Commercial oil and gas flow was discovered in the Jurassic sandstones of Moxizhuang and Yongjin areas in the central Junggar Basin. The reservoir porosity and permeability are very low in some areas where the buried depth of the Jurassic reaches 6 000 m， belonging to ultra-low porosity and ultra-low permeability reservoirs. In such tight reservoirs， the charging power of crude oil， the source of oil and gas， and the migration process need to be systematically discussed. Biomarkers were used to determine the source of oil and the main source rocks of crude oil in tight sandstone reservoirs to reveal the enrichment process and main controlling factors of crude oil in tight sandstone reservoirs. The slip distances of the faults in horizontal and vertical directions and the sealing properties of the faults were studied by quantitative and semi-quantitative methods， and the relationship between fault size and oil test production was clarified by combining statistics and geological analysis. Moreover， the influence of fault size and fault distance on oil and gas accumulation was explored. Micro-CT scanning， constant-rate mercury injection， nuclear magnetic resonance （NMR）， and physical simulation were applied to systematically describe and characterize the pore structure parameters of sandstones and fluid accumulation models in these tight reservoirs. The results show that the crude oil is mainly from the Permian source rocks. The faults with large sizes and high activity intensity are vertical migration channels of oil and gas， and the faults with horizontal and vertical movement distances of more than 700 m and a size of more than 20 km at geological time have better oil and gas transport capacity. However， the faults with small sizes and short movement distances are mostly closed. The pore sizes of the tight reservoirs are small， mainly in the range of 163.8-207.7 μm， and the throats are 0.5-8.1 μm. The heterogeneity of the micro-pore structures leads to the differential charging and enrichment of crude oil. The oil content of reservoirs with medium porosity and coarse throat is the highest， followed by the reservoirs with large porosity and small throat， and the oil content of reservoirs with small porosity and micro throat is the lowest. The accumulation of Jurassic tight sandstone reservoirs is mainly influenced and controlled by high activity intensity， large faults， and pore structures of tight sandstone.

Neoproterozoic glaciogenic diamictite and cap carbonate couplets have played a pivotal role in understanding glacial-interglacial cycles and establishing regional stratigraphic correlation. The Alxa Block in northwestern China preserves a sequence of Neoproterozoic diamictites along its southern margin, but the age and origin of the succession remain debatable due to the lack of discovery of cap carbonate. We report newly discovered cap carbonates that overlie the diamictites of the Shaohuotonggou Formation in the Longshoushan region in the southern Alxa Block. Based on detailed geological investigations, we identified massive diamictites, stratified diamictites, and cap carbonates in the lower part of the formation. The presence of ice-rafted dropstones, bullet- shaped and facetted clasts, glacial striations, and relatively low chemical index of alteration values of sedimentary matrix support a glaciogenic origin of the diamictites. The 2- to 2.6-m-thick cap carbonates are mainly composed of thinly laminated microcrystalline dolomites and show sheet cracks, cemented breccias, and tepee-like structures at the basal part of the unit. These features and their consistently negative delta 13 C values (-5.2 parts per thousand to-2.2 parts per thousand) are characteristic of Marinoan-age cap carbonates (ca. 635 Ma). The quasi-continuous deposition of the massive diamictites, stratified diamictites, and cap carbonates suggests that the formation of this couplet was closely related to the Marinoan glaciation and subsequent deglaciation. We propose a three-stage depositional model for the glaciogenic succession and recommend that the diamictite and cap carbonate couplet in the Alxa Block provides a credible mark of the Cryogenian-Ediacaran boundary for further stratigraphic correlation and investigation.

