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Article

Pore Characteristics and Influencing Factors of Marine and Lacustrine Shale in the Eastern Sichuan Basin, China

by
Jianglin He
1,2,*,
Lixia Zhu
1,2,
Ankun Zhao
1,2,
Dong Wang
1,2,
Zhen Qiu
3 and
Ping Yang
1,2
1
Chengdu Center of Geological Survey, China Geological Survey, Chengdu 610082, China
2
Key Laboratory for Sedimentary Basin and Oil and Gas Resources, Ministry of Natural Resources, Chengdu 610081, China
3
PetroChina Research Institute of Petroleum Exploration and Development, China National Petroleum Corporation, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Energies 2022, 15(22), 8438; https://doi.org/10.3390/en15228438
Submission received: 20 October 2022 / Revised: 5 November 2022 / Accepted: 9 November 2022 / Published: 11 November 2022
(This article belongs to the Special Issue Sedimentary Organic Matter in Shale Oil/Gas Systems)
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Abstract

:
Although almost all the shale gas in China is exploited from marine shale (Wufeng–Longmaxi Formation) in Sichuan Basin and several prolific wells, it has also been obtained in Jurassic lacustrine shale. However, the reservoir conditions of the lacustrine shale are not well understood, which has impeded a breakthrough regarding lacustrine shale gas in Sichuan Basin. To probe the reservoir conditions of the lacustrine shale in Sichuan Basin, we take the Wufeng–Longmaxi shale and Dongyuemiao shale sampled from wells and outcrops as examples. A series of experiments were conducted, including TOC, XRD, FE-SEM, N2 adsorption, Micro-CT, vitrinite reflectance and bitumen reflectance. The results show that the pores in marine shale are mainly composed of organic-matter-hosted pores (OM pores). However, in the lacustrine shale, the pores are mainly composed of dissolution pores and intergranular pores. The marine shale is characterized by small-caliber and large-volume pores in which cluster pores are levitated in the shale as kites and connected by past channels. However, in the lacustrine shale, the cluster pores and the past channels are mainly arranged according to the flow channels in the vertical direction. The arrangement of the pores in the marine shale is obviously deformed by compaction. The lacustrine shale is characterized by under-compaction. It can be deduced that the sweet spots for lacustrine shale gas are likely located at the areas characterized by under-compaction resulting from fluid pressure conducted upward, such as the hinge zone of syncline or the core of anticline overlap on the gas reservoirs.

1. Introduction

Shale gas, as an alternative resource awaiting exploitation, has received much attention for its large reserves [1]. Marine shale gas and terrestrial shale gas have been successfully developed in North America and China [2]. In China, shale gas exploration began in 2005 [3]. Up to 2021, the annual output of shale gas has amounted to 230 × 108 m3 in China [4], which is the world’s second most. Meanwhile, more than 90% of the shale gas was derived from Sichuan Basin [5,6] (Figure 1a). The Fuling shale gas field is also located in Eastern Sichuan Basin (Figure 1b), which is the largest shale gas field outside North America [7]. Up to now, almost all the shale gas from Sichuan Basin has been exploited from marine shale (Wufeng–Longmaxi Formation) [5]. However, with the development of shale gas, most of the unexploited marine shale gas is buried deeper than 3500 m in Sichuan Basin. A major challenge is being faced to maintain the increase in shale gas production in China because of more and more expensive [8] and harsh technical requirements for the hydraulic fracturing of deep shale gas (burial depth > 3500 m).
Since 2011, some authors have been focused on the lacustrine shale gas exploration in Sichuan Basin [9,10]. Up to now, industrial shale gas flow has been obtained in some areas of Sichuan Basin, such as Fuling [10,11], Yuanba [9] and Pingchang [12]. Several prolific wells (>20 × 104 m3/d) were discovered in the Jurassic lacustrine shale. Especially, the daily production of well YL-21 is 50.9 × 104 m3/d [13], whose commercial value is comparable to a marine shale gas well. Previous studies deduced that the area is about 4 × 104 km2 [14], favorable for lacustrine shale gas exploration in Sichuan Basin. The Jurassic lacustrine shale has seemed to be a key player for increasing the shale gas and oil production in Sichuan Basin in the next several years [15]. However, the practices show that the large resource (10 × 108 t to16 × 108 t) is not matching the small geological reserves of shale gas and oil (up to now, merely 0.8 × 108 t has been demonstrated) in the lacustrine shale. The poor reservoir properties are not matching the relatively high production of oil and gas [15]. These factors indicate that the reservoir conditions of the lacustrine shale have not been well understood, which has impeded obtaining a breakthrough regarding lacustrine shale gas in Sichuan Basin [15].
In this work, based on geochemical, mineral, imaging and physical tests, we compared the difference of pore characteristics and influencing factors for marine shale (Wufeng–Longmaxi shale) and lacustrine shale (Dongyuemiao shale) in the Eastern Sichuan Basin (Figure 1d), where industrial shale flow has been achieved both in marine and lacustrine shale. The pore characteristics and influencing factors will be considered, such as the formation mechanism, spatial arrangement and tectonic evolution of the pores, with an aim to probe the reservoir conditions of the lacustrine shale in Sichuan Basin. This research is expected to provide a useful reference for the commercial development of lacustrine shale gas in Sichuan Basin.

