Pore Evolution Characteristics and Accumulation Effect of Lower Jurassic Continental Shale Gas Reservoirs in Northeastern Sichuan Basin

Abstract

The Sichuan Basin is a key area for shale gas energy exploration in China. However, the pore evolution mechanism and accumulation effect of the Lower Jurassic continental shale gas in the northeastern Sichuan Basin remain poorly understood. In this study, the pore structure characteristics of shale reservoirs and the dynamic accumulation and evolution of shale gas in the northern Fuling and Yuanba areas were systematically analyzed by adsorption experiments, high-pressure mercury injection joint measurement, and thermal simulation experiments. The results indicate the following: (1) The continental shale in the study area is predominantly composed of mesopores (10–50 nm), which account for approximately 55.21% of the total pore volume, followed by macropores (5–50 μm) contributing around 35.15%. Micropores exhibit the lowest proportion, typically less than 10%. Soluble minerals such as clay minerals and calcite significantly promote pore development, while soluble organic matter may block small pores during hydrocarbon generation, which facilitates the enrichment of free gas. (2) The thermal simulation experiment reveals that pore evolution can be divided into two distinct stages. Prior to 450 °C, hydrocarbon generation leads to a reduction in pore volume due to the compaction and transformation of organic matter. After 450 °C, organic matter undergoes cracking processes accompanied by the formation of shrinkage fractures, resulting in the development of new macropores and a significant increase in pore volume. This indicates that thermal energy input during the thermal evolution stage plays a key role in reservoir reconstruction. (3) The early Jurassic sedimentary environment controls the enrichment of organic matter, and the Cretaceous is the key period of hydrocarbon accumulation. Hydrocarbon generation and diagenesis synergistically promote the formation of gas reservoirs. The Cenozoic tectonic activity adjusted the distribution of gas reservoirs, and finally formed the enrichment model with the core of source–reservoir–preservation dynamic matching. For the first time, combined with dynamic thermal simulation experiments, this study clarifies the stage characteristics of pore evolution of continental shale and identifies the main controlling factors of shale gas accumulation in the Lower Jurassic in northeastern Sichuan, which provides a theoretical basis for continental shale gas exploration and energy resource development, offering important guidance for optimizing the selection of exploration target areas.

1. Introduction

As a clean and low-carbon energy resource, shale gas is playing an important role in the global energy transition [1,2,3,4,5,6]. In recent years, the successful development of marine shale gas (such as Barnett and Haynesville shale in North America) has promoted the unconventional oil and gas revolution, while continental shale gas still faces many challenges due to its complex geological conditions and accumulation mechanism [7,8,9,10]. China possesses tremendous potential in continental shale gas resources. The evaluation of its resources is 3.7 × 10¹² m³, accounting for more than 15% of the total shale gas resources in China [11,12,13,14,15]. The Sichuan Basin represents a core region for shale gas exploration in China. While the commercial development of marine shale gas has been achieved in blocks such as Jiaoshiba, the exploration of continental shale gas remains at a stage of key technical challenges, particularly in the Lower Jurassic artesian well group in the northeastern part of the basin. The characteristics of the reservoir pore evolution and accumulation effects are not yet fully understood, which hinders the efficient development and utilization of energy resources in this area.

Domestic and foreign scholars have conducted systematic research on the pore structure of shale and the mechanisms of hydrocarbon accumulation within these structures. Marine shale is dominated by organic matter pores, and its evolution is controlled by thermal maturity [16,17]. Due to the high input of terrigenous debris and complex types of organic matter (mainly type III kerogen), the pore development of continental shale is affected by multiple factors such as mineral composition, diagenesis, and energy conversion processes during hydrocarbon generation [18,19]. In recent years, scanning electron microscopy, gas adsorption–mercury intrusion porosimetry, and other technologies have been widely used in shale pore characterization, revealing the characteristics of micro–macro pore co-development [20,21,22]. However, obvious deficiencies remain in the study of continental shale in northeastern Sichuan. First, there is no experimental evidence for the dynamic evolution mechanism of pores in the thermal evolution process. Second, the quantitative characterization of reservoir reconstruction by diagenesis-hydrocarbon generation coupling is insufficient. Third, the “source–reservoir–preservation” dynamic matching model of continental shale gas has not yet been established, resulting in unclear main controlling factors of accumulation.