To investigate the heterogeneous characteristics of the shale pore size distribution (PSD) of the Daanzhai Member in the Ziliujing Formation in the Sichuan Basin and its influencing factors, an analysis of its shale components, pore structure, and morphology was conducted. The analysis methods included the determination of total organic carbon (TOC), field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), LP-CO2GA, and LT-N2GA. The heterogeneity of the PSD was further analyzed via multifractal theory. The results indicate that the PSDs of both micropores and mesopores in shale exhibit multifractal features. The heterogeneity of mesopores is higher than that of micropores, but the pore connectivity is lower in mesopores than in micropores. Additionally, the degree of dispersion is higher in mesopores than in micropores. The PSD of micropores is influenced mainly by pores in the range of 0.30~0.70 nm in diameter. The distribution of mesopores is significantly affected by pores within the range of 2~10 nm in diameter. The pore connectivity and heterogeneity of mesopores are influenced primarily by the specific surface area (SSA) of the shale. In the case of micropores, both the SSA and pore volume (PV) contribute to the pore connectivity and heterogeneity. The effects of the rock components on the pore heterogeneity and connectivity vary significantly, with mineral components being the primary factors influencing pore heterogeneity. Compared with those of the mature Bakken Formation and the overmature Wufeng–Longmaxi Formation, the shale of the high-maturity Daanzhai Member has higher small-scale pore heterogeneity but weaker mesopore heterogeneity.

Two low-latitude glacial events (i.e. the Sturtian and Marinoan glaciations) occurred in the Cryogenian, both of which were known as the “Snowball Earth”. The sedimentary environment and continental chemical weathering recorded in the syn-glacial, inter-glacial, and post-glacial strata are of great significance to investigate the global glaciations and to establish the Earth’s environment evolution in the Neoproterozoic. Well-preserved middle-late Neoproterozoic sedimentary sequences (including the Yalaguzi, Bolong, Kelixi, Yutang, Kuerkake, and Kezisuhumu Formations from bottom to top) have been identified at the southwestern margin of the Tarim Craton. Nevertheless, there was ongoing debate regarding the evolution of sedimentary environments and their comparison to global glaciers. Furthermore, the sedimentary processes of glacial strata and continental chemical weathering remained obscure. This study conducted detailed field investigation, detrital zircon geochronology, and whole-rock major-element composition analysis. The sedimentary environment evolved from a continental to marine environment. The purplish red conglomerate of the Yalaguzi Formation represents alluvial fan facies, and the combination of diamictites and ice-raft debris depositions in the Bolong and Yutang Formations indicates glacial facies. After the deglaciation of the Bolong glaciation, the Kelixi Formation experienced the transition from neritic-littoral to fan delta-littoral. Afterwards, the sedimentary environment changed to the glacial facies of the Yutang Formation and evolved from a littoral-neritic to shore environment of the Kuerkake Formation. By using the mean age of the youngest two or more grains that overlap in age at 1σ (YC1σ (2+)), the maximum deposition age of the Kelixi Formation has been determined to be 662 Ma, which means that the Bolong and Yutang glaciations correspond to the Sturtian and Marinoan glaciations, respectively. The presence of relatively high corrected chemical index of alteration (CIAcorr) values in otherwise low CIA values documented in the Bolong Formation implies the existence of warm-humid intervals during the overall cold climate. The low CIAcorr values recorded in the Yutang Formation are consistent with a cold event in the Marinoan glaciation. The large variety of the CIAcorr values within the Kelixi Formation suggests that continental weathering during the interglacial period exhibited a tendency of (weakly)-strong-moderately-weakly-strong evolution. Notably, the dramatic fluctuation (from weak to strong continental weathering) recorded in the upper member of the Kelixi Formation implies the arrival of the Marinoan glaciation. A sharp CIAcorr increase is observed during the transition periods between the Bolong and Kelixi Formations, as well as the Yutang and Kuerkake Formations, indicating a huge increase in the chemical weathering intensity in the aftermath of the Sturtian and Marinoan glaciations. This suggests that the Sturtian and Marinoan deglaciation might have been associated with intense continental chemical weathering. This work was financially supported by the National Natural Science Foundation of China (grants 42272249 and 41972237) and the Hong Kong Research Grants Council General Research Fund (17307918).