2. Geological Setting

Sichuan Basin is located on the western margin of Yangtze Craton in southwestern China [16], which covers an area of 180,000 km2 [17]. Sichuan Basin comprises a 6000 to 12,000 m thick [18] Ediacaran to Cenozoic sedimentary succession overlying the pre-Sinian Proterozoic basement (Figure 1d). According to the difference in tectonic evolution of various regions in Sichuan Basin, it can be divided into the Eastern Sichuan Basin, the North Sichuan Basin, the Western Sichuan Basin, the Central Sichuan Basin, the Southwestern Sichuan Basin, and the Northwest Sichuan Basin (Figure 1a) [4]. The Eastern Sichuan Basin is a prominent Mesozoic NE-trending fold-and-thrust belt [9], where the high and steep structural zones were formed dominated by the intense compressing and thrusting force during later Yanshan Movement and Himalayan movement (Figure 1b,c) [19]. In the Eastern Sichuan Basin, the synclines are wider than the anticlines and are flat-bottomed [20].
Sichuan Basin has experienced a complex tectonic and sedimentary history since the breakup of Rodinia at approximately 750 Ma [21]. It consists of four main stages: (1) the Neoproterozoic rift stage (750–635 Ma), (2) the passive continental margin stage (635–488 Ma), (3) the Proto-Tethys ocean closure stage (488–420 Ma) and (4) the Orogeny stage (420 Ma to present) [22]. During the late Ordovician to early Silurian period, the Wufeng–Longmaxi Formation was deposited as the Middle-Upper Yangtze evolved into a shallow restricted basin [23,24] and a huge and thick marine shale was deposited in the deep inner shelf basin [24], which is the key production layer of shale gas in China now [22]. As the upper Yangtze collided with the Tethys block or the arc-basin collisional zone to the west [25] during the late Triassic to early Jurassic, Sichuan Basin was dominated by shores, shallow lakes and semi-deep lake facies during the early Jurassic period [26]. The Ziliujing Formation was deposited in this period, which is 200–900 m thick (Figure 1d) and consisted of Zhenzhuchong (J1z), Dongyuemiao (J1–2z1), Ma’anshan (J1–2z2) and Da’anzhai (J1–2z3) members in the ascending stratigraphic order [27]. The maximum flooding surface in the early Jurassic period occurred in the middle part of the Dongyuemiao member [18]. The area of the lacustrine shales in Ziliujing Formation is around 78,000 km2 [14] in Sichuan Basin.

3. Sample and Methods

3.1. Samples

In this work, both the core samples and outcrops data are taken into consideration to evaluate the influence of weathering action. The marine shale samples are both collected from the well HD-1 and SBT outcrops (Figure 1b). Previous studies have reported a great deal of test data of core samples from lacustrine shale, which are available for comparative study. The lacustrine shale samples were mainly collected from the LMS outcrops (Figure 1b), which are excavated more than 2 m under the surface. As shown in Table 1 and Table 2, 55 samples were collected for TOC and XRD testing; 19 marine shale samples were collected for bitumen reflectance test, and 10 lacustrine samples were collected for vitrinite reflectance test. Further, 55 samples were collected for porosity and permeability test, but merely 35 samples were tested successfully; 41 samples were collected for N2 adsorption test. According to the porosity test results, the samples HD1-17, SBT-R10 and LMS-T9 were collected for Micro-CT test. The porosity values of HD1-17 and SBT-R10 are close to the average porosity of all the marine shale samples. The porosity of sample LMS-T9 is also close to the average porosity of all the lacustrine shale samples (Table 1).

3.2. Analytical Methods

The total organic carbon (TOC) was tested at Key Laboratory of Shale Gas Exploration, Ministry of Land and Resources in China, Chongqing Municipality, China according to the Chinese National Standard methods GB/T 19145-2003 [30]. The shale samples were powdered to less than 200 mesh particles. Appropriate amount of the powder was weighed by removing the inorganic carbon with excess hydrochloric acid. Then, they were diluted to neutral with distilled water. Finally, they were dried to determination, with 17 °C and 59% RH humidity [4]. The TOC can be expressed as the weight percent, which is calculated by the formula:
TOC = (WR/WT) × 100
where TOC is the total organic carbon, %; WR and WT are the residual weight after dried and the total weight of the powder, respectively, g.
The thermal evolution of the organic matter was tested in the Engineering Research Center for Shale Gas Resource and Exploration, Chongqing Institute of Geology and Mineral Resources, China. The thermal evolution of the lacustrine shale was determined by the vitrinite reflectance (Ro). Vitrinite reflectance was measured by a Leitz MPV-3 micro-photometer under the oil immersion reflected optical light according to the Chinese National Standards GB/T 6948-1998 [31]. For each sample, about 30 different vitrinite observation points were randomly selected for measurements and the averaged data were used. The thermal evolution of marine shale was determined by the bitumen reflectance (Rb). The bitumen reflectance was measured with a polarizing microscope Germany Lycra Axioscope A1 and American CRAIC spectrophotometer +MSP 400. All tests were conducted in accordance with Chinese Industrial Standard SY/T 5124-2012 [5].
X-ray diffraction (XRD) was tested at Key Laboratory of Shale Gas Exploration, Ministry of Land and Resources in China, Chongqing Municipality, China. The shale samples were powdered less than 200 mesh (i.e., <75 lm). Then, the powder was tested with a Bruker D8 DISCOER diffractometer (Co Kα-radiation, 45 kV, 35 mA) according to the two independent processes of the CPSC procedure. The diffracted beam was tested with scintillation at 0.02° 2 θ step size and 20 s step time. Diffractograms are derived from 2° to 76° 2 θ [32].
The field emission scanning electron microscopy (FE-SEM) was tested in Engineering Research Center for Shale Gas Resource and Exploration, Chongqing Institute of Geology and Mineral Resources, China. The shale samples were polished with argon ion milling instrument (Hitachi High-Tech IM4000). After the argon-ion-beam milling, the vertical sectioned samples were coated with carbon. The samples were inserted into an FEI Helios NanoLabTM 650 DualBeam FIB-SEMTM for imaging and SEM Maps [1]. Images were taken using back-scattered electron at 5 kV acceleration voltage and a working distance of 4 mm [33].
Nitrogen adsorption was tested by the Micromeritics ASAP 2420 instrument. The samples were pulverized to about 60–80 mesh. About 1.5–2.5 g pulverized samples was used for nitrogen adsorption tests. Then, degassing was performed at an ambient temperature of 25.0 °C before testing N adsorption and desorption amounts. After degassing, the N2 adsorption amount was tested at −195.8 °C, with an increase in the P/Po from 0.01 to 0.99. Then, keep the equilibration about 10 s. During the desorption process, P/Po was decreased from 0.99 to 0.14. The N2 desorption isotherms were approximately obtained [34].
Helium porosity was tested at Sichuan Kelite Oil and Gas Technology Service Company Limited (Chengdu, China), China. The samples were drilled to a diameter of 2.54 cm and length of 5 cm for porosity and permeability tests. The porosity was calculated by dividing the pore volume from the total volume according to Chinese national standard GB/T 29172-2012 [1].
Pulse permeability was tested at Sichuan Kelite Oil and Gas Technology Service Company Limited (Chengdu, China), China with YCS-II briquette and a rock permeability tester at 17 °C temperature and 55–60% humidity [5]. The permeability was measured by pressure pulse attenuation method, which is a non-steady permeability test method, according to the Chinese National Standard GB/T 34533-2017 [1,35].
The Micro-CT scans were tested by a compact laboratory CT system (GE phoenix nanotom m 180). First, core plugs were drilled with 50 mm length. Then, the samples were scanned over 360° by a large field of view mode, with an exposure time of 120 s, and 1600 slices were collected from this scanning process [1]. Finally, the 3D image of the sample was generated from Avizo Fire 8.1 by importing data into it. The quantitative analyses of the pore length and width were conducted on the rendered volumes in Avizo by assigning different gray scale values to the microstructural features of interest. The pore throat connectivity was qualitatively determined based on the gray scale values of different segmentations [36].

