Delineation of coaly source rock distribution and prediction of organic richness from integrated analysis of seismic and well data
Introduction
Identification and delineation of petroleum source rocks is a key aspect of petroleum systems evaluation, yet most basin modelling studies lack reliable information on the distribution, volume and richness of the source rock. Typically, source rocks have been delineated by analysing well data, seismic attributes and depositional environments, and source rock richness (i.e., total organic carbon content) has been estimated by taking averages from sparse well calibrations (e.g., Peters et al., 2005; Badics et al., 2015; Kroeger et al., 2015). However, there remains large uncertainty in the delineation and characterisation of source rocks in the absence of well data calibration of seismic facies, especially since different facies can show similar amplitude responses. Simple averages of total organic carbon (TOC) at well locations within a stratigraphic interval typically do not provide sufficiently representative information on the distribution of source rock richness for the target petroleum system. Not only are wells rarely drilled through source rock intervals, but wells are generally widely spaced and cannot adequately capture the typically large lateral variation in source rock richness (e.g., Bohacs and Suter, 1997), especially in terrestrial, coaly source rock systems.
Several new seismic inversion data techniques have recently been developed for characterising source rocks (Løseth et al., 2011; Chopra et al., 2012; Ogiesoba and Hammes, 2014; Badics et al., 2015; Amato del Monte et al., 2018). Most of these were developed for shale gas exploration and conventional shale source rocks. In contrast, few studies have focused on identifying and delineating conventional coaly source rocks, and even fewer have attempted to use seismic inversion techniques to estimate the organic richness of coaly source rocks (Løseth et al., 2011) using acoustic properties such as P-impedance (i.e., the product of density and P-wave velocity).
Cretaceous–Cenozoic coaly source rocks have generated large volumes of oil and gas in many sedimentary basins, particularly in Australasia and southeast Asia (Field and Browne, 1989; Noble et al., 1991; Moore et al., 1992; Cook et al., 1999; Sykes et al., 2014a). Mapping and characterising coaly source rock facies are therefore key issues for ongoing exploration and exploitation in many terrestrial-sourced basins. This paper presents a study to better delineate coaly facies and estimate their organic richness within the Maari 3D seismic data volume, offshore southern Taranaki Basin, New Zealand (Fig. 1). The term coaly facies is used in this study to refer to the group of three broad coaly lithologies – coal, shaly coal and coaly mudstone – that collectively constitute the continuum of coaly source rocks (Sykes and Raine, 2008, Sykes and Snowdon, 2002). Well data show that the coaly facies display wide variability in their vertical and lateral distributions, locally and regionally. An integrated analysis of well logs, image logs, seismic data, well cuttings descriptions and geochemical data was carried out to map the distribution and quantify the organic richness of coaly facies within the coal-bearing Paleocene–Eocene interval imaged in the Maari 3D seismic volume. The first step was to determine the density and velocity cut-off values for the three coaly lithologies so that impedance could be used to define coaly facies in seismic inversion-derived impedance volumes. Seismic reflection character, depositional environment and P-impedance were then integrated to identify the spatial distribution of coaly facies within the Paleocene–Eocene coastal plain depositional setting. Lastly, the variability in organic richness within this setting was mapped by developing a proxy TOC volume from inverted P-impedance data for the Maari 3D area.
It is important to note that Paleocene–Eocene coal measures are not the source interval for the black oil reserves within Maari Field. Biomarker studies indicate that the oil is derived primarily from Late Cretaceous coaly rocks of the Rakopi and possibly North Cape formations (Killops et al., 1994; Sykes et al., 2012). Rather, the Paleocene–Eocene interval in the Maari 3D volume was selected for study because it provides recent, high-quality seismic and well data with which to develop and test our methodology. This methodology could be equally applied to the Cretaceous and other coaly source rock intervals in other areas of Taranaki and other basins with suitable well and seismic data.
