stratigraphic cyclicity in the deep-water setting reflects the interplay of accommodation and sedimentation on the shelf, which controls shoreline trajectories, sediment supply to the shelf edge, and the timing of all elements of the sequence stratigraphic framework. Stratigraphic trends defined by changes in the types, volume, and composition of gravity flows during the shoreline transit cycles on the shelf provide the diagnostic criteria for the identification of deep-water systems tracts and bounding surfaces. Non-diagnostic variability in the sedimentological makeup of systems tracts reflects the unique tectonic and depositional settings of each sedimentary basin, and needs to be rationalized on a case-by-case basis. Contour currents may further modify the sedimentological makeup of deep-water sequences, but do not provide diagnostic elements for the definition of systems tracts and bounding surfaces. The application of sequence stratigraphy to the deep-water setting relies on the construction of composite profiles that illustrate the relative chronology of the different types of gravity-driven processes at regional scales. The cyclicity relevant to the definition of sequences is described by the composite rather than local profiles. The place of accumulation of depositional elements depends on the location of sediment entry points along the shelf edge, the types of gravity-driven processes, and the seafloor morphology. The allocyclic and/or autocyclic lateral shifts of deep-water depositional elements further enhance the offset between local trends and the regional composite profile in terms of timing and frequency of cycles, timing of coarsening- and fining-upward trends, and timing of the coarsest sediment. The sedimentological cycles defined by local trends must not be confused with the stratigraphic cycles defined by regional composite profiles.

    更新日期:2020-01-15
  • Origin of sulfate-rich fluids in the early Triassic Montney Formation, Western Canadian Sedimentary Basin
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-11
    Mastaneh H. Liseroudi; Omid H. Ardakani; Hamed Sanei; Per K. Pedersen; Richard A. Stern; James M. Wood

    This study investigates diagenetic and geochemical processes that control regional distribution and formation of sulfate minerals (i.e., anhydrite and barite) in the Early Triassic Montney Formation in the Western Canadian Sedimentary Basin. The generation of H2S in hydrocarbon reservoirs is often associated with the dissolution of sulfate minerals, as a major source of sulfate required for sulfate-reducing reactions. The formation of pervasive late diagenetic anhydrite and barite in the high H2S zone of the Montney Formation is therefore contrary to the normal paragenetic sequence of sour gas reservoirs. Petrographic observations revealed early and late anhydrite and barite cement. The early fine-crystalline anhydrite cement is dominant in northeastern British Columbia (low H2S zone), while the late-stage coarse-crystalline cement and fracture/vug-filling anhydrite are dominant in Alberta (high H2S zone). The bulk isotopic values (δ34S: +2.9 to +24.7‰ V-CDT, δ18O: −11.2 to +15.7‰ V-SMOW) suggest that sulfate-rich fluids originated mainly from modified Triassic connate water was the origin of early anhydrite. In contrast, the SIMS isotopic values of late anhydrite (δ34S: +18.5 to +37‰ V-CDT, δ18O: +12 to +22‰ V-SMOW) and barite cement (δ34S: +23.3 to +39‰ V-CDT, δ18O: +13.2 to +18.7‰ V-SMOW) as well as fracture/vug-filling anhydrite (δ34S: +23.5 to +24.7‰ V-CDT, δ18O: +13.3 to +14.7‰ V-SMOW) from Alberta represents a mixed isotopic signature of Triassic connate water and contribution of dissolved sulfate-rich fluids derived from dissolution of Devonian evaporites. The 87Sr/86Sr isotope ratios of the fracture/vug-filling anhydrite (0.7092–0.7102) are highly radiogenic suggesting extensive water/rock interactions between sulfate-rich fluids and siliciclastic and basement rocks. The similar isotopic composition of the late anhydrite/barite and fracture/vug-filling anhydrite in western Alberta with Devonian evaporites isotopic signature, and the highly radiogenic 87Sr/86Sr ratio further supports sulfate-bearing fluids were mainly originated from underlying Devonian evaporites and migrated upwards through deep-seated faults/fractures to the Montney Formation.

    更新日期:2020-01-13
  • Origin, composition and relative timing of seaward dipping reflectors on the Pelotas rifted margin
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-11
    Caroline Harkin; Nick Kusznir; Alan Roberts; Gianreto Manatschal; Brian Horn