4. Results

4.1. Organic Geochemical Characteristics

4.1.1. Total Organic Carbon Contents

As shown in Table 1, the TOC of marine shale samples ranges from 0.95% to 4.43%, with an average of 3.39% in well HD-1 at about 1330 m depth (Figure 2), which is similar to the TOC of the marine shale at SBT outcrops in the range of 0.97% to 6.00% (average 3.05%). The TOC of the lacustrine shale ranges from 1.02% to 3.61%, with an average of 1.66% at the LMS outcrops, which is similar to the TOC of the Dongyuemiao shale drilled at the depth of 2268–2810 m as well. In well FY-10, the range is from 0.16% to 4.04%, with an average of 1.74% [11], and the average of TOC is 1.26% and 3.74% in wells XL101 and HF-1, respectively [29].

4.1.2. Maturity of Organic Matter

Considering that there is no vitrinite in the marine shale kerogen, the maturity of the marine shale was tested by bitumen reflectance (Rb). Then, the equivalent vitrinite reflectance (eq Ro) was calculated by the empirical formula eq Ro = 0.618 Rb + 0.4 [28]. As shown in Table 1, the eq Ro of marine shale samples ranges from 1.71% to 2.08%, with an average of 1.96% in well HD-1 at about 1300 m depth, which is larger than the eq Ro of samples from SBT outcrops that range from 1.14% to 1.89%, with an average 1.50%. The Ro of lacustrine shale ranges from 1.23% to 2.09% (av. 1.64%) sampled from the wells at a depth of 2268–2810 m, which is much more mature than the Ro of samples from LMS outcrops, which are in the range of 0.51% to 0.84% (average 0.68%).

4.2. Mineralogy

The mineral component of marine and lacustrine shale samples is presented in Table 1 and Figure 3. It shows that all the shale samples are mainly composed of quartz, feldspar, carbonates, pyrite and clay. The quartz content in the marine shale is much richer than that in the lacustrine shale. In the marine shale, the quartz contents range from 3.5% to 100%, with an average of 49.7%. Most of them are larger than 35%. The quartz content is smaller than 10% in the bentonite which is a millimeter-scale interlayer in marine shale, such as in samples HD1-15 and SBT-R2 in Table 1. Most of the quartz content in marine shale is larger than 35%. Especially, the quartz content is 100%, as observed in the siliceous shale in the Wufeng Formation (Table 1). The quartz contents range from 11% to 46%, with an average of 25.8% in the lacustrine shale samples, which is obviously smaller than in the marine shale. Both the marine and lacustrine shale also contain abundant clay minerals. In the marine shale, the clay mineral content ranges from 0 to 87.7%, with an average 33.23%, which is smaller than in the lacustrine shale (39–75%, average 57.4%).
It is generally accepted that, the richer the content of brittle minerals, the more favorable they are for later reservoir reconstruction. The brittleness index has been used to characterize the brittleness of shale, and brittle minerals mainly include carbonate, quartz, feldspar and pyrite [37]. According to the brittleness index calculation formula [38]:
BI = (WQ + WF + WC + WD)/WT
where BI is the brittleness index, WQ, WF, WC and WD are the weights of quartz, feldspar, calcite and dolomite, respectively, and WT is the total mineral weight.
The brittleness index of all the shale samples was calculated (Table 1). The results show that the distribution span of the brittleness index of marine shale is large (12.3–100), and the average value is 66.8. The brittleness index of lacustrine shale ranges from 25 to 61, with an average value of 42.3 (Figure 3).

