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The assessment of gas storage capacity is crucial to furthering shale gas exploration and development in the eastern Sichuan Basin, China. Eleven organic-rich shale samples were selected to carry out the high pressure methane sorption, low-pressure N2/CO2 gas adsorption, and bulk and skeletal density measurements to investigate the methane storage capacity (MSC). Based on the relative content of clay, carbonates, quartz + feldspar, we grouped the 11 samples into three lithofacies: silica-rich argillaceous shale (CM-1), argillaceous/siliceous mixed shale (M-2), and clay-rich siliceous shale (S-3). The total porosity of the shale samples varies from 3.4% to 5.6%, and gas saturation ranges from 47% to 89%. The measured total gas amount ranges from 1.84 mg/g to 4.22 mg/g with the ratio of free gas to total gas amount ranging from 52.7% to 70.8%. Free gas with high content in the eastern Sichuan Basin may be the key factor controlling amount of shale gas production. The TOC content critically controls the MSC of shales, because micropore, mesopore volumes and the specific surface areas associated with organic matter provide the storage sites for the free and adsorbed gas. The methane sorption capacities of samples from different lithofacies are also affected by clay minerals and moisture content. Clay minerals can provide additional surface areas for methane sorption, and water can cause a 7.1%–42.8% loss of methane sorption capacity. The total porosity, gas-bearing porosity, water saturation, free gas and adsorbed gas number of samples from different lithofacies show subtle differences if the shale samples had similar TOC contents. Our results suggest that, in the eastern Sichuan Basin, clay-rich shale lithofacies is also prospecting targets for shale gas production. [ABSTRACT FROM AUTHOR]

The vertical geochemical variations in total organic carbon (TOC) content and major and trace elements of the Lower Cambrian Shuijingtuo Formation from the Yichang Slope in the Upper Yangtze were investigated to assess the environmental conditions (redox conditions, water mass restriction, terrigenous input, relative sea-level terrestrial, and paleoproductivity) and to determine the primary controlling factors for organic matter enrichment. The Shuijingtuo Shale is divided into three intervals. The lithofacies of interval I are mainly black siliceous shale with high TOC content, and interval II is mainly black siliceous shale with moderate TOC content. Interval III consists of black, clay-rich siliceous shale and dark-gray calcareous shale and is characterized by a low TOC content. The effects of diagenesis and hydrothermal activity on the elements were evaluated prior to analyzing the environmental condition. There are good positive correlations between TOC and U/Al or Mo/Al ratios, suggesting that major/trace elements still retain the geochemical signature of the sedimentary environment. Meanwhile, the geochemical proxies consisting of Al, Fe, Mn, and Ti indicate that the study area did not experience hydrothermal deposits. The redox proxies (U/Th, Corg:Ptot, and MoEF–UEF) indicate that the interval I samples were formed under a strong reducing condition. The diagram of TOC vs. Mo indicates that the water mass was moderately restricted during the deposition of interval I shales. Proxies of terrigenous input and relative sea-level (Zr/Al and Zr/Rb) suggest that the organic-rich shales at the bottom were deposited under a relatively high sea-level and experienced minimal input of terrigenous debris from the source area. Additionally, the paleoproductivity indicated by Sibio and (Ni + Cu + Zn)/Al was high for interval I samples. During the interval II period, the relative sea-level began to decline, the seawater still remained in reducing conditions, and there was no change in the productivity, but the input of terrigenous debris increased significantly. In the interval III depositional period, the relative sea-level continued to decrease, the seawater shifted to a dysoxic condition, and the paleoproductivity was also at a lower level. The evolution of the sedimentary environment indicates that the high TOC content in the interval I samples is mainly attributed to the strong reducing condition, the preservation condition and debris dilution together control the organic matter content within the interval II samples, and the low TOC content within the interval III samples is constrained by a combination of the poor preservation conditions and lower paleoproductivity. [ABSTRACT FROM AUTHOR]

To evaluate the effects of quartz precipitation on the abundance and preservation of organic matter pores in marine shale reservoirs, the type of authigenic quartz and the source of silica, as well as the corresponding relation of the Lower Cambrian Shuijingtuo Formation shale in South China were investigated. Quartz in the Shuijingtuo shale occurs as four different types: detrital quartz, replacement of biosiliceous debris, euhedral quartz filled in interparticle pores, and microquartz dispersed in a clay matrix. Euhedral quartz (1–5 μm) and matrix-dispersed microquartz (100–400 nm) are found to be the dominant forms of authigenic quartz. The euhedral quartz accumulates along the interparticle pores, and the porous organic matter fills the interior of the space. Microquartz is mainly wrapped in porous organic matter. Two silica sources were revealed: biogenic silica and clay-derived silica. Biogenic Si is most likely the major source for authigenic quartz in the organic-rich (total organic carbon (TOC) > 2.55 wt.%) samples, which accounts for 23–57 wt.% (average 35 wt.%) of the total Si. Based on petrographic observations, we posit that the precipitation of large-sized euhedral quartz in the interparticle pores most likely originated from biogenic silica in the early stage of diagenesis and that the silica for the clay matrix-dispersed microquartz is provided by biogenic silica and clay-derived silica. The observation of SEM images indicates that the precipitation of early diagenetic euhedral quartz in the interparticle pores enhances rock stiffness, and the buttressing effect can protect the organic matter pores from compaction during the late-stage burial diagenesis. In contrast, the precipitation of late diagenetic microquartz in the clay matrix can lead to a reduction in the capacity of the accommodation space to host retained petroleum, consequently leading to a reduction in the development of organic matter pores and the generation of shale gas. [ABSTRACT FROM AUTHOR]