The Ediacaran-age glaciation is the last of three major glacial events during Cryogenian-Ediacaran time. Unlike the earlier Cryogenian-age Sturtian and Marinoan glaciations that deposited worldwide diamictite and cap carbonate couplets, the Ediacaran-age glacial cap carbonates have rarely been documented, which hinders the understanding of Ediacaran glaciation and glacial stratigraphic correlations. Here, we newly identified an Ediacaran glacial cap carbonate overlying the stratified diamictites in the upper part of the Shaohuotonggou Formation in the Neoproterozoic Hanmushan Group along the southwestern margin of the Alxa Block. The stratified diamictite with unsorted clasts and widespread ice-rafted dropstones indicates its glacial origin. The absence of glacial-interglacial depositional cycle suggests that no glacier advance-retreat process occurred during this glaciation, which likely represents a short-lived glacial event with rapid deglaciation. Carbon isotope values of the cap carbonate fluctuated between ca. -2.6 %o and ca. + 1.5 %o, comparable to other Ediacaran glacial cap carbonates (ca. 560 Ma) from Arabia, Qaidam, northwestern Australia and Iran. Taking into account recently discovered Marinoan-age diamictite and cap carbonate couplet at the basal Shaohuotonggou Formation, we suggest that the southern Alxa Block most likely preserves two Neoproterozoic glacial events, i.e. the Marinoanage glaciation and the late Ediacaran-age glaciation. Thus, the Shaohuotonggou Formation most likely represents a late Cryogenian-late Ediacaran succession that contains two intervals of glaciogenic deposits. The occurrence of Marinoan and Ediacaran glacial cap carbonates thus merits a reconsideration of the paleogeographic and paleoclimatic evolution in the Alxa Block during late Neoproterozoic time.

Zircon U-Pb dating results (Table S1) and whole-rock major-element compositions of sedimentary rocks (Table S2).

A series of terrestrial shale samples with different thermal maturities were subjected to hydrous artificial pyrolysis to study the evolution of terrestrial shale pores. The original shale was obtained from the terrestrial interval of a core sample, the total organic carbon (TOC) content was 8.34 wt%, and the vitrinite reflectance (Ro) was 5.31%. The original shale core was cut into eight parts, which were heated at temperatures of 300, 350, 400, 420, 450, 500, 550, and 600 °C to obtain samples with different thermal maturities. The pore size distribution (PSD), pore volume (PV), specific surface area (SSA), and pore types were investigated via CO2 and N2 adsorption tests and field emission scanning electron microscopy (FE-SEM). Many organic matter (OM) pores and mineral pores were observed via FE-SEM with increasing thermal maturity. The total PV and SSA increased until the sample reached 500 °C and then decreased, and the mesopore volume followed this trend. The micropore volume first decreased, increased until the sample reached 500 °C, and then decreased; the macropore volume increased to a peak in the sample pyrolyzed at 420 °C and then remained stable. Pores with sizes ranging from 10 to 30 nm were the predominant contributors to the shale pore volume. The SSA was affected by pores with diameters less than 20 nm, which accounted for approximately 54% of the SSA. The rate of OM conversion influenced pore creation.

The pore-fracture structure of deep coal reservoirs is highly important for evaluating, exploring, and developing coalbed methane (CBM) resources. This study considers three coal samples from the DJ57 well in the Benxi Formation in the Daning-Jixian block on the eastern margin of the Ordos Basin as the research object. Based on the coal quality parameters of the coal samples, field emission scanning electron microscopy (FE-SEM), gas adsorption experiments, high-pressure mercury intrusion porosimetry (MIP), and microcomputed tomography (micro-CT) scanning were used to quantitatively characterize the nanoscale pores and microscale fractures in deep coal reservoirs and to evaluate the pore-fracture structure at different scales. The results reveal that the pore types in the Benxi Formation coal samples are diverse and include mainly organic matter (OM) pores, inorganic pores (intraparticle and interparticle pores), and microfractures. The organic pores are diverse in shape and mainly exhibit round, oval, and wedge shapes, while the microfractures exhibit slender stripes or serrated curves. The multiscale quantitative characterization of deep coal reservoir pores and fractures is based on a variety of pore characterization methods, and the pore and fracture volume distributions are mainly U-shaped, revealing the coexistence of micropores and microfractures. The volumes of micropores (0.3-2 nm), mesopores (2-50 nm), macropores (50 nm to 10 mu m), and microfractures (>10 mu m) account for 78.00%, 6.78%, 2.08%, and 13.14%, respectively, of the total pore volume (PV). Based on a full-scale pore-fracture splicing calculation, the total permeability of the Benxi Formation coal samples ranges from 5.77 to 28.22 mD. The observation results indicate that the microfractures are connected to each other, forming a network structure with strong connectivity. The microfractures are mainly associated with pore diameters>100 mu m, accounting for approximately 95% of the total permeability. Moreover, micropores in deep coal reservoirs provide a large space for CBM adsorption, and microfractures enhance the seepage capacity of CBM.