4.3. Pore Morphology Analyses Based on FE-SEM Images

An FE-SEM image is mainly used to qualitatively characterize the pore structure characteristics of shale, such as the distributions, shapes, sizes and arrangement of pores [39]. The FE-SEM images show that many micro-nano pores are in marine and lacustrine shale samples. According to the relationship between pores and minerals, the pores can be mainly divided into the following three categories: organic-matter-hosted pores (OM pores), dissolution pores and intergranular pores.
The OM pores are mainly in the organic matter (Figure 4a–d). Previous studies recognized that the formation of OM pores is mainly controlled by kerogen type, organic matter abundance and thermal evolution degree [40,41]. In the Eastern Sichuan Basin, many OM pores are developed in both marine and lacustrine shale samples. The OM pores possess strong microscopic heterogeneity, and their pore diameters vary between several nanometers and hundreds of nanometers. The OM pores in the lacustrine shale (Figure 4b) are obviously more circuitous and jagged than in the marine shale (Figure 4c). The diameter of the OM pores in lacustrine shale is smaller than in marine shale. In the lacustrine shale, all the OM pores are arranged in a disorderly manner in the organic matter (Figure 4b,d). In the marine shale, most of the OM pores are arranged in a disorderly manner in the organic matter (Figure 4a) as well. However, part of the OM pores involves directional alignment in postmature organic matter, which is obviously influenced by extrusion stress (Figure 4c). Previous studies concluded that OM pores are mainly formed during the kerogen thermal cracking stage [41]. The diameter of OM pores likely increases as kerogen thermally cracks, which may lead to the strength of the organic matter decreasing. Hence, the OM pores are more likely to be affected by burial compaction or tectonic stress.
In this work, the dissolution pores are mainly observed in the organic matter (Figure 4c,d) and the clearance between the minerals (Figure 4e,f). The shape of the pores is mainly round or approximately round. The wall of holes is smooth, which indicates that the pores are likely connected with other pores as a migration method for the fluid, such as water or hydrocarbon fluid. The diameter of these pores is mainly wider than 2 μm, which is obviously larger than the OM pores. However, the dissolution pores in the lacustrine shale are much wider than those in the marine shale. The diameter of the dissolution pores in lacustrine shale is commonly larger than 50 μm, which is likely correlated to the discharge of the formation water in the burial process. Meanwhile, the dissolution pores in the lacustrine shale are commonly isolated from each other (Figure 4f). The dissolution pores in the marine shale are partly connected with each other, which may indicate that the fluid is not discharged from the shale easily after the diagenesis of shale. The dissolution pores are connected with each other by the micro-fractures in the overpressure unit (Figure 4e).
The intergranular pores are also observed in the marine and lacustrine shale (Figure 4g,h). In the marine shale, the intergranular pores are mainly observed between the particles of pyrite. They are mainly an angular shape, with a diameter smaller than 1μm. In the lacustrine shale, the intergranular pores are not only observed in the crystal druse of pyrite but also between the clay mineral and quartz (Figure 4h). The diameter of the intergranular pores ranges from several nanometers to several micrometers, with various shapes. The diameter and the shape of the intergranular pores are dominated by the arrangement of the minerals and the diameter of the minerals. As the diagenesis progressed, the intergranular pores gradually reduced for the mineral crystallization, tending to parallel arrangement and the cementation of the strata fluid. Hence, the intergranular pores are much fewer than in the lacustrine shale.

4.4. Microscopic Pore Structure Characteristics

It is known that the gas in shale is mainly in the adsorption and free state [42]. The gas in shale is mainly adsorbed in the mesopores (the diameter is 2–50 nm) and micropores (diameter < 2 nm). The free gas is mainly trapped in the macropores (diameter > 50 nm) [29]. The N2 adsorption can well characterize the content of the mesopores and micropores. Hence, N2 adsorption is commonly used to study the mesopores and micropores of shale.
As shown in Figure 5 and Table 2, the specific area of marine shale ranges from 1.68 to 35.95 m2/g, with an average of 16.75 m2/g. In the lacustrine shale, the specific area ranges from 8.42 to 23.18 m2/g, with an average of 15.75 m2/g, which is smaller than the marine shale. The total volume of the pores ranges from 0.0044 to 0.0867 cc/g, with an average of 0.027 cc/g in the marine shale, which is smaller than that in the lacustrine shale. In the lacustrine shale, the total volume of the pores ranges from 0.0237 to 0.0739 cc/g, with an average of 0.0418 cc/g. The mean pore diameter of the marine shale ranges from 3.74 nm to 33.99 nm, with an average of 7.5 nm, which is narrower than that in the lacustrine shale (7.37–13.27 nm, av. 10.647 nm).
The adsorption isotherms of each shale sample are different, which is mainly caused by the strong heterogeneity of shale [43]. As shown in Figure 6, the N2 adsorption amount gradually increases with increasing pressure, and the phenomenon of saturated adsorption is not shown until the relative pressure is close to 1.0, reflecting the presence of macropores in both marine and lacustrine shales [44]. Although the adsorption isotherms of each sample are slightly different in morphology, they all exhibit anti-S type with increasing relative pressure. This means that the adsorption isotherms of shales belong to typical IV-type isotherms, indicating that the pores in the shales are a complete pore system range continuously from micropores to macropores. In all the shale samples, the adsorption and desorption curves overlap at a relatively low pressure (0–0.4); thus, no adsorption loop would form, while a large adsorption loop would form at a relatively high pressure (0.4–1.0). When the pressure is approaching 0.5, there is a turning point at which a sharp drop could occur in the desorption curve. It is indicated that the samples are of small-caliber and large-volume pores, where microcracks are better developed. These kinds of pores are beneficial for adsorption of gas but are not conductive to desorption and diffusion due to their bad ventilation [45]. Although the total pore volume of sample HD1-M10 is smaller than that of sample LMS-M5, the desorption curve of sample HD1-M10 drops much more sharply than the desorption curve of sample LMS-M5, which indicates that marine shale is more obviously characterized by small-caliber and large-volume pores.

4.5. Reservoir Physical Property

The porosity of marine shale samples varies from 1.08% to 7.16%, with an average of 2.76% in well HD-1, which is a little smaller than in the SBT outcrops. The porosity of the sample ranges from 2.23% to 5.19%, with an average of 3.69%. The porosity of the lacustrine shale is larger than that of the marine shale. The porosity of the lacustrine shale varies from 2.84% to 6.94%, with an average of 4.47% in the outcrops LMS and in the range 1.64–6.68%, with an average of 4.5% in the wells (Table 1).
An extreme variation is observed in the test results of shale permeability. In the SBT outcrops, the permeability of the marine shale varies from 0.002 to 7.18 md, with an average 0.872 md, which is much larger compared to the core samples from well HD-1. The permeability of the marine shale from well HD-1 ranges from 0.001 to 0.328 md, with an average 0.052 md (Table 1). In the lacustrine shale, the permeability in the outcrops is larger than from the wells as well. In the outcrops LMS, the permeability of the shale varies from 0.003 to 1.23 md, with an average 0.513 md, which is larger compared to the core samples (0.44–0.729 md, av. 0.134 md). As shown in Figure 7, the reservoir physical property of the lacustrine shale is better than marine both in porosity and permeability.