• The Ediacaran Doushantuo shale is analyzed for organic matter accumulation mechanism. • Geochemical indicators suggest the paleoenvironment evolution characteristics of sedimentary. • Organic matter accumulation of the Doushantuo Formation is mainly controlled by redox conditions. • The Doushantuo shale is the oldest and a promising shale gas play in South China. The organic-rich Ediacaran Doushantuo Shale have been identified as a high-quality source rock and a main shale gas play in the Yichang area, South China, which was characterized by high TOC content, high thermal maturity, enormous cumulative thickness, and relatively high contents of brittle minerals. As the oldest shale gas stratum, the paleo-environmental characteristics and organic matter (OM) accumulation mechanism remain unclear in this special marine setting. In this study, core descriptions, TOC content, X-ray diffraction (XRD) mineralogical analysis, scanning electron microscopy (SEM) observation, and elemental analysis, including the major, trace, and rare earth elements, were combined to identify lithofacies, reconstruct paleo-environmental conditions, and document organic matter (OM) accumulation mechanisms for this potential Ediacaran Doushantuo Shale in the Yichang area, South China. Four major shale lithofacies were identified based on TOC content and mineral composition. The geochemical redox proxies, such as the enrichment factors of Uranium (U-EF) and ratio of organic carbon to total phosphorus (C org /P), indicate that the Doushantuo Shale was probably deposited in a dysoxic-anoxic conditions, which is consist with the interpretation from size distribution of pyrite framboids. The terrigenous influxes suggest the Doushantuo Shale was deposited in arid climate in absence of chemical weathering. Meanwhile, the productivity proxies (biogenic SiO 2 , Ba, and P/Al) indicate that the organic-rich Doushantuo Shale was deposited under a relatively low primary productivity. In addition, hydrothermal input has limited influence on organic matter (OM) accumulation of the Doushantuo Shale, which was also reported in the Qinglinkou Section of Yichang area. It can be observed that the organic matter (OM) accumulation mechanism of the Doushantuo Shale, deposited after the Marinoan "Snow Earth" glaciation, in the Yichang area, South China, was mainly controlled by the redox conditions, which is much different from typical marine shales and transitional shales. [ABSTRACT FROM AUTHOR]

The Lower Cambrian Niutitang and Sinian Doushantuo shales are the most important and widespread source rocks and target layers in South China. Reliable data of the thermal maturity of organic matter (OM) is widely used to assess hydrocarbon generation and is a key property used in determining the viability and hydrocarbon potential of these new shales. Nevertheless, traditional thermal maturity indicators are no longer suited to the vitrinite-lack marine shales. This study aims to combine high throughput Raman and infrared spectroscopy analysis to confirm and validate the thermal maturity in comparison with the bitumen reflectance (Rb). Raman parameters such as the differences between the positions of the two bands (VG-VD) are strong parameters for calculating the thermal maturity in a large vitrinite reflectance (Ro) ranging from 1.60% to 3.80%. The infrared spectroscopy analysis indicates that the aromatic C-C bands and CH2/CH3 aliphatic groups both are closely correlated with thermal maturity. The calculated Ro results from Raman and infrared spectroscopy are in strong coincidence with the Rb. The relationships between Rb and pore volumes/surface areas (calculated from N2 adsorption) indicate that the sample with Rb of 3.40% has the lowest pore volumes and surface areas. Focused ion beam scanning electron microscopy (FIB-SEM) observations of OM pores indicate that Ro of approximately 3.60% may be an upper limit for OM porosity development. Obviously, kerogen Raman and infrared spectroscopy can indicate methods for reducing the risk in assessing maturity with practical, low-cost accurate results. Exploration of shale gas in the high maturity (>3.40%–3.60%) region carries huge risks. [ABSTRACT FROM AUTHOR]

Reservoir properties and shale gas potential of the Upper Jurassic continental shales from the Mohe Foreland Basin were investigated based on organic geochemical analysis, X‐ray diffraction, scanning electron microscopy, low pressure nitrogen adsorption, high pressure mercury injection, and porosity and permeability analyses. The results indicate that the shallow lacustrine shales of the Emuerhe Formation are characterized by high total organic carbon content (0.43–22.83 wt.%), low hydrocarbon generative potential (less than 5.0 mg HC/g rock), type II kerogen, and a moderate thermal maturity stage (Ro of 1.05–1.46%). The inorganic pores dominate the pore system, and pores associated with clay minerals largely contribute to the surface area. The inorganic pore‐dominated lacustrine shale reservoir is characterized by dense, low porosity, and poor connectivity. The shales of the Emuerhe Formation show strong heterogeneity in mineralogy and are relatively enriched in clay minerals (33–66 wt.%). Due to the lack of biogenic silica, the effect of brittle minerals on hydraulic stimulation is ambiguous in the shallow lacustrine shales. The widely distributed, extensionally unfilled fractures can not only enhance hydrocarbon storage and migration capacity, but also play an important role in the generation of artificial reservoir fracturing. The pore network of lacustrine shales contributed by inorganic pores and unfilled micro‐fractures could also generate a relatively high porosity and permeability. The high‐mature thick shales in the northwest area of the Xingan Town would be the favourable zone for the shale gas exploration and exploitation in the Mohe Foreland Basin. [ABSTRACT FROM AUTHOR]

Organic matter (OM) nanopores developed in transitional facies shales, i.e., the Upper Permian Longtan and Dalong Formations in the Yangtze Platform, China, were investigated to determine the corre- sponding influence of thermal maturity and OM types within the geological conditions. A suite of 16 core samples were taken from Type-III Longtan shales and Type-II Dalong shales from two wells covering a ma- turity (Ro, vitrinite reflectance) ranging from 1.22% to 1.43% and 2.62% to 2.97%, respectively. Integrated analysis of the shale samples was carried out, including field-emission scanning electron microscopy (FESEM), low-pressure N2 and CO2 adsorption, high-pressure CH4 adsorption, and mercury intrusion capillary pressure (MICP) analysis. The fluid inclusions of liquid and gas hydrocarbons trapped in calcite vein samples in Dalong shales of two wells were studied using laser Raman and fluorescence spectroscopy. FE-SEM images indicated that OM pores in different formations varied substantially in terms of shape, size, and distribution density. OM pores in Type-II Dalong shales of Well XY1 were mainly micropore, sparsely distributed in the gas-prone kerogen with a spot-like and irregular shape, while bitumen rarely developed observable pores. In contrast, the morphology of OM pores in Type-III Longtan shales were significantly different, which was due to differences in the OM type. The primary OM pores in some terrestrial woody debris in Longtan shales had a relatively larger pore diameter, ranging from hundreds of nanometers to a few micrometers and were al- most all rounded in shape, which might be one of the factors contributing to larger pore volume and gas adsorption capacity than Dalong shales of Well XY1. Comparing Dalong shales of Well XY1 with relatively lower thermal maturity, there were abundant spongy-like pores, densely developed in the pyrobitumen in Type-II Dalong shales of Well EY1, with an irregular shape and diameter ranging from several to hundreds of nanometers. Many blue fluorescent oil inclusions and a small number of CH4 inclusions mixed with C2H6 could be observed within calcite veins in Dalong shales of Well XY1, whereas only CH4 inclusions could be identified within calcite veins in Dalong shales of Well EJ1. Therefore, thermal maturity not only controlled the type of hydrocarbons generated, but also makes a significant contribution to the formation of OM pores, resulting in larger pore volumes and adsorption capacity of Type-II shale samples in the dry gas window. [ABSTRACT FROM AUTHOR]