Objective To explore the enrichment conditions and main controlling factors of shale gas in the Lower Wuerhe Formation of the Permian System in the Junggar Basin, the Lower Wuerhe Formation in the Dongdaohaizi Sag was selected as the research object. Methods Based on the data of outcrop, core, well logging, well-calibrated seismic reflections and the technologies of total organic carbon (TOC) content determination, whole-rock X-ray diffraction, gas adsorption (N2, CO2), the distribution characteristics, organic matter development characteristics, reservoir characteristics, and gas bearing characteristics of the Lower Wuerhe Formation shale were studied. Results The results show that: (1) The organic matter of the Lower Wuerhe Formation shale is dominated by Ⅱ2 and Ⅲ types and shows an average TOC content of 1.58%. The average vitrinite reflectance (Ro) of organic matter is 1.46%, which indicates the mature stage. The average thickness of the source rock is 75 m. Summarily, the source rock is good and has a high gas potential. The basin simulation results show an average shale gas content of 1.89 m3/t in the Lower Wuerhe Formation. (2) The pores and microfractures in shale reservoirs are highly developed, and gas is primarily adsorbed in micropores and mesopores. The average porosity and permeability are 6.10% and 0.27×10-3 μm2 respectively, which are favourable for shale gas accumulation. (3) The shale has a high clay mineral content, with an average of 29.6%, providing a significant specific surface area and enhancing the gas adsorption capacity of the shale. Additionally, the average brittle mineral content is 50.9%, indicating good frackability. (4) Moreover, the shale reservoir exhibits a relatively large pressure coefficient of 1.58, indicating the favourable conservation conditions. The analysis of the regional tectonic-sedimentary environment and geochemical parameters indicates that the main factors controlling shale gas accumulation in the Lower Wuerhe Formation of the Dongdaohaizi Sag are geochemical parameters and preservation conditions. The key factors influencing shale gas accumulation include the high thermal evolution maturity of organic matter, large shale thickness, high TOC content, and good preservation conditions. These conditions suggest that the favourable area for shale gas exploration and development in the Dongdaohaizi Sag is located in the northeastern slope area of the sag's abdomen. Conclusion The results of this research reveal the enrichment conditions and main controlling factors of shale gas in the Lower Wuerhe Formation in the Dongdaohaizi Sag, which has reference value for deep oil and gas exploration in the abdominal area of the Junggar Basin.