5. Discussion

5.1. Difference in Formation Mechanism of the Pores

Shale is defined as a type of tight rock. During the burial process, a large number of the pores in it are compacted by the compaction of overlapped strata. As mentioned above, the resident pores in the shale are mainly of three types: OM pores, dissolution pores and intergranular pores.
The OM pores are commonly in nanometer-scale, with the protection of an organic matter framework, which is a benefit in terms of not being destroyed by the compaction. Meanwhile, the kerogens are cracked to oil and gas with thermal evolution, which not only contributes to the formation of OM pores but also the formation of overpressure in the shale, which can offset part of the compaction in shale. Hence, it is generally acknowledged that a higher TOC content means a stronger shale gas storage capacity [46]. However, with kerogen cracking progression, the diameter of the OM pores becomes more and more wide. The OM pores may be compacted by extrusion stress (Figure 4c). Hence, the porosity of over-mature shale may increase first and then decrease slightly or even present a negative correlation directly with the increase in TOC content [41]. Compared to lacustrine shale, marine shale is much more organic-rich (Figure 2). Meanwhile, there is a positive correlation between content of TOC and quartz and clay mineral in marine shale (Figure 8), which indicates that the endogenous inputs and the adsorption of the clay mineral contribute to the enrichment of organic matter in marine shale [47]. It is obviously different from lacustrine shale, in which the organic matter is coming from both the terrestrial organic matter and algae in the paleo-lake [18]. There is no obvious correlation between content of TOC and quartz and clay mineral in lacustrine shale (Figure 8). Meanwhile, there is a positive correlation between TOC content and porosity in marine shale (Figure 9), which is consistent with the character of the pore structures based on the N2 adsorption results (Figure 6). However, there is no obvious correlation between TOC content and porosity in lacustrine shale (Figure 9). This may be caused by the small contribution of OM pores to the porosity in lacustrine shale regarding the low TOC content (Figure 2) and low thermal evolution stage (Table 1) of the lacustrine shale.
In the early diagenesis stage, the water in sediment is expelled through a large number of isolate pores by compaction. With the fluid flowing out, a few macropores are formed in sediments (Figure 4f) by the fluid pressure and the dissolution of part of the pore wall. In the middle diagenesis stage, framework minerals undergo corrosion for generation of organic acids, forming intraparticle dissolution pores. However, part of the pores may be cemented with the diagenesis. The fluid in the strata cannot be expelled and is entrapped in the local area. As the fluid becomes entrapped more and more, the overpressure is formed. The pores are connected in some local area with the corrosion curves (Figure 4e). As shown in Figure 4e,f, the dissolution pores in lacustrine shale are mainly formed by the expulsion of fluid in the shale. The dissolution pores in marine shale are mainly formed by the corrosion of organic acids.
The intergranular pores are associated with the framework of mineral particles, especially best developed in pressure shadows of brittle particles, which can protect these pores from collapse. In addition, due to the strong plasticity of clay minerals [48], it is easy to be compacted and deformed; thus, rigid particles have a significant effect on the preservation of intergranular pores. As shown in Figure 3, the brittle index of marine shale is larger than in lacustrine shale. However, merely a few intergranular pores are observed in the marine shale (Figure 4g). The intergranular pores are commonly observed in the lacustrine shale samples. The contribution of the intergranular pores is obviously smaller in the marine shale than in the lacustrine shale.

5.2. Arrangement Differences of the Pores

As shown in Figure 9, the TOC is positively correlated to the porosity (R = 0.84), but there is no obvious correlation between TOC and permeability. Meanwhile, the porosity of the marine shale is smaller than that of the lacustrine shale (Figure 7), while the permeability is similar. This indicates that the permeability of the shale is not only influenced by the porosity but also by the arrangement of the pores. As shown in Figure 10, the distribution of the pores is obviously different in marine and lacustrine shale. In the marine shale (Figure 10a), a few pores are connected in the horizontal direction as cluster pores. The past channels are characterized by the linear parallelism pores, which had been the migration path for the fluid in the early to middle diagenesis stage. They are oblique in arrangement. Part of the past channel is blocked by the cemented or squeezed action during the diagenesis process. The pores in the past channel are mainly in the nanometer-scale and intermittently connected. The cluster pores are levitated in the shale as kites and connected by the past channels. In the cluster pores, the pores are connected with each other, with the overpressure to offset the vertical extrusion stress. It is consistent with the N2 adsorption results as well (Figure 6), which are characterized by small-caliber and large-volume pores. In the lacustrine shale (Figure 10b), the cluster pores and the past channels are mainly arranged according to the flow channels in the vertical direction. Although there are more past channels in lacustrine shale than in marine shale, merely a few past channels are connected with the cluster pores. Hence, although the porosity of the lacustrine shale is larger than the marine shale, the permeability is similar to marine shale (Figure 7).