Marine shales in South China, such as the Upper Ordovician Wufeng (O 3 w) and Lower Silurian Longmaxi (S 1 l) shales, have generally experienced a long-term burial and rapid uplift process. The variations of temperature and pressure caused by the complex tectonic movements have significant effects on the shale gas storage capacities. Based on the supercritical Dubinin-Radushkevich (SDR) equation and high-pressure methane (CH 4) adsorption experiments, equations to calculate the shale gas storage capacities under geological conditions were established in this work. Using the derived equations, the gas storage capacities in two basic geological processes were modeled, namely, the burial process, which has an increased gas content, and the uplift process, which has a constant gas content. The two basic models were applied to investigate the evolution of the shale gas content of the O 3 w-S 1 l shales in the JY 1 Well in the Fuling shale gas field, Sichuan Basin, South China. Geological models of the gas storage capacities of the shales during burial show that the storage capacity of adsorbed gas (SC adsorbed gas) initially rapidly increases and then slowly decreases with the increasing burial depth. The storage capacities of free gas (SC free gas) and total gas (SC total gas) increase with the increasing burial depth. Compared with SC adsorbed gas , SC free gas significantly increases as the pore pressure increases. Such an obvious difference implies that free gas is the predominant gas form in the initial stage of shale gas production. During uplift, the gas content remains unchanged, resulting in a linear decrease in SC free gas and a linear increase in SC total gas. Nonetheless, SC free gas during uplift is still higher than SC free gas during burial at the same burial depth. The modeling of the shale gas content evolution of the O 3 w-S 1 l shales in the JY 1 shows that from the maximum burial depth of 6200 m to the present-day burial depth of 2415 m, the free gas content declines by 0.89 m3/t, from 2.30 m3/t to 1.41 m3/t, while the adsorbed gas content only increases by 0.58 m3/t, from 0.47 m3/t to 1.05 m3/t. This difference indicates that only a small amount (0.31 m3/t) of shale gas was lost during the uplift and that most (0.58 m3/t) of the reduced free gas was converted into adsorbed gas. A unique aspect of this study is that we illustrate how adsorbed and free gas are converted when the total gas content remains constant. Moreover, the two basic geological models established in this study can also be applied to other shale gas systems with complex tectonic movements. • Two basic geological models of shale gas storage capacities were established. • During burial, adsorbed gas initially rapidly increases and then slowly decreases. • The effect of pore pressure on free gas is greater than that on adsorbed gas. • During uplift, it is the conversion from free gas to adsorbed gas that keeps total gas content constant. • Most of the reduced free gas has been converted into the adsorbed gas in JY 1 Well during uplift. [ABSTRACT FROM AUTHOR]

Quartz in shales has been demonstrated to exert a significant effect on shale gas production by influencing rock mechanical behavior and reservoir porosity; however, quartz types and origins in shales are complicated and challenging to study due to their fine-grained nature. In this study, we propose combining thin section images, scanning electron microscopy (SEM) images, energy dispersive spectroscopy (EDS) maps, cathodoluminescence (CL) images and geochemical data to investigate quartz types and origins in shale formations. Shale samples from the Ordovician Wufeng-Silurian Longmaxi Formations, Eastern Sichuan Basin, China were used to study quartz types, origins and the effect of quartz cementation on porosity development. Samples from the Wufeng-Longmaxi Formations have TOC contents in the range of 0.5–4.7 wt%, averaging 2.7 wt%, and are currently in the thermally overmature stage. Five types of quartz, including silt-size detrital quartz, siliceous skeletal fragments, quartz overgrowth, clay matrix-dispersed microquartz, and aggregates of euhedral quartz, were identified. The ternary diagram of Fe–Al–Mn, the cross-plot of SiO 2 versus Zr, and the cross-plot of Si versus Al suggest that the dissolution and reprecipitation of siliceous skeletons may provide the major source of silica for quartz cementation in the lower part of the Wufeng-Longmaxi Formations, and released silica as a result of smectite to illite transformation is also important for authigenic quartz formation in the upper part of the Wufeng-Longmaxi Formations. Our SEM images suggest that the rigid framework formed by quartz cementation not only prevents primary interparticle pores from compaction but also favors the generation of organic matter-associated pores. • Five types of quartz are identified in the Wufeng-Longmaxi Formation. • Biogenic silica provides the major source for quartz cementation in the Lower member. • Aggregates of euhedral quartz can protect organic pores and interparticle pores from compaction. [ABSTRACT FROM AUTHOR]

Organic nanopores in shale gas reservoirs are the main storage space for free gas and adsorbed gas and directly affect the occurrence mode and movement of shale gas. In this study, field emission-scanning electron microscopy (FE-SEM), low-pressure gas (N 2 and CO 2) adsorption and mercury injection capillary pressure (MICP) analyses were performed and interpreted with statistical and fractal analyses to investigate the organic pore structure in the Upper Ordovician Wufeng shale (O 3 w) and the Lower Silurian lower member of Longmaxi shale (S 1 l). It was found that organic pores in 15 samples from 8 layers of the Wufeng-Longmaxi shale section are usually developed in discrete organic matter particles, organic matter associated with clay minerals and organic matter associated with pyrite framboids. The size, shape and quantity of organic pores vary greatly. The organic pores in Wufeng Formation (layer 1) are densely developed in organic matter particles with relatively small sizes and irregular shapes. The organic pores in the lower part of the lower member of Longmaxi Formation (layers 3, 4 and 5) are less developed and have relatively larger pore sizes with an elliptical shape. In contrast, the organic pores in the upper part of the lower member of Longmaxi Formation (layers 6, 7, 8 and 9) are the least developed, with elliptical shapes and pore sizes in between the two cases above. A higher value of fractal dimension refers a more complex form and this value of whole organic pores with full range of sizes (D wop) is the largest and followed by organic macropores (D op>50), organic pores with pore sizes between 4 nm and 50 nm (D op4-50), and organic pores with pore sizes less than 4 nm (D op<4). The fractal dimension of the upper part of the lower member of Longmaxi Formation (layers 6, 7, 8 and 9) is lower than that of both the Wufeng Formation (layer 1) and the lower part of the lower member of Longmaxi Formation (layers 3, 4 and 5). Possible causes leading to pore complexity and heterogeneity include the TOC content, pore size, mineral composition and tectonic effect. Wufeng shale is located at the bottom of Wufeng-Longmaxi shale weakness zone and had experienced more tectonic compression, nappe-slip and reconstruction, which may be the main reason that organic nanopores in Wufeng shale (layer 1) are more complex than other layers. • This study focuses on 8 layers of the Upper Ordovician Wufeng-Lower Silurian lower member of Longmaxi shale section. • We summarized the developmental characteristics of organic pores in the study area. • The results distinguish the organic nanopore structure and fractal dimension in each layer. • The possible causes of difference in organic pore structure and complexity are discussed. [ABSTRACT FROM AUTHOR]