Tuff reservoirs,as a type of tight oil and gas reservoir,are significantly influenced by their micro and nano-pores in terms of hydrocarbon storage.To understand the pore structure and heterogeneity characteristics of tuff reservoirs in the Perm-ian Section 2 Feng in Hashan area,Junggar Basin,six tuff samples from Well Ha 11 in Hashan area were selected.These samples underwent total organic carbon(TOC)content measurement,whole rock mineral composition analysis,and character-ization of pore structure through CO2 and N2 adsorption experiments.Using multifractal theory,the pore heterogeneity and con-nectivity were analyzed.The results show that the average TOC content of the tuff samples is 0.931%,and the mineral compo-nents are mainly feldspar,quartz,clay minerals,and dolomite.Micropores are mainly developed in the 0.33-0.38,0.50-0.68,and 0.72-0.86 nm pore size ranges,while meso-macropores are mainly developed in the 2.94-16.09 nm range.The generalized fractal dimension Dq decreases with increasing q,and the singular fractal spectrum shows a convex asymmetric parabola.The pores in tuff reservoirs have multifractal characteristics.Micropores(0-2 nm)have smaller width of the singular spectrum(Δα)values and larger Hurst index(H)values,while meso-macropores(2-100 nm)have larger Δα values and smaller Hurst index(H)values,indicating that micropores have better homogeneity and connectivity.The heterogeneity of meso-macropores is affected by the pore volume.With the increase in pore volume,pore heterogeneity increases,and connectivity decreases.TOC content and mineral composition have different effects on pore heterogeneity and connectivity.An increase in TOC content improves the connectivity of meso-macropores.An increase in plagioclase content increases the heterogeneity of meso-macropores.An increase in clay mineral content increases the heterogeneity of micropores,while an increase in dolomite and calcite content reduces the heterogeneity of meso-macropores.

In order to clarify the pore structure and fractal characteristics of the deep continental shale of the Lower Wuerhe Forma-tion in the central Junggar Basin,the Lower Wuerhe Formation shale in the Dongdaohaizi Sag was taken as the research object.On the basis of an in-depth analysis of shale minerals and geochemical characteristics,the pore structure characteristics of the Lower Wuerhe Formation shale were quantitatively characterized by using field emission scanning electron microscopy and low-temperature N2 adsorption experiments.The fractal dimension of shale pores was calculated based on the FHH model,and the rela-tionships among TOC content,mineral composition,pore structure parameters,fractal dimension,and its geological significance were revealed.The results show that the Lower Wuerhe Formation shale mainly develops inorganic pores and micro-fractures,and the pore size distribution is multi-peak type,mostly parallel plate or narrow slit pores.The pore development of shale is controlled by TOC and the content of quartz,feldspar,and clay minerals,which results in significant differences and strong heterogeneity among pore structures.Shale pores of the Lower Wuerhe Formation in the study area have double fractal characteristics,in which the surface fractal dimension D1 varies from 2.452 2 to 2.594 8,with an average value of 2.540 9.The fractal dimension D2 of the structure ranges from 2.604 5 to 2.774 8,with an average of 2.705 6.TOC is negatively correlated with fractal dimension,while pore structure parameters(specific surface area and pore volume)and mineral composition(quartz,feldspar,and clay mineral con-tent)are positively correlated with fractal dimension.An increase in the content of brittle minerals such as quartz,feldspar and clay minerals contributes to the development of micro-and nano-scale pores and micro-fractures.This results in an increase in specific surface area,pore volume,and fractal dimension.As pore heterogeneity strengthens,the complexity of the pore structure also in-creases.

Nanopore heterogeneity significantly affects the adsorption, desorption, and diffusion processes of coalbed methane. Here, the medium- and high-rank coals of the Benxi Formation in the eastern margin of the Ordos Basin were taken as the research object, and low-pressure CO2 adsorption and low-temperature N-2 adsorption experiments were used to study the pore characteristics; combined with multifractal theory, the multifractal characteristics of micropores (0.3-2.0 nm) and meso-/macropores (2-300 nm) in MRC and HRC are characterized and explored, and the evolutionary trends and influential factors during metamorphism are explored. The results show that the pore distribution of micropores and meso-/macropores in MRC to HRC coal has multifractal characteristics. Compared with meso-/macropores, micropores exhibit smaller Delta alpha values and larger H values; that is, micropores have weaker heterogeneity and better pore connectivity. As the coal rank increases, the percentage of the micropore volume in the coal increases, and the pore size distribution of micropores and meso-/macropores tends to be more homogeneous, which leads to a decrease in the pore structure uniformity and improvement in the pore connectivity in coal reservoirs. An increase in the vitrinite content leads to better connectivity and a more uniform pore structure of micropores and meso-/macropores. An increase in the mineral content and A(d) leads to a decrease in the connectivity of micropores and meso-/macropores and tends to increase the complexity of pore structures. With increasing micropore volume and specific surface area, the degree of pore aggregation and heterogeneity decreases, and the pore connectivity increases. However, there is no significant correlation between the pore structure and the multifractal parameters of the meso-/macropore structure. The HRC samples have more micropores than the MRC samples and thus exhibit greater pore structure homogeneity and better pore connectivity. A uniform distribution and good connectivity of the pores help to improve the methane adsorption capacity.