5.3. Difference in Tectonic Evolution

As mentioned above, the lacustrine shale is obviously different from the marine shale in the formation mechanism and arrangement of the pores, which is not only controlled by the material basis but also the thermal degradation and the compaction during their burial process. The equivalent vitrinite reflectance of the marine shale is 1.50% and 1.96% in the outcrops and well, which is more mature than the lacustrine shale, with Ro 0.68% and 1.64% in the outcrops and well, respectively (Table 1). The lacustrine shale has entered into the mature to wet gas stage [49]. According to the thermal model based on the test results of fluid inclusions [11], the lacustrine shale experienced the maximum formation temperature of about 140 °C, with a burial depth of about 4500 m (Figure 11a). The kerogen in lacustrine shale is mainly converted to oil and gas condensate in the Eastern Sichuan Basin. In this process from solid kerogen to oil gas mixture, the volume is not increasing as sharply as in the marine shale. Meanwhile, the TOC in lacustrine shale is smaller than that in marine shale. The hydrocarbon generation pressurization is not as obvious as in the marine shale either. Hence, the cluster pores are mainly in a vertical direction along the fluid flow channel. The marine shale experienced a maximum formation temperature of about 210 °C, with a burial depth of about 6000 m (Figure 11b). The marine shale progressed to the high to over-mature stage. It is indicated that residual liquid hydrocarbon and heavy hydrocarbon gas had been completely converted to gas before being uplifted [47]. Hydrocarbon generation pressurization is more easily formed, which is a benefit to the pores to offset compaction. The lateral permeability is commonly larger than the vertical permeability of marine shale [50]. Its TOC is richer than that of lacustrine shale. The OM pores are more easily connected with each other at the adjacent organic matter, with the action of hydrocarbon generation pressurization. Hence, the cluster pores in marine shale are approximately parallel to the bedding (Figure 10).
In the Eastern Sichuan Basin, the lacustrine shale (Dongyuemiao Member) is overlapped on the marine shale (Wufeng–Longmaxi Formation). The maximum burial depth of the marine shale experienced is larger than that of the lacustrine shale (Figure 11). According to the burial history model (Figure 11), the lithostatic pressure is about 117 MPa at the deepest burial stage of lacustrine shale, estimated with the average density 2.6 kg/m3 for the upper formation. The lithostatic pressure is about 156 MPa at the deepest burial stage of marine shale, which is about 39 MPa larger than the lacustrine shale experienced. Meanwhile, the marine shale that has undergone compaction action time is much longer than lacustrine shale (Figure 11). Hence, the past channels are mainly low-angle-inclined in the marine shale and high-angle-inclined in the lacustrine shale (Figure 10). It is indicated that the influence of compaction action on the pores is more obvious in the marine shale than in the lacustrine shale.

6. Conclusions

According to geochemical, mineral, imaging and physical tests in Wufeng–Longmaxi shale and Dongyuemiao shale, this work compared the differences in pore characteristics and influencing factors regarding the marine and lacustrine shale in the Eastern Sichuan Basin. Following are the main conclusions drawn from the study:
(1)
The TOC of the marine shale is in the range of 0.97–6.00%, with an average 3.39%, which is richer than the lacustrine shale, with an average 1.74%. Meanwhile, the thermal evolution of the marine shale (av. eq Ro = 1.96%) is also more mature than the lacustrine shale (av. Ro is 1.64% in well and 0.68% in outcrops). These are benefits of OM pores formation in marine shale. In marine shale, TOC is positively correlated to porosity. Few intergranular pores are observed in the FE-SEM images. It is indicated that the pores in the marine shale are mainly composed of OM pores. However, there is little correlation between TOC content and porosity in the lacustrine shale. Dissolution pores and intergranular pores are commonly observed in the FE-SEM images. It is indicated that the pores in the lacustrine shale are mainly composed of dissolution pores and intergranular pores. This difference is not only correlated to the difference in TOC content but also the difference in thermal evolution. In the over-mature stage, the OM pores would be expanded, with the kerogen cracking and hydrocarbon generation pressurization.
(2)
The arrangement of the pores is obviously different in marine and lacustrine shale. In the marine shale, the cluster pores are levitated in the shale as kites and connected by past channels. The marine shale is characterized by small-caliber and large-volume pores. However, in the lacustrine shale, the cluster pores and the past channels are mainly arranged according to the flow channels in the vertical direction. Although the porosity of the marine shale is smaller than that of the lacustrine shale, the permeability is similar in each. No obvious correlation has been observed between porosity and permeability in the shale. The permeability of the shale is likely dominated by the arrangement of the pores.
(3)
The arrangement of the pores in marine shale is obviously deformed by compaction. Hydrocarbon generation pressurization is a benefit to the pores to offset the compaction, which promotes formation of lateral cluster pores. However, in lacustrine shale, although the porosity of the core samples is smaller than in the outcrops, the pores are not obviously compacted with the characteristic of lateral arrangement. The lacustrine shale is characterized by under-compaction, which commonly results from fluid pressure conducted upward by the vertical channels. The compaction action is stronger in marine shale than in lacustrine shale in the Eastern Sichuan Basin, China.
Hence, it can be deduced that the sweet spots for lacustrine shale gas are likely located at the areas characterized by under-compaction resulting from the fluid pressure conducted upward, such as the hinge zone of syncline or the core of anticline overlap on gas reservoirs.