Recently, much attention has been paid to Permian shale from the Yangtze Platform because of the high–quality hydrocarbon source rock. This research investigates fracture veins derived from conventional cores of the Permian Leping Formation in the Pingle Depression, Lower Yangtze Platform. Based on petrographic observations, micro–Raman spectroscopy, and fluorescent spectrometry, hybrid CH 4 –C 2 H 6 gas inclusions, pure CH 4 inclusions, dissolved CH 4 aqueous inclusions, and different abundant blue-green fluorescence petroleum inclusions were discovered in quartz and calcite. The pressure–temperature–time–composition properties of the CH 4 –bearing inclusions were obtained by quantitative Raman analysis and thermodynamic models. The pore fluid pressure (59.05–87.78 MPa) calculated from Raman shift of C–H symmetric stretching (v 1) band of methane indicates inclusions trapped at close to lithostatic pressures, which can provide a key evidence for understanding the formation and evolution of overpressure. PT entrapment conditions, reconstruction of hydrocarbon generation and simulation of burial history permit to constrain accumulation time sequence of paleo-fluid in shale veins. Blue-green fluorescence with high-mature petroleum fluid trapped in late Cretaceous (~81–71 Ma); hybrid CH 4 –C 2 H 6 fluid trapped in early Paleogene (~68–61 Ma); and CH 4 fluid trapped in Paleogene (~62–51 Ma). Dry gas generation period close to the age of maximum burial depth at early Himalayan uplift period. • Quartz and calcite cements were found in near-horizontal and high-angle Permian shale veins. • Raman quantitative analysis recovery paleo-pressure of CH 4 -bearing inclusions. • Hydrocarbon-generating pressurization caused overpressure state. • Distinction of different components fluid charging time in fracture veins. [ABSTRACT FROM AUTHOR]

Organic pores, one of the significant pore types in shale reservoir, can be formed in both primary organic matter (kerogen) and secondary organic matter such as solid bitumen and pyrobitumen. Compared to the primary organic pores that are mainly observed in immature kerogen, secondary organic pores in migrated organic matter (solid bitumen and pyrobitumen) are more abundant and well connected to the matrix. In this study, the petrographic characteristics between the organic matter and matrix as observed in field emission-scanning electron microscopy (FE-SEM) images were used to characterize the pore system in the overmature Lower Cambrian Shuijingtuo (Niutitang-equivalent) Shale in the south of Huangling anticline, Yichang area, China. Pyrobitumen-hosted pores were observed to be the predominant pore type in the organic-rich Shuijingtuo Shales. The porous pyrobitumen occurs primarily in the original interparticle pores filled with microcrystalline quartz, which is the primary petrographic evidence to identify the migrated organic matter. Pore-filling organic matter and precipitation of authigenic quartz, rather than mechanical compaction, resulted in further loss of a large number of interparticle pores. The porous organic matter that filled in the intraparticle space within the early-formed framboidal pyrite and the pre-existing dissolution pores within the quartz and pyrite grains is pyrobitumen. This pyrobitumen had migrated as a mobile phase into the aforementioned pores spaces in the initial phase of hydrocarbon emplacement during the petroleum expulsion and migration process. With increasing thermal maturity, this migrated organic matter thermally transformed into pyrobitumen, and nanoscale pores were developed with thermal cracking into gas. The results show that the pore volume and surface area are positively correlated with the total organic carbon (TOC) content, indicating that organic matter primarily controls shale porosity for the Shuijingtuo Shales. • Pyrobitumen-hosted pores are the predominant pore type in the organic-rich Shuijingtuo Shale. • The porous pyrobitumen primarily occurred in the original intergranular pores. • The lack of interparticle pores is mainly caused by the late cementation and filling by organic matter. • The organic matter is the prominent factor controlling the shale porosity for the Shuijingtuo Shale studied in this work. [ABSTRACT FROM AUTHOR]

(1) The images have been replaced, and the associated OM porosity result for JY11-4-03 contained in Tables 3-4 and Figure 11 has been corrected thereafter. (a) Microstructural overview in large range of sample JY1-08; (b) OM pores in a single OM grain; (c) OM pores within the pyrite framboids; (d) Heterogeneously distributed OM pores with irregular and faveolated shapes; (e)-(f) Organic matter grains with no OM pores; (g) Elliptical OM pores; (h) Irregular OM pores; (i) Faveolated OM pores in the organic matter grain; (j)-(k) Pyrite framboids and OM pores; (l) Heterogeneously distributed OM pores with irregular shapes. In the right plot, the solid squares represent the OM porosity calculated from FE-SEM images, while the blank squares denote the porosity from helium porosimetry. [Extracted from the article]

This study evaluates the pore network systems in the Neoproterozoic Sinian Doushantuo (Z 1 d) shale from the Yichang area in the Middle Yangtze region. All samples were classified based on their lithologies and examined by geochemical and petrographic analysis, assisted with carbon dioxide/nitrogen (CO 2 /N 2) adsorption, mercury injection capillary pressure (MICP) and focused ion beam-scanning electron microscopy (FIB-SEM) to fully characterize the nanoscale porosity of the different shale lithofacies. The calcareous shale lithofacies group (I-C), mixed shale lithofacies group (II-M) and siliceous shale lithofacies group (III-S) were identified via ternary mineralogy diagrams. Clearly, different lithofacies had different porosities, pore volume, surface area and pore-size distribution (ranging from 0.3 nm to 36 μm). The mixed shale lithofacies group (II-M) had the highest total pore volumes, followed by the calcareous shale lithofacies group (I-C) and the siliceous shale lithofacies group (III-S). Peak values (2–20 nm, 20–50 nm, 50–60 nm and 90–200 nm) obtained from N 2 adsorption were identified as organic matter (OM) pores, dissolution intraparticle (intraP) pores and intercrystalline pores (using SEM images analysis). Porosity shows a non-monotonic trend with TOC content and a maximum at 4.5–5.5 wt% TOC, similar relationships between TOC and mesopore/macropore volumes, due to pore collapse and compaction. The FIB-SEM images revealed solid bitumen and two types of kerogen in the examined shale samples; these different organic matter types had clearly distinct pore characteristics. While the smaller OM particles mobilized and converted into solid bitumen in which OM pores were larger and more abundant, the large-scale OM particles were most likely non-porous inert maceral and sponge-like porous amorphous kerogen. Overall TOC content and organic types are the key controlling factors to the nanoscale porosity development in the Doushantuo Shale, and the Sinian Doushantuo shale (in the Yichang area) has great potential for shale gas exploration and exploitation in South China. • Sinian Doushantuo Shale reservoir was analyzed for lithofacies classification and porosity characteristics. • Multiple methods (CO 2 and N 2 sorption, FIB-SEM and MICP) indicate the existence of μm-nm pore size spectrum. • TOC content, mineral support and organic types are ascribed as crucial parameters to reveal porosity development of shales. • Doushantuo Shale in Yichang area is a brand new shale gas formation and base in South China. [ABSTRACT FROM AUTHOR]