    The mechanism by which seaward dipping reflectors (SDRs) are formed is a topic of debate. Two contrasting models exist for their formation, the volcanic-faulting model and the volcanic-loading model. Each of these models has important implications for the processes which control the structure and formation of magma-rich rifted continental margins. We have examined high-quality deep-seismic reflection data across the Pelotas Basin, offshore Brazil. These data reveal a remarkable set of SDRs, for which we have investigated the likely nature of their formation. The total package of SDRs has an across-strike width of ∼200 km and a variable vertical thickness of ∼10–17 km, previously interpreted as volcanic flows. Detailed observations, however, show changes in seismic character and geometry within the SDR package, which suggest a complex and varied evolution. We have used gravity anomaly inversion and seismic observations together to investigate the likely composition of the SDRs by determining the proportion of basaltic material to sedimentary/volcaniclastic material (basalt fraction) within the SDRs. This has been achieved by minimising the difference between the depth of the gravity Moho and seismic Moho in order to quantify the lateral variation in basalt fraction, taken to be proportional to the bulk density of the package. The density of the SDR package together with seismic interpretation is then used to infer the composition, depositional environment, source and time of formation relative to breakup. Our analysis suggests that the overall SDR basalt fraction and bulk density decrease oceanwards, possibly due a change in the type of volcanic deposits from predominantly subaerial to volcaniclastics, possibly deposited subaqueously. The SDRs can be split into three sub-packages. The two inner SDR packages are interpreted to consist of lava flows sourced from syn-tectonic, subaerial eruptions, associated with the onshore Paraná Large Igneous Province, flowing eastwards into an extensional basin. The outer SDR package shows reflector geometries that progressively offlap oceanwards, interpreted as extrusives sourced from an eastwards-migrating, newly formed ocean ridge. Our analysis suggests that both the volcanic-faulting and volcanic-loading models for SDR formation are applicable to the Pelotas rifted margin, recording distinct syn-rift and syn-breakup magmatic events. We show that both SDR formation models can be recognised in a naturally occurring example and can coexist on the same margin.

    更新日期:2020-01-13
  • A new approach to discern the hydrocarbon sources (oil vs methane) of authigenic carbonates forming at marine seeps
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-11
    Yuedong Sun; Shanggui Gong; Niu Li; Jörn Peckmann; Meng Jin; Harry H. Roberts; Duofu Chen; Dong Feng

    Numerous marine hydrocarbon seeps have been discovered in the past three decades, the majority of which are dominated by methane-rich fluids. However, an increasing number of modern oil seeps and a few ancient oil-seep deposits have been recognized in recent years. Oil seepage exerts significant control on the composition of the seep-dwelling fauna and may have impacted the marine carbon cycle through geological time to a greater extent than previously recognized. Yet, distinguishing oil-seep from methane-seep deposits is difficult in cases where δ13Ccarb values are higher than approximately −30‰ due to mixing of different carbon sources. Here, we present a comparative study of authigenic carbonates from oil-dominated (site GC232) and methane-dominated (site GC852) seep environments of the northern Gulf of Mexico, aiming to determine the geochemical characteristics of the two types of seep carbonates. We analyzed (1) major and trace element compositions of carbonates, (2) total organic carbon (TOC), total nitrogen (TN) and carbon isotope (δ13CTOC) of residue after decalcification, (3) sulfur isotope signatures of chromium reducible sulfur (CRS, δ34SCRS) and residue after CRS extraction (δ34STOS), as well as (4) sulfur contents (TOS) of residue after CRS extraction. Carbonates from the studied oil seep are dominated by aragonite and exhibit lower δ34SCRS values, suggesting carbonate precipitation close to the sediment surface. In addition, oil-seep carbonates are characterized by higher TOC and TOS contents and higher TOC/TN ratios, as well as less negative δ13CTOC values compared to methane-seep carbonates, probably reflecting a contribution of residual crude oil enclosed in oil-seep carbonates. Very low δ13CTOC values (as low as −68.7‰, VPDB) and low TOC/TN ratios of methane-seep carbonates indicate that the enclosed organic matter is derived mainly from the biomass of methanotrophic biota. This study presents new geochemical data that will allow the discrimination of oil-seep from methane-seep deposits. Although some of the geochemical patterns are likely to be affected by late diagenesis, if applied with caution, such patterns can be used to discern the two end-member types of seepage – oil seeps and methane seeps – in the geological record.

    更新日期:2020-01-13
  • Shallow gas in the Holocene mud wedge along the inner East China Sea shelf
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-10
    Yufeng Chen; Bing Deng; Jing Zhang

    High-resolution seismic surveys were carried out in the inner shelf of the East China Sea to investigate the distribution of shallow gas in Holocene sediments. Shallow gas is distributed extensively in the Holocene strata of the shelf which is dominated by the Yangtze River. Organic-rich, fine-grain sediments deposited throughout the Holocene are essential for gas generation and accumulation. The thickness of the Holocene sediment is the main controlling factor determining the distribution of shallow gas. Gas seepages were found mostly at a water depth up to 20 m, where sediment erosion mainly occurs. Coincidence between shallow gas seepage and high methane concentration in the water column suggests that gas emission from these sediments is a potentially significant source of greenhouse gas. The interaction between shallow gas migration and coastal erosion would therefore potentially lead to increased greenhouse gas emission and accelerated sediment erosion. Such results can be applied to other costal locations around the world.

