To identify the factors controlling high-quality deep shale gas reservoirs and the exploration and development potential of the Lower Paleozoic marine shale in the Sichuan Basin, the sedimentary environment of deep shale was comprehensively analysed using core thin sections, scanning electron microscopy, gamma ray spectrometry logging, and elemental logging data. In addition, the geological conditions of deep shale gas accumulation and the effect of tectonic processes on the preservation conditions are discussed based on the experimental data of mineral composition analysis, geochemical features, and reservoir spatial characteristics. (1) The sedimentary environment changes from an anoxic water environment to an oxygen-rich oxidizing environment from bottom to top in the Wufeng-Longmaxi Formation in southern Sichuan. The deep shale gas reservoir shows overpressure and rich gas characteristics, namely, high formation pressure (2.0~2.2), high porosity (20%~55%), and high gas content (4.0~5.0 m3/t). (2) The favourable sedimentary environment has a higher hydrocarbon generation potential and deposits of rich organic matter and siliceous particles. During the hydrocarbon generation process, the rich organic matter generates a large number of organic pores and a large specific surface area, which provides the main reservoir and adsorption space for free and adsorbed shale gas. A large number of biogenic siliceous particles provide a solid rock support framework for the shale reservoir, thereby maintaining excellent reservoir physical properties. (3) Late and small stratigraphic uplifts result in a short shale gas escape time and favourable preservation conditions. Additionally, the small-scale faults and a high-angle intersection between the fracture strike and the geostress direction are conducive to the preservation of shale gas. (4) A high formation pressure coefficient, a sedimentary environment rich in organic siliceous deep-water continental shelf microfacies, and a relatively stable tectonic structure are conducive to the accumulation of deep shale gas.

The Lower Paleozoic marine shale in the Sichuan Basin has undergone a complex tectonic evolution process. Affected by the deep burial, strong uplift, strong denudation, and strong deformation, most organic-rich marine shale is buried deep [16]. The area with a burial depth less than 3500 m at the bottom of the Lower Silurian Longmaxi Formation is approximately 6.3×104 km2, and the area with a burial depth greater than 3500 m is 12.8×104 km2 [7, 8]. Hence, the area with a deep burial depth is twice that of the area with shallow to intermediate depths. Three national shale gas demonstration areas, namely, Fuling, Weiyuan–Changning, and Zhaotong, have been established in the southern Sichuan area, all of which are shale gas reservoirs with a shallow to intermediate burial depths less than 3500 m [911]. In recent years, the exploration and development of deep shale gas has also been vigorously pursued. For example, gas was obtained at 45.67×104 m3/d in the gas production test of the well Zu202-H1 in the high and steep Dazu anticline, gas was obtained at 22.37×104 m3/d in the gas production test of the well Huang202 in the high and steep Huangguashan anticline, and 137.9×104 m3/d gas was obtained in the gas production test of well Lu203 on the Luzhou anticline tectonic belt. The high initial production rates of natural gas from deep shale gas wells with a vertical depth greater than 3500 m represent a strategic breakthrough in the exploration and development of deep shale formations. Recently, the test production rates of nine horizontal wells in the deep shale in the Luzhou area were between 20.0850.69×104 m3/d. The shale gas wells showed large deviations in test gas production from the estimated ultimate recovery (EUR). The analysis of factors affecting the enrichment and high yield of shale gas in this area is the focus of research on the exploration and development of shale gas in this area [1217].

The marine shale in southern China has the characteristics of old age, complex genesis, strong tectonic activity, and large differences in gas-bearing properties [18]. Many scholars have proposed the theory of “binary enrichment” [19] and the oil and gas exploration idea of “source rock and caprock controlling oil and gas accumulation” [20, 21]. The marine shale in southern China has undergone multistage tectonic movements, and shales at different evolution stages differ significantly in their gas content and preservation conditions. Late tectonic uplift and denudation can reduce the pressure and diffusivity of shale gas [22, 23]. Compared with shallow shale gas, there are many differences in the geological characteristics and controlling factors of deep shale gas enrichment. Normally, deep shale gas is mostly located in more complex structural areas under larger in situ stress and stress differences and higher temperatures and pressures, so the exploration and development technology of deep shale gas is different. Meanwhile, due to the influence of sedimentation, diagenesis, and tectonism, deep shale gas reservoirs have higher contents of brittle minerals, more developed natural fractures, and more complex pore evolution, resulting in different shale geological characteristics and controlling factors for shale gas enrichment [24]. Therefore, we comparatively analysed shale gas with a shallow to intermediate burial depth in the Weiyuan area and deep shale gas in the Luzhou area and clarified the characteristics, differences and main controlling factors of shale gas enrichment conditions in the two areas by comprehensively analysing the drilling, elemental logging, logging, and analysis data. The research results and methods are of great significance for the exploration of deep shale gas in the study area and similar areas.