The Lower Sinian Doushantuo Shale comprises the oldest Neoproterozoic shale gas reservoir in the Middle Yangtze Yichang area of China, which makes the study of their pore characteristics critical to future shale gas exploration and exploitation. Geochemical, petrographical, and pore characteristics are studied in detail, with focused ion beam scanning electron microscopy (FIB-SEM) image analysis to further support and confirm the interpretations of the nano-scale pore structure characteristics. High total organic carbon (TOC) content, high thermal maturity, and considerable brittle mineral content indicate promising potential of the Doushantuo shale. High TOC samples suggest elevated micro-meso porosity, and H2(b) and H3 type hysteresis loops indicate inkbottle, slit-like, and flake-like pore types. Significant inorganic pores are commonly observed in dolomite, pyrite, and apatite, which are characterized by diamond or irregular shapes, with pore diameters ranging from 5 nm to 1000 nm as a result of the interaction between the minerals and the acidic organic fluids. Clustering organic matter (OM) pores are observed with pore diameters ranging from 5 nm to 50 nm; they are characterized by sponge-like, elliptical, and alveolate shapes, which indicate minor organic matter (OM) pores. More substantial organic matter (OM) pores can also be found around or between the pyrite and apatite. Fractures can be clearly observed between the organic matter and minerals with lengths of approximately 10 μm and 50 nm in width, which serve as conduits for shale gas migration. The pores in the Doushantuo Shale are dominated by inorganic pores and minor organic matter (OM) pores, which are different from the pore structure of the Wufeng-Longmaxi Shales in the Sichuan Basin. The Doushantuo Shale organic matter (OM) pores are much smaller and more densely distributed than those in the Wufeng-Longmaxi Shales. The different organic matter origins of these two shales have caused distinct organic matter (OM) pore-development mechanisms. Organic matter (OM) pores and inorganic pores could contribute to the volume of the adsorbed gas and free gas, respectively, which provide significant reservoir space in the Lower Sinian Doushantuo Shale in the Yichang area. • The oldest Neoproterozoic Sinian Doushantuo shale gas reservoir isanalyzed for pore characteristics. • Multiple methods (CO2 and N2 sorption and FIB-SEM) indicate theexistence of µm-nm pore size spectrum. • Inorganic pores are dominated in this dolomite-rich Sinian shale.Pore development is different with typical shales in US. • Although this shale is not as good as the typical shales, it still beascribed as a new promising shale layer in worldwide. [ABSTRACT FROM AUTHOR]

Deposited in different sedimentary settings, four leading shale formations (Late Ordovician Wufeng, Early Silurian Longmaxi, Late Permian Dalong, and Early Jurassic Dongyuemiao Shales) are currently the most promising zones for shale gas development in the Middle Yangtze Platform of South China. Based on complementary tests [low pressure gas physisorption, mercury injection capillary pressure (MICP), contact angle measurement, fluid imbibition into initially dry shale, and tracer diffusion into initially fluid-saturated shale followed by tracer mapping with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)], this work investigates their pore structure (both geometry and connectivity) and wettability characteristics, as well as the coupled effects of these characteristics on fluid flow and tracer migration. Pores <50 nm account for the majority of the pore volume in these organic shales. The shapes of hysteresis loop from gas physisorption show that the pores are mostly inkbottle-shaped in Wufeng, Longmaxi and Dongyuemiao Shales, while they are mostly narrow plate- or slit-like shaped pores in Dalong Shale. These four organic shales are strongly oil-wetting and moderately water-wetting. According to the imbibition behaviors toward aqueous (deionized water) and oleic (n-decane) phases, hydrophobic pores are better connected than hydrophilic pore networks, which is consistent with the measured contact angles. Diffusion of nano-sized nonsorbing (perrhenate [ReO 4 − ]) and sorbing (cesium [Cs + ]) tracers into brine-saturated shales indicates a high spatial variability and limited pore connectivity in these organic shales. The effective diffusion coefficient values are on the order of 10 −13 m 2 /s with an associated geometric tortuosity ranging from 11.1 to 41.4. Due to the limited edge-connected pore spaces and low diffusion coefficients, the migration of hydrocarbons will be very slow from the shale matrix to hydraulically created fractures, with the initially high production of hydrocarbons from connected pore spaces near the sample edge (e.g., fracture face). [ABSTRACT FROM AUTHOR]

The nano-scale pore systems of organic-rich shale reservoirs were investigated from Upper Ordovician Wufeng and Lower Silurian Longmaxi Formations in southeast Sichuan Basin. These two formations are the most important target plays of shale gas development in China. The purpose of this article is to assess the geometry and connectivity of multi-scale pore systems, and to reveal the nature and complexity of pore structure for these over-mature gas shales. To achieve these objective, total organic carbon, mineralogy, image analyses by focused ion beam-SEM, low pressure nitrogen adsorption, mercury injection capillary pressure (MICP) and spontaneous fluid [deionized (DI) water and n-decane] imbibition were performed. Most of the visible pores from SEM work in Wufeng and Longmaxi shales are within nm- and μm-size regimes and belong to organic matter (OM) pores. The shapes of OM pore in Longmaxi samples are elliptical, bubble-like, irregular or rounded. Wufeng pores are mainly irregular, linear and faveolated, even though two shales have small depth difference, as well as similar thermal maturity, kerogen type and TOC content. Nano-scale pores in Longmaxi are mainly associated with narrow platelike or slitlike pores with pore size of 3–50 nm; while inkbottle pores are dominant in Wufeng samples and over 88% of the pore volume is contributed by pores with diameter <20 nm. Overall, porosity, pore volume and surface area values from Wufeng samples are much higher than those in Longmaxi, which is mainly correlated with the different TOC contents and mineral compositions. MICP tests show that a total of 5 inflection points (indicative of different connected pore networks) are identified in all pressure regions for Longmaxi, while only 2 for Wufeng in high pressure region with the associated permeability at nano-darcy range. Imbibition curves of n-decane are divided into three stages: the initial stage (Stage Ⅰ), linear imbibition stage (Stage Ⅱ) and late imbibition stage (Stage Ⅲ), and the slopes of linear imbibition stage are around 0.5, suggesting well-connected pore spaces for n-decane. In contrast, imbibition curves for DI water are divided in two stages with linear slopes of between 0.25 and 0.5, indicating moderately-connected pore networks for the movement of DI water. This is consistent with the mixed-wet nature of these shales, with observed weak wettability for hydrophilic, while complete wetting for hydrophobic fluids. [ABSTRACT FROM AUTHOR]

