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The sedimentary environment and organic matter (OM) accumulation are vital indicators for shale gas exploration. However, research on deep shale gas systems is relatively limited; moreover, the exploration of deep shale gas in the southeastern Sichuan Basin has entered a period of stagnation. In this study, systematic geochemical analysis of Wufeng (WF) and the first member of the Longmaxi (Long-1) deep shale samples from the recently drilled DY7 well in the Dingshan area of the Sichuan Basin is carried out, and the longitudinal variations in major and trace elements are revealed. The differences in the WF, lower section of the Long-1 (Long-11) and upper section ofthe Long-1 (Long-12) shales are studied in terms of redox conditions, paleoproductivity, terrigenous detrital input, sedimentation rate and paleoclimate, and the different main controlling factors of OM accumulation for these three layers are discussed. The WF shale has a higher TOC content (mean: 5.73%), the Long-11 shale has a high TOC content (mean: 2.89%), while the Long-12 shale has a low TOC content (mean: 1.44%). For the WF shale, due to complex geological events and large fluctuations in element contents, its TOC content is poorly correlated with these indices, redox and paleoproductivity proxies have a positive association with the Long-11 shale's TOC content, but negatively correlated with terrigenous input and sedimentation rate indices. The formation of these two sets of organic-rich shales (TOC > 2%) is jointly controlled by good preservation conditions. In contrast, the TOC content of the WF shale is higher than that of the Long-11 shale as the result that terrigenous input and sedimentation rate of the Long-11 shale represent the dilution and destruction of OM, which is different from the former. During the Long-12 depositional period, the water column experienced weak reducing conditions and low productivity, and its high terrigenous debris input further diluted the OM, leading to a low TOC content. [ABSTRACT FROM AUTHOR]

Creep behavior is a very important attribute of shale and is crucial in the design of hydraulic fracturing schemes to ensure the long-term stable development of shale gas. However, how different shale minerals, organic matter, bedding planes, and pores affect the micro-creep behavior of Upper Ordovician Wufeng and Lower Silurian Longmaxi (WF–LMX) Formation shales is poorly understood. In this study, we employed a nanoindentation mechanical testing technique alongside rock mineralogical, major elemental, and pore analyses to investigate the creep behavior and influencing factors of WF–LMX shales at the microscale. The results show that (1) the creep displacement (Δh) and indentation creep parameter (CIT) are each positively correlated with clay, total pore volume, and clay + total organic carbon (TOC) contents but negatively correlated with the content of quartz, excess SiO2, and TOC. We found weak or no correlation between the occurrence of minor rock constituents, such as feldspar, carbonates, and pyrite, and the shale creep properties; (2) the creep parameters (Δh, CIT, and stress exponent (n)) exhibit anisotropy due to the layering of shale, with values 7.3%–24.2% higher in the plane perpendicular to bedding (X1) than those in the plane parallel to bedding (X3). The creep displacement exhibits negative correlations with Young's modulus, hardness, and stress exponent (n), especially for the X1 direction; (3) compared with those of China's Yanchang shale, the stress exponents of WF–LMX shale are relatively high (8.5–30), indicating that the average creep capacity of WF–LMX shale is relatively weak. Overall, nanoindentation technology has shown great potential in studying shale creep and provides quantitative data support for macroscopic shale creep research.

The study of deep shale gas (>3500 m) has become a new research hotspot in the field of shale gas research in China. In this study, 16 representative deep shale samples were selected from different layers of the Wufeng–Longmaxi Formation in the Z-3 well in the western Chongqing area to conduct low-field nuclear magnetic resonance (NMR) tests, field-emission scanning electron microscopy (FE-SEM) observation, and fractal modeling. By comparing the differences in pore structure and their influencing factors in representative samples from different layers, the particularities of high-quality reservoirs have been revealed. The results show that the Z-3 well shales mainly develop micropores and mesopores, with pore sizes of 1 nm–200 nm. The fractal dimensions of bound fluid pores D1 (1.6895–2.3821) and fractal dimension of movable fluid pores D2 (2.9914–2.9996) were obtained from T2 spectra and linear fitting, and the pores were divided into three sections based on the NMR fractal characteristics. TOC content was one of the major factors affecting the gas content in the study area. The shale samples in the bottom S1l1-1 sub-layer with a higher TOC content have larger porosity and permeability, leading to enhanced homogeneity of the pore structure and favorable conditions for shale gas adsorption. A comparative understanding of the particularities of pore structure and influencing factors in high-quality reservoirs with higher gas content will provide the scientific basis for further exploration and exploitation of the Wufeng–Longmaxi Formation deep shale reservoirs in the western Chongqing area. [ABSTRACT FROM AUTHOR]