The southern Sichuan area is located to the west of the Huayingshan fault and to the east of the Qiyueshan fault. It is in the low and steep Chuannan fold belt, with broom-like surface structural traces [25, 26]. The regional structure of the Luzhou area is located in the low and steep tectonic belt in southern Sichuan, surrounded by the gentle tectonic belt in central Sichuan, the high and steep tectonic belt in eastern Sichuan, the low tectonic fold belt in southwestern Sichuan, and the Loushan fold belt (Figure 1(a)). NE-trending structures and broom and similar structures from northeast to southwest are mainly developed in the Luzhou area, the strength of tectonic folds gradually weakens from northeast to southwest, and the faults are formed mainly in the large-scale compressional orogenic environment of the Himalayan period. The Luzhou area is characterized mainly by broad synclines and narrow anticlines, with five low and steep fault anticlines and three synclines. The anticlines are the Gufoshan structure, the Jiukuishan structure, the Longdongping structure, the Yanggaosi structure, and the Haichao structure. The synclines are the Desheng syncline, the Baozang syncline, and the Yunjin syncline from west to east. Two groups of main faults, a NE–SW-trending fault and an EW-trending fault, are developed, with a large distance between them (Figure 1(b)). The main faults control the tectonic morphology. There are a total of 32 grade II faults, of which 14 are distributed in the L203-Y101 well area. The grade II and grade III faults are developed in the central and southern regions, with a displacement of generally 20 to 170 m and an extension length of generally 5 to 10 km. The burial depth of the Longmaxi Formation is in the range of 3000-4500 m, of which the area with a burial depth greater than 3500 m accounts for 85% of the total area. The Weiyuan area is tectonically located in the low and steep fold belt in the southwestern part of the central plain zone of the paleouplift in central Sichuan, which mainly includes the slope area on the eastern wing of the Weiyuan anticline. The main structure is NE-trending and gently dipping. The dip angle of the Longmaxi Formation gradually decreases from northwest to southeast, and it is a relatively gentle tectonic area. The maximum fault displacement in this area is distributed in the range of 0-100 m, and the preservation conditions of the shale gas reservoirs are excellent. The burial depth of the Longmaxi Formation is distributed mainly in the range of 1800 m to 3700 m, and the area with a burial depth greater than 3,500 m accounts for 48.8% of the total area (Figure 1(c)).

3.1. Shale Samples

In this study, cores were sampled from the Wufeng-Longmaxi Formation from four wells, namely, wells L206 and Y101H3-8 in the Luzhou area and wells W202 and W208 in the Weiyuan area, and the total organic carbon (TOC) content, degree of thermal evolution (Ro), whole-rock X-ray diffraction, shale porosity, pore type, and microscopic pore structure are experimentally tested (Table 1).

3.2. Experimental Methods

3.2.1. Analysis and Testing of Major and Trace Elements

A total of 458 drill cutting samples were collected from the Lower Silurian Longmaxi Formation of wells W202, Lu206, Y101H3-8, and Y101H41-2 and were tested with a CIT-3000SY X-ray fluorescence spectrometer for the major and trace elements. The tested major elements included aluminium (Al), silicon (Si), iron (Fe), calcium (Ca), potassium (K), magnesium (Mg), manganese (Mn), sodium (Na), phosphorus (P), sulfur (S), and titanium (Ti). The main tested trace elements included arsenic (As), silver (Ag), cadmium (Cd), copper (Cu), vanadium (V), chromium (Cr), cobalt (Co), nickel (Ni), uranium (U), molybdenum (Mo), barium (Ba), zinc (Zn), lithium (Li), strontium (Sr), and rubidium (Rb). The main rare earth elements include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), yttrium (Y), and scandium (Sc). Hence, the elemental chemical profile was established for the Wufeng-Longmaxi Formation in the Luzhou and Weiyuan areas.

3.2.2. Characteristics of the Whole-Rock Mineral Composition

The contents of the shale mineral components were measured by the whole-rock X-ray diffraction with an X’Pert MPD PRO X-ray diffractometer (PANalytical, Netherlands) using an X-ray tube with a Cu target, a maximum voltage of 60 kV, a maximum current of 55 mA, and a maximum output power of 3 kW. Prior to the experiment, the samples were ground to a particle size of 320 mesh (approximately 40 μm). Each sample was horizontally placed in the goniometer, with a 2θ angle ranging from 1° to 160°. During the test, the position of the sample remained unchanged, and the X-ray tube and the detector rotated relative to the sample. The phases and lattice parameters can be quantitatively analysed with the built-in software and the PDF-2 database of the International Centre for Diffraction Data.

3.2.3. Determination of Geochemical Parameters

(1) TOC Content Analysis. The TOC content of the samples was measured using a CS230SH carbon and sulfur analyser (Leco, USA). The carbon and sulfur in the sample were oxidized to carbon dioxide and sulfur dioxide, respectively, by heating under oxygen-rich conditions to absorb infrared radiation. Before the experiment, the samples were cleaned by ultrasonication. The cleaned and dried shale samples were ground in an agate grinder to a size of 200 mesh or more. The carbonate mineral components were removed by acid washing and then dried at a low temperature. The reference sample was replaced with a new sample for every twenty samples tested.

(2) Analysis of the Thermal Evolution of Organic Matter. The test was performed using an Axio Scope.A1 polarizing microscope and an MSP.400 microspectrophotometer. Each test sample was placed on a stage, and the asphalt reflectance was measured under nonpolarized light. From the measured asphalt reflectance of the test sample, the relationship between vitrinite reflectance and asphalt reflectance (Ro=0.6569×Rob+0.3364, natural series) established by Feng et al. (1988) was used to calculate the equivalent vitrinite reflectance.

3.2.4. Nuclear Magnetic Resonance (NMR) Determination of the Pore Type

In this study, a low-field nuclear magnetic resonance instrument was used to determine the pore type of the shale. The T2 spectra of the undisturbed shale samples were measured. After the samples were dried at a high temperature, their T2 distributions were measured. The T2 spectra were measured under vacuum after the samples were saturated with dodecane. The samples were then placed in a MnCl2 solution (brine with no nuclear magnetic signal) for self-absorption, and the dodecane in the inorganic pores of the core was driven out by the MnCl2 solution. The T2 spectrum was measured every 24 h until the T2 spectrum stabilized. Then, the experimental data were processed and interpreted. Finally, ρ2=2μm/s was selected as the empirical coefficient, and the conversion coefficient C was obtained using the above conversion equation. Then, the relaxation time spectra were converted to pore-throat radius frequency distribution curves. The signal corresponding to the T2 distribution was converted to the corresponding pore volume percentage.