    更新日期:2020-01-11
  • The Baiyun Slide Complex, South China Sea: A modern example of slope instability controlling submarine-channel incision on continental slopes
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-09
    Wei Li; Tiago M. Alves; Michele Rebesco; Jie Sun; Jian Li; Shuang Li; Shiguo Wu

    The Baiyun Slide Complex is one of the largest submarine landslides on the northern margin of the South China Sea. Newly acquired high-resolution bathymetric data, 2D and 3D seismic data permitted the systematic investigation of the Baiyun Slide Complex in terms of its seafloor morphology and associated sedimentary processes. The headwall region of the Baiyun Slide Complex, located at a water depth between 1000 m and 1700 m, is U-shaped and opens towards the east. It was efficiently and almost completely evacuated, generating pronounced headwall and sidewall scarps. Submarine channels, sediment waves, migrating channels, sediment drifts and moats are observed within and around the headwall region, illustrating the effects of both downslope and along-slope sedimentary processes. Submarine channels are 16–37 km-long 800-1500 m-wide, and 20 to 50 m-deep. As a modern example of the interplay between slope instability and subsequent incision, submarine channels were generated after the formation of the Baiyun Slide scar to suggest intensified downslope sedimentary processes after the slope collapsed. The initiation and formation of these submarine channels result from the evacuation of the Baiyun Slide scar, which provided the necessary space of the continental slope to accommodate subsequent turbidity and mass wasting flows. Our results are an important example of how submarine landslides can influence erosional and depositional processes on continental margins.

    更新日期:2020-01-09
  • Catalytic hydropyrolysis of asphaltenes in marine oil from the Tarim Basin, NW China: Implications to complicated oil charging histories in an old composite basin
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-09
    Jian Chen; Wanglu Jia; Zhongyao Xiao; Ping'an Peng

    Oil asphaltenes have been used to characterize the source(s) and timing of expulsion of early charged oils. However, the application on asphaltene in a complex petroleum basin have been limited. Marine oils from the Tarim Basin, NW China, were selected to release asphaltene-bound molecules by the catalytic hydropyrolysis technique (Hypy). The analyzed samples were 10 heavy oils from two major oil-production regions (the Tabei and Tazhong uplifts) in the basin. The asphaltenes may be mixed products from multiple hydrocarbon charging stages, if we consider the main oil filling events and the regional geothermal histories of corresponding reservoirs. However, the similar molecular and isotopic characteristics of the hydropyrolyzates indicate contributions from one major source rock (possibly lower Cambrian mudstone) during relatively early periods, based on the presently known source characteristics. The preservation of asphaltenes probably reflects a low reservoir temperature after charging. The asphaltenes were derived from marine algae deposited in a strongly reducing environment, given the biomarker data (e.g., relatively abundant aryl isoprenoids and low Pr/Ph) and carbon isotopic data (−35.5‰ to −34.4‰). Asphaltenes were expelled around the oil generation peak. Moreover, molecular ratios indicative of maturity (e.g., tricyclic terpanes/hopanes), derived by Hypy of asphaltenes, suggest that the asphaltenes of the Tazhong Uplift are generally more mature than those of the Tabei Uplift. Moreover, this is mainly related to different maturities when the oil was expelled. The free hydrocarbons in the Tazhong Uplift show greater molecular and isotopic variations than those in the Tabei Uplift. This might be attributed to more complex accumulation processes in the former. Late oils with a range of maturities, mixing proportions with early biodegraded oils, and degrees of alteration by thermochemical sulfate reduction could all have played a role in generating this variability. The results indicate that the hydrocarbons released from oil asphaltenes by Hypy can provide insights into multiple charging processes in an old composite petroleum basin.

    更新日期:2020-01-09
  • How to deal with missing overburden - Investigating exhumation of the fragment of the Mid-Polish Anticlinorium by a multi-proxy approach
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-09
    Katarzyna Łuszczak; Michał Wyglądała; Michał Śmigielski; Marta Waliczek; Bronisław A. Matyja; Andrzej Konon; Mirosław Ludwiniak

    Tectonic inversion and erosion conceal basin geological history. As simple stratigraphic reconstructions can be inaccurate, thermal modelling techniques are often applied. In this paper, we show the advantages of a multi-proxy approach for exhumation studies, which explores full range of all possible solutions. The part of the Mesozoic Central European Basin that was inverted into the Mid-Polish Anticlinorium in the Late Cretaceous is chosen as a case study. The analysis consists of three steps: First, a wide range of possible geological scenarios is constructed, accounting for two alternatives of the overburden sequence, southern and northern, that result from the impact of the Holy Cross Fault on Mesozoic deposition. The second stage comprises apatite fission track and vitrinite reflectance analyses carried out on a ∼1500 m long vertical profile, inverse modelling of these data to obtain thermal histories, and 1D thermal modelling of burial-erosion using the PetroMod software. In the third step, results of modelling exercises are combined with probabilities of the overburden variants to define an ensemble of the most likely geological scenarios. By applying this workflow we conclude that exhumation of the Mid-Polish Anticlinorium began in the latest Turonian–early Campanian and that ∼1.7–2.3 km of uppermost Triassic–Cretaceous rocks were removed. The heat flow was similar or slightly higher than the present-day value and during the Jurassic the study area was located on the northern side of the Holy Cross Fault, where deposition was faster. We also investigate the impact of elevated Late Palaeozoic heat flow on our samples and find that it was overprinted by high Late Cretaceous temperatures. All this is achieved by presenting the results as an ensemble of the most likely solutions within the wide modelling space.