The marine shale within the Sichuan Basin constitutes China's significant shale gas production, featuring old formation age, high degree of thermal evolution, multiple tectonic movements, and complex structural conditions. However, there are significant differences in the shale gas preservation conditions and reservoir quality in different areas, limiting future large-scale exploration and development. Pore structure significantly influences shale reservoir quality, gas content, and exploration of shale gas occurrence, migration, and enrichment mechanisms. The influence of structural-dominated preservation conditions on shale pore structures is essential to comprehend for effective shale gas exploitation. This study employs field-emission scanning electron microscopy in conjunction with other techniques (low-temperature N2 adsorption, low-temperature CO2 adsorption, and nuclear magnetic resonance) for detailed analyses of the pore structure across varied structural zones, revealing the influence of structural attributes, fault systems, depth of burial, and formation pressure on pore architecture, and examining the relationship between pore structure and shale gas preservation conditions. The results show that stable structural condition is conducive to the development and preservation of shale pores. Structural compression causes inorganic and organic pores to become narrow and elongated due to shrinkage, with a significant increase in microfractures. The porosity of shale with stable structural conditions exhibits markedly increased porosity compared to samples under structural compressions. Under conditions of similar TOC and mineral composition, the pore size distribution (PSD), pore volume (PV), and specific surface area (SSA) of shale after structural compression are significantly lower than those of samples with stable structural conditions. As the burial depth increases, the shale porosity shows a decreasing trend, but the decrease is limited. Burial depth significantly impacts the SSA and PV of high-TOC samples (3%-6%). As the burial depth increases, both SSA and PV show a significant decreasing trend. When the burial depth reaches 4000 m, SSA and PV tend to concentrate. The formation pressure coefficient is an important factor for the development and preservation of shale pores, and porosity is positively correlated with the formation pressure coefficient. Increased formation pressure coefficient indicates superior preservation conditions and enhanced pore development. [ABSTRACT FROM AUTHOR]

In order to reveal the restriction in shale gas enrichment of the Wufeng-Longmaxi Formation in the northern Guizhou province, the influence model of detachment layer was established through field geological investigation, core observation, logging, sample analysis, and geological background data. The response relationship between the detachment layer and the shale gas enrichment model in different structural formats was analyzed. The results show that the thickness of the Wufeng-Longmaxi Formation’s detachment layer is influenced by the conditions near the fault zones and mineralogical characteristics. The lithofacies of the detachment layer shows mainly a combination of clay-rich shale facies. This indicates that lithofacies type is one of the main factors influencing the variation in slip layer thickness. The detachment layer exhibits distinct well logging response characteristics and is influenced by nitrogen enrichment. The development of detachment fractures allows atmospheric nitrogen to infiltrate shale gas. It leads to poor gas saturation in the shale gas. In addition, the overall tectonic deformation in the northern Guizhou province was found to gradually intensify from Northwest to Southeast, and there were two tectonic models: a slot-shift tape transition belt and a spacer type deformation belt. The influence of decollements on shale gas preservation was barely found in the northern Guizhou province. It is mainly controlled by buried depth of the target layer, conditions of the cover layer, structural type, and deformation intensity.