The tectonic activities in northeastern Sichuan are complex. The early Jurassic sedimentary environment and the late Yanshan-Himalayan tectonic movement jointly shaped the current shale gas reservoir pattern. Although Yuanba, northern Fuling and other blocks have received commercial gas flows [23,24]. However, the problems of strong reservoir heterogeneity and poor pore connectivity lead to significant differences in single well production. Previous studies have shown that the shale organic matter abundance (total organic carbon (TOC) average 1.04%) and maturity (vitrinite reflectivity (R_o) average 1.27%) of the Ziliujing Formation are at a medium level, but the high clay mineral content (average 52.7%) and low brittle mineral ratio significantly affect the fracturing performance [19]. In addition, the tectonic uplift and fluid activity since the Cretaceous may have had adverse effects on the preservation of gas reservoirs, and it is urgent to clarify the spatial–temporal coupling relationship between pore evolution and the accumulation process.

In this study, the Lower Jurassic continental shale within the northeastern Sichuan Basin was selected as the focal research object to elucidate the evolutionary characteristics of pore structure and the underlying mechanisms of accumulation within these structures. The total pore size distribution was quantitatively characterized through integrated adsorption and mercury intrusion porosimetry experiments. Utilizing open-system thermal simulation, the dynamic evolution of pore structure during hydrocarbon generation was systematically monitored. Furthermore, the synergistic influence of mineralogical composition, diagenetic processes, and tectonic activity on reservoir quality and hydrocarbon enrichment was comprehensively analyzed. The experimental results revealed a distinct two-stage threshold effect in pore evolution, leading to the formulation of a mineral–hydrocarbon synergistic pore-enhancement model. A dynamic coupling model of “hydrocarbon generation–reservoir formation–preservation mechanism” was established, with the Cretaceous period identified as the critical phase for hydrocarbon accumulation. Based on these findings, a conceptual dynamic accumulation model for continental shale gas of “early rich source–Cretaceous high-quality reservoir–Freshman strong guarantee” was proposed, providing a theoretical basis for predicting favorable exploration zones of continental shale gas. These findings significantly advance our understanding of continental shale gas enrichment mechanisms and offer substantial implications for guiding future exploration and development strategies in the northeastern Sichuan Basin.

2. Geological Background

The northeastern Sichuan Basin is located on the northern margin of the Yangtze plate, encompassing the Tongnanba tectonic belt, the central Sichuan gentle tectonic belt, and the eastern Sichuan fault–fold belt in the northern depression of the Sichuan Basin (Figure 1) [25,26]. The study area includes the Yuanba block in northern Sichuan and the northern Fuling block in eastern Sichuan.

The northern part of the Fuling area is located within the arcuate high-steep structural belt of eastern Sichuan. To the east, it borders the Dachiganjing–Zhaigouwan structural belt, while to the west, it is bounded by the Nanmenchang–Datianchi structural belt [25].

The northern part of Fuling experienced crustal uplift and subsidence during the Caledonian period, forming paleo-uplifts and paleo-slopes. In the Indosinian period, influenced by crustal movement and fault activity, the Kaijiang paleo-uplift was formed in eastern Sichuan. During the Yanshan-Himalayan period, the region was affected by the Jiangnan Xuefeng orogenic belt and the Longmenshan fault zone. From the crustal movement in the Caledonian period to the tectonic change in the Indosinian period, and then to the far-reaching influence brought by the two orogenic belts in the Yanshan-Himalayan period [28,29]. These processes work together, resulting in the present East Sichuan region mainly showing the characteristics of NE-trending structures, faults, and synclines. The lithology in the northern Fuling area is dominated by shale. The Middle–Upper Jurassic is characterized by purple mudstone, which is frequently interbedded with siltstone and silty mudstone. The Lower Jurassic develops gray-black shale with a high organic matter content [30,31].

The Yuanba area is located in the northern part of the basin, adjacent to the front edge of the Longmen Mountains. The north and south are connected to the Micangshan–Dabashan foreland thrust belt and the central Sichuan low-lying tectonic belt [19,22,24,31,32,33]. The region was subjected to strong lateral compression from the middle Yanshanian to the early Himalayan, resulting in the Micangshan–Dabashan uplift in the NW-SE direction [34,35,36]. The Middle–Upper Jurassic in the Yuanba area is characterized by brown-red or purple mudstones interbedded with lithic sandstones of unequal thickness, while the Lower Jurassic is composed of gray mudstones interbedded with lithic sandstones and black shales (Figure 1c).