    更新日期:2020-01-09
  • Origin of chert nodules in the Ediacaran Doushantuo Formation black shales from Yangtze Block, South China
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-07
    Ping Gao; Zhiliang He; Gary G. Lash; Shuangjian Li; Rongqiang Zhang

    Chert nodules are widely developed in Ediacaran deposits of South China, yet the mechanism responsible for their formation remains disputed. Petrological and geochemical studies of chert nodules and host shale of the Ediacaran Doushantuo Formation of the Yangtze Block, South China, were carried out to more fully assess silicification of these deposits. Doushantuo chert nodules display a concentrically layered internal structure dominated by quartz and lesser concentrations of calcite, carbonate fluorapatite, organic matter, pyrite, and sphalerite. Individual layers become thinner from nodule centers to edges. Our results suggest that (1) Doushantuo host black shale accumulated under persistent anoxic bottom water conditions and (2) chert nodules formed as a consequence of the anaerobic oxidation of methane focused along sulfate-methane transition zones at shallow burial depth. Nodules likely originated from porous proto-nodules that consisted of 13C-depleted authigenic calcite produced by the anaerobic oxidation of methane. Decaying of organic matter within nodules helped to create porosity that was filled by authigenic calcite and also released phosphate and zinc ions to pore water, thus favoring precipitation of carbonate fluorapatite and sphalerite. Silica supplied principally from the diagenesis of clay minerals of the host shale cemented the nodules and replaced early formed calcite. The progressive infilling of residual porosity by 18O-depleted pore fluids associated with increasing burial depth is reflected in the strongly negative δ18Ocarb values documented from the nodules. Thus, the formation of Doushantuo chert nodules reflects a protracted diagenetic history initiated by bacterial sulfate reduction and anaerobic oxidation of methane close to and within the sulfate-methane transition zone and continued in association with the diagenetic release of silica from host shale as well hydrocarbon generation with increasing burial depth.

    更新日期:2020-01-07
  • Facies associations, detrital clay grain coats and mineralogical characterization of the Gironde estuary tidal bars. A modern analogue for deeply buried estuarine sandstone reservoirs
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-07
    Maxime Virolle; Hugues Féniès; Benjamin Brigaud; Raphaël Bourillot; Eric Portier; Patricia Patrier; Daniel Beaufort; Isabel Jalon-Rojas; Hervé Derriennic; Serge Miska

    Estuarine tidal bar sandstones are complex reservoir geobodies commonly exploited by the oil and gas industry. In order to better predict the reservoir potential of these geobodies, this study provides a modern-day reservoir analogue, describing tidal bars in the inner and outer Gironde estuary from the microscopic to the macroscopic scale. The originality of this work lies in the multi-scale study of modern estuarine tidal bars based on numerous piston cores extracted in a high-energy environment. This work demonstrates that these tidal bars are composite sedimentary bodies made up of individual reservoir sand units separated by thick muddy layers. Their vertical facies associations and internal architectures are controlled by local hydrodynamic variations and seasonal river influxes. Detrital clay grain coats are notably characterized using a portable and handheld mineral spectrometer from the base to the top of the tidal bars. X-ray diffraction and electron microscopes reveal that these coats are mainly composed of di-octahedral smectite, illite, chlorite and kaolinite associated with other components such as diatoms or pyrite. The best reservoir geobodies are those with the minimum clay permeability barriers at the macro and mesoscale. An optimum coated grain content and clay fraction volume is also needed for generating authigenic clay coatings and inhibiting quartz overgrowth. These conditions are met within the tidal sand bars of the outer estuary funnel that are expected to be the best reservoir geobodies in deeply buried sandstones.

    更新日期:2020-01-07
  • The pore structural evolution of the Marcellus and Mahantango shales, Appalachian Basin
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-07
    Liaosha Song; Timothy R. Carr

