Pore structure evaluation in ultra-deep tight sandstones using NMR measurements and fractal analysis

doi:10.1016/j.petrol.2022.110180

Journal of Petroleum Science and Engineering

Volume 211, April 2022, 110180

https://doi.org/10.1016/j.petrol.2022.110180 Get rights and content

Highlights

•
NMR T₂ spectrum is used to divide pore structure type of ultra-deep sandstones.
•
Fractal behaviors of various pore structure are unraveled using NMR and fractal analysis.
•
Complex pore assemblages result in the high fractal dimension of Type II and III pore structure.

Abstract

The reservoir quality, pore spaces and pore size distributions of Bashijiqike sandstones in Kuqa depression were described by thin sections, SEM (scanning electron microscopy) analysis and nuclear magnetic resonance (NMR) tests. In order to quantitatively characterize the complexity of pore structure, NMR T₂ (transverse relaxation time) spectrum was used for fractal analysis. This study unravels the relationships between petrophysical parameters, NMR parameters and fractal dimension of the ultra-deep tight sandstones. Primary intergranular pores are the main component of pore spaces, followed by secondary dissolution pores and micropores, and there contain microfractures. The NMR T₂ spectra are either bi-modal or uni-modal distribution. The uni-modal T₂ spectrum reflects uniform pore spaces. The coexistence of intragranular pores and intergranular pores leads to the bi-modal T₂ spectrum. There is a positive correlation between fractal dimensions and T_2gm (geometric mean of the T₂ distribution). Consequently, four pore structure types are determined according to irreducible water content, T_2gm, RQI (reservoir quality index), and the characteristics of individual pore structure are summarized. Because of the uniform pore space, Type I and Type IV have the lowest fractal dimensions. The structure of Type II and Type III are most heterogeneous due to their combination of intergranular and intragranular pores. The results help clarify the internal relationships between petrophysical parameters and microstructure, and have implications for pore structure evaluation of ultra-deep sandstones worldwide.

Introduction

Tight sandstones contain abundant oil and gas reserves, however, they have low porosity (<10%) and low permeability (intrinsic permeability<0.1mD) (Desbois et al., 2011; Gao and Li, 2015; Zou et al., 2012; Lai et al., 2018a). In addition, the tortuous systems containing intergranular, intragranular pores and micropores are the main features of tight sandstones. The pore throats have a wide range from nano-scale (<1 mm) to micro-scale (Rezaee et al., 2012; Desbois et al., 2011; Anovitz and Cole, 2015; Lai et al., 2018a; Liu and Ostadhassan, 2019; Wu et al., 2020). Pore structure, which includes geometry, size, connectivity and distribution of pore throat (Fu et al., 2015), determines the hydrocarbon migration and accumulation, and it plays a dominant control in reservoir quality evaluation (Anovitz et al., 2015; Xi et al., 2016; Lai et al., 2019; Gao et al., 2020; Yuan et al., 2021). The wide variations in reservoir quality of tight rocks are mainly attributed to the heterogeneous pore networks (Anovitz et al., 2015). Therefore the characterizations of pore structure in tight sandstones are important for enhancing hydrocarbon recovery (Schmitt et al., 2015; Lai et al., 2018a).

The Kuqa Depression is rich in natural gas resources (Liu et al., 2020). Many giant gas fields have been found in the Kuqa Depression recently, including the Kela 2, Awa, Bozi, Dabei, Keshen gas fields (Jin et al., 2008). However, the Kuqa depression is a reactivated foreland basin, and had undergone complex tectonic evolution histories (Liu et al., 2020; Zheng et al., 2020). Additionally, the main gas bearing formation is ultra-deeply buried (>6000 m), leading to a complex assemblage of pores spaces and tortuous pore throat structure. Therefore the application of various complementary methods and fractal theory in pore throat structure characterization are of great importance.