3. Materials and Methods

3.1. Experimental Samples

The continental shale samples were collected from Well YL4 of Ziliujing Formation in the Yuanba area, and Well FY1 and Well XL101 in the northern Fuling area. Additionally, low-maturity samples from the Qiaoting section in Nanjiang (R_o = 0.7%, TOC = 1.14%) were also included. Among them, the Nanjiang Qiaoting profile samples are mainly used for thermal simulation experiments. Samples of the YL4 well in the Yuanba area, and wells FY1 and XL101 in the northern Fuling area, were selected for comprehensive conventional and unconventional experimental analyses. These included total organic carbon content, rock pyrolysis, vitrinite reflectance measurements, and petrographic studies using ordinary thin sections and optical microscopy. Advanced techniques such as cathodoluminescence, argon ion polishing combined with scanning electron microscopy (SEM) under the same field of view, open thermal simulation experiments, and adsorption–mercury intrusion porosimetry were also applied to characterize the full pore structure of the shale reservoir. Based on the “source–reservoir” spatial–temporal configuration relationship, this study investigates the accumulation mechanism of continental shale gas (Figure 2).

3.2. Experimental Methods

3.2.1. Thermal Simulation Experiment

In this experiment, a series of temperature points were set to simulate the state of shale at different maturation stages. The temperature was increased by 10 °C per hour and maintained for 72 h at each temperature point. After reaching the preset temperature, the sample was polished by argon ions and observed by field emission scanning electron microscopy to track and analyze the evolution of pore structure with temperature. The experiment was carried out under open conditions without fluid and pressure, so that the samples could be taken out at any time for the same field-of-view observation [37].

3.2.2. Adsorption–Mercury Porosimetry Experiment

The full-aperture experiment of adsorption–mercury injection combined measurement of shale reservoir adopts China’s Jingwei Gaobo JW-BK222 (Origin: Beijing, China, Supplier: Beijing Jingwei Gaobo Instrument Co., Ltd.) nitrogen adsorption instrument. After crushing the sample to 60–80 mesh, the sample was vacuum-dried at 110 °C for 14 h, and then the nitrogen isothermal adsorption–desorption experiment was carried out in a liquid nitrogen environment. The test process was based on the GB/T 21650.2-2008 standard [38]. The specific surface area was calculated by the BET multi-point model, and the adsorption curve was analyzed by the BJH model to determine the pore size distribution.

The high-pressure mercury injection experiment used the American Mac AutoPore IV 9520 automatic mercury injection instrument (Origin: Xiamen, China, Supplier: Xiamen Mingda Technology Co., Ltd.). The sample was a block parallel sample, dried at 60 °C for 48 h to remove free water and bound water, and comprehensive data analysis was performed according to the NB/T 14008-2015 standard [39].

4. Results

4.1. Organic Geochemistry and Mineral Composition Characteristics

The geochemical and mineral composition characteristics of the Lower Jurassic continental shale in the northeastern part of the Sichuan basin are the core basis for reservoir evaluation and accumulation research. The characteristics of organic geochemistry and mineral composition systematically reveal the differences in the mineral composition, organic matter type, abundance, and maturity of shale in the Da’anzhai section and the Dongyuemiao section, and clarify their control mechanism in reservoir pore development and hydrocarbon generation potential.

4.1.1. Mineral Components

The shale mineral composition in the study area is dominated by clay minerals, silicate minerals (quartz and feldspar), and carbonate minerals (calcite, dolomite), and the mineral assemblages of different layers are significantly different (Figure 3).

Clay minerals account for 28.8%–67.4% (average 52.7%), and the Dongyuemiao section (average 58.7%) is higher in clay minerals than the Da’anzhai section (average 47.5%). It is dominated by illite and an illite–smectite mixed layer, and the smectite content is less. The content of silicate minerals is 0.7%–44.9%, with an average content of 9.98%. The quartz content is 20%–50% (average 29.9%), and the feldspar content is 0.9%–10% (average 3.47%). The carbonate mineral content is 0.7%–44.9%, with an average content of 9.98%. The content of calcite in the Da’anzhai section is 11.7%, and the content of dolomite is less than 1%. The content of calcite in the Dongyuemiao section is 3.03%. The development degree of carbonate interlayer in the Da’anzhai section of the Yuanba area is more significant than that in the Dongyuemiao section of northern Fuling. This comparison not only reflects the differences in geological characteristics between the two places, but also further confirms the relative enrichment of carbonate minerals in the Da’anzhai section.