    The generation and primary migration of hydrocarbons in organic-rich shale leaves void space in organic matter, which is the porosity associated with organic matter commonly observed under scanning electron microscope (SEM). In this study, Middle Devonian black shale core samples were collected from three wells penetrating the organic-rich Marcellus Shale and the organic-lean Mahantango Formation in Pennsylvania and West Virginia. Pyrolysis, ion milled SEM and low-pressure nitrogen adsorption analysis were conducted to investigate the organic richness and the properties of the pore system. Vitrinite reflectance (Ro) values in the range of 1.36%–2.89% represent a maturity spectrum covering the wet-gas to post-mature zones. In general, the pore system is composed of organic matter-hosted pores and mineral-hosted pores. However, the dominant pore types and pore sizes vary stratigraphically across lithology and abundance of organic matter. All the organic matter observed in this study shows an amorphous occurrence. Pore space between mineral grains (both silt-size and clay-size) can be filled by organic matter, which contains secondary porosity generated by thermal cracking of kerogen. Mineral-hosted pores are concentrated in organic-lean samples in which secondary organic matter could not fill most of the primary pore space. The destruction of primary mineral-hosted pores and the generation of secondary organic matter-hosted pores were observed. TOC values show positive correlations with the porosity, specific surface area, and the abundance of micropores. Increasing thermal maturity correlates with a significant decrease of pore volume and surface area, primarily through diminishing or vanishing of micropores. The richness and thermal maturity of organic matter in organic-rich Devonian shale can be effective parameters for evaluation of reservoir quality and upscaling the appraisal.

    更新日期:2020-01-07
  • Insights in the pore structure, fluid mobility and oiliness in oil shales of Paleogene Funing Formation in Subei Basin, China
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2020-01-07
    Xiaoping Liu; Jin Lai; Xuechun Fan; Honglin Shu; Gaocheng Wang; Xiaoqiang Ma; Mengcai Liu; Ming Guan; Yufeng Luo

    Pore structure is an important factor influencing reservoir properties of oil shales. Routine core analysis, thin section, scanning electron microscope (SEM), mercury injection capillary pressure (MICP) tests, Nuclear Magnetic Resonance (NMR), and computed tomography (CT) were used to provide insights into the pore throat distribution in oil shales of member 2 of Paleogene Funing Formation (E1f2) in Subei Basin, China, with the special aim to unravel the effect of pore structure on fluid mobility and oiliness. The relationships between NMR parameters, petrophysical property and capillary parameters are investigated. The results show that rock composition in the oil shales consist of quartz, feldspar, carbonate particles, clay minerals and organic matters. Pore systems consist of large-scale interparticle pores, intragranular dissolution pores, and small-scale micropores within clay minerals and the organic matter pores. Four types of pore structure (Type Ⅰ, Type Ⅱ, Type Ⅲ and Type Ⅳ) are divided according to the patterns of capillary curves, capillary parameters, NMR T2 (transverse relaxation time) spectrum and BVI (bulk volume of immovable fluid) values. From Type Ⅰ to Type Ⅳ pore structure, the maximum mercury saturation (SHgmax) and mercury extrusion efficiency are decreasing, and the SHgmax are less than 50% averagely, indicating the oil shales are characterized by very poor pore connectivity. The T2 spectrum changes from bi-modal behavior to uni-modal and the right peaks become lower or even disappearing from Type Ⅰ to Type Ⅳ pore structure. Fluid mobility is not primarily controlled by pore size, but dependent on the content of short T2 components (<1 ms). The micro-fractures as well as particles (mainly dolomites) filling them emit strong blue fluorescence. The edges of particles emit strong blue fluorescences indicate that the interparticle pores are oil bearing. The carbonate particles are oil-wet, and intragranular dissolution pores within carbonate particles are fluorescent. Authigenic clay minerals abundant in intercrystalline pores, and organic matters abundant in organic pores, emit scattered strong blue fluorescence. Pore structure controlled fluid mobility and determined the microscopic oiliness and macroscopic oil bearing property or hydrocarbon productivity. The comprehensive study above gains insights into the microscopic pore structure and their controls on fluid mobility and oiliness in shale oil reservoirs, and this may have implications for resource potential evaluation and effective exploitation of oil shales.

    更新日期:2020-01-07
  • Organic geochemical characteristics and organic matter enrichment of mudstones in an Eocene saline lake, Qianjiang Depression, Hubei Province, China
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2019-12-19
    Xiangxin Kong; Zaixing Jiang; Youheng Zheng; Meng Xiao; Chen Chen; Hao Yuan; Fengling Chen; Shiqiang Wu; Jianguo Zhang; Chao Han; Shengqian Liu

    The Qianjiang Depression is a typical Eocene hypersaline lake basin developing good source rocks in the Qianjiang Formation, which have recently been identified as a potential target for saline lacustrine shale oil exploration. Previous studies have only focused on estimates of source rock potential and reservoir quality. However, studies on organic matter enrichment mechanisms in evaporative environments are limited. Based on organic geochemical and mineralogical data, as well as sedimentological analyses, we researched the factors controlling differences in organic matter enrichment in the basin. The results show that total organic carbon (TOC) contents are closely associated with lithofacies with distinct variations. Three main lithofacies associations (LAs) were identified: (1) glauberite-rich LA1 with medium TOC; (2) carbonate-rich LA2 with high TOC; and (3) anhydrite-rich LA3 with low TOC. The distribution of biomarkers from 14 samples indicate both higher land plants and aquatic organisms were involved in the organic matter accumulations. Combining experimental results on gammacerane/C30 hopane, extended tricyclic terpane ratio (ETR), Sr/Ba, V/Ni, and sedimentary characteristics, we analyzed the sedimentary environments of the three LAs. LA1 was deposited in a salinity stratified waterbody with strong evaporation, LA2 was deposited in a still, stratified deep waterbody, whereas LA3 was deposited in a disturbed, saline waterbody. Sedimentary environments not only controlled the formation of the lithofacies, but also influenced the organic matter sources and preservation. Relatively low salinity, strong anoxic condition without physical disturbance, appropriate nutritional terrigenous material inputs, and algal blooms produced the best organic matter enrichment in the saline lake system.