Section snippets
Geological setting
The Taranaki Basin is primarily located offshore west of the North Island of New Zealand (Fig. 1) and for more than five decades has been the main focus area for oil and gas exploration and production in New Zealand. The evolution of this basin has been well studied (Knox, 1982; Holt and Stern, 1994; King and Thrasher, 1996; Giba et al., 2010; Reilly et al., 2015; Strogen et al., 2017). The basin developed during the Cretaceous–Cenozoic and has an early rift-drift history associated with the
Data
Active petroleum exploration in the Taranaki Basin over the last five decades has generated a large amount of openfile 2D and 3D seismic reflection data, well data and associated reports that are available from New Zealand Petroleum and Minerals (https://data.nzpam.govt.nz). In this study, we used data from nine wells (Matuku-1, Pukeko-1, Te Whatu-2, Kea-1, Maari-1, Moki-1, Manaia-2/2A, Whio-1 and Maui-4), the Maari 3D seismic data (PGS Data Processing, 2009), eight horizon grids (Thrasher et
Delineation and characterisation of coaly facies from well log data
Density, velocity, resistivity and gamma ray values characteristic of different coaly lithologies are summarised in Table 1. In general, they are characterised by low density, low velocity and high resistivity (>7 ohmm), whereas gamma ray does not appear to be particularly discriminatory for these facies. The density cut-off values of coal (<1.5 g/cc) and shaly coal (1.5–2 g/cc) are adopted from Sykes and Snowdon (2002) and Sykes (2014). Cross-plot analysis of well logs in the Paleocene and
Depositional controls on distribution of coaly facies
Accumulation of significant volumes of coaly facies in a sedimentary succession primarily depends on (1) growth of vegetation, (2) preservation of organic matter, (3) restricted dilution by clastic sediments, and (4) creation of accommodation space (Bohacs and Suter, 1997; Peters et al., 2000; Bohacs et al., 2005). The depositional environment of the Paleocene–Eocene section in the Maari 3D area was a predominantly terrestrial coastal plain setting (Fig. 12e) (King and Thrasher, 1996; Strogen,
Conclusions
Integrated analysis of well log, geochemical and seismic data has enabled the delineation of coaly facies and estimation of average organic richness (TOC) within the Paleocene–Eocene interval of the Maari 3D seismic volume. We think our approach can help predict source rock distribution where good quality seismic data is available and is supported by geochemistry, conventional log and image log analysis. Coal and shaly coal lithologies are best defined at well sites using a combination of
Credit author statement
Tusar R. Sahoo: Conceptualization, Data curation, Formal analysis, Methodology, Visualization, writing- original draft and subsequent editing, Robert H. Funnell: Supervision, Writing – review & editing, Stephen W. Brennan: collection and analysis of coal samples, Richard Sykes: Funding acquisition, analysis of coal samples, Writing – review & editing, Glenn P. Thrasher: Supervision, Ludmila Adam: review, Mark J.F. Lawrence: image log analysis and review, Richard L. Kellett: review, Xiajing Ma:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was primarily funded by the Ministry of Business, Innovation and Employment (MBIE), New Zealand, as part of the GNS Science-led programme “Understanding petroleum source rocks, fluids, and plumbing systems in New Zealand basins: a critical basis for future oil and gas discoveries” (Contract C05X1507). We would like to thank several mine operators which provided access to the outcrop samples. CRL Energy (now Verum Group; Lower Hutt, New Zealand) undertook density analyses of outcrop
References (86)
- et al.
Methods for source rock identification on seismic data: an example from the Tanezzuft Formation (Tunisia)
Mar. Petrol. Geol.
(2018) - et al.
Predicting distribution of total organic carbon (TOC) and S2 with Δ log resistivity and acoustic impedance inversion on talang akar formation, Cipunegara sub basin, West Java. Engineering physics international conference
EPIC 2016. Procedia Engineering
(2017) - et al.
Sandstone provenance and sediment dispersal in a complex tectonic setting: Taranaki Basin, New Zealand
Sediment. Geol.
(2018) - et al.
Sequence stratigraphy and controls on reservoir sandstone distribution in an Eocene marginal marine-coastal plain fairway, Taranaki Basin, New Zealand
Mar. Petrol. Geol.
(2012) - et al.
An interdisciplinary approach to reservoir characterisation; an example from the early to middle Eocene Kaimiro Formation, Taranaki Basin, New Zealand
Mar. Petrol. Geol.
(2017) - et al.