4.1.2. Geochemical Characteristics

The vitrinite group has the highest content in this study area, reaching an average of 67%, while the inertinite group is second, accounting for 28%. In contrast, the proportion of the sapropel group and exinite group was the lowest, accounting for only 1.7% (Figure 4). Based on previous studies on kerogen carbon isotopes (δ¹³C = −24.5‰–−22.8‰) and the H/C atomic ratio, combined with these data, it can be shown that the main type of kerogen in the study area is type III, its parent material is mainly terrestrial higher plant debris, and it has a medium level of hydrocarbon generation potential.

The TOC content of the Ziliujing Formation shale in the study area is 0.14%–1.97% (Figure 4c), with an average of 1.04%. The Dongyuemiao section (average 1.52%) is slightly higher than the Da’anzhai section (average 0.63%).

The vitrinite reflectance R_o is between 0.7% and 1.97% (average 1.27%). Due to the small burial depth (average 3000 m), the maturity of the Da’anzhai section is slightly lower, and the vitrinite reflectance of the Dongyuemiao section reaches 1.57% (Figure 4d). The mature stage (R_o is between 0.7% and 1.2%) is dominated by oil generation, and the liquid hydrocarbon fills the pores. In the high-maturation stage (R_o is between 1.2% and 1.5%), it enters the wet gas–condensate oil stage, and the organic matter is cracked to form nanopores. In the over-mature stage (R_o > 2.0%), the macropore volume increases significantly (verified by thermal simulation experiments).

4.2. Reservoir Pore Type

The pore types of Lower Jurassic continental shale reservoirs in northeastern Sichuan are complex, diverse, and divided into three main categories: organic matter pores, mineral matrix pores, and microcracks. Through argon ion polishing–field emission scanning electron microscopy (FE-SEM) observation, and nitrogen adsorption experiments and high-pressure mercury injection combined measurements, combined with mineral composition and organic geochemical parameters, the morphological characteristics, distribution law, and control effect on the reservoir performance of various pores were systematically revealed [18,40].

4.2.1. Organic Pores

Organic matter pores are the key space for shale gas occurrence, and their development degree is controlled by the organic matter type, abundance (TOC), and maturity (R_o). The pores of solid bitumen are mainly developed in hydrogen-rich vitrinite and solid bitumen, with various shapes (oval, honeycomb). The pore size is concentrated within 10–50 nm (Figure 5a,b), accounting for 60%–75% of the total organic matter pores. The kerogen pores are mainly developed in the vitrinite and plastids (<5%), and the pore size is mostly less than 10 nm, showing an isolated distribution (Figure 5c,d).

In low-maturation stages, the soluble organic matter (asphaltene) fills the micropores, and the pore volume accounts for a small proportion. In high-maturation stages, the organic matter cracks to form shrinkage cracks (1–5 μm) and nanopores (10–50 nm), and the pore volume increases (Figure 5c).

4.2.2. Mineral Matrix Pores

The pores of mineral matrix are controlled by the mineral type, diagenesis, and dissolution effect, mainly including the following four categories (Figure 6).

It is formed by the accumulation of rigid mineral particles such as quartz and feldspar. The pore size range is 50–500 nm, the shape is irregular, and the connectivity is good (Figure 6c). The Da’anzhai section has a high content of brittle minerals (quartz and calcite > 45%), and intergranular pores account for 25%–30%. Due to the plastic deformation of clay minerals, the intergranular pores in the Dongyuemiao section are compressed to 10%–15%. The pores between illite and smectite (2–10 nm) are mainly slit-like. The conversion of smectite to illite releases interlayer water, and the pore volume increases, but the directional arrangement leads to a decrease in permeability. Intergranular pores are developed between pyrite framboid aggregates, which are often filled with organic matter (Figure 6b), and the pore volume is small. Pyrite is a sign of a reducing environment, and the contribution of pyrite intercrystalline pore to reservoir is limited. Soluble minerals such as calcite and feldspar are formed by the dissolution of acidic fluids, with a pore size of 5–50 μm and irregular shape (Figure 6d). The average content of calcite in the Da’anzhai section is 11.7%, dissolution pores account for 15%–20%, and dissolution pores in the Dongyuemiao section are rare (<5%).