The sedimentary environment and organic matter (OM) accumulation are vital indicators for shale gas exploration. However, research on deep shale gas systems is relatively limited; moreover, the exploration of deep shale gas in the southeastern Sichuan Basin has entered a period of stagnation. In this study, systematic geochemical analysis of Wufeng (WF) and the first member of the Longmaxi (Long-1) deep shale samples from the recently drilled DY7 well in the Dingshan area of the Sichuan Basin is carried out, and the longitudinal variations in major and trace elements are revealed. The differences in the WF, lower section of the Long-1 (Long-11) and upper section of the Long-1 (Long-12) shales are studied in terms of redox conditions, paleoproductivity, terrigenous detrital input, sedimentation rate and paleoclimate, and the different main controlling factors of OM accumulation for these three layers are discussed. The WF shale has a higher TOC content (mean: 5.73%), the Long-11 shale has a high TOC content (mean: 2.89%), while the Long-12 shale has a low TOC content (mean: 1.44%). For the WF shale, due to complex geological events and large fluctuations in element contents, its TOC content is poorly correlated with these indices, redox and paleoproductivity proxies have a positive association with the Long-11 shale’s TOC content, but negatively correlated with terrigenous input and sedimentation rate indices. The formation of these two sets of organic-rich shales (TOC > 2%) is jointly controlled by good preservation conditions. In contrast, the TOC content of the WF shale is higher than that of the Long-11 shale as the result that terrigenous input and sedimentation rate of the Long-11 shale represent the dilution and destruction of OM, which is different from the former. During the Long-12 depositional period, the water column experienced weak reducing conditions and low productivity, and its high terrigenous debris input further diluted the OM, leading to a low TOC content.

The deep shale of the Wufeng–Longmaxi Formation in the southern Sichuan Basin has high gas potential. The development characteristics of lamination could significantly impact reservoir property. Samples were investigated using microscopic observation, element analysis, organic petrology analysis, mineralogy analysis, and pore structure analysis to determine the types of laminae and laminasets, clarify the formation conditions of argillaceous lamina and silty lamina as well as their relationships with the sedimentary environment, and explore the influence of laminae on shale reservoir property. Results indicate that the Wufeng Formation shale exhibits weak development of laminae due to bioturbation, while the Longmaxi Formation shale develops continuous, parallel, and plate-like laminae. Compared with light silty lamina-rich shale, dark argillaceous lamina-rich shale usually develops in an anoxic reduction environment, with higher total organic carbon content, porosity, pore volume, specific surface area, and more developed organic matter pores, which can provide greater space for shale gas adsorption and storage. Shales in the middle section of the Longmaxi Formation are characterized by the development of silty-argillaceous interbedded type laminaset, which have good reservoir performance, making them the primary target for deep shale gas exploration and development. [ABSTRACT FROM AUTHOR]

The marine shale within the Sichuan Basin constitutes China’s significant shale gas production, featuring old formation age, high degree of thermal evolution, multiple tectonic movements, and complex structural conditions. However, there are significant differences in the shale gas preservation conditions and reservoir quality in different areas, limiting future large-scale exploration and development. Pore structure significantly influences shale reservoir quality, gas content, and exploration of shale gas occurrence, migration, and enrichment mechanisms. The influence of structural-dominated preservation conditions on shale pore structures is essential to comprehend for effective shale gas exploitation. This study employs field-emission scanning electron microscopy in conjunction with other techniques (low-temperature N2 adsorption, low-temperature CO2 adsorption, and nuclear magnetic resonance) for detailed analyses of the pore structure across varied structural zones, revealing the influence of structural attributes, fault systems, depth of burial, and formation pressure on pore architecture, and examining the relationship between pore structure and shale gas preservation conditions. The results show that stable structural condition is conducive to the development and preservation of shale pores. Structural compression causes inorganic and organic pores to become narrow and elongated due to shrinkage, with a significant increase in microfractures. The porosity of shale with stable structural conditions exhibits markedly increased porosity compared to samples under structural compressions. Under conditions of similar TOC and mineral composition, the pore size distribution (PSD), pore volume (PV), and specific surface area (SSA) of shale after structural compression are significantly lower than those of samples with stable structural conditions. As the burial depth increases, the shale porosity shows a decreasing trend, but the decrease is limited. Burial depth significantly impacts the SSA and PV of high-TOC samples (3%–6%). As the burial depth increases, both SSA and PV show a significant decreasing trend. When the burial depth reaches 4000 m, SSA and PV tend to concentrate. The formation pressure coefficient is an important factor for the development and preservation of shale pores, and porosity is positively correlated with the formation pressure coefficient. Increased formation pressure coefficient indicates superior preservation conditions and enhanced pore development.