3.2.5. Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) Scan

A Helios 650 dual beam electron microscope (FEI) was used for the FIB-SEM scan. In FIB-SEM, a gallium ion beam was introduced at an angle of 52° to the electron beam in the field emission electron microscope. The ion beam perpendicularly cut the sample surface, and the sample surface was scanned with the electron beam at an angle of 38°. The cutting area was 10μm×15μm, the pixel size was 2-10 nm, and the slice thickness was 10 nm or 20 nm. The ion beam voltage and current were 500 V-30 kV and 1.1 pA-65 nA, respectively. The electron beam voltage and current were 1-24 kV and 0.78 pA-26 nA, respectively. Scanning and imaging micron-scale samples are suitable for 3D scanning and imaging of cores of various lithologies to establish a three-dimensional network model for quantifying the porosity, organic matter content, and connectivity.

The enrichment of shale gas requires more than simply a good material basis, including reservoir characteristics (rock and mineral characteristics, geochemical characteristics, reservoir spatial characteristics, physical property characteristics, etc.) and gas-bearing characteristics (gas saturation, total gas content, and analytical gas content). It is also necessary to clarify the effects of the burial depth of the target layer and the formation pressure coefficient on the preservation of shale gas reservoirs under effective preservation conditions. Deepening the understanding of the shale accumulation mechanism in deep shale gas complex tectonic areas and clarifying the characteristics and differences in enrichment conditions between deep shale gas and conventional shale gas areas have important reference value and guiding significance for the prediction of favourable areas for deep shale gas exploration and development.

4.1. Analysis of Paleooxygen Facies and Sedimentary Environment of Water Bodies

We further analysed vertical differences in the paleooxygen facies and sedimentary environment of the shale sedimentary water through gamma ray spectrometry logging and elemental logging. Redox-sensitive elements V, Mo, Ni, U, and thorium (Th) and the U/Th, V/(V+Ni), Ni/Co, and V/Cr ratios can be used as good indicators to judge the redox environment of ancient marine waters [2632]. Among them, the U/Th ratio is the most commonly used. A U/Th ratio>1.25 indicates an anoxic environment, a U/Th ratio<0.75 indicates an oxic environment, and a U/Th ratio between 0.75 and 1.25 indicates an oxygen-deficient environment [3336]. The calculation method of excess silicon proposed by Holdaway and Clayton was introduced to calculate the biosilica content index (SiBio) of the evaluation wells in the study area and to analyse the sedimentary environment of shale [37]. The U/Th ratio, TOC content, and SiBio in the Wufeng-Longmaxi Formation in the southern Sichuan Basin first increased and then decreased overall. The U/Th ratio of the layer 1 of the 1st Longyi submember (hereafter Longyi11) at the bottom of the Longmaxi Formation was generally above 1.25, the TOC content was greater than 5%, and the SiBio was greater than 20% (Figure 2). The U/Th ratio, TOC content, and SiBio in well L203 were bimodal in the Longyi11 and layer 3 of the 1st Longyi submember (hereafter Longyi13), with a maximum U/Th of 2.9, a maximum TOC content of 5.2%, and a maximum SiBio of 25.3%. The U/Th ratio in well Yang101H3-8 was unimodal, with a maximum value of 3.4 in the Longyi11. The U/Th ratio, TOC content, and SiBio gradually decreased from the bottom up in well Y101H3-8 (Figure 2(c)). The thickness, U/Th ratio, and TOC content of the Longyi14 in well Y101H3-8 were all greater than those of wells W202 and L203, and the TOC content was greater than 2.0% throughout Long14 sublayer (Figures 2(a) and 2(b)). The results indicate that the bottom of the Wufeng-Longmaxi Formation was an anoxic water environment, and the U/Th ratio gradually decreased from the bottom up, indicating the change from an anoxic water environment to an oxygen-rich oxidizing environment. The U/Th ratio of well W202 reached 5.0 in Longyi11, the TOC content reached 7.1%, and the SiBio reached 29.0% (Figure 2(a)). Subsequently, the U/Th ratio and TOC content rapidly decreased in the layer 2 of the 1st Longyi submember (hereafter Longyi12) and slightly increased in Longyi13.

In the Luzhou area, well L203H79-4 was used as an example. Core observation showed that the high-quality shale interval of the Longmaxi Formation has developed bedding structures, which have manifested mainly as horizontal bedding, rhythmic bedding, massive bedding, and thick parallel bedding. Thin layers of silty sand and black peaty form the horizontal bedding with alternating light and dark planes, and a small amount of dolomite and needle-like mica are found in the silty sand. The high-quality shale interval is rich in graptolite fossils. Monograptidae and Rastrites are located in the core. The graptolite individuals are relatively large and intact. In addition, there are relatively abundant skeletal fossils of siliceous organisms, such as sponges, radiolarians, and spicules, which reflect a deep-water continental shelf sedimentary environment with deeper water bodies. Vertically, in the evolutionary sequence of sedimentary facies from bottom to top, the clay content gradually increased, the siliceous content gradually decreased, the carbonate mineral content showed two cycles of increasing and then decreasing, and the organic matter content and U/Th ratio gradually decreased. The Wufeng Formation-Long1 submember with high-quality shale can be divided into six sedimentary microfacies. The well depth at the bottom of the Longyi11 to layer 4 of the 1st Longyi submember (hereafter Longyi14) was 3833.16-3847.40 m, and there was high organic matter abundance and SiBio. The main vertical sedimentary microfacies included organic-rich siliceous muddy shelf microfacies and organic-rich silicon-containing muddy shelf microfacies. The TOC contents were between 2.5% and 7.1%, the U/Th ratios were between 0.5 and 2.7, and the SiBio was between 10.0% and 33.9%. The well depth ranged from 3788.0 m at the top to 3833.16 m at the bottom of the Longyi14, and the variation in organic matter abundance was cyclic. The main sedimentary microfacies from bottom to top were organic-rich calcium-bearing silty deep-water continental shelf microfacies, calcium-bearing clayey deep-water continental shelf microfacies, calcareous clayey deep-water continental shelf microfacies, and clay-rich deep-water continental shelf microfacies, with TOC contents of 1.0% to 3.5% (Figure 3). Hence, the basic conditions for the formation of shale gas were satisfied.