    更新日期:2020-01-04
  • Quantitative evaluation of the transporting capacity of unconformities: A Case Study from the Zhongguai Area, Junggar Basin, NW China
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2019-12-19
    Kuaile Zhang; Zhenliang Wang; Yiqin Jiang; Aiguo Wang; Changyu Fan; Baoli Xiang; Ni Zhou

    Unconformities are special geologic bodies with a three-layer structure including the rock above the unconformity, a weathered clay layer, and a leached rock. The Upper Wuerhe (Upper Permian) bottom unconformity in Northwest Junggar Basin is the main pathway system linking the Zhongguai (ZG) lift with hydrocarbon source kitchens. Based on the log response characteristics of the unconformity's vertical structure and core calibration, a principal component analysis (PCA) was applied to extract and identify the evaluation parameters of the unconformity's vertical structure. On the basis of Darcy's Law, the quantitative characterization formula of the unconformity's transport capacity with an actual geological significance was established after taking several factors into consideration. The migration probability of the unconformity was used to evaluate the possibility that the unconformity acted as a migration pathway. Consequently, a relationship between the migration probability and the evaluation parameters was developed to analyze the effectiveness of these parameters in characterizing the transporting capacity of the unconformity. This study shows that the quotient of the two principal components extracted from the conventional logging curves (Pc1/Pc2) have different reference values in the leached and parent rocks. The distribution of thicknesses of the weathered clay layer and the leached rock is well matched with the distribution of paleogeomorphology. The parameters representing only a single geological factor are not enough to determine whether the unconformity acted as a migration pathway. The flow coefficient (Cf) containing multiple factors can effectively and clearly identify the transporting capacity of the unconformity, considering the positive correlation between the parameter Cf and the migration probability of the unconformity. The oil and gas shows of the area with a parameter Cf greater than 10 × 10−4 cm2/s are abundant, which is the superior transporting area of the unconformity, and there is a good matching relationship between the distribution of the superior transporting area of the unconformity and that of the hydrocarbon distribution.

    更新日期:2020-01-04
  • Fine-grained volcanic-hydrothermal sedimentary rocks in Permian Lucaogou Formation, Santanghu Basin, NW China: Implications on hydrocarbon source rocks and accumulation in lacustrine rift basins
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2019-12-23
    Xin Jiao; Yi-qun Liu; Wan Yang; Ding-wu Zhou; Bin Bai; Tian-shu Zhang; Min-ru Zhao; Zhe-xuan Li; Zi-yuan Meng; Yi-yao Yang; Zhi-xin Li

    Subaqueous volcanic-hydrothermal field is a complex hybrid geological system, where sediments originated from volcanic, hydrothermal, lacustrine, terrestrial, and biotic sources are mixed. Recent petrological studies on such sediments have indicated a potential relationship between volcanic-hydrothermal activities and oil generation and accumulation. Here, we explore the mechanisms of sedimentation and oil accumulation in well-laminated organic-rich fine-grained sedimentary rocks in Permian Lucaogou Formation deposited in the lacustrine Santanghu rift basin, NW China. We conducted petrographic, carbon and oxygen isotopic, and organic geochemical analyses of fifty-four core samples from three wells (forty-eight samples from Well W1, two from W2, four from W3) designed for tight oil exploration. Four main observations and interpretations are: 1) The fine-grained sediments are a mixture of volcanic-hydrothermal detrital feldspar grains, authigenic quartz crystals, and primary lacustrine dolomite, forming interbedded tuffaceous shale and dolostone; 2) authigenic quartz crystals were precipitated from acidic hydrothermal fluids, whereas fine-grained angular alkaline feldspars were derived from subaqueous volcanic eruption; 3) the TOC content and oil generation potential increase with increased mixing of feldspars and carbonate minerals. Hence, the dolomite-bearing tuffaceous shale and tuffaceous dolostone are the best source rocks; and 4) the fine-grained sedimentary rocks are interpreted as a subaqueous volcanic-hydrothermal deposit that controls the source potential. The sedimentary processes of an ancient sediment-hosted geothermal system and its implications on hydrocarbon source rocks and accumulation were first identified. Algal bloom induced by subaqueous eruptions is the main cause for episodic deposition of organic matter and extreme heterogeneity in both lithofacies and source-rock distribution. The results suggest that cone-shaped buildups on seismic sections, which were formed by volcanic-hydrothermal deposits and contain highly-mixed feldspar and carbonate sediments, are the best hydrocarbon reservoirs, and demonstrate a strong relationship between volcanic-hydrothermal activities and hydrocarbon accumulation in an extensional tectonic setting.