The nano-scale pore systems in shale reservoirs control shale gas transportation and aggregation, which is of great significance for the resource evaluation of shale oil and gas and the selection of a "sweet spot". Taking twelve marine shale samples from the Wufeng–Longmaxi Formation in the Zigong area, southwest Sichuan Basin, as the research target, we carried out a series of experiments, including total organic carbon (TOC) analysis, X-ray diffraction (XRD), gas adsorption (CO2 + N2), and mercury intrusion porosimetry (MIP), to study the full-scale pore structure characterization and controlling factors of pore volume and specific surface area. The results presented the following findings. (1) Marine shale samples from the target area are rich in organic matter, with an average TOC value of 3.86%; additionally, the mineral composition was dominated by quartz and clay minerals, with average contents of 44.1% and 31.4%, respectively. (2) The full-scale pore size distribution curves of pore volume developed multimodally, with the main peaks at 0.5 nm–2 nm, 3 nm–6 nm, and 700 nm–2.2 um; moreover, the full-scale pore size distribution curves of a specific surface area developed unimodally, with the main peak ranging from 0.5 nm to 1.2 nm. (3) Pore volume was mainly contributed by mesopores and macropores, with an average contribution of 46.66% and 42.42%, respectively, while the contribution of micropores was only 10.91%. The specific surface area was mainly contributed by micropores and mesopores, with an average contribution of 64.63% and 29.22%, respectively, whereas the contribution of micropores was only 6.15%. (4) The TOC content mainly controlled the pore volume and specific surface area of micropores and mesopores, while the clay and feldspar content generally controlled the pore volume and specific surface area of macropores. Additionally, the quartz content had an inhibitory effect on the development of all pore types. These results will help researchers understand the laws of gas accumulation and migration. [ABSTRACT FROM AUTHOR]

In recent years, significant efforts have been directed towards extracting deep shale gas (SG) in Luzhou. Understanding the factors influencing the accumulation and preservation of SG in the section extending from the Upper Ordovician Wufeng (WF) Formation (Fm) through the Lower Silurian Longmaxi (LMX) Fm is crucial for its large‐scale, profitable development. The shale layers in the LMX Fm located in the Luzhou‐Zigong area in the southern part of the Sichuan Basin have undergone unique geological processes, including deep deposition, intensive uplift, erosion and deformation. The Luzhou block, situated in the low‐fold zone in southern Sichuan between the southern slope of the paleo‐uplift in central Sichuan and the down‐warping fold zone in south‐eastern Sichuan, was studied using seismic, well‐log, drilling and core analysis and test data to understand its tectonic, sedimentary, reservoir and fracture characteristics and identify the principal factors controlling gas accumulation in this area. The study found that SG in the Luzhou block is primarily distributed in the WF Fm through the Longyi1 sub‐member. Class I continuous reservoirs, 2.6–20.3 m in thickness, are mainly found in the Longyi11–13 sublayers and those with large continuous thickness produced high yields during tests. Gentle anticlinal zones and wide, gentle tectonic structures with relatively high‐pressure coefficients have formed in this area. High‐angle fractures (HACs) in this area tend to accumulate free gas and, therefore, facilitate the accumulation and production of SG. The study concludes that the continuous thickness of Class I reservoirs and natural HACs controlled by the centre of the deep‐water continental shelf are the primary factors controlling SG accumulation in the Luzhou block. [ABSTRACT FROM AUTHOR]