4.2. Characteristics of Deep Shale Gas Reservoirs

4.2.1. Characteristics of Mineral Components

The lithology of the Wufeng Formation-Longyi1 submember in southern Sichuan is mostly dark grey and grey shale, and the bottom is dark grey, black, and greyish black shale, which is rich in graptolite fossils. From the top to the bottom of the high-quality shale interval, the amount of organic matter increases, and the amount of paleobiomass increases. Quartz particles are distributed in the form of sandy laminations parallel to the beddings, are interlayered with organic matter and clay, and contain calcite and dolomite fragments and a small amount of plagioclase and pyrite. The whole-rock X diffraction results show a large difference between the contents of different mineral components in the Wufeng-Longmaxi Formation. Specifically, the contents of quartz and clay minerals are highest, followed by the contents of feldspar, calcite, and dolomite. The content of pyrite is lowest. The average contents of other minerals, such as analcime, apatite, muscovite, biotite, rutile, and gypsum, are all less than 1%.

In Longyi11 to Longyi13 in the Luzhou area, the contents of brittle minerals are between 64% and 83%, with an average of 77%; the quartz contents are between 46% and 60%, with an average content of 52%; the feldspar contents are between 4% and 5%, with an average of 4%; and the clay mineral contents are between 15% and 33%, with an average of 22%. In Longyi14 in the Luzhou area, the average content of brittle minerals is 57%, the average quartz content is 37%, the average feldspar content is 8%, and the average clay mineral content is 39%. In Longyi11 to Longyi13 in the Weiyuan area, the contents of brittle minerals are between 69% and 83%, with an average of 75%; the quartz contents are between 28% and 57%, with an average of 44%; the feldspar contents are between 4% and 11%, with an average of 7%; and the clay mineral contents are between 14% and 28%, with an average of 22%. In Longyi14 in the Weiyuan area, the average content of brittle minerals is 64%, the average quartz content is 36%, the average feldspar content is 4%, and the average clay mineral content is 35% (Table 2). Overall, the contents of brittle minerals in the two areas are high in the vertical direction and gradually decrease from bottom to top. The content of brittle minerals is slightly higher in the Luzhou area than in the Weiyuan area.

4.2.2. Geochemical Characteristics

The experimental data (Table 3) show that the abundance of organic matter in the Longyi1 submember in the Luzhou and Weiyuan areas gradually increases from top to bottom, with the highest TOC in Longyi11 to Longyi13, followed by the organic matter contents in Longyi14 and the Wufeng Formation. The TOC content at the bottom of the Longmaxi Formation is slightly higher in the Luzhou area than that in the Weiyuan area. In Longyi11 to Longyi13 in the Luzhou area, the TOC contents are between 3.30% and 4.63%, with an average of 4.04%, and the equivalent vitrinite reflectance is between 3.19 and 3.31%, with an average of 3.25%. In Longyi14 of the Luzhou area, the average TOC content is 2.47%, and the average equivalent vitrinite reflectance is 3.21%. In the Wufeng Formation in the Luzhou area, the average TOC content is 3.14%, and the average equivalent vitrinite reflectance is 3.30%. In Longyi11 to Longyi13 of the Weiyuan area, the TOC contents are between 2.28% and 4.70%, with an average of 3.22%, and the equivalent vitrinite reflectance is between 2.84 and 2.92%, with an average of 3.22%. In Longyi14 of the Weiyuan area, the average TOC content is 2.31%, and the average equivalent vitrinite reflectance is 2.82%. In the Wufeng Formation in the Weiyuan area, the average TOC content is 1.23%, and the average equivalent vitrinite reflectance is 2.84%. Vertically, the TOC content of the Longmaxi Formation is relatively high at the bottom and gradually decreases from bottom to top. The TOC contents are higher in Longyi11 to Longyi13 and lower in the Longyi14. The degree of thermal evolution of organic matter reaches the highly mature to overmature stage.

The microscopic components of the organic matter in the reservoir interval of the Luzhou area are dominated by sapropelinite and inertinite. The core samples had an average sapropelinite content above 90%, indicating type I kerogen. The microscopic components of the organic matter in the reservoir section of the Weiyuan area are dominated by sapropelinite and exinite, of which sapropelinite is the main carrier for the formation and occurrence of organic pores. The kerogens in this interval are mainly types I and II1.

4.2.3. Physical Characteristics

The pore structure and pore type of shale reservoirs have an important impact on the reservoir capacity. In Longyi11 to Longyi13 in the Luzhou area, the porosity is between 4.7% and 6.7%, with an average of 5.6%, and the water saturation is between 18.4% and 35.9%, with an average of 27.2%. In Longyi14 of the Luzhou area, the average porosity is 4.5%, and the average water saturation is 42.1%. In the Wufeng Formation in the Luzhou area, the average porosity is 4.9%, and the average water saturation is 24.0%. In Longyi11 to Longyi13 in the Weiyuan area, the porosity is between 4.1% and 7.9%, with an average of 6.0%, and the water saturation is between 33.5% and 69.0%, with an average of 50.0%. In Longyi14 in the Weiyuan area, the average porosity is 5.0%, and the average water saturation is 56.2%. In the Wufeng Formation in the Weiyuan area, the average porosity is 5.3%, and the average water saturation is 67.0%. In the vertical direction, the reservoir performance of the Longyi1 submember in the Luzhou and Weiyuan areas is equally good in Longyi14 and Longyi11 to Longyi13. The porosity of Longyi14 is generally greater than 4% at the bottom, does not change greatly in the middle to upper part, and decreases at the top (Table 4).