• Water vapor adsorption of over-mature Longmaxi shales are investigated using gravimetric measurement. • Five classical isotherm models for water adsorption isotherm are evaluated. • Physical meaning behind the related parameters in the five classical isotherm models are discussed. • Four parameter double log polynomial (DLP) model is the optimal model to fit WVA isotherm on Longmaxi shale. Mathematical characterization of water vapor adsorption (WVA) isotherms on organic-rich shale is important for both modeling processes such as water flow and mass transport, and accurate evaluation of moisture content in shale. Although several theoretical and empirical models have been proposed to describe WVA on porous materials (e.g, coal, food and carbon black), the applicability of these proposed models for over-mature gas shale (e.g., >2.0%R o) is not well understood, which requires both experimental data and a theoretical description of downhole shale. In this work, WVA isotherms were measured at 50℃ (323.1 K) using a dynamic vapor sorption apparatus for four over-mature Lower Silurian Longmaxi shales, the leading gas-producing shale reservoir in the Sichuan Basin, China. To describe water adsorption behavior mathematically, five models, including the Guggenheim-Anderson-de Boer (GAB), double log polynomial (DLP), Oswin, Freundlich, and Frenkel-Halsey-Hill (FHH), for WVA isotherm are evaluated for their ability to match the experimental WVA isotherm data. In general, all the models are suitable for fitting the experimental data, with R2 values larger than 0.95. However, the transformations of non-linear isotherm equations to their linear forms could implicitly alter the error structure and may violate the error variance and normality assumptions. To address this deficiency, six more statistical parameters (AAD, MSE, SEE, RSS, ARE and χ2) were used to evaluate the goodness-of-fit results for different models. Comparative studies show that the four-parameter DLP model is the optimal to predict the WVA isotherms on Longmaxi shale, with the smallest fitting errors, rather than the most versatile GAB model documented in the literature. Considering the WVA behavior, the distribution of water (free and adsorbed) in hydrophobic and hydrophilic pores in shales was also discussed, which can provide a reference point for theoretical analysis of water flow and retention. [ABSTRACT FROM AUTHOR]

The wettability of reservoir rocks is a critical factor for the petroleum exploration and recovery, which plays an important role on relative permeability, capillary pressure, and ultimate recovery. This study aims to characterize the difference in wettability before and after soxhlet extraction, and reveal the influences of mineral composition, pore structure and extractable organic components on wettability of shales with different lithofacies. Several organic shale samples from the Dongyuemiao (DYM) section of the Jurassic Ziliujing (ZLJ) formation were collected to investigate the mineral composition, pore-size distribution, wettability characteristics and determine the main controlling factors on wettability alteration. Pore type and abundant are observed using argon ion polishing scanning electron microscopy. The pore volume (PV) and pore size distribution (PSD) characteristics before and after extraction were obtained using a combination of nitrogen physisorption and high-pressure mercury intrusion porosimetry (MIP). Spontaneous imbibition (SI) and nuclear magnetic resonance (NMR) experiments were used to investigate the wettability characteristics before and after extraction. Results show that clay, quartz and calcite are dominant minerals in the DYM shales, and three typical lithofacies are identified, including mixed shale (M), argillaceous shale (CM) and calcareous shale (C). Clay mineral interP pore is the dominant pore type in the DYM shales, followed by organic matter (OM) pore. Before the extraction, the DYM shale is more oil lipophilic, while its wettability alters slightly after the extraction. The M shales and C shales maintain lipophilic, while as the CM shales alters from oil wet to mixed wet. Samples with higher quartz have better connectivity of the hydrophilic and lipophilic pore networks, and carbonate minerals play an opposite role. During the extraction process, polar solvents could react with clay minerals to cause pore space expansion, which enhances the water wettability after extraction. Due to the low OM content and poor organic clay complex related pores in the study area, clay minerals have not significant correlation with oil wettability, but influence the water wettability. No correlation occurs between total organic carbon (TOC) content and oil/water wettability of shales before and after extraction. In comparison, macropore has a significant impact on water wettability, while micropore and mesopore play significant impact on oil wettability. • Wettability is evaluated for original and extracted continental shale oil samples using fluid imbibition and NMR experiments. • Mercury intrusion porositmetry, gas physisorption and field emission-SEM are conducted to characterize pore structure. • The effects of mineral composition, pore structure and extractable organic component on continental shale wettability are discussed. [ABSTRACT FROM AUTHOR]

Water is ubiquitously present within organic-rich shale and can exert an important effect on the gas-in-place and gas adsorption capacity of shale reservoirs. Thus, understanding the water adsorption characteristics of gas shale reservoir is critical for optimizing shale-gas productivity. In this work, the water vapor adsorption characteristics of organic-rich Wufeng and Longmaxi Shales in the Sichuan Basin were studied over a wide range of relative humidities (RHs, 5–95%) utilizing a gravimetric method. All the water vapor adsorption isotherms are categorized as type II, suggesting similar water adsorption behaviors between samples. Being pronounced at approximately 70–80% RH, hysteresis loops from water adsorption curves are characterized by calculating areal hysteresis index (AHI) values. The Wufeng Shales have higher AHI values than those of Longmaxi Shales, which are mainly related to the variation of TOC content and pore structure characteristics. Pore size distribution obtained from water vapor adsorption show unimodal distribution of pore sizes, which are generally comparable to that derived from nitrogen physisorption. The mineral content (e.g., quartz and clay contents) and pore structure characteristics (e.g., pore type, porosity and surface area) have great effects on the water vapor uptake in the Wufeng and Longmaxi Shales. Compared to the shales with low surface area and porosity, Wufeng and Longmaxi Shale samples with higher surface area and porosity exhibit much higher water vapor uptake, suggesting that moisture sorption is mainly controlled by the amount of adsorption sites and volumes. When RH increases from low to high values, methane capacity of partially moisture-equilibrated shale could have experienced rapid decline, moderate decrease and slow decrease stages, finally reaching the minimum adsorption capacity for full moisture-equilibrated shale. • Water vapor adsorption characteristics of overmature Wufeng and Longmaxi Shales are investigated. • The hysteresis loop obtained from water adsorption test is quantitatively characterized using the areal hysteresis index method. • The formation mechanisms for hysteresis loops are discussed, which provides insight into the various methane adsorption capacity of partially moisture-equilibrated shale. • Pore structure analyses via water vapor adsorption are performed and compared to those via nitrogen physisorption. • The water vapor adsorption capacity is collectively affected by the TOC content, mineral composition and pore structure. [ABSTRACT FROM AUTHOR]