4.2.3. Microfractures

Microfractures are the core carrier of shale gas seepage and enrichment. From FE-SEM observations, fractures within the studied samples can be divided into two categories, macrofractures (>1 mm) and microfractures (<1 mm) (Figure 7), according to their apparent length.

The organic matter shrinkage fracture is formed by the volume shrinkage of organic matter during hydrocarbon generation, with a width of 0.1–1 μm and an extension length of 10–50 μm (Figure 7a). It is mainly developed in highly mature shale (R_o > 1.2%). Diagenetic microfractures are produced by diagenetic compaction and mineral shrinkage, with a width of 0.05–0.2 μm and poor connectivity, accounting for less than 10%. The structural microfractures are affected by the tectonic stress of the Yanshan–Himalayan period, with a width of 0.5–5 μm, an extension length of >100 μm (Figure 7b), and an increase in permeability. The density of structural microfractures in the Da’anzhai section is high, which is conducive to the accumulation of free gas. Microfractures and dissolution pores form a “pore–fracture” network, and the fracture complexity index increases after fracturing.

4.3. Full Aperture Experiment of Adsorption-Mercury Injection Joint Measurement of Shale Reservoir

In this study, the total pore size (0.3 nm–500 μm) of shale samples from the Dongyuemiao and Da’anzhai sections of the Lower Jurassic Ziliujing Formation in northeastern Sichuan was quantitatively characterized by nitrogen (N²) adsorption experiments and high-pressure mercury injection techniques, revealing the heterogeneity of the pore structure of continental shale reservoirs and its main controlling factors.

4.3.1. Micropore and Mesopore Characteristics

Through the liquid nitrogen isothermal adsorption–desorption test of the samples, the specific surface area was calculated according to the BET model, and the pore size distribution was analyzed by the BJH model (Figure 8).

The adsorption–desorption curve of the Da’anzhai section (H3 type hysteresis loop) is parallel, and the hysteresis loop is wide and slow (Figure 8a), indicating that the open parallel plate pores coexist with some ink bottle pores. The significant range of the bimodal pore size distribution is 10–30 nm.

The adsorption–desorption curve of the Dongyuemiao section (H4-type hysteresis loop) is gentle, and the hysteresis loop is closed to P/P ≈ 0.45 (Figure 8b), reflecting that the closed slit-shaped pores at one end are dominant. The pore size is concentrated within 2–50 nm.

The Dongyuemiao section has a high clay content (average 58.7%) and developed interlayer pores (2–10 nm), resulting in H4-type loopback characteristics. The content of quartz and calcite in the Da’anzhai section reaches 40%–55%, supporting open pores and forming H3-type loops.

4.3.2. Macropore Characteristics

Bulk samples were analyzed using a AutoPore IV 9520 mercury porosimeter utilizing the Washburn equation for the calculation of pore size distribution (Figure 9).

The mercury intrusion curve of the Dongyuemiao section has a small volume proportion in the low-pressure section (<0.2 MPa). The mercury input in the middle and the high-pressure section (>10 MPa) increased sharply, reflecting the poor connectivity of mesopores (10–50 nm). The main peak of pore size is located in the range of 5000 to 20,000 nm, and the development of dissolution pores is relatively limited. In the low-pressure section (0.1–0.5 MPa) of the Da’anzhai section, mercury inflow increases rapidly, indicating a significant increase in macropores. This suggests that the dissolution pores in the limestone interlayer contribute significantly to the pore structure (Figure 6a). Here, the main pore size peak shifts to the range of 5000–50,000 nm, and the proportion of dissolution pores increases accordingly.

The content of calcite in the Da’anzhai section is 11.7%, and the secondary macropores are formed by the dissolution of acidic fluid (Figure 6d), and the pore volume is 2–3-times higher than that of the Dongyuemiao section. The Dongyuemiao section has a shallow burial depth (2269–2736 m) and weak compaction, but the plastic deformation of clay leads to macropore compression.

4.3.3. Comprehensive Comparison of Full Aperture Distribution

Integrating N² adsorption and mercury intrusion data (Figure 10), the shale pores in the study area show the characteristics of “mesopores dominated, macropores followed, and micropores limited”.