    更新日期:2020-01-04
  • The heterogeneity of pore structure in lacustrine shales: Insights from multifractal analysis using N2 adsorption and mercury intrusion
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2019-12-13
    Ming Guan; Xiaoping Liu; Zhijun Jin; Jin Lai

    A series of parameters from a variety of techniques, such as pore size distribution, surface area, pore volume, etc., have been used to characterize the pore structure of unconventional shale reservoirs. However, the investigation on the quantification of pore structure heterogeneity is still scarce. In this aspect, a multifractal theory combined with N2 adsorption and mercury intrusion was used to obtain pore structure characteristics and quantify the heterogeneity of pore size distribution of nine lacustrine shale samples from the second member of Kongdian Formation (Ek2) in Cangdong Sag, Bohai Bay Basin, China. The results show that the Ek2 shale contains complex pore network, and pores with small pore size predominate in pore system. Both N2 adsorption branches and pore throat size distributions from mercury intrusion demonstrate multifractal nature. The width of singularity spectra from multifractal analysis is used to quantitatively characterize the heterogeneity degree of pore size distribution and shows good consistency with pore size distributions derived from N2 adsorption and mercury intrusion. The pore structure from mercury intrusion is much heterogeneous than that from N2 adsorption, indicating that an increased number of large pores will increase the heterogeneity of pore size distributions. Compared with other rock compositions, TOC has an obvious impact on the heterogeneity of pore structure in small pore size range.

    更新日期:2020-01-04
  • Alternating of aggradation and progradation dominated clinothems and its implications for sediment delivery to deep lake: The Eocene Dongying Depression, Bohai Bay Basin, east China
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2019-12-19
    Jianping Liu; Benzhong Xian; Youliang Ji; Chenglin Gong; Junhui Wang; Zhen Wang; Peng Chen; Delin Song; Wenzong Wei; Ximeng Zhang; Luxing Dou

    Sediment routing in deep-water settings can be significantly different in lacustrine basins from their well-studied marine counterparts, which has been largely overlooked during the past decades. The seismically well-imaged lacustrine clinothems in the Eocene Dongying Depression provide a good opportunity to bridge the deep-water sediment partitioning with clinothem architecture and shelf-edge trajectories, which have been studied through the integration of 3D seismic data and well-logging data. In the study area, the Eocene delta-complexes are architecturally manifested as alternating aggradation dominated (AC) and progradation dominated clinothems (PC). The Eocene Dongying clinothem architecture has been documented in terms of shelf–edge trajectory angle (θsht), edge-to-toe distance (ETT), aggradation height (Ah), progradation distance (Pd) and maximum foreset angle (γmfa). Aggradation dominated clinothems are characterized by ascending shelf-edge trajectories with thick and widespread sand-prone bottomsets, suggesting higher efficiency of sediment delivery into the outlying lake floor. Progradation dominated clinothems, in contrast, are characterized by flat to slightly descending trajectories and generally by a lack of bottomsets. Alternations of rising and flat to slightly descending trajectories accompanied with different clinothem types probably indicate frequent relative lake level changes (on time-scales of 100s of kyr). Further analysis of seismic images and extensive well-logs suggest the distal aggradational bottomsets are generally consist of sub-lacustrine channel-lobe systems, whereas sediment slide, slump blocks occur in slope to slope toe areas of progradation dominated clinothems, which indicate varied processes of deep-water sediment transport associated with different clinothem types and shelf-edge trajectories. Lake-level, sediment supply, water salinity and density change induced by humid-arid climate cycle are suggested to be main factors controlling clinothems stacking pattern and sediment dispersal into deep-lacustrine. Humid climate are considered to be related with strongly aggradational clinothems, during which sediment can be directly transported to the distant bottomset areas due to increased sediment concentration and energy in river discharge. Whereas during dry periods, progradation dominated clinothems start to build under stillstand or falling lake level conditions, sediment generally accumulates at river mouth and collapses due to sediment overload or fault activity. Therefore, a relationship between deep-lacustrine depositional process and climatic signature was established which may provide new thoughts in both the study of deep-water sequences and deep-water exploration in lacustrine basins.