The shale in the Wufeng-Longmaxi Formation in the southern Sichuan Basin has a relatively good porosity. The pore space composition of shale in different wells mainly includes inorganic pores, inorganic fractures, organic pores, and organic fractures. In the shale of the Luzhou area, the proportions of inorganic pores and fractures are between 25.8% and 98.5%, and the proportions of organic pores and fractures are between 5.1% and 74.2%. Overall, the contribution rate of organic pores and fractures to the areal porosity is higher than that of inorganic pores and fractures in the Luzhou area. In the shale of the Weiyuan area, the proportions of organic pores and fractures in the areal porosity are between 15.8% and 89.0%, and the proportions of inorganic pores and fractures are between 10.5% and 84.2%, indicating that the proportions of organic pores and fractures are comparable to those of inorganic pores and fractures. The degree of development of organic and inorganic pores is lower in the Luzhou area than in the Weiyuan area (Figure 4). Due to differences in the burial depth, the deep shale gas areas exhibit slightly decreased total porosity, organic pore porosity, and inorganic pore porosity, which is the most distinguishable feature of pore development in the deep shale gas areas compared with the shallow shale gas areas. Under overpressure conditions, the correlation between shale porosity and current burial depth is not obvious. Overpressure in the shale strata can partially offset the mechanical compaction of pores by the overburden pressure, delaying or even changing the decrease in porosity with increasing burial depth, which is conducive to the preservation of shale gas.

4.2.4. Reservoir Spatial Characteristics

The microscopic characteristics of shale pores in the Wufeng Formation and Longyi1 submember of the Luzhou and Weiyuan areas were analysed with casting thin sections and argon ion beam cross-section polishing-SEM. The main pore types include intergranular pores of detrital minerals, intercrystalline pores of clay minerals, intercrystalline pores in raspberry-shaped pyrite aggregates, organic pores, and intragranular dissolution pores [3840]. In the deep shale in the Luzhou area, the organic pores are mainly large bubble-like and honeycomb-shaped pores that are interconnected, forming an excellent pore structure; the inorganic pores are mainly intragranular dissolution pores, and the degree of development of intergranular pores is relatively low; the microfractures are commonly due to compaction or intergranular shrinkage of the clay minerals, with local development of mineral edge fractures [4143]. In shale found at shallow to intermediate depths in the Weiyuan area, organic pores occur mainly inside the organic matter and are evenly distributed, and some occur sporadically in the organic matter associated with clay minerals and raspberry-shaped pyrite. The organic pores are mostly elliptical or vesicular. The inorganic pores are mainly interlayer pores, intergranular pores, dissolution pores, and intercrystalline pores of clay minerals. The degree of development of intragranular dissolution pores is relatively low, and a small number of dissolution pores are developed in feldspar minerals. The microfractures are mostly mineral edge fractures and shrinkage fractures, with fewer organic matter edge fractures (Figure 5).

A comparative analysis of the characteristics of the shallow to intermediate and deep shale reservoirs in the Sichuan Basin showed that the mechanical compaction caused by the high overburden pressure affects the shale micropores (<2 nm), mesopores (2-50 nm), and macropores (>50 nm) differently. Micropores are often more susceptible to compaction. As the burial depth increases, the overburden pressure also significantly increases, which to a certain extent will cause the specific surface area and volume of shale micropores to develop towards uniformity, but the overpressure is conducive to the development and preservation of micropores and may change the trend of micropore reduction. In contrast, the specific surface area, volume, and total pore volume of mesopores may differ considerably due to changes in shale composition and overpressure state [44, 45]. In the Luzhou area, the radius of the organic pores in Longyi1 submember is between 100 nm and 200 nm. Organic mesopores are well developed in the Wufeng Formation, Longyi11, and Longyi13. In Longyi12, micropores and mesopores are relatively well developed, while macropores are poorly developed. In Longyi14, micropores are well developed pores in segment a, mesopores are well developed in segment b, and micropores and macropores are relatively developed in segment c. In the Weiyuan area, the radius of organic pores in Longyi1 submember exceeds 300 nm, and they are mainly mesopores. In Longyi11 to Longyi13, the organic pores are predominantly mesopores, followed by micropores. The pore radius was smaller in Longyi14, where both micropores and mesopores are distributed, and the radius of some pores is less than 2 nm.

5.1. The Overpressure and Rich Gas Characteristics of Deep Shale Gas

The shale formation pressure coefficient of the Wufeng-Longmaxi Formation in southern Sichuan and the corresponding shale gas well production rates vary greatly. The formation pressure coefficient in the Weiyuan area has a large span, ranging from 1.2 to 2.0. In the Weiyuan region, the shallow buried gentle area and the high-angle slope area have relatively shallow burial depths. In addition, the shale strata are often uplifted and denuded, mainly under low pressure and normal pressure, the preservation conditions are poor, and the possibility of horizontal and longitudinal dissipation is large. For well W201, the pressure coefficient is low (1.0), the shale gas content is 2.6 m3/t, and the single-well gas production rate is 0.26×104 m3/d. For the central low and gentle slope area and deep-buried gentle area, the structure is relatively stable, the formation is overpressured, the preservation conditions are good, and the possibility of shale gas dissipation is small. For wells W202H18-1, W208, and W204 with large burial depths, the pressure coefficients are 1.48, 1.58, and 1.90, respectively; the shale gas contents are 3.4 m3/t, 3.0 m3/t, and 3.0 m3/t, respectively; and the single-well gas production rates are 22.32×104 m3/d, 16.5×104 m3/d, and 22.73×104 m3/d, respectively.