Coal and shale reservoirs are characterized by low porosity, low permeability, and complex pore structure, which are the limiting factors for dual gas production. Studying the pore structure of shale and coal informs gas migration, adsorption mechanisms, and storage modes, guiding the assessment of reservoir quality and development plans, crucial for dual gas recovery and underground H2 and CO2 storage. Hence, in this study, we aimed to comparatively analyze the pore structures of different rock types. Gas adsorption and field-emission scanning electron microscope coupled with multiple fractal theory analyses were performed to characterize the porosity and pore structure of the over-mature Longmaxi Formation (Lower Silurian system) and coal of the Benxi Formation (Upper Carboniferous system). The organic pores in coal were more developed than in shale. Micropores provided pore volume and specific surface area in coal, whereas meso-macropores were dominant in shale. Micropores and meso-macropores exhibited multifractal pore-size distributions in coal and shale. Micropores had higher connectivity but lower heterogeneity than meso-macropores. The permeability, connectivity, and heterogeneity of coal rock were related to vitrinite reflectance, ash content, and moisture content, respectively, whereas the permeability, connectivity, and heterogeneity of shale were influenced by total organic carbon content and mineral composition. In the shale of the Longmaxi Formation, gas molecules were predominantly adsorbed in microscopic pores, whereas in the coal of the Benxi Formation, they were primarily free, with a few in microscopic pores. Desorption of adsorbed gas into free gas in the microcavities of coal beds was key to coal bed methane development, whereas migration of shale gas from the matrix reservoir space to natural or engineered fracturing was controlled by pore connectivity. Thus, the study reveals the differences between coal and shale rocks that impact gas distribution and emphasize the importance of pore evaluation for effective development and storage strategies in both reservoirs. Collectively, this research has important implications for future dual-gas and tri-gas co-extraction and selection layers as well as underground hydrogen storage.

Accurate description of micro and nano pores in coal reservoir plays an important role in evaluating the reservoir and gas production capacity of coalbed methane, we used 6 continuous samples of high rank coal in Daning-Jixian area on the eastern margin of Ordos Basin as the research object, and combined with high pressure mercury injection, low temperature N2 adsorption and low pressure CO2 adsorption experiments to characterize the pore structure characteristics. The aim of this study is to have an in-depth understanding of the full-size pore size distribution and explore the influencing factors of coal reservoir pore structure. The results show that the pore volume and specific surface area of coal sample in the study area have obvious differences with the pore size distribution, and the contribution rate of coal sample micropores to pore volume and specific surface area is significant, especially the diameter less than 1.5 nm micropore provides storage space for most of the adsorptive gas. The contribution of mesoporous and macroporous to pore volume and surface area is relatively small, which is not conducive to coalbed methane seepage. The vitrinite content of coal samples in the study area is positively correlated with pore volume and specific surface area, which contributes significantly to pore development. The relationship between inertinite content and pore volume and surface area is not obvious. With the increase of Mad, more adsorption pores are developed in coal samples, and pore volume and specific surface area are negatively correlated with Ad, indicating that the increase of Ad will lead to the decrease of pore volume and specific surface area.