Laboratory NMR measurements, which commonly measure T₂ transversal relaxation time distribution, provide continuous pore size distributions (Dillinger and Esteban, 2014; Daigle et al., 2014; Sigal, 2015; Wang et al., 2018; Yuan et al., 2018; Zhang and Zhang, 2021). Besides NMR porosity, FFI (free fluid index), BVI (bulk volume of irreducible water), T_2cutoff (T₂ value separating FFI and BVI), and T_2gm can be derived from NMR measurements (Pape and Clauser, 2009; Meng et al., 2016; Wang et al., 2018, 2020). Therefore, a large number of parameters are provided directly or indirectly by NMR tests, including permeability, porosity, and fluid saturation. These parameters are important in reservoir characterization (Müller-Huber et al., 2016). In addition, NMR gives the insights into the fluid states and types (movable/producible fluids, bound fluids) (Müller-Huber et al., 2016; Wang et al., 2018; Fan et al., 2019; Zhang et al., 2020). Nondestructive and quantitative are two advantages of NMR measurements, and consequently NMR analysis is one of the most commonly used methods for pore structure evaluation (Golsanami et al., 2016; Li et al., 2017; Wang et al., 2018; Wu et al., 2020).

Fractals are virtual, self-similar geometrical objects (Cai et al., 2010; Lai and Wang, 2015; Kulesza and Bramowicz, 2014). Fractal property is one of the characteristics of sandstone pore throat systems, which appears independent of scales (Lai et al., 2016; Yuan and Rezaee, 2019). Fractal theory helps build up the bridges between microscopic morphology and macroscopic performances (Kleinberg et al., 1994; Cai et al., 2010; Wang et al., 2012, 2018; Daigle and Johnson, 2016; Zhang et al., 2018). It is widely used for quantitatively characterizing heterogeneity of pore throat structure (Li and Horne, 2006; Li, 2010a, b; Wang et al., 2018; Yuan and Rezaee, 2019; Lai et al., 2019; Zhang et al., 2020). Lai et al. (2018b) firstly proposed a new model from NMR T₂ spectrum, which can be used to derive the fractal dimensions. This model is widely used to quantitatively characterize the pore size and distribution. (Zhang et al., 2020; Zhang and Hu, 2020; Lu and Liu, 2021).

In this study, the pore structure of Bashijiqike ultra-deep tight sandstones is described by means of petrophysical measurements and fractal theory. The NMR T₂ spectrum, NMR parameters as well as fractal dimension are used for pore structure evaluation. The study characterizes the connection between microscopic pore structure and fractal dimensions, and unravels the link between microscopic pore structure and macroscopic behaviors. In addition, the characteristics of various pore structures are summarized. The work is helpful for improving the scientific understanding of pore structure distribution and has practical implications for pore structure evaluation of ultra-deep tight sandstones, and is hoped to provide reference for pore structure evaluation and fractal analysis of sandstones worldwide.

Section snippets

Geological setting

The Kuqa depression is situated in the transition zone between Tarim Basin and southern Tianshan (Fig. 1) (Shi et al., 2004; Zou et al., 2006; Zeng et al., 2010; Feng et al., 2018; Zheng et al., 2020; Lai et al., 2021). The Kuqa depression is a foreland depression, which is formed during the Mesozoic to Cenozoic time (Shi et al., 2004; Zhang and Huang, 2005; Zeng et al., 2010; Nian et al., 2016). Due to the complex evolutionary history (Nian et al., 2016; Ju and Wang, 2018; Neng et al., 2018),

Methodology

In the model of calculating fractal dimensions using NMR measurement fist proposed by Lai et al. (2018b), it is based on the assumption that the pore systems are spherical. Thus, the equation containing the pores numbers (N_i) and size of pores r_i (T_2i value) can be written in (Eq. (1)) (Sigal, 2015; Zhang et al., 2020). $N_{i} = \frac{V_{p i}}{\frac{4}{3} π r_{i}^{3}} = \frac{V_{p i}}{36 π {(ρ T_{2 i})}^{3}}$

V_pi (%) is the incremental pore volume amplitudes at specific T₂ (T_2i, ms), r_i is the pore size, and ρ is surface relaxivity (μm/ms).