Similar to shale gas with shallow to intermediate burial depths, deep shale gas has good material bases for reservoir formation and basic geological conditions for enrichment and high yield. In wells L203 and Y101 in the Luzhou area, the formation pressure coefficient reaches 2.2. The formation pressure coefficient decreases southeastward to 1.8 at well H201 (Figure 6). Wells T201H, L201, and L210 are located in the high part of the anticlines, fault folds have developed in the corresponding area, and the burial depth is relatively shallow. Their formation pressure coefficients are 1.94, 1.97, and 1.95, respectively; their porosities are 4.2%, 4.9%, and 4.6%, respectively; and their shale gas contents are 3.3 m3/t, 4.6 m3/t, and 3.0 m3/t, respectively. Wells L205, L206, and Y101H10-3 are in the broad and gentle regions of the Fuji syncline, Desheng syncline, and Baozang syncline, respectively. The fault development is poor, and the burial depth is relatively deep. Their formation pressure coefficients are 2.24, 2.32, and 2.20, respectively; their porosities are 4.6%, 4.6%, and 4.2%, respectively; their shale gas contents are 5.0 m3/t, 3.5 m3/t, and 3.6 m3/t, respectively; and their single-well gas production rates are 20.3×104 m3/d, 30.5×104 m3/d, and 60.84×104 m3/d, respectively. In addition, organic pores are developed in the deep shale with a high areal porosity, which is generally between 20% and 55% and can reach 60% locally. The pore radii are mainly between 100 and 200 nm, and the pores are mainly honeycomb-shaped and elliptical. Overall, the deep shale gas in southern Sichuan has overpressure and rich gas characteristics, namely, high formation pressure, high porosity, and high gas content [46, 47].

5.2. Control of the Quality and Thickness of the Reservoir by the Deep-Water Strong Reducing Environment

In southern Sichuan, the deep-water continental shelf sedimentary area further enlarged during the Late Ordovician to Early Silurian period, and the sedimentary environment of the Wufeng-Longmaxi Formation in the early depositional stage was a euxinic-semieuxinic retention sea basin environment (Figure 7). In the Katian to Hirnantian, extensive transgression occurred in the depositional period, the sea level continued to rise, the water body was deep, and the representative lithology was dark organic-rich silty shale. In terms of the characteristics of the geochemical elements, the ratios of Ni/Co, V/Cr, and U/Th, TOC content, SiBio, and Ti content increased rapidly, showing a weak reducing environment with an increase in organic carbon content.

In the Rhuddanian, the sedimentary water body of the entire Longmaxi Formation was the deepest. In terms of the characteristics of the geochemical elements, the ratios of Ni/Co, V/Cr, and U/Th, TOC content, SiBio, and Ti content were all gradually decreased from bottom to top, indicating that the reducing environment was the strongest at the bottom and strong in deep water far from the provenance area. During the depositional period, the sea level continued to rise. The marine environment was the sea area with the lowest water energy in the outer continental shelf and was a strong reducing environment with relatively weak hydrodynamic conditions and little disturbance by sea currents and storm currents. Thus, it was suitable for the accumulation and preservation of abundant organic matter. Planktons were prosperous, graptolite fossils were rich, and most benthic fossils were characterized by different degrees of silicification. High SiBio and high organic carbon content are usually conducive to the development and preservation of shale pores. In the early Rhuddanian, the Weiyuan area was affected mainly by the southeast compression, and the central Sichuan paleouplift and the central Guizhou uplift were the two underwater uplifts at that time, which made the sedimentary basement high on both sides and low in the middle [48]. The sedimentary environment of well W204 in the central Weiyuan area was in the deep-water continental shelf microfacies rich in biosilica. In the east, wells W212, W205, and W232 were affected by the Neijiang paleouplift, the water depth was shallower than that in the depressed areas in the basin, and the environment was a weak reducing environment with organic-rich silty deep-water continental shelf microfacies. In the south, the high-quality facies zones of wells L203 and Yang101 were all organic-rich silty deep-water continental shelf microfacies. The sedimentary thickness of high-quality shale reaches 40-80 m in the southern Sichuan area, is 56.5 m in the Changning area, increases to 74.5 m in the Luzhou area in the middle of the basin, and decreases northwards to 48.1 m in the Weiyuan area, where the central Sichuan uplift caused stratigraphic overlap.

After entering the Aeronian, under the effect of global sea level decline and regional tectonic uplift, sea levels began to decline, water bodies became shallower, the mud content was higher, the sediments in the provenance area changed to some extent, sediments were relatively coarse-grained, and the sedimentary environment changed to a shallow-water continental shelf sedimentary environment [49]. In terms of the characteristics of the geochemical elements, the ratios of Ni/Co, V/Cr, and U/Th, TOC content, SiBio, and Ti content apparently decreased from bottom to top, with Ni/Co<5, V/Cr between 2 and 4.25, U/Th<0.75, TOC<2%, SiBio<2%, and Ti>0.8%, indicating the input of many terrigenous clastics and a relatively humid climate, which is not conducive to the formation of high-quality reservoirs. The sedimentary environment evolved from oxidizing to weakly reducing, the organic carbon content in the shale decreased, the reducibility of the water body was poor, and the organic matter was difficult to preserve. The sedimentary centre is located in the Luzhou area of southern Sichuan, and the sedimentary thickness was 350-370 m, which decreased to 120-350 m towards the Weiyuan area of the central Sichuan uplift.

A favourable sedimentary environment has higher hydrocarbon generation potential and abundant shale gas resources. It is dominated by an oxygen-deficient and strong reducing environment, where abundant organic matter and siliceous particles are deposited. In the process of hydrocarbon generation, abundant organic matter generates a large amount of organic pores and a large specific surface area, which provide the main storage and adsorption space for the free and adsorbed shale gas. A large number of biogenic and detrital siliceous particles provide a solid rock support framework for the shale reservoirs, which reduces the damage to micro- and nanoscale pores caused by diagenesis and late tectonic processes, thereby maintaining good reservoir physical properties. Affected by the sedimentary environment, shale organic matter enrichment exhibits certain regional differences. The closer to the sedimentary centre the shale is, the higher the TOC content is. Therefore, the TOC content, ratios between trace elements, and SiBio of the Luzhou area are higher than those of the Weiyuan area (Table 5). The relationship between TOC, paleoproductivity, and redox conditions indicates that the TOC content of the gas-bearing shale interval of the Wufeng-Longmaxi Formation in the study area has good correlations with the U/Th ratio, Ni/Co ratio, and SiBio, and these correlations are positive in high-quality gas-bearing segments (Figure 8). The paleoredox conditions are the main factors controlling the enrichment of organic matter in the shale in the study area.