Significant shale gas sources have been discovered in Wufeng-Longmaxi Formation in the Yiling Block of Western Hubei. This paper aims to investigate the palaeoenvironmental conditions and is shale gas geological significance of the Wufeng-Longmaxi shales from well YD1 by integrating data on total organic carbon (TOC) content, major and trace elements, and scanning electron microscope and so on. The results show that the average TOC value of the Wufeng Formation and the lower part of Long-maxi Formation are greater than 2. 00%, and thus they are organic-rich shales which are rich in Si, Ca, Vr, Co, Ni, Zn, Mo, Ba and U. The Wufeng-Longmaxi shales were deposited under highly restricted conditions, which are the main reasons for the organic matter accumulation. Due to the tectonic uplift caused by the Kwangsian Orogeny in the Upper Longmaxi Formation, the palaeoenvironment was characterized by a bottom water oxic conditions, and the insufficient nutrients caused by the weak upwelling, which is the main factor for the turnover of graptolite biozones from LM5 to LM6. The organic rich shale, especially the lower part of Longmaxi Formation, has better physical properties and higher gas content. Compared with the Jiaoshiba Block in Southeastern Sichuan, although the organic-rich shale of Wufeng-Longmaxi Formation in Yiling Block is thinner (10-20 m) and the total gas content is relatively lower, the TOC content is high and the tectonic activity is weak due to the protection by the Huanging rigid basement. It still has good shale gas exploration potential in the northeastern of study area close to the Qinling Ocean. This research has direct meaning to the development of shale gas in Yichang Area, Western Hubei. [ABSTRACT FROM AUTHOR]

The study of deep shale gas (>3500 m) has become a new research hotspot in the field of shale gas research in China. In this study, 16 representative deep shale samples were selected from different layers of the Wufeng–Longmaxi Formation in the Z-3 well in the western Chongqing area to conduct low-field nuclear magnetic resonance (NMR) tests, field-emission scanning electron microscopy (FE-SEM) observation, and fractal modeling. By comparing the differences in pore structure and their influencing factors in representative samples from different layers, the particularities of high-quality reservoirs have been revealed. The results show that the Z-3 well shales mainly develop micropores and mesopores, with pore sizes of 1 nm–200 nm. The fractal dimensions of bound fluid pores D1 (1.6895–2.3821) and fractal dimension of movable fluid pores D2 (2.9914–2.9996) were obtained from T2 spectra and linear fitting, and the pores were divided into three sections based on the NMR fractal characteristics. TOC content was one of the major factors affecting the gas content in the study area. The shale samples in the bottom S1l1-1 sub-layer with a higher TOC content have larger porosity and permeability, leading to enhanced homogeneity of the pore structure and favorable conditions for shale gas adsorption. A comparative understanding of the particularities of pore structure and influencing factors in high-quality reservoirs with higher gas content will provide the scientific basis for further exploration and exploitation of the Wufeng–Longmaxi Formation deep shale reservoirs in the western Chongqing area.

After more than 10 years of exploration, development, research, and practical efforts, China has opened up new perspectives for the commercial exploitation of marine shale gas. While high shale gas production is a main driver for energy security and economic development in China, there have been few attempts to systemically scientific analysis the challenges, prospect, development strategies, and goals for shale gas. Here, we present a detailed comparison of the differences in shale gas between the Sichuan Basin and North America from multiple dimensions, explain how and to what extent recent advances have been made, discuss the current challenges, and provide strategies to deal with these challenges. We demonstrate that a total of 13 graptolite zones developed in the Wufeng–Longmaxi Formations, achieved by representative cores from 32 coring wells and 7 outcrop profiles, can establish the chronostratigraphic framework in the Sichuan Basin, which leads to the potential impact of high-quality reservoir distribution and shale gas production. Shale gas is still faced with the challenges of complex underground and surface conditions, low single-well EUR, and immature deep development engineering technology. To circumvent these issues, here, we propose several strategies, including sweet-spot optimization, low-cost drilling techniques, and efficient fracturing technologies. Our results strengthen the importance of adopting fundamental theoretical research and practical and feasible development goals to realize more commercial discoveries of shale gas of diverse types and higher growth of shale gas reserves and production. [ABSTRACT FROM AUTHOR]