Factors influencing the methane adsorption capacity (MAC) of marine-continental transitional facies shales have been identified applying a variety of techniques (e.g., total organic carbon (TOC) content, X-ray diffraction mineralogy, low-pressure CO 2 and N 2 adsorption, and methane adsorption analyses) on samples from the Upper Permian Longtan Formation, northern Guizhou Province, Southwest China. The TOC contents of the Longtan shale samples ranged between 1.2 and 9.9 wt% (average = 3.5 wt%). The results of the bulk XRD analysis suggested that the mineralogical composition of the studied samples was different from that of typical marine shales: the samples primarily consisted of clay minerals, followed by quartz and feldspar. The Langmuir volumes (V L) of the 14 shale samples ranged from 1.02 ml/g to 5.25 ml/g (average = 2.52 ml/g); moreover, their MAC was positively correlated with the pore volume, surface area, and TOC content, suggesting that organic matter and pore structure were the most critical factors influencing the adsorption capacity of these transitional shales. Our results showed that the MAC was not positively with the clay content; additionally, the MAC tended to increase with increasing pressure, but to decrease with increasing temperature. The presence of moisture greatly reduced the MAC. Overall, the MAC of the transitional Longtan Formation shales resulted to be quite different from that of typical marine shales (e.g., the Lower Silurian Longmaxi Formation in the Sichuan Basin) in terms of mineralogical component and abundance of organic pores. The particularly high abundance of (hydrophilic) clay minerals in the Longtan Formation transitional shales resulted in a higher number of adsorption sites occupied by water molecules than in the Longmaxi Formation and a lower MAC. Finally, the abundance of organic pores in marine shales resulted in a higher MAC than that of transitional shales. • TOC content dominantly influence methane adsorption capacity in transitional Longtan shale. • Moisture greatly reduces the methane adsorption capacity in transitional Longtan shale. • Adsorption capacity of Longtan shale is lower than that of typical marine Longmaxi shale. [ABSTRACT FROM AUTHOR]

Spontaneous imbibition (SI) is a capillary-force controlled process, which has been identified as one of the most important mechanisms to affect hydrocarbon recovery from unconventional shale reservoirs. In this work, we focus on the mineral composition, total organic carbon (TOC) content, pore geometry characteristics and fluid imbibition behaviors of the terrestrial Lower Jurassic Dongyuemiao shale reservoirs. The pore morphology is directly imaged and observed via field emission-scanning electronic microscope (FE-SEM). Nitrogen (N 2) physisorption and mercury intrusion porosimetry (MIP) are applied to investigate the pore geometry characteristics. The slopes of deionized (DI) water/ n -decane imbibition from fluid imbibition experiments are used to evaluate the pore connectivity characteristics of hydrophilic and hydrophobic pore spaces, respectively. Finally, the relationships between shale dominant mineral composition, TOC content and pore structure parameters on fluid imbibition behavior are discussed. Experimental results show that clay and quartz are the two dominant minerals in the Dongyuemiao shales. In this study, four typical lithofacies are identified in the Dongyuemiao shales, including laminated or massive mudstone, silty mudstone, shell limestone and argillaceous siltstone. Organic matter (OM) pores and interP pores are the dominant pore types in the Dongyuemiao shales, and most of the pores with sizes smaller than 50 nm. The imbibition slopes of n -decane are generally higher than that of DI water, indicating better pore connectivity of hydrophobic pores. Due to protection by quartz grains, samples with higher abundances of quartz are beneficial for forming a connected pathway for DI water migration, resulting in a better-connected water-wet pathway. The hydrophobic and hydrophilic pore networks in samples with higher TOC content are better connected, which are speculated to be related to the high volume of connected organic matter in shale matrix. Positive correlations occur between porosity/pore volume and fluid (including DI water and n -decane) imbibition slopes, suggesting that the connectivity of pore networks become better as porosity and pore volume increase, regardless of whether the pores are hydrophobic or hydrophilic. • The effects of pore structure, TOC content and mineral composition on imbibition characteristics are studied. • Nano-scale OM pores and interP pores are the two dominant pore types. • Pore connectivity is investigated using the imbibition slope in a plot between cumulative imbibition (in log scale) and imbibition time (in a log-log scale). • Oil-wet pore networks are better connected compared to water-wet ones, as indicated by higher imbibition slopes. • The main mineral composition and complex pore structure have important effects on fluid imbibition behavior. [ABSTRACT FROM AUTHOR]

To characterize the degree of organic matter graphitization in the over mature lower Paleozoic marine shales from South China, a suite of kerogen samples covering different maturity (R o % ranging from 1.2% to 4.2%) were analyzed by Laser Raman microprobe (LRM), Transmission electron microscope (TEM) and Fourier transform infrared spectroscopy (FTIR) techniques. Chemical structures and related major changes caused by thermal evolution history were systematically discussed. The results indicate that with the thermal maturity increasing, the aliphatic side chains fall off gradually, and the aromatic rings increase continuously. The ordered structure of shales continues to enhance, leading to the increasing of graphitization degree. The parameters of LRM (i.e. position of G and D bands, peak position difference (RBS), full width at half maximum (FWHM) and intensity ratio (I D /I G)) highlight the "turning point" at maturity level around Rmc R o % = 3.5%, which represents the chemical structure jump of the kerogen. This chemical structure jump can be characterized by the continuous decreasing of amorphous carbon before the Rmc R o % exceeded 3.5% and the sharp increasing degree of aromatic conjugation after that point. TEM analysis indicates that the carbon layers of shale samples with Rmc R o % exceeds 3.5% show better ordering and continuity than that of low maturity shale samples. The FTIR analysis suggests that there are relatively high aromatics C C chains and very few aromatic C-H chains in the high maturity samples. This critical chemical structure jump at Rmc R o % = 3.5% could be interpreted as the structure of organic matter begins to transform from amorphous carbon to crystalline graphite in shales. Due to the increasing degree of graphitization, graphitized shale is characterized by stable chemical structure, condensed and orderly carbon layer constructure, and ultra-low resistivity. • Chemical structure "turning point" of organic matter is around Rmc R o % = 3.5%. • After the "turning point", the degree of graphitization is enhanced. • The aromaticity, orderliness and crystallinity of carbon layers increase abruptly. • Structure of OM starts to transform from amorphous carbon to crystalline graphite. [ABSTRACT FROM AUTHOR]