    更新日期:2020-01-04
  • Geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the first offshore gas-hydrate production test region in the Shenhu area, Northern South China Sea
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2019-12-16
    Wei Zhang; Jinqiang Liang; Jiangong Wei; Jing'an Lu; Pibo Su; Lin Lin; Wei Huang; Yiqun Guo; Wei Deng; Xiaolu Yang; Zhifeng Wan
    更新日期:2020-01-04
  • Seafloor depressions on the Nigerian margin: Seabed morphology and sub-seabed hydrate distribution
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2019-12-14
    F. Taleb; M. Lemaire; S. Garziglia; T. Marsset; N. Sultan

    Gas hydrate quantification using acoustic data requires proper knowledge of the mineralogy of their host sediment. In this paper, a petrophysical model allowing GH quantification at sites where mineralogy profiles are absent is proposed. This approach is applied to a high gas flux pockmark system in the Gulf of Guinea where in-situ acoustic and geotechnical measurements together with core measurements could have been correlated and tied to seismic data. Projections of the in-situ measurements on seismic profiles have shown that the study area not only accommodates zones of shallow and dense GH; but also zones where solid hydrate and free gas coexist as well as pockets of free gas. Further analysis of several seismic profiles has allowed illustrating the detailed GH occurrence zone within the study area, estimate its volume and its occupancy ratio of the pockmark. Correlations between GH content and 3D bathymetry sections have allowed to draw a link between different GH contents and the morphology of the pockmark, which also shares similarities with the morphology of the GH occurrence zone it accommodates.

    更新日期:2020-01-04
  • The influence of variable progradation to aggradation ratio and facies partitioning on the development of syndepositional deformation in steep-walled carbonate platforms
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2019-12-13
    Andrea Nolting; Christopher K. Zahm; Charles Kerans; Yaser Alzayer

    One of the remaining questions in carbonate geology and reservoir studies is the origin of steep-walled carbonate platform margins and the role of gravity versus other structural processes in controlling the distribution of fracture networks and failure surfaces. This study is an attempt to document the role of gravity, interacting with stratigraphic architecture in controlling fracture patterns and margin collapse. We employ finite-discrete element models to illustrate that progradation/aggradation (P/A) ratio and facies tract partitioning in steep-walled carbonate platforms affects the development, distribution, and intensity of syndepositional deformation. All models are under the influence of only gravitational loading, where pore pressure is held constant. We utilize a modified Mohr-Coulomb constitutive law with a Rankine Rotating crack tensile corner to capture both tensile and shear brittle failure. Our results illustrate that P/A ratio affects the distribution and intensity of discrete fractures that form in steep-walled carbonate platforms. Our results suggest that deformation is more extensive in a progradational carbonate platform, where shelf edge angles are greatest and the clinoform is thickest. Alternatively, aggradational carbonate platforms experience localized deformation in front of the antecedent shelf edge where the clinoform is thickest and steepest. The introduction of mechanical heterogeneity associated with facies tract partitioning affects the intensity of fracturing in both progradational and aggradational models, with the greatest number of fractures developing in reef facies in both scenarios. Development of brittle deformation under the sole application of gravity and lack of seaward lithostatic confinement is consistent with the syndepositional nature of deformation in these settings. This work illustrates the interplay between carbonate platform geometry, facies distribution, and the resulting syndepositional deformation.

    更新日期:2020-01-04
  • Petrofacies prediction and 3-D geological model in tight gas sandstone reservoirs by integration of well logs and geostatistical modeling
    Mar. Petrol. Geol. (IF 3.538) Pub Date : 2019-12-26
    Binfeng Cao; Xiaorong Luo; Likuan Zhang; Yuhong Lei; Jinsong Zhou

    These effective reservoir rocks that can have sufficient porosity and permeability and hydrocarbon accumulations determine economic reserves and high yield of tight oil and gas. Understanding the origin and distribution of these effective reservoir rocks is important for sorting out “sweet spots” in tight reservoirs. Multiple-scale analyses from the microscopic, via drill core to single wellbore have been conducted to examine reservoir heterogeneity of the Upper Paleozoic Shanxi Formation in the southeastern Ordos Basin. Five sandstone petrofacies have been defined in terms of texture and framework composition, detrital matrix, diagenesis and pore types: quartz arenite, tuffaceous quartz arenite, ductile lithic-lean sub-litharenite, ductile lithic-rich sandstone and tightly carbonate-cemented sandstone. The various petrofacies show fundamentally different pathways of diagenetic and reservoir-quality evolutions, due to the complexity of texture and detrital composition in the Shanxi Formation, directly related to the provenance and depositional environment. Quartz arenites and ductile grain-lean sub-litharenites both form effective reservoir rocks. By translating petrofacies to well-log signatures corrected from petrographic and core analysis data, a model based on factor analysis has been built to predict petrofacies at the well scale. For investigating the distribution of each petrofacies at the field scale, sequential indicator simulation method has been used to construct a 3-D petrofacies model with facies conditioning. The isopach maps of quartz arenites and ductile lithic-lean sub-litharenites have been made from the 3-D petrofacies model to quantify their areal distributions, and to define the “sweet spots” by combining isopach maps of these effective reservoir rocks and gas production data from well tests. The concept of reservoir petrofacies is an important tool for the characterization of tight sandstone reservoirs.

    更新日期:2020-01-04
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