The Wufeng-Longmaxi Formation shales with burial depths > 3 500 m in the southern Sichuan Basin are believed to have large shale gas potentials. However, the reservoir properties and gas-in-place (GIP) contents of these shales exhibit variations across different sublayers. In this study, a set of Wufeng-Longmaxi shales with burial depths of 4 000–4 200 m was derived from the Well Y101H2 in the Luzhou area, and the differences in geological and geochemical characteristics, porosity, water saturation and GIP content of various sublayer shales were investigated. The results indicate that the TOC content and effective porosity of the upper (LMX1-4) and lower (LMX1-3) sublayer shales of the first member of the Longmaxi Formation are better than those of the Wufeng Formation (WF) shales, which results in the LMX1-4 and LMX1-3 shales having higher GIP contents than the WF shales. The GIP contents of the LMX1-3 shales are higher than those of the LMX1-4 shales, and this is likely because the organic matter of the LMX1-3 shales had more aquatic organisms and was preserved in a stronger reductive environment, which leads to a stronger gas generation potential. In contrast to the middle-shallow LMX1-4 shales in the Sichuan Basin, the deep LMX1-4 shales in the Luzhou area have advantageous reservoir properties and GIP contents, and thus it is promising to synchronously exploit the deep LMX1-4 and LMX1-3 shales in some regions of the southern Sichuan Basin. [ABSTRACT FROM AUTHOR]

Based on the identification of visual kerogen assessment, a subdivision method of organic matter types was proposed. Combined with GR curve, the distribution was further studied, the relationship between shale reservoir parameters and organic matter types was analyzed, and the significance was discussed. The results show that the kerogen types of organic rich shale in the lower part of Wufeng-Longmaxi Formation in the south of the Sichuan Basin are mainly of type I, II1a and II1b, while type II2 and type III are not developed. In Wufeng Formation, the shale organic matter is mainly of type II1a and II1b. From Long111 to Long114, the shale displays a trend of "Type I-II1b-II1a-II1b". Type I was characterized by higher TOC, porosity and gas bearing capacity of shale reservoirs than the other organic matter. The reservoir parameter of type II1a was better than type II1b, indicating that the organic matter type control on the quality of shale reservoirs. Type difference is closely related to the bio-precursors component, among which type I and type II1a are related to the content of planktonic algae, while type II1b is related to the content of benthic algae. This study provides a good reference for the subdivision of marine shale organic matter types and the evaluation and prediction of gas bearing properties. [ABSTRACT FROM AUTHOR]

Deeply buried (>3500 m) marine shale has become a focus point for the future exploration and exploitation of shale hydrocarbon in China. Low-temperature nitrogen adsorption (LTNA), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and other experiments were combined to characterize the pore structure and fluid division in deep-marine shale of the southern Sichuan Basin in this study. The results suggest that the deep-marine shale had a relatively developed nanopore network, especially with honeycomb organic pores. These organic pores were largely macropores with good connectivity in three-dimensional space and constituted the major reservoir space of the deep-marine shale gas. Microfractures were predominantly clay-mineral-related fractures, and the development degree of microfractures connected with organic pores was low, which contributed to the preservation of organic pores. Within the deep-marine shale interval, the pore volumes of Section 1 and Section 3 were higher. Pore volume was predominantly contributed by pores above 10 nm, where macropores accounted for a large proportion. Based on a combination of high-speed centrifugation and gradient temperature drying, the pore fluid of deep-marine shale reservoirs was quantitatively classified into four types: clay-bound fluid, capillary-bound fluid, free-flowing fluid, and closed-pore fluid. The clay-bound fluid existed in pores of less than 4.25 nm, which cannot be exploited. Quantitative division of the shale pore system could be realized by using the pore space differences of different types of fluids. [ABSTRACT FROM AUTHOR]

The deep shale of the Wufeng–Longmaxi Formation in the southern Sichuan Basin has high gas potential. The development characteristics of lamination could significantly impact reservoir property. Samples were investigated using microscopic observation, element analysis, organic petrology analysis, mineralogy analysis, and pore structure analysis to determine the types of laminae and laminasets, clarify the formation conditions of argillaceous lamina and silty lamina as well as their relationships with the sedimentary environment, and explore the influence of laminae on shale reservoir property. Results indicate that the Wufeng Formation shale exhibits weak development of laminae due to bioturbation, while the Longmaxi Formation shale develops continuous, parallel, and plate-like laminae. Compared with light silty lamina-rich shale, dark argillaceous lamina-rich shale usually develops in an anoxic reduction environment, with higher total organic carbon content, porosity, pore volume, specific surface area, and more developed organic matter pores, which can provide greater space for shale gas adsorption and storage. Shales in the middle section of the Longmaxi Formation are characterized by the development of silty-argillaceous interbedded type laminaset, which have good reservoir performance, making them the primary target for deep shale gas exploration and development.