Depositional environment and diagenesis are important factors affecting pore characteristics, whereas how differential depositional and diagenetic processes affect pore development and preservation in shale remains controversial. In this study, based on geochemical analysis, low-temperature gas (including N 2 and CO 2) physisorption, core observation, thin section and field emission-scanning electron microscopy, the influences of various paleoenvironmental conditions and diagenetic pathways on pore development of the Wufeng and Longmaxi Formations are investigated. The results indicate that paleoenvironmental conditions determine primary mineral composition and content of organic matter, resulting in the various shale lithofacies. Subsequently, minerals in different shale lithofacies experienced various diagenetic events and evolution pathways, finally affecting pore generation and evaluation. Three system tracts, including transgressive systems tract (TST), early highstand systems tract (EHST) and late highstand systems tract (LHST) are identified in the Wufeng and Longmaxi Formations. Siliceous shale, mainly deposited in the TST, contains large amounts of biogenic authigenic quartz and abundant organic matter. Authigenic quartz aggregates effectively inhibit compaction and protect abundant pores. The contents of extrabasinal components increase during the EHST, while biogenic quartz and organic matter decrease to mainly develop mixed shale. Insufficient microcrystalline authentic quartz and high content of clay minerals lead to higher degree of compaction in mixed shale than siliceous shale, resulting in fewer residual pores. In contrast, clay minerals in clay-rich shale, deposited in the LHST, undergo strong compaction and illitization of smectite during diagenesis, resulting in strong deformation and loss of primary pores. As a result, higher amount of OM pores is generated and preserved in the organic-rich siliceous shales from the bottom of Wufeng and Longmaxi Formation, and gradually decreases in the mixed shale, while clay-rich shale in the upper section of the Longmaxi has the lowest amount of pore volume. • Paleoenvironmental condition and its effects on primary composition of three system tracts in the Wufeng and Longmaxi Formations are analyzed. • Micropore system of the Wufeng and Longmaxi Shales is characterized using multiple approaches. • Primary components of three types of shale lithofaciescontrol the various diagenetic events. • Coupled effects of depositional environment and diagenesis on pore development and preservation are discussed. [ABSTRACT FROM AUTHOR]

The structural preservation conditions are crucial for shale gas accumulation and preservation. In this study, the Emuerhe thrust nappe structural belt in the Mohe Basin is taken as an example to investigate the influence of the thrust nappe structure on shale reservoir quality. According to the rock metamorphic characteristics and deformation degree, the thrust nappe belt is divided into the root zone, middle zone, and front zone. Based on the observations of field outcrops, cores, and thin sections, the hydrocarbon generation capacity and reservoir properties of shale in different units were investigated through an integrated analysis of organic geochemical data, X-ray diffraction, low-pressure CO 2 and N 2 adsorption, and helium porosity measurements. Furthermore, implications for shale gas preservation are discussed. Owing to the influence of high temperature and pressure caused by dynamic metamorphism, the R o value of shales in the root zone exceeds 5.5%, which is far beyond the theoretical upper limit of hydrocarbon generation potential. Thus, the hydrocarbon generation potential of shales exhibits an opposite trend to the deformation strength of the Emuerhe thrust nappe belt. The S 1 +S 2 value of shales within the root zone and middle zone is generally lower than 0.3. The primary pore structure of shales in the root zone was changed or destroyed by ductile shearing and mylonitization. Furthermore, dynamic metamorphism led to a significant decrease in the adsorption capacity of shales. Both the micropores and mesopores in these shales are very limited, and the storage space is relatively low. The strong compaction caused by the thrust nappe tectonic led to a reduction in the pore size. Thus, pores with diameters <10 nm dominated the main reservoir space, and the porosity of shales in the middle zone was lower than that in the root zone. The influence of thrust nappe tectonics on the front zone was relatively weak. Overall, the shales exhibited good hydrocarbon generation potential, high specific surface area, and pore volume. The pore–fracture system consists of relatively large pores, and the extensional fractures are favorable for enhancing the physical properties of shale reservoirs in the front zone. The strong thrust nappe resulted in caprock that experiences broad uplift-erosion and fault failure. The original structure of the shale can be changed or destroyed by ductile shearing and strong compaction. Heating-up caused by tectonics could also promote the rapid desorption of shale gas, which can lead to poor self-sealing performance of shale. The hydrocarbon generation capacity, reservoir property, and preservation conditions of shale display an improving trend along the thrust direction of the Emuerhe thrust nappe structural belt. • Impact of thrust nappe structure on shale gas reservoir was studied. • Original structure of shale could be changed by ductile shearing and compaction. • Dynamic metamorphism led to a rapidly decrease in adsorption capacity of shales. • The thrust nappe structure led to a reduction of pore size. • The thrust nappe structure could lead to poor self-sealing performance of shale. [ABSTRACT FROM AUTHOR]

The relative sea-level fluctuations play a vital role in the organic matter enrichment of black shales across the world. The Wufeng-Lower Longmaxi shales deposited during the Late Ordovician to Early Silurian stage are accompanied by the global transgression and regression, whereas the role of relative sea-level changes on organic matter accumulation has been rarely discussed. In this study, we integrated TOC content, X-ray diffraction (XRD) mineralogical composition, thin section and scanning electron microscope (SEM) images, elemental geochemical data, and petrographic features of pyrite framboids to investigate the varying paleoenvironmental conditions and the effect of relative sea-level changes on organic matter accumulation under a robust sequence stratigraphic framework. Based on detailed petrographic observations and mineralogical analysis, six major lithofacies are identified in the Wufeng-Lower Longmaxi formations, including siliceous mudstone, clay-rich siliceous mudstone, siliceous-argillaceous mixed shale, silty mudstone, siltstone, and bentonite. TOC content is generally higher in the transgressive systems tracts (relative sea-level rise stage) than in the highstand systems tracts and falling stage systems tract (relative sea-level fall stage). Multiple geochemical proxies indicate contrasting paleoenvironmental changes between relative sea-level rise and fall stage. Relative sea-level rise resulted in elevated productivity, predominantly anoxic conditions, low detrital flux, and moderate water mass restriction, as indicated by high biogenic Si content, high Mo-EF (enrichment factor of element Mo), small-sized pyrite framboids, low Al content and moderate Mo/TOC ratio. On the contrary, relative sea-level fall generally resulted in decreased productivity, dysoxic conditions, increased detrital flux, and strongly restricted conditions. The correlations between TOC content and geochemical proxies suggest that organic matter accumulation was a combined effect of paleoproductivity, paleoredox conditions, detrital influx, and water mass restriction. The variation of these factors was strongly influenced by relative sea-level fluctuations related to Hirnantian glaciation and regional tectonic movements during the Late Ordovician-Early Silurian transitional stage. • Biogenic barium is not a reliable paleoproductivity proxy for the Longmaxi Formation. • The paleoenvironmental conditions are contrasting between relative sea-level rise and fall stage. • Relative sea-level rise is favorable for OM accumulation. • OM accumulation model is established within sequence stratigraphic framework. [ABSTRACT FROM AUTHOR]