The total pore size of Dongyuemiao section has a bimodal distribution (10–30 nm and 5000–50,000 nm), with a high proportion of slit pores and low permeability, which is conducive to the occurrence of adsorbed gas. The full pore size of the Da’anzhai section shows a bimodal distribution (10–30 nm and 5000–50,000 nm). Open pores and dissolution pores synergistically increase permeability, which is conducive to the occurrence of free gas. The study area is mainly dominated by mesopores, accounting for 55.21%, followed by macropores, accounting for 35.15%, and micropores accounting for the lowest proportion, less than 10%.

The causes of pore structure heterogeneity in the study area are mainly mineral composition and diagenesis–hydrocarbon generation coupling. The clay minerals control the nanopore structure, and the carbonate minerals dominate the macropore development. The soluble organic matter in the Dongyuemiao section blocks the micropores, and the dissolution of the Da’anzhai section expands the macropores.

5. Discussion

5.1. Effect of Hydrocarbon Generation and Mineral Composition on Pore Development of Continental Shale Reservoir

The development of pores in the Lower Jurassic continental shale reservoirs is controlled by both mineral composition and hydrocarbon generation. The two jointly shape the pore structure characteristics through a synergistic or competitive mechanism. Combined with experimental data and geological analysis, the constraints of mineral components on the pore types, the dynamic driving of hydrocarbon generation in pore evolution, and the coupling effect of the two are systematically elucidated.

5.1.1. Differentiation Control of Mineral Composition on Pore Development

The mineral composition of shale directly affects the pore type, pore size distribution, and connectivity through the differences in particle accumulation, dissolution response, and mechanical properties. The mineral composition of the study area is dominated by clay minerals (28.8%–67.4%, average 52.7%), silicate minerals (22.7%–57.2%, average 33.3%), and carbonate minerals (0.7%–44.9%, average 9.98%, Figure 3). Their roles in pore development are as follows:

(1)

Clay minerals control the development of meso–macropores

Clay minerals (illite, smectite) contribute to the main pore space through interlayer pores (pore size 2–10 nm) and intergranular pores (pore size 50–500 nm). Interlayer water is released during the conversion of smectite to illite, forming slit-like pores. The total pore volume is positively correlated with clay mineral content. The plastic deformation of clay minerals leads to the increase in pore loss rate in the compaction stage.

(2)

Brittle minerals (quartz and carbonate) control the development of micro–mesopores

Brittle minerals (quartz, calcite) affect the pore structure through rigid support and dissolution. The volume of micropores is positively correlated with the content of quartz, and the proportion of intercrystalline pores (pore size < 10 nm) is 30%–45% (Figure 11). Calcite dissolution forms secondary macropores (pore size 5–50 μm). The average content of calcite in the Da’anzhai section reaches 11.7%, and the volume ratio of dissolution pores increases (Figure 6d). The fracture complexity index and permeability of samples with a brittle mineral content > 40% (Da’anzhai section) increase after fracturing.

(3)

The evolution of organic matter has a dynamic adjustment effect on pores

The organic matter abundance (TOC = 0.14%–1.97%) and maturity (R_o = 0.7%–1.97%) indirectly influence the pores through the hydrocarbon generation process. In low-maturation stages, soluble organic matter (asphaltene) fills micropores, resulting in a decrease in micropore volume. In high-maturation stages, the organic matter is cracked to form nano-scale organic pores (pore size 10–50 nm), and the pore volume increases (Figure 5c).

5.1.2. Two-Stage Driving of Hydrocarbon Generation on Pore Evolution

The dynamic effect of hydrocarbon generation on pore structure was revealed by thermal simulation experiments (300–550 °C) (

Table 1. Thermal simulation experiment temperature point design table.

Thermal Simulation Experiment Temperature Point/°C	Original Sample	300	350	400	450	500	550
Ro/%	0.7	0.8	1.15	1.6	2.0	2.4	3.1
Mature stage of organic matter	Mature			High maturity		Over-mature

, Figure 12 and Figure 13).

At the initial stage of hydrocarbon generation (300–450 °C), the liquid hydrocarbon filled the primary pores (pore size 50–500 nm), and the total pore volume decreased (Figure 13).