Copyright of Journal of Northeast Petroleum University is the property of Journal of Northeast Petroleum University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Wufeng–Longmaxi Formation in northern Guizhou Province is the dominant shale gas exploration and development strata in China. This study investigates the Shixi and Daozhen synclines, which are located in the basin margin transition zone. This region experienced complex tectonic stages and superimposed transformations, resulting in heterogeneous preservation conditions and shale reservoir pressure evolution in different tectonic units of the Wufeng–Longmaxi Formation. Based on fluid inclusion analysis, the types of fluid inclusion and the fluid filling stages of the Wufeng–Longmaxi shale reservoir were determined by laser Raman analysis and homogenization temperature and salinity tests. It was found that the fluid inclusion density and paleopressure in each filling stage were then restored. The results confirm that: ① Both the Shixi syncline and Daozhen syncline Wufeng–Longmaxi shale reservoirs experienced two fluid filling phases, in the early Yanshanian and late Yanshanian–Himalayan, respectively. In the course of tectonic evolution, the Wufeng–Longmaxi Formation in the Shixi and Daozhen synclines experienced pressure relief to some extent during the two aforementioned key fluid charging periods but remained in overpressurized states overall; ② The Wufeng–Longmaxi Formations of the Shixi and Daozhen synclines are characterized by high-density methane inclusions; ③ Based on the differences in the structural preservation conditions, the pressure evolution mechanisms of the Wufeng–Longmaxi Formation Shales during the important tectonic evolution period in northern Guizhou Province were revealed. Furthermore, differential enrichment and accumulation models of shale gas were established and named "facing thrust structural sealing type (Shixi syncline)" and "reverse fault lateral blocking type (Daozhen syncline)". [ABSTRACT FROM AUTHOR]

Copyright of Journal of Northeast Petroleum University is the property of Journal of Northeast Petroleum University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The shale gas exploration and development potential of the Wufeng-Longmaxi formation in southeast Sichuan is good. Based on experimental, seismic, logging, drilling and fracturing data, taking the evaluation of shale gas sweet spot target as the main line, the integration of geology-geophysical exploration and geology-engineering, was studied combining with the practice of exploration and development, a series of target evaluation techniques had been defined, such as geological comprehensive evaluation, sweet spot target evaluation system and geology-engineering integration design of horizontal wells. The sweet spot target evaluation system was guided by the geological knowledge of “three factors controlling reservoir”, to establish the evaluation system and standard of the sweet spot target, and select the favorable target. The geology-engineering integration design of horizontal wells is through optimizing the deployment of horizontal wells and fracturing design, allowing the trajectory to travel better through the formation and create more fractures, to improve single well production. Through the integration and application of target evaluation techniques, to form fixed practices and mature technologies, the exploration and development of shale gas in Wufeng-Longmaxi formation in southeast Sichuan can be efficiently guided. [ABSTRACT FROM AUTHOR]