To study the influence of coal metamorphism on the pore system development characteristics of deep coal reservoirs, coal samples of different ranks were collected as research objects. Based on maceral identification and industrial analysis, the pore structure of the coal samples of different ranks was characterized on multiple scales by combining low-pressure CO2 adsorption (L -PA), low-temperature N2 adsorption (L -TA) and high -pressure mercury intrusion porosimetry (MIP) experiments. In this paper, the distribution and variation in pores in coal samples of different ranks were investigated, the relationship between the pore structure characteristics and coal metamorphism was examined, and the influence of coal metamorphism on the pore structure was analysed. CH4 isothermal adsorption experiments of the coal samples of different ranks were conducted to reveal the relationship between the Langmuir parameters and Ro,max. According to the shape of the hysteresis loop of the nitrogen adsorption-desorption curves, the pores in the low-rank coal samples mainly included slit pores and cylindrical pores, while those in the medium- and high -rank coal samples mainly included slit pores, semiclosed wedge pores and ink-bottle pores. There were significant differences in the pore structures of the coal samples of different ranks. With increasing metamorphism, the pore volume (PV) and specific surface area (SSA) first decreased and then increased according to a U-shaped trend, reaching minimum values at a Ro,max of 1.7 %. According to the characterization results of the coal sample pore diameter, the pore size distribution (PSD) type was mainly the unimodal micropore-dominated type. Coal micropores with pore diameters smaller than 1.5 nm largely contributed to the SSA and PV. In addition, the coal rank linearly increased with increasing Langmuir volume (VL), while the Langmuir pressure (PL) first decreased and then increased, indicating that the CH4 adsorption capacity increased with increasing metamorphism.

Shale pore systems are the result of the geological evolution of different matrix assemblages, and the composition of gas shale is considered to affect the pore systems in shale reservoirs. This study aimed to investigate the impact of both organic and inorganic constituents on the shale pore system, including specific surface area (SSA) and pore volume in Wufeng–Longmaxi Shale. Multiple linear regression (MLR) was employed to examine the contributions of different components to shale pore structure. The pore structure parameters, including pore SSA and pore volume, were obtained by gas adsorption experiments in 32 Wufeng–Longmaxi Shale (Late Ordovician–Early Silurian) samples. Both pore SSA and pore volume were calculated by the density functional theory (DFT) model on shale samples, and the pore types were determined by high-resolution field emission scanning electron microscopy (FE-SEM). The results of the X-ray diffractometer (XRD) analysis indicate that the Wufeng–Longmaxi Shale is dominated by quartz, clays, carbonates, feldspar, pyrite, and organic matter. Four models were made using SPSS software, all of which showed significant correlation between shale pore size and organic matter (OM) and clays. The content of organic matter played the biggest role in determining the size and structure of the pores. Although the content of quartz is the highest and serves as a rigid skeleton in shale reservoirs, it has complicated effects on the pore structure. In this study, most of the quartz is biogenetic and part of it is transformed from clays in deep shale. Therefore, these two parts of quartz are, respectively, related to organic matter and clays. In essence, the pores related to these two parts of quartz should be attributed to organic matter and clays, which also support the conclusion of the MLR models.

页岩孔隙结构是控制与影响海相页岩储层质量的主要参数之一,对于页岩气资源评估、勘探和开发极其重要.为阐明泸州地区深层五峰-龙马溪组页岩孔隙结构,基于氦气膨胀法测孔隙度、矿物组分分析、TOC测试,采用CO2和N2吸附、高压压汞联合表征页岩全孔径,并与氧化-还原条件、水分、TOC等因素藕合,探讨页岩孔径分布的主控因素.结果表明:页岩中以介孔为主,微孔次之,宏孔较少,平均分别占总孔隙的73％、23％、4％;孔径分布与孔隙体积从五峰组向上先增加后降低;TOC与低孔径孔体积相关性高;水分占据页岩孔隙,大幅降低页岩孔隙体积和表比面积.因此,页岩孔径分布受沉积条件、TOC、水分共同控制.