Petroleum generation and migration from Talang Akar coals and shales offshore N.W. Java, Indonesia
Org. Geochem.
(1991) - et al.
Evolution of faulting and plate boundary deformation in the Southern Taranaki Basin, New Zealand
Tectonophysics
(2015) - et al.
Integrated TOC prediction and source rock characterization using machine learning, well logs and geochemical analysis: case study from the Jurassic source rocks in Shams Field, NW Desert, Egypt
J. Petrol. Sci. Eng.
(2019) Application of Rank(Sr), a maturity index based on chemical analyses of coals
Mar. Petrol. Geol.
(2002)- et al.
Guidelines for assessing the petroleum potential of coaly source rocks using Rock-Eval pyrolysis
Org. Geochem.
(2002)
Marine influence helps preserve the oil potential of coaly source rocks: Eocene Mangahewa Formation, Taranaki Basin, New Zealand
Org. Geochem.
Petroleum Geochemistry of the Taranaki Basin
AVO classification of lithology and pore fluids constrained by rock physics depth trends
Lead. Edge
Quantitative Seismic Interpretation — Applying Rock Physics Tools to Reduce Interpretation Risk
Seismic screening for hydrocarbon prospects using rock-physics attributes
Lead. Edge
Long-wave elastic anisotropy produced by horizontal layering
J. Geophys. Res.
Assessing source rock distribution in Heather and Draupne Formations of the Norwegian North Sea: a workflow using organic geochemical, petrophysical, and seismic character
Interpretation
The velocity of compressional waves in rocks to 10 kilobars, Part 1
J. Geophys. Res.
Sequence stratigraphic distribution of coaly rocks: fundamental controls and paralic examples
AAPG (Am. Assoc. Pet. Geol.) Bull.
Production, destruction, and dilution – the many paths to source-rock development
Dynamic elasticity and the controlling physical properties of New Zealand's coaly source rocks
Depth structure maps, isopach maps and a regional velocity model from the southern Taranaki Basin (4D Taranaki project)
GNS Science data series
Rock physics templates for clay-rich source rocks
Geophysics
Shale gas reservoir characterization workflow
Cretaceous-Cenozoic Geology and Petroleum Systems of the Great South Basin, New Zealand
Stratigraphic controls on water quality at coal mines in southern New Zealand
N. Z. J. Geol. Geophys.
Cretaceous and Cenozoic sedimentary basins and geological evolution of the Canterbury region, South Island, New Zealand
New Zealand Geological Survey Basin Studies
Maari 3D Simultaneous Inversion Report, PMP 38160
Evolution of faulting and volcanism in a back-arc basin and its implications for subduction processes
Tectonics
Hampson-Russell STRATA User Manual Guide
Making interpretable images from image logs
Depositional Age, Facies, and Cyclicity within the Mangahewa Reservoir Fairway, Middle to Late Eocene, Taranaki Basin
A Reservoir and Seal Petrology Study of the Paleocene to Miocene Succession, Well Whio-1, Offshore Taranaki Basin
Sedimentology and structure of the malvern hills coal mine, Canterbury, New Zealand
N. Z. J. Geol. Geophys.
Subduction, platform subsidence, and foreland thrust loading: the late Tertiary development of Taranaki Basin, New Zealand
Tectonics
Organic metamorphism and the generation of petroleum
AAPG (Am. Assoc. Pet. Geol.) Bull.
Geology and lignite resources of east Southland group, New Zealand
New Zealand Geological Survey Bulletin
A geochemical appraisal of oil generation in the Taranaki Basin, New Zealand
AAPG (Am. Assoc. Pet. Geol.) Bull.
Post-Eocene development of the Taranaki Basin, New Zealand: convergent overprint of a passive margin
Cretaceous-Cenozoic geology and petroleum systems of the Taranaki Basin, New Zealand
Institute of Geological and Nuclear Sciences Monograph 13
Cretaceous to Recent sedimentary patterns in New Zealand
A 2-D, oblique-dip outcrop transect through a third-order, progradational, deep-water clastic succession, Urenui–Mount Messenger Formations, New Zealand
Taranaki Basin, structural style and tectonic setting
N. Z. J. Geol. Geophys.
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