Then N(>r_i), which

Pore spaces, porosity and permeability

According to core analysis, the minimum porosity is 1.26% and the maximum porosity is 14.69%, and the average porosity is 7.30%, while 0.017 to 133.0 mD is the wide range of permeability, with an average of 2.042 mD (Fig. 2). Most of the data points in the permeability versus porosity crossplot fall in the area with air permeability less than 1 mD and porosity less than 10% (Fig. 2). Therefore the Bashijiqike Formation in the Bozi wellblock of Kuqa depression is ultra-deeply buried tight

Discussion

In this section, the internal relationships between pore structure, reservoir quality and fractal dimension are unraveled using NMR tests and fractal analysis for the ultra-deeply buried (>6000 m) in Kuqa depression.

The permeability is commonly related to total porosity, indicating that the pore spaces of the samples using in this study are dominantly pores, but not fractures (Fig. 2). The NMR T₂ distribution also confirm this due to the rareness in tail distribution and long T₂ components

Conclusions

The porosity of Bashijqike sandstones in Bozi Wellblock of Kuqa depression is between 1.26% and 14.69%, and the average value is 7.30%, while permeability is from 0.017 to 133.0 mD, and the average value is 2.042 mD. The pore system includes residual intergranular pores, grain intragranular dissolution pores, micropores, fractures or microfractures. The NMR T₂ spectra have two types: bi-modal and uni-modal distribution. The uni-modal T₂ spectrum behaviors indicate that pore spaces are mainly

Author contribution statement

Yi Xin: Conceptualization, Methodology, Software, Writing- Reviewing and Editing; Guiwen Wang: Conceptualization, Methodology, Software, Software, Validation, Writing- Reviewing and Editing; Bingchang Liu: Data curation, Writing – original draft preparation; Yong Ai: Data curation, Writing – original draft preparation; Hongkun Liu: Data curation, Writing – original draft preparation; Deyang Cai: Data curation, Writing – original draft preparation; Shuwen Yang: Software, Validation; Yuqiang Xie:

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.

Acknowledgments

This work is supported by National Science & Technology Major Project of China (No. 2017ZX05008-004-001). We thank the three anonymous reviewers for their constructive comments, which improve the paper significantly. The authors thank the handling editors of JPSE for their kind work on this paper. We also thank Rouhi Farajzadeh, the Executive Editor for his enthusiasm, patience, and tireless efforts.

References (69)

L.M. Anovitz et al.
Effect of quartz overgrowth precipitation on the multiscale porosity of sandstone: a (U)SANS and imaging analysis
Geochem. Cosmochim. Acta
(2015)
J. Cai et al.
Fractal analysis of invasion depth of extraneous fluids in porous media
Chem. Eng. Sci.
(2010)
G. Desbois et al.
High-resolution 3D fabric and porosity model in a tight gas sandstone reservoir: a new approach to investigate microstructures from mm-to nm-scale combing argon beam cross sectioning and SEM imaging
J. Petrol. Sci. Eng.
(2011)
A. Dillinger et al.
Experimental evaluation of reservoir quality in Mesozoic formations of the Perth Basin (Western Australia) by using a laboratory low field Nuclear Magnetic Resonance
Mar. Petrol. Geol.
(2014)
X. Fan et al.
Pore structure evaluation of tight reservoirs in the mixed siliciclastic-carbonate sediments using fractal analysis of NMR experiments and logs
Mar. Petrol. Geol.
(2019)
J. Feng et al.
Quantitative prediction of fracture distribution using geomechanical method within Kuqa Depression, Tarim Basin, NW China
J. Petrol. Sci. Eng.
(2018)
H. Fu et al.
Investigation of the factors that control the development of pore structure in lacustrine shale: a case study of block X in the Ordos basin, China
J. Nat. Gas Sci. Eng.
(2015)
A. Giri et al.
Fractal pore structure of sedimentary rocks: simulation in 2-d using a relaxed bidisperse ballistic deposition model
J. Appl. Geophys.
(2012)
N. Golsanami et al.
A review on the applications of the nuclear magnetic resonance (NMR) technology for investigating fractures
J. Appl. Geophys.
(2016)
Z.J. Jin et al.
The tectonics and petroleum system of the Qiulitagh fold and thrust belt, northern Tarim basin, NW China
Mar. Petrol. Geol.
(2008)

W. Ju et al.

A preliminary study of the present-day in-situ stress state in the Ahe tight gas reservoir, Dibei Gasfield, Kuqa Depression

Mar. Petrol. Geol.