Author Contributions

J.H.: experiment, data evaluation, conceptualization, methodology and writing of original draft manuscript. A.Z.: sampling and experiment. L.Z.: drawing and experiment. D.W.: sampling and field geological survey. Z.Q.: data evaluation and supervision. P.Y.: experiment and data evaluation. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant No. 41772126) and the China Geological Survey (Grant No. DD20221661).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Geological overview of Sichuan Basin and the distribution of the study area. (a) The shape and structure units of Sichuan Basin. (b) Map of the Eastern Sichuan Basin showing the geomorphological features in the study area and the location of wells and outcrops. (c) The structural profile of Eastern Sichuan Basin showing the structure style and the location of well HD-1 and well FY-10. (d) Stratigraphic column of Eastern Sichuan Basin showing the vertical configuration relationship between marine and lacustrine shale.
Figure 1. Geological overview of Sichuan Basin and the distribution of the study area. (a) The shape and structure units of Sichuan Basin. (b) Map of the Eastern Sichuan Basin showing the geomorphological features in the study area and the location of wells and outcrops. (c) The structural profile of Eastern Sichuan Basin showing the structure style and the location of well HD-1 and well FY-10. (d) Stratigraphic column of Eastern Sichuan Basin showing the vertical configuration relationship between marine and lacustrine shale.
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Figure 2. The TOC content of the marine and lacustrine shale samples from the wells and outcrops.
Figure 2. The TOC content of the marine and lacustrine shale samples from the wells and outcrops.
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Figure 3. The mineral characteristics of the marine and lacustrine shale samples.
Figure 3. The mineral characteristics of the marine and lacustrine shale samples.
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Figure 4. The pore morphological characteristics in the FE-SEM images. (a) The OM pores in the marine shale. (b) The OM pores in the lacustrine shale. (c) The dissolution pores in the organic matter are deformed by the extrusion stress in the marine shale. (d) Part of the OM pores were dissolved in the lacustrine shale. (e) The dissolution pores in the marine shale are partly connected with each other by the micro-fractures. (f) The dissolution pores in the lacustrine shale are isolated from each other. (g) In marine shale, the intergranular pores in the clusters of pyrite crystals. (h) The lacustrine shale; the intergranular pores are commonly observed between the mineral particles.
Figure 4. The pore morphological characteristics in the FE-SEM images. (a) The OM pores in the marine shale. (b) The OM pores in the lacustrine shale. (c) The dissolution pores in the organic matter are deformed by the extrusion stress in the marine shale. (d) Part of the OM pores were dissolved in the lacustrine shale. (e) The dissolution pores in the marine shale are partly connected with each other by the micro-fractures. (f) The dissolution pores in the lacustrine shale are isolated from each other. (g) In marine shale, the intergranular pores in the clusters of pyrite crystals. (h) The lacustrine shale; the intergranular pores are commonly observed between the mineral particles.
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Figure 5. The specific area, total pore volume and mean pore diameter from the N2 adsorption test results.
Figure 5. The specific area, total pore volume and mean pore diameter from the N2 adsorption test results.
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Figure 6. The adsorption and desorption curves of the typical marine and lacustrine shale samples based on the N2 adsorption tests.
Figure 6. The adsorption and desorption curves of the typical marine and lacustrine shale samples based on the N2 adsorption tests.
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Figure 7. The reservoir physical properties of the marine and lacustrine shale samples. (a) The porosity tested results of the marine and lacustrine shale; (b) The permeability tested results of the marine and lacustrine shale.
Figure 7. The reservoir physical properties of the marine and lacustrine shale samples. (a) The porosity tested results of the marine and lacustrine shale; (b) The permeability tested results of the marine and lacustrine shale.
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Figure 8. The correlation between TOC and mineral content. (a) The correlation between TOC and quartz content. (b) The correlation between TOC and clay content.
Figure 8. The correlation between TOC and mineral content. (a) The correlation between TOC and quartz content. (b) The correlation between TOC and clay content.
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Figure 9. The correlation between TOC and reservoir physical property. (a) The correlation between TOC and porosity. (b) The correlation between TOC and permeability.
Figure 9. The correlation between TOC and reservoir physical property. (a) The correlation between TOC and porosity. (b) The correlation between TOC and permeability.
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Figure 10. The arrangement characteristics of the pores in the Micro-CT images. (a) The pores distribution in marine shale sample HD1-17. (b) The pore distribution in lacustrine shale sample LMS-T9.
Figure 10. The arrangement characteristics of the pores in the Micro-CT images. (a) The pores distribution in marine shale sample HD1-17. (b) The pore distribution in lacustrine shale sample LMS-T9.
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Figure 11. The tectonic evolution characteristics of the marine and lacustrine shale. (a) The burial and thermal history of the lacustrine shale in Ziliujing Formation in Fuling area, modified from [10]. (b) The burial and thermal history of the marine shale in Wufeng–Longmaxi Formation in well JY1 (Figure 1), modified from [47].