At high temperature, kerogen cracked into methane, forming shrinkage joints (pore size of 1–5 μm) and organic pores (pore size of 10–50 nm), and the macropore volume increased (Figure 13). The organic acids produced by hydrocarbon generation and CO₂ dissolved calcite, the proportion of secondary dissolved pores increased (Figure 6d), and the total porosity increased.

The temporal and spatial coupling of mineral composition and hydrocarbon generation determines the “dynamic balance” of reservoir pores. The combination of high calcite content and hydrocarbon generation dissolution in the Da’anzhai section can increase the macropore volume. The combination of high clay content and low maturity in the Dongyuemiao section leads to a decrease in porosity [37].

5.2. Effect of Diagenesis on Pore Development of Continental Shale Reservoir

Diagenesis significantly affects the development and evolution of continental shale reservoir pores in the northeastern Sichuan Basin through compaction, cementation, dissolution, and mineral transformation. The specific manifestations are as follows:

(1)

Compaction action

The buried depth of shale in the study area is significantly different (2269–2736 m in the north of Fuling, and 3748–3790 m in the Yuanba area), and the compaction effect increases with the burial depth. Due to the larger buried depth (500–1000 m deeper than Fuling), the porosity in the Yuanba area decreased by 1.0% on average. During the early diagenesis stage (buried depth < 2000 m), the primary intergranular pores (pore size 5–50 μm) were rapidly compressed and the porosity was lost. In the middle-late diagenetic stage, quartz cementation and carbonate mineral precipitation further reduce pore connectivity, resulting in lower porosity in the Da’anzhai section than in the Dongyuemiao section.

(2)

Clay mineral transformation

Clay minerals account for 28.8%–67.4% (average 52.7%) of the total mineral content of shale in the study area, and their transformation process has a dual effect on pore development. Smectite → illite conversion: With the increase in maturity, the interlayer water of smectite is released to form interlayer pores (pore size 2–10 nm), and the porosity increases locally. Illite directional arrangement: In the high-maturation stages (R_o > 1.2%), the directional arrangement of illite flake particles leads to the change in pore morphology from open type to slit shape, and the permeability decreases. Due to the low content of clay minerals in the Da’anzhai section (47.5% in the Da’anzhai section and 58.7% in the Dongyuemiao section), the conversion’s pore-increasing effect is more significant and the porosity is improved.

(3)

Dissolution

The dissolution of soluble minerals by acidic fluids (organic acids, CO₂) is the main controlling factor for the development of secondary pores. The dissolution of carbonate minerals is the main controlling factor for the development of secondary pores. The content of calcite in the Da’anzhai section is 11.7%, and the dissolution forms macropores (pore size 5–50 μm). The proportion of pore volume increased by 15%–20%. The calcite content in the Dongyuemiao section is only 3.03%, and the contribution of the dissolution pores is weak. Feldspar and debris are dissolved, and feldspar dissolution forms mesopores (10–50 nm), which makes the proportion of mesopore volume in the Yuanba area significantly higher than that in northern Fuling. The dissolution increases the total porosity of the Da’anzhai section.

The diagenesis has a compound control mode of “compaction inhibition–transformation adjustment–dissolution efficiency” on the pore development of continental shale. Among them, dissolution and clay mineral transformation are the key to improving reservoir performance, but they need to be matched with a moderate burial depth (2000–3500 m) to avoid excessive compaction. In exploration practice, priority should be given to the selection of the calcite-enrichment area and smectite–illite transformation transition zone in the Da’anzhai section, so as to take into account the development of secondary pores and reservoir brittleness and to support an efficient fracturing transformation.

5.3. Source–Reservoir Evolution Process and Accumulation Effect of Continental Shale

The accumulation effect of the Lower Jurassic continental shale in northeastern Sichuan is controlled by the dynamic matching of “source–reservoir–preservation”, and its evolution process can be divided into three stages (Figure 14):

(1)

Early Jurassic: Organic matter enrichment and reservoir prototype formation

During the early Jurassic sedimentary period, the northeastern Sichuan Basin was in a stable subsidence environment, and two sets of organic-rich shales were developed in the Dongyuemiao and Da’anzhai sections. The Dongyuemiao section is dominated by gray-black shale, with an average TOC of 1.52%. The type of organic matter is dominated by type III kerogen (vitrinite accounts for 67%), and terrigenous higher plant debris input is dominant. The Da’anzhai section is affected by the shell limestone interlayer, with a slightly lower TOC (0.14%–1%) and carbonate mineral content of 13.2%. At this stage, the primary intergranular pores are developed, but the organic matter is immature, and the hydrocarbon generation has not yet started. The pores are mainly slit-like structures dominated by compaction, and the reservoir capacity is limited.