(2018)

R.L. Kleinberg et al.

Mechanism of NMR relaxation of fluids in rock

J. Magn. Reson., Ser. A

(1994)

S. Kulesza et al.

A comparative study of correlation methods for determination offractal parameters in surface characterization

Appl. Surf. Sci.

(2014)

J. Lai et al.

Fractal analysis of tight gas sandstones using High-Pressure Mercury Intrusion techniques

J. Nat. Gas Sci. Eng.

(2015)

J. Lai et al.

A review on pore structure characterization in tight sandstones

Earth Sci. Rev.

(2018)

J. Lai et al.

Spectrum of pore types and networks in the deep cambrian to lower ordovician dolostones in Tarim basin, China

Mar. Petrol. Geol.

(2020)

K. Li et al.

Fractal modeling of capillary pressure curves for the Geysers rocks

Geothermics

(2006)

K. Li

Analytical derivation of Brooks–Corey type capillary pressure models using fractal geometry and evaluation of rock heterogeneity

J. Petrol. Sci. Eng.

(2010)

K. Li

More general capillary pressure and relative permeability models from fractal geometry

J. Contam. Hydrol.

(2010)

P. Li et al.

Pore throat structure and fractal characteristics of tight oil sandstone: a case study in the Ordos basin, China

J. Petrol. Sci. Eng.

(2017)

Xiaoping Liu et al.

Insights in the pore structure, fluid mobility and oiliness in oil shales of Paleogene Funing Formation in Subei Basin, China

Mar. Petrol. Geol.

(2020)

Xiaoping Liu et al.

Fractal behaviors of NMR saturated and centrifugal T2 spectra in oil shale reservoirs: the Paleogene Funing formation in Subei basin, China

Mar. Petrol. Geol.

(2021)

M. Meng et al.

Research on the auto-removal mechanism of shale aqueous phase trapping using low field nuclear magnetic resonance technique

J. Petrol. Sci. Eng.

(2016)

E. Müller-Huber et al.

Pore space characterization in carbonate rocks—approach to combinenuclear magnetic resonance and elastic wave velocity measurements

J. Appl. Geophys.

(2016)

Y. Neng et al.

Effect of basement structure and salt tectonics on deformation styles along strike: an example from the Kuqa fold–thrust belt, West China

Tectonophysics

(2018)

T. Nian et al.

The in situ stress determination from borehole image logs in the Kuqa Depression

J. Nat. Gas Sci. Eng.

(2016)

T. Nian et al.

Characterization of braided river-delta facies in the Tarim Basin Lower Cretaceous: application of borehole image logs with comparative outcrops and cores

Mar. Petrol. Geol.

(2018)

R. Rezaee et al.

Tight gas sands permeability estimation from mercury injection capillary pressure and nuclear magnetic resonance data

J. Petrol. Sci. Eng.

(2012)

A. Sakhaee-Pour et al.

Fractal dimensions of shale

J. Nat. Gas Sci. Eng.

(2016)

M. Schmitt et al.

Characterization of Brazilian tight gas sandstones relating permeability and angstrom-to micron-scale pore structures

J. Nat. Gas Sci. Eng.

(2015)

Y. Shen et al.

Effective evaluation of gas migration in deep and ultra-deep tight sandstone reservoirs of Keshen structural belt, Kuqa depression

J. Nat. Gas Sci. Eng.

(2017)

G. Shi et al.

The use of artificial neural network analysis and multiple regression for trap quality evaluation: a case study of the Northern Kuqa Depression of Tarim Basin in western China

Mar. Petrol. Geol.