5.3. Effect of Tectonic Processes on Shale Gas Preservation Conditions

Preservation conditions constitute a key factor in deep shale gas enrichment in the complex tectonic region of southern Sichuan. The preservation conditions of shale gas depend on tectonic type and deformation strength, the development of faults and reservoir fractures, multistage tectonic movements and burial depth conditions, and regional caprock conditions. The formation pressure coefficient and shale gas content reflect the shale gas preservation effects. Among them, the time and amplitude of tectonic uplift and the development characteristics of faults and reservoir fractures have the greatest influences on preservation conditions, and they affect the preservation conditions of shale gas differently.

5.3.1. Time and Amplitude of Tectonic Uplift

The time and magnitude of tectonic uplift can be obtained by analysing the burial and thermal history. The Luzhou and Weiyuan areas in southern Sichuan differ greatly in tectonic conditions and the history of stratigraphic uplift [50]. The stratigraphic uplift occurred increasingly late from the northern part of the basin towards the centre of the basin. The tectonic uplift time was approximately 68-70 Ma for well W202 in the Weiyuan area, approximately 70 Ma-78 Ma for the Wufeng-Longmaxi Formation shale in well Y101 in the Desheng syncline in the Luzhou area, and approximately 85 Ma-90 Ma for the Wufeng-Longmaxi Formation shale in well L210 in the Baozang and Yunjin synclines in the Luzhou area. Studies have shown that the thermal evolution history of typical wells in the Desheng syncline in Luzhou can be divided into four stages. The first stage is the rapid burial stage from the Late Ordovician to Early Silurian, with a burial depth exceeding 2 km, and a formation temperature of approximately 90-110°C. This stage is the early-to-mid hydrocarbon generation stage. The second stage is the period of slow uplift and subsidence from the Early Silurian to the Middle Triassic, with a burial depth of approximately 3 km, a formation temperature of approximately 130-170°C, and a paleopressure coefficient of 1.5. This stage is the late hydrocarbon generation stage. The third stage is the rapid deep burial stage from the Middle Triassic to the Late Cretaceous, with a burial depth of approximately 6 km, a formation temperature exceeding 200°C, and a paleopressure coefficient of 2.1. This stage is the main gas generation stage. The fourth stage is the uplift stage since the Late Cretaceous, with an uplift time of approximately 70 Ma-78 Ma, an uplift amplitude of 3010 m, and a paleopressure coefficient of 2.7 (overpressure). The thermal evolution histories show higher uplift amplitudes and much faster uplift rates in typical wells in the slope area of Weiyuan than in those in the Desheng syncline of Luzhou. The uplift stage in the Weiyuan area since the Late Cretaceous occurred in 68-70 Ma, with an uplift amplitude of 3680 m.

The tectonic evolution of shale is a dynamic process that determines the final reservoir space of the shale interval. During the deep burial period, the maximum burial depth of typical wells in the Weiyuan area was 5720 m, the Ro value was between 2.0% and 2.5%, corresponding to the peak period of gas generation, and the TOC content was 1.0-5.0%. During the deep burial period, the maximum burial depth of typical wells in the Luzhou area was 6740 m, the Ro value was between 2.5% and 3.0%, corresponding to the peak period of gas generation, and the TOC content was between 1.7 and 5.0%. During the deep burial period, organic pores were mostly mesopores. At this time, the total porosity and the pore specific surface area were the largest, and the shale adsorption capacity was enhanced, indicating that the deep burial period was the most suitable period for thermal evolution (Figure 9). After the Wufeng-Longmaxi Formation reached the maximum burial depth, tectonic uplift controlled the escape and preservation of shale gas. The earlier the uplift time is, the longer the shale gas loss time, which is less conducive to the preservation of shale gas. The earlier the uplift time is, the longer the duration of gas escape and the more unfavourable the shale gas preservation conditions. The tectonic uplift and preservation conditions determine whether shale gas can be enriched and accumulated. The greater the stratigraphic uplift is, the lower the shale gas content. Well W202 has a maximum uplift magnitude of 3926 m, a daily gas production rate of 2.75×104 m3/d, a shale gas content of 4.5 m3/t, a TOC content of 3.5%, and a porosity of 6.3%. Well W204 has a maximum uplift magnitude of 2950 m, a daily gas production rate of 16.50×104 m3, a shale gas content of 6.6 m3/t, a TOC content of 3.5%, and a porosity of 7.6%. In the Luzhou area, the late uplift time, deep burial depth, and small uplift magnitude resulted in the favourable shale gas preservation conditions. Well Y101 in the Luzhou area has an uplift magnitude of 3010 m, a shale gas content of 4.5 m3/t, a TOC content of 3.9%, and a porosity of 4.7%. The daily gas production of well Y201-H2 in the area of well Y101 is 43×104 m3.

5.3.2. Faults and Reservoir Fracture Development

Faults control the development of fractures, which may become escape channels for shale gas. The nature, scale, assemblage, period, and derived tectonic fractures of faults are important factors affecting the preservation conditions of shale gas reservoirs [5153]. The faults in the Luzhou area were formed mainly under the large-scale compressional orogenic environment of the Himalayan period and are mostly reverse faults (Figure 10). According to the extension length and displacement, the faults in the study area can be divided into four levels. Faults of different scales affect the preservation conditions of shale gas to different extents. The first-level faults are basin-controlling faults, which are deep and large faults that break up to the surface and have a controlling effect on the structures. There is no such fault in the study area. The second-level faults have a destructive effect on the oil and gas preservation conditions, and the preservation conditions of shale gas wells within 3 km of a second-level faults are greatly affected. For example, well G205-H1 is located on the hanging wall of the Hai-1 fault of the Gufoshan structure, 1.29 km away from the fault; the total gas content of Longyi11 to Longyi13 is low (2.7 m3/t), and the test yield is 16.0×104 m3. The destructive effect of the third-level fault on oil and gas preservation is relatively limited. The preservation conditions of shale gas within 2 km of a third-level fault are affected, and the gas-bearing capacity is weakened. For example, well L208 is 1.96 km from the Luo-24 fault; the total gas content of the Longyi11 to Longyi13 is 3.4 m3/t, and the test yield is 26.5×104 m3. Fourth-level faults can be used as channels for oil and gas migration without causing damage to the oil and gas preservation conditions.