(2)

Cretaceous: Hydrocarbon generation–diagenesis synergy and key period of hydrocarbon accumulation

The Cretaceous is the core stage of shale gas accumulation. With the increase in buried depth (the buried depth of the Dongyuemiao section is 2269–2736 m), the maturity of organic matter increases significantly (R_o average is 1.59%), and it enters the stage of kerogen pyrolysis. Thermal simulation experiments show that 450 °C (corresponding to R_o ≈ 2.0%) is the threshold of pore evolution. Hydrocarbon generation and pore-increasing effectt, before 450 °C, the liquid hydrocarbon generated by hydrocarbon generation fills the primary pores, resulting in a decrease in pore volume. After 450 °C, the organic matter cracks into methane, accompanied by the development of shrinkage joints, and the macropore volume increases. Diagenetic dissolution efficiency, soluble minerals such as calcite dissolve under acidic fluids, and the pore network is synergistically amplified with the transformation of clay minerals (smectite → illite). At this stage, the proportion of free gas is more than 60%, and the porosity of the reservoir is improved. Due to the high content of brittle minerals (quartz and carbonate minerals > 40%), the Da’anzhai section is more conducive to fracturing to form a complex fracture network [41].

(3)

Cenozoic: Structural adjustment and current gas reservoir pattern finalization

The tectonic activity in the Himalayan period led to regional uplift, but the uplift range was limited (<1000 m). The gas reservoir was not exposed to the surface and the preservation conditions were good. The adjustment of tectonic stress makes the microfractures expand, the fracture density increases, and the free gas migrates to and enriches the high porosity and permeability zone. Due to the high content of clay minerals (average 58.7%) in the Dongyuemiao section, the compaction effect continued to inhibit the porosity (decreased by about 1.0%), but still maintained the advantage of adsorbed gas. The Da’anzhai section is dominated by dissolved pores and free gas, forming a vertical differentiation pattern of “upper adsorption and lower dissociation” [42].

The model reveals that the accumulation of continental shale gas needs to meet the time–space coupling of “source–reservoir–preservation” [43,44], guiding the exploration to preferentially select the dissolution pore development area and structural stability zone in the Da’anzhai section, and provides theoretical support for the efficient development of shale gas in northeastern Sichuan.

6. Conclusions

(1)

The continental shale reservoirs in northeastern Sichuan are dominated by mesopores (10–50 nm), followed by macropores (5–50 μm), and micropores (<2 nm) account for the lowest proportion. Clay minerals and calcite synergistically control pore development: Interlayer pores (2–10 nm) of clay minerals contribute to high specific surface area, while calcite dissolution forms macropores. The “clay-carbonate synergistic pore-increasing” model was established to reveal the differential control of mineral assemblages on pore type and distribution, providing a mineralogical basis for “dessert” prediction.

(2)

Thermal simulation experiments show that the pore evolution has a threshold effect of 450 °C (corresponding to R_o ≈ 2.0%). Below 450 °C, the liquid product of hydrocarbon generation fills the pores, the micropore volume decreases, and the total pore volume decreases. Above > 450 °C, the organic matter is cracked to form shrinkage cracks (1–5 μm) and nanopores (10–50 nm), and the macropore volume increases. The two-stage dynamic characteristics of the pore evolution of continental shale are clarified, which provides theoretical support for the evaluation of deep shale gas reservoirs.

(3)

The Cretaceous is the key period of hydrocarbon accumulation. Hydrocarbon generation and dissolution diagenesis synergistically form high porosity and permeability reservoirs, and free gas accounts for more than 70%. The Cenozoic tectonic fine-tuning (uplift amplitude < 1000 m) maintains the integrity of the gas reservoir and forms a vertical differentiation pattern of “upper adsorption and lower free”. A dynamic accumulation model of “early rich source–Cretaceous high-quality reservoir–Freshman strong guarantee” is proposed, and the spatial and temporal coupling mechanism of structure–hydrocarbon generation–preservation is clarified, which guides the exploration to give priority to the development area of dissolution pores in the Da’anzhai section and the structural stability zone with R_o between 0.7% and 1.33%.