(2004)

S. Sun et al.

Fracture zones constrained by neutral surfaces in a fault-related fold: insights from the Kelasu tectonic zone, Kuqa depression

J. Struct. Geol.

(2017)

V. Tavakoli et al.

Diagenetic controlled reservoir quality of South Pars gas field, an integrated approach

Compt. Rendus Geosci.

(2011)

Cited by (23)

Study on the effect of wood admixture on the physical and mechanical properties of corn cob ecological recycled concrete
2024, Journal of Building Engineering
This paper investigates the physical and mechanical properties of ecological concrete using corn cob pellets, shavings, and wood chips. Wood chips and shavings were partially used as substitutes for corn cob particles to enhance the physical and mechanical properties of the eco-concrete. The impact of substitution on the macroscopic physical and mechanical properties is examined based on various pore structures. The pore size distribution was analyzed using the non-destructive testing technique of nuclear magnetic resonance (NMR). The pore complexity is analyzed through fractal dimension analysis. The pore distribution was assessed using magnetic resonance imaging (MRI). The roughness of pore distribution was analyzed by combining gray theory and counting the box dimension. The addition of biomass particles results in a 22-fold increase in the content of harmful pores, which can be quantitatively and visually observed using NMR and MRI techniques. This is the reason of compressive strength deterioration. Additionally, the optimal substitution of wood chips is determined. With a maximum biomass admixture of 50%, the uniaxial compressive strength increased by 18.9% (to 2.565 MPa, which meets the strength requirements of self-bearing wall insulation blocks). This study aims to establish a foundation for utilizing ecological concrete in agricultural facilities and rural wall materials.
A novel method for wireless telemetry during air drilling based on air pressure pulses
2024, Geoenergy Science and Engineering
Utilization of air drilling and horizontal well drilling is theorized to be one of the most effective methods for drilling in tight sandstone reservoirs. However, only a few horizontal wells were successfully drilled with air over the decades. One major setback is the lack of an effective downhole wireless telemetry technique. This study offered an innovative solution to wireless telemetry in air drilling, which is called as air pressure pulse telemetry, wherein the downhole decoded information is transmitted via air pressure pulses signals. For the method to work, an air pulse generator was designed to create sufficiently strong positive pulses at the bottom of the well for telemetry. The generator can totally cut off air flow when closed. Theoretically, pressure pulses of any intensity can be generated on demand. Unipolar return to zero (Unipolar RZ) modulation was applied herein for signal modulation. Baseband filter and differential processing technique were used to cancel the noise and enlarge the weak signals, and a method of threshold line is used to decode the signals. To evaluate the performance of the method, a laboratory experimental set was built, and all the sub-methods, such as air pulses generator, signal modulation, noise cancellation, data decoding methods, were tested. The results show that the pulse generator can create sufficiently strong pressure pulses on demand, while signals can be successfully captured and decoded after transmission 2000 m. The experimental results have confirmed the functionality of the novel method of air pressure pulse telemetry. Increasing operating pressure can both create stronger pressure pulses and reduce the attenuation of pulse signals. Reducing the time slot can improve the transmission rate, but it can also cause severe signal attenuation, ultimately leading to telemetry failure. Based on our results, it is recommended that the time slot be no less than 4 s. The proposed method provides a new pathway to achieving reliable wireless telemetry in air drilling. The method was compared with the existing wireless telemetry, and some potential challenges and future research directions were also discussed.
Fault-karst systems in the deep Ordovician carbonate reservoirs in the Yingshan Formation of Tahe Oilfield Tarim Basin, China
2023, Geoenergy Science and Engineering
Fault-karst systems in deep and ultra-deep carbonate reservoirs have a huge potential for hydrocarbon exploration. However the characteristics of reservoir space and the main controlling factors of fault-karst reservoirs are unclear due to strong heterogeneity. Therefore, the exploration and development of deep and ultra-deep carbonate reservoirs are restricted. To fill the gap, core, thin section and well log (conventional logs and image logs) data are used to analyze the lithology and main reservoir space in the deep Ordovician reservoirs in the Tahe Oilfield, Tarim Basin, China. The lithologies are dominated by intraclastic limestone, dolomitic limestone and silt to fine-sized crystalline dolostone. The reservoir space mainly includes dissolution vug, fracture, fracture-vug and cave. Among them, caves and their surrounding fractures are the main storage spaces for hydrocarbons in the Ordovician reservoir. Four fault-karst systems are divided including “V” shape, “U” shape, “layer” shape and compound type according to the morphology of the seismic section. In addition, the reservoir space of different fault-karst systems is clarified using image logs. Combined with drilling data, it is found that the reservoir scale of “U” shape fault-karst system is larger, while that of “layer” shape fault-karst system is smaller. In addition, the direction of maximum horizontal in-situ stress is NE-SW determined by induced fractures picked up by image logs. The NE orientation faults and fractures are increased and the effect of karst fluid on reservoirs is enhanced. However, the NW direction faults and fractures are restricted. Therefore, the fault-karst system is mainly observed along the NE main fault. The fluid alteration has two influences on the reservoir quality. On the one hand, the fluid alteration can dissolve the carbonate rocks and expand the reservoir scale; on the other hand, the fluid will carry mud and sand, which will reduce the reservoir space scale. The distance from the fault will affect the types of reservoir space of the fault-karst system. Away from the fault, the reservoir space will change from caves and fractures to facture-vug, and finally into small-scale dissolution vugs. Therefore, it should not only pay attention to seismic beads features but also to the size of available reservoir space on logging during drilling. This study can provide insight into the different types of fault-karst system models, and help clarify the main controlling factors of the fault-karst system scale.