Previous studies have shown that the angle between the fault strike and the geostress direction affects the fault sealing performance (Figure 11) [5457]. Specifically, the fault sealing performance is the best when the angle exceeds 67.5°, is good when the angle is between 45° and 67.5°, is poor when the angle is between 22.5° and 45°, and basically fails when the angle is less than 22.5°. In the Luzhou area, the highly conductive fractures in well L 211 have dominant strikes of ENE 70°-80° and dip angles of 70°-80°, while the highly resistive fractures have dominant strikes of ENE 50°-60° and NE 90°-110° and dip angles of 40°-60°. The geostress direction is NE 98.3°-100.6°, forming angles of 20°-30° with dominant strikes of the highly conductive fractures and small angles with dominant strikes of the highly resistive fractures. Well L211 has a total gas content of 3.4 m3/t in the Longyi11 to Longyi13, indicating poor fault sealing performance. The high-conductivity fractures in well Lu208 have a dominant strike of ENE 60° and dip angles of 10°-30°, while the high-resistivity fractures have no obvious dominant strike. The geostress direction is NE 100°, forming angles of 40°-50° with the highly conductive fractures. Well L208 has a total gas content of 3.4 m3/t in the Longyi11 to Longyi13and a test yield of 26.5×104 m3, indicating moderate fault sealing performance. In well Y101H10-3, the highly conductive fractures have dominant strikes of NNE 40-50°, while the highly resistive fractures have dominant ENE strikes and dip angles of 10°-30°. The geostress direction is NE 110°, forming angles of 60°-70° with the highly conductive fractures. Well Y101H10-3 has a total gas content of 3.7 m3/t in the Longyi11 to Longyi13 and a test yield of 43.0×104 m3, indicating good fault sealing performance. The fractures of the gas reservoir section of well W204H10-2 in the Weiyuan area are mainly the ENE, striking the highly resistive fractures; they have dominant strikes of ENE 60-70° and dip angles of 20°-70°. The geostress direction forms angles of 30°-50° with the high-resistivity fractures. Well W204H10-2 has a total gas content of 2.4 m3/t in the Longyi11 to Longyi13 and a test yield of 10.3×104 m3, indicating moderate fault sealing performance. The highly conductive fractures in well W202H18-1 have dominant strikes of NNE 40-50° and WNW 290°-300° and dip angles of 40-60° and 80-90°. The geostress direction is approximately WE 90-100°, forming angles of 50°-60° with the highly conductive fractures. Well W202H18-1 has a total gas content of 3.7 m3/t in the Longyi11 to Longyi13 and a test yield of 23.3×104 m3, indicating good fault sealing performance.

  • (1)

    The U/Th ratios, TOC contents, and SiBio values in the Wufeng-Longmaxi Formation in southern Sichuan initially increase and then generally decrease. The black organic-rich siliceous shale at the bottom of the Wufeng-Longmaxi Formation has a U/Th ratio exceeding 1.25, a TOC content exceeding 5%, and SiBio values exceeding 20%. The water body is anoxic, and the U/Th ratios gradually decrease from bottom to top, indicating a change to an oxygen-rich environment. The organic matter in the high-quality parts of the Wufeng-Longmaxi Formation shale in the deep shale gas reservoirs is composed of mainly amorphous sapropelinite and inertinite. The organic matter content exceeds 3.0%, and the degree of thermal evolution Ro generally exceeds 3.0. The kerogens are mainly type I in a highly mature to overmature stage (dry gas production) and have great potential for hydrocarbon generation. The pore types of the reservoir are mainly inorganic pores and organic pores, and the porosity exceeds 4.5%. Due to differences in the burial depth, deep gas reservoirs exhibit decreasing porosity compared with shallow gas reservoirs

  • (2)

    Horizons with higher hydrocarbon generation potential and abundant shale gas resources are oxygen-deficient and strongly reducing, with abundant organic matter and siliceous grains. The sedimentary thickness of high-quality shale reaches 40-80 m in the southern Sichuan area, is 56.5 m in the Changning area, increases to 74.5 m in the Luzhou area in the middle of the basin, and decreases northwards to 48.1 m in the Weiyuan area, where the central Sichuan uplift caused stratigraphic overlap. The TOC content shows good correlation with the U/Th ratios, Ni/Co ratios, and SiBio values in the high-quality deep shale gas reservoir intervals of the Wufeng-Longmaxi Formation

  • (3)

    Tectonic processes have an important impact on the preservation of shale gas. Faults control the development of fractures, which may become escape channels for shale gas. Second- and third-order faults have a destructive effect on the oil and gas preservation conditions, whereas fourth-order faults act as channels for oil and gas migration. When fracture strikes and the present-day principal horizontal stress intersects at a high angle, good fault seal performance inhibits the escape of shale gas. The main factors controlling deep shale gas accumulation and good preservation conditions include late and small-amplitude tectonic uplift, small fault scales, high fault sealing performance, a moderate degree of fracture development, and high shale gas content

All the data that support the findings of this study are included within the manuscript.

The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.

The authors declare no conflicts of interest.

This study was financially supported by the Open Funds of Shale Gas Evaluation and Exploitation Key Laboratory of Sichuan Province (No. YSK2022002) and Natural Gas Geology Key Laboratory of Sichuan Province (No. 2021trqdz05).

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