Figure 11. The tectonic evolution characteristics of the marine and lacustrine shale. (a) The burial and thermal history of the lacustrine shale in Ziliujing Formation in Fuling area, modified from [10]. (b) The burial and thermal history of the marine shale in Wufeng–Longmaxi Formation in well JY1 (Figure 1), modified from [47].
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Table
Table 1. The organic geochemistry, mineralogical composition and reservoir physical properties of the marine and lacustrine shale in the study area.
Table 1. The organic geochemistry, mineralogical composition and reservoir physical properties of the marine and lacustrine shale in the study area.
Shale
Type		Sample NO.FormationDepth (m)/
Area		TOC
(%)		Rb
(%)		Ro
(%)		Mineral Composition (%)Brittleness IndexPorosity
(%)		Permeability
(md)
QuartzFeldsparCarbonatesPyriteClay
Marine ShaleHD1-1O3w1337.120.952.622.0238.96.315.40.738.761.3--
HD1-2O3w1336.262.72.682.0635.4516.16.836.763.37.160.328
HD1-3O3w1335.303.282.632.0241.18.314.28.328.171.9--
HD1-4O3w1334.262.992.581.9940420.44.131.568.53.270.027
HD1-5O3w1333.503.762.571.9953.654.33.533.666.4--
HD1-6O3w1333.203.982.722.0889.605.64.80100--
HD1-7O3g1331.814.312.571.99851.402.810.889.22.10.049
HD1-9S1l1329.663.22.581.9953.111.36.13.725.874.22.50.001
HD1-10S1l1328.794.32.471.934713.18.16.725.174.91.880.001
HD1-11S1l1327.744.432.521.9639.51312.86.428.371.7--
HD1-12S1l1326.633.69--39.711.613.64.13169--
HD1-13S1l1325.654.23--36.610.224.52.226.573.51.90.001
HD1-14S1l1324.693.62--42.812.88.1432.367.7--
HD1-15S1l1323.823.39--3.50.80.349.446541.830.224
HD1-16S1l1322.783.032.221.7743.51710.93.325.374.7--
HD1-17S1l1321.803.54--33.711.925.63.325.574.53.260.017
HD1-18S1l1320.883.06--24.54.77.32.361.238.8--
HD1-19S1l1320.073.282.531.9637.37.27.8344.755.33.860.005
HD1-20S1l1319.003.35--32.56.215.83.142.457.63.920.051
HD1-21S1l1317.773.47--40.16.711.52.739611.990.007
HD1-22S1l1316.763.432.561.98396.66.43.844.255.8--
HD1-23S1l1315.823.82--45.36.410.24.733.466.62.390.007
HD1-24S1l1314.783.4--41.87.16.8638.361.7--
HD1-25S1l1313.913.7--43.45.48.32.540.459.62.450.006
HD1-26S1l1312.823.82--46.56.463.637.562.5--
HD1-27S1l1311.843.442.121.7143.46.35.52.242.657.4--
HD1-28S1l1310.842.51--61.66.59.41.920.679.41.150.001
HD1-29S1l1309.912.81--59.36.43.62.328.471.6--
HD1-30S1l1308.782.92--58.44.51.81.334661.080.001
SBT-R1O3wSBT3.442.041.6648.71.101.149.150.95.190.861
SBT-R2O3wSBT2.60--4.2008.187.712.34.127.182
SBT-R3O3wSBT3.401.641.4187.700012.387.7--
SBT-R4O3wSBT4.31--87.600012.487.62.620.005
SBT-R5O3wSBT4.281.191.1410000001003.300.010
SBT-R6O3wSBT5.311.611.4830001783--
SBT-R7O3gSBT6.00--548.401.136.563.5--
SBT-R8S1lSBT2.37--83.700016.383.74.770.002
SBT-R9S1lSBT1.832.411.8972.48.30019.380.72.460.002
SBT-R10S1lSBT0.97--458.101.545.454.63.390.915
SBT-R11S1lSBT1.94--41.33.804.250.749.34.160.532
SBT-R12S1lSBT1.78--41.76.400.651.348.74.720.680
SBT-R13S1lSBT1.39--42.77.603.746542.230.007
Lacustrine ShaleLMS-T1J1–2z 1LMS1.86-0.8225513057432.900.003
LMS-T2J1–2z 1LMS1.09-0.644270051493.600.380
LMS-T3J1–2z 1LMS1.02-0.593744055456.350.045
LMS-T4J1–2z 1LMS1.72-0.634680046542.840.500
LMS-T5J1–2z 1LMS1.88-0.512860066343.870.167
LMS-T6J1–2z 1LMS1.41-0.571852075253.830.063
LMS-T7J1–2z 1LMS1.53-0.8417334046543.001.221
LMS-T8J1–2z 1LMS3.61-0.731748368326.940.396
LMS-T9J1–2z 1LMS1.4-0.821658071294.761.127
LMS-T10J1–2z 1LMS1.13-0.6711049139616.631.230
FY-10J1–2z 12810.121.74-1.23–2.0916–354.3–5.20–23.80–5.146–68404.5 (1.64–6.68)0.134 (0.044–0.376)
XL101J1–2z 12268.951.26-1.4225.64.79.54.555.744.32.860.664
HF-1J1–2z 12710.023.74--27.44.13.6460.939.14.60.7269
1 Where Rb is bitumen reflectance test results for marine shale, Ro is the test results of vitrinite reflectance for lacustrine shale and the calculated results of marine shale by the empirical formula eq Ro = 0.618 Rb + 0.4 [28]. The data of FY-10 are from [11]. The data of XL101 and HF-1 are from [10,29].
Table
Table 2. The N2 adsorption test results.
Table 2. The N2 adsorption test results.
Shale TypeSample NO.FormationSpecific Area (m2/g)Total Pore Volume (cc/g)Mean Pore Diameter (nm)Shale TypeSample NO.FormationSpecific Area (m2/g)Total Pore Volume (cc/g)Mean Pore Diameter (nm)
Marine shaleHD1-M1S1l35.950.059536.623Marine shaleHD1-M22S1l13.430.016154.811
HD1-M2S1l3.9910.0075637.58HD1-M23S1l11.570.012354.271
HD1-M3S1l21.410.0866816.19HD1-M24S1l17.230.020274.707
HD1-M4S1l18.510.027685.981HD1-M25S1l17.730.021924.945
HD1-M5S1l35.740.053255.96HD1-M26S1l21.30.021113.965
HD1-M6S1l18.510.027685.981HD1-M27S1l34.350.043555.071
HD1-M7S1l4.7280.0078486.639HD1-M28S1l23.10.026374.565
HD1-M8S1l24.580.040766.634HD1-M29S1l16.80.021485.115
HD1-M9S1l26.920.0676110.04HD1-M30S1l10.180.013135.157
HD1-M10S1l17.670.033587.602Lacustrine ShaleLMS-M1J1–2z118.70.044179.447
HD1-M11S1l5.7980.0492733.99LMS-M2J1–2z122.750.053549.415
HD1-M12S1l11.60.0294910.17LMS-M3J1–2z122.260.0738613.27
HD1-M13S1l12.740.015874.98LMS-M4J1–2z119.020.0475510
HD1-M14S1l3.4550.010712.39LMS-M5J1–2z114.780.0427411.57
HD1-M15S1l1.6780.00437910.44LMS-M6J1–2z113.220.0371111.23
HD1-M16S1l4.5110.0117210.39LMS-M7J1–2z123.180.0626210.81
HD1-M17S1l18.40.018313.98LMS-M8J1–2z18.8890.0245411.04
HD1-M18S1l15.820.019054.817LMS-M9J1–2z18.7360.0256311.73
HD1-M19S1l18.230.017063.744LMS-M10J1–2z113.310.02457.365
HD1-M20S1l19.320.019624.063LMS-M11J1–2z18.4220.0236711.24
HD1-M21S1l17.490.018614.257
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He, J.; Zhu, L.; Zhao, A.; Wang, D.; Qiu, Z.; Yang, P. Pore Characteristics and Influencing Factors of Marine and Lacustrine Shale in the Eastern Sichuan Basin, China. Energies 2022, 15, 8438. https://doi.org/10.3390/en15228438

AMA Style

He J, Zhu L, Zhao A, Wang D, Qiu Z, Yang P. Pore Characteristics and Influencing Factors of Marine and Lacustrine Shale in the Eastern Sichuan Basin, China. Energies. 2022; 15(22):8438. https://doi.org/10.3390/en15228438

Chicago/Turabian Style

He, Jianglin, Lixia Zhu, Ankun Zhao, Dong Wang, Zhen Qiu, and Ping Yang. 2022. "Pore Characteristics and Influencing Factors of Marine and Lacustrine Shale in the Eastern Sichuan Basin, China" Energies 15, no. 22: 8438. https://doi.org/10.3390/en15228438

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