Logging evaluation of pore structure and reservoir quality in shale oil reservoir: The Fengcheng Formation in Mahu Sag, Junggar Basin, China
2023, Marine and Petroleum Geology
The Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin is characterized by frequent changes of mineral compositions and lamina structure. The alkali lacustrine shale reservoir shows strong heterogeneity and anisotropy, it is particularly important to clarify the relationship between pore structure and reservoir quality. The pore characteristics were investigated by using thin sections, Scanning Electron Microscope (SEM), and Nuclear Magnetic Resonance (NMR) analyses. The pore types are mainly composed of inorganic pores, organic pores, and microfractures. The storage space is dominated by inorganic pores and microfractures. The pore structure was divided into four types by NMR parameters including T₂ spectral morphology, T_2gm, total porosity, and movable fluid porosity. Combined with core NMR experiments and two-dimensional (2D) NMR logs, 21 ms was adopted as the T_2cutoff value to represent the demarcation between movable and bound fluids. From Type I to Type IV, the energy clusters reflect the movable fluids including movable oil and movable water gradually decreasing in the upper right corner. The energy clusters reflect the bound fluids including clay-bound water and bitumen gradually increasing in the lower left corner in the T₁-T₂ map. The results show that the heterogeneity of mineral compositions and lamina structures are the key factors directly leading to the complexity and distribution of pore structure. Quartz with strong compaction resistance helps keep a large number of primary interparticle pores, and feldspar dissolution pores caused by organic acids are present. Dolomite intercrystalline pores and dissolution pores provide a large amount of storage space for the shale, while the cementation of calcite reduces pore spaces. The increase of felsic mineral content is conducive to improving the pore structure and oil-bearing potential of oil shale reservoir, while clay minerals and calcite are contrary. There are abundant primary interparticle pores and dissolution pores in the layered siliceous (felsic) shales, and they consist of the upper sweet spot in the Fengcheng Formation. The longitudinal time (T₁) and transverse time (T₂) spectra are bimodal with high total porosity and movable oil content, reflecting high reservoir quality and oil-bearing property. Laminated dolomite shales contain dolomite intercrystalline pores and dissolution pores, and they consist of the lower sweet spot in the Fengcheng Formation. The results above provide a new perspective on the evaluation of pore structure and reservoir quality of shales in the context of an alkaline lake.
Experimental study on stress-dependent multiphase flow in ultra-low permeability sandstone during CO<inf>2</inf> flooding based on LF-NMR
2023, Energy
Oil production performance in the unconventional reservoirs is significantly affected by the flooding agent and occurrence environment. In this study, the in-situ stress and displacement pressure effects are considered, the response of CO₂/Oil and pore structure was monitored by online low field nuclear magnetic resonance (LF-NMR). The results show that the pore structure was composed of adsorption pore (AP＜3 m s), percolation pore (3 m s＜PP＜30 m s), and migration pore (30 m s＜MP). The stress sensitivity of adsorption pore is more obvious than that of percolation pore and gradually weakened responding to the increase of in-situ stress. The increased in-situ stress narrowed and closured pore throats and limited the migration of CO₂/Oil, and the pore transformed from gas-channeling type into homogeneous type and the hydraulic conductivity of CO₂ and oil decreased non-linearly responding to the pore shrinkage. The percolation pore and migration pore dominate the oil recovery, and the decreased MP resulted in the non-linearly and dynamic reduction of oil recovery. The higher relative permeability was presented and the end-point oil saturation non-linearly increased with the increasing in-situ stress. The increased displacement pressure effectively improved the oil recovery and dynamic gas channeling, and the incremental oil recovery was primarily contributed by the PP and MP due to the increased CO₂ swept area. The positive effect of displacement pressure on oil recovery gradually reduced with the increase of in-situ stress. The findings provide a significant insight on the application of CO₂-EOR in ultra-low permeability reservoirs.
Localized host-rock porosity near sealed fractures at 7 km depth: Geochemical evidence of deep-seated porosity controls
2023, Journal of Structural Geology
In Cretaceous Bashijiqike Formation sandstone from 6.5 to 7 km depth in the Kelasu tectonic zone, Kuqa depression thin section observation, quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), nano CT scanning and digital core reconstruction reveals localized host-rock porosity near sealed fractures. Primary interparticle pores are the main type, followed by some secondary (dissolved) pores at the edge of grains, interparticle secondary pores, intercrystalline pores and microfractures. Dolomite and calcite are the common fracture-filling minerals, while a few fractures are filled with gypsum and quartz. The phases that exist in fractures are progressively more common farther from the fracture. Thus there is a sort of halo around the fracture that is depleted in the phases that exist in the fracture. Although the types of pores are the same, the farther from the fracture, the tighter the rock, the lower the porosity and average pore size, and the weaker the inter-pore connectivity with local preservation of interparticle pores. We propose a mechanism for how this pore structure arises. This study enhances the understanding of the physical properties and pore-fracture interaction of deep tight reservoirs, and contributes to further accurate evaluation of reservoirs.