This study investigated the effect of overpressure on the deep shale pore system in the Wufeng-Longmaxi Formation (WLF), a well-established shale gas reservoir in the southern Sichuan Basin, China. The Y1 well was drilled to explore deeper overpressured sections of the basin. Organic geochemistry, mineralogy analysis, field emission scanning electron microscopy (FE-SEM), and gas physisorption experiments were conducted to analyze the pore system. Results showed that despite strong compaction, deep organic-rich shale retained large pores. Compared to shallow shale, deep shale had a larger organic porosity with a smaller average pore size, although some pore sizes exceeded those in shallow shale. Nitrogen (N2) adsorption indicated that the abundance of organic matter (OM) affected mesoporous volume and specific surface area (SSA), while carbon dioxide (CO2) adsorption experiments suggested that micropores were not influenced by OM abundance. Comparing calculated pore SSA and volume via gas adsorption of shallower and deep shale samples revealed that, under similar OM abundance, pore SSA was nearly identical, but deep shale had a larger pore volume than shallow shale. The preservation of pores, particularly in deep shale, is attributed to overpressure, which protects against collapse; additionally, generated shale gas during thermal evolution of OM serves as pore support.

选取泸州区块龙马溪组页岩为研究对象,基于核磁共振(nuclear magnetic resonance,NMR)实验、场发射扫描电子显微镜(field emission scanning electron microscope,FE-SEM)等手段,对泸州区块深层页岩的孔隙结构进行了分析,并利用分形几何近似方程计算了NMR分形维数,讨论了分形维数与总有机碳(total organic carbon,TOC)、热演化程度、矿物成分含量、孔隙度和渗透率等之间的关系.结果表明:泸州区块页岩广泛发育有机孔、粒间孔、粒内孔及微裂缝;有机孔孔径为 10～40 nm,无机孔孔径为 20～70 nm;NMR分形维数具有明显两段式特征,束缚流体孔隙分形维数(Dmin)与TOC、成熟度、石英含量呈正相关关系,与黏土矿物含量呈负相关关系,可动流体孔隙分形维数(Dmax)与石英含量、孔隙度、渗透率呈负相关关系.分形维数将矿物组分、储层物性与储层非均质性联系,研究结果可为深层页岩表征提供依据.

In this study, the microscopic pore characteristics of shale in marine strata are evaluated. Based on field emission scanning electron microscopy (FE-SEM), low-temperature N 2 adsorption (LT-N 2 GA), low-pressure CO 2 adsorption (LP-CO 2 GA) and high-pressure methane adsorption (HPMA) experiments, the pore characteristics of 12 shales from the Wufeng–Longmaxi Formations in northern Yunnan and Guizhou are characterized qualitatively and quantitatively. Fractal Frenkel–Halsey–Hill (FHH) theory is used to analyse the fractal characteristics, and the adsorption pore characteristics of shale are discussed. The correlation between the fractal dimension and pore structure and adsorption performance is determined. The results show that the total organic carbon (TOC) contents of the 12 shales are in the middle–low level, ranging from 0.43% to 5.42%, and the shales are generally in the highly mature to overmaturity stage (vitrinite reflectance ( R o ) values between 1.80% and 2.51%). The mineral composition is mainly quartz and clay minerals. The average clay mineral content is 40.98% (ranging from 24.7% to 63.3%), and the average quartz content is 29.03% (ranging from 16.8% to 39.6%), which are consistent with those of marine shale in the Sichuan Basin. FE-SEM and LT-N 2 GA isotherms reveal a complex shale pore structure and open pore style, mainly ink bottle-shaped and parallel plate-like pores. The total pore volumes (PVs) range from 0.012–0.052 cm 3 /g, and the specific surface area (SSA) values range from 18.112–38.466 m 2 /g. All shale samples have abundant micropores and mesopores, accounting for >90% of the total SSA. The fractal dimensions, D1 and D2, were obtained from N 2 adsorption data, with different adsorption characteristics at 0–0.5 and 0.5–1.0 relative pressures. The fractal dimensions increase with increasing BJH PV and BET SSA and decrease with decreasing average pore diameter (APD). The fractal dimensions are positively correlated with the TOC and clay mineral contents and negatively correlated with the quartz content. The fractal dimension can be used to evaluate the methane adsorption capacity; the larger the fractal dimension is, the larger the methane adsorption capacity is. Fractal analysis is helpful to better understand the pore structure and adsorption capacity of shale gas reservoirs.