View all citing articles on Scopus

View full text

Pore structure evaluation in ultra-deep tight sandstones using NMR measurements and fractal analysis

Highlights

Abstract

Introduction

Section snippets

Geological setting

Methodology

Pore spaces, porosity and permeability

Discussion

Conclusions

Author contribution statement

Declaration of competing interest

Acknowledgments

Geochem. Cosmochim. Acta

Chem. Eng. Sci.

J. Petrol. Sci. Eng.

Mar. Petrol. Geol.

Mar. Petrol. Geol.

J. Petrol. Sci. Eng.

J. Nat. Gas Sci. Eng.

J. Appl. Geophys.

J. Appl. Geophys.

Mar. Petrol. Geol.

Mar. Petrol. Geol.

J. Magn. Reson., Ser. A

Appl. Surf. Sci.

J. Nat. Gas Sci. Eng.

Earth Sci. Rev.

Mar. Petrol. Geol.

Geothermics

J. Petrol. Sci. Eng.

J. Contam. Hydrol.

J. Petrol. Sci. Eng.

Mar. Petrol. Geol.

Mar. Petrol. Geol.

J. Petrol. Sci. Eng.

J. Appl. Geophys.

Tectonophysics

J. Nat. Gas Sci. Eng.

Mar. Petrol. Geol.

J. Petrol. Sci. Eng.

J. Nat. Gas Sci. Eng.

J. Nat. Gas Sci. Eng.

J. Nat. Gas Sci. Eng.

Mar. Petrol. Geol.

J. Struct. Geol.

Compt. Rendus Geosci.