The nano-scale pore systems in shale reservoirs control shale gas transportation and aggregation, which is of great significance for the resource evaluation of shale oil and gas and the selection of a “sweet spot”. Taking twelve marine shale samples from the Wufeng–Longmaxi Formation in the Zigong area, southwest Sichuan Basin, as the research target, we carried out a series of experiments, including total organic carbon (TOC) analysis, X-ray diffraction (XRD), gas adsorption (CO2 + N2), and mercury intrusion porosimetry (MIP), to study the full-scale pore structure characterization and controlling factors of pore volume and specific surface area. The results presented the following findings. (1) Marine shale samples from the target area are rich in organic matter, with an average TOC value of 3.86%; additionally, the mineral composition was dominated by quartz and clay minerals, with average contents of 44.1% and 31.4%, respectively. (2) The full-scale pore size distribution curves of pore volume developed multimodally, with the main peaks at 0.5 nm–2 nm, 3 nm–6 nm, and 700 nm–2.2 um; moreover, the full-scale pore size distribution curves of a specific surface area developed unimodally, with the main peak ranging from 0.5 nm to 1.2 nm. (3) Pore volume was mainly contributed by mesopores and macropores, with an average contribution of 46.66% and 42.42%, respectively, while the contribution of micropores was only 10.91%. The specific surface area was mainly contributed by micropores and mesopores, with an average contribution of 64.63% and 29.22%, respectively, whereas the contribution of micropores was only 6.15%. (4) The TOC content mainly controlled the pore volume and specific surface area of micropores and mesopores, while the clay and feldspar content generally controlled the pore volume and specific surface area of macropores. Additionally, the quartz content had an inhibitory effect on the development of all pore types. These results will help researchers understand the laws of gas accumulation and migration.

The three-dimensional (3D) pore structures and permeability of shale are critical for forecasting gas production capacity and guiding pressure differential control in practical reservoir extraction. However, few investigations have analyzed the effects of microscopic organic matter (OM) morphology and 3D pore nanostructures on the stress sensitivity, which are precisely the most unique and controlling factors of reservoir quality in shales. In this study, ultra-high nanoscale-resolution imaging experiments, i.e. focused ion beam-scanning electron microscopy (FIB-SEMs), were conducted on two organic-rich shale samples from Longmaxi and Wufeng Formations in northern Guizhou Depression, China. Pore morphology, porosity of 3D pore nanostructures, pore size distribution, and connectivity of the six selected regions of interest (including clump-shaped OMs, interstitial OMs, framboidal pyrite, and microfractures) were qualitatively and quantitatively characterized. Pulse decay permeability (PDP) measurement was used to investigate the variation patterns of stress-dependent permeability and stress sensitivity of shales under different confining pressures and pore pressures, and the results were then used to calculate the Biot coefficients for the two shale formations. The results showed that the samples have high OM porosity and 85% of the OM pores have the radius of less than 40 nm. The OM morphology and pore structure characteristics of the Longmaxi and Wufeng Formations were distinctly different. In particular, the OM in the Wufeng Formation samples developed some OM pores with radius larger than 500 nm, which significantly improved the connectivity. The macroscopic permeability strongly depends on the permeability of OM pores. The stress sensitivity of permeability of Wufeng Formation was significantly lower than that of Longmaxi Formation, due to the differences in OM morphology and pore structures. The Biot coefficients of 0.729 and 0.697 were obtained for the Longmaxi and Wufeng Formations, respectively.

After more than 10 years of exploration, development, research, and practical efforts, China has opened up new perspectives for the commercial exploitation of marine shale gas. While high shale gas production is a main driver for energy security and economic development in China, there have been few attempts to systemically scientific analysis the challenges, prospect, development strategies, and goals for shale gas. Here, we present a detailed comparison of the differences in shale gas between the Sichuan Basin and North America from multiple dimensions, explain how and to what extent recent advances have been made, discuss the current challenges, and provide strategies to deal with these challenges. We demonstrate that a total of 13 graptolite zones developed in the Wufeng–Longmaxi Formations, achieved by representative cores from 32 coring wells and 7 outcrop profiles, can establish the chronostratigraphic framework in the Sichuan Basin, which leads to the potential impact of high-quality reservoir distribution and shale gas production. Shale gas is still faced with the challenges of complex underground and surface conditions, low single-well EUR, and immature deep development engineering technology. To circumvent these issues, here, we propose several strategies, including sweet-spot optimization, low-cost drilling techniques, and efficient fracturing technologies. Our results strengthen the importance of adopting fundamental theoretical research and practical and feasible development goals to realize more commercial discoveries of shale gas of diverse types and higher growth of shale gas reserves and production.

Deeply buried (>3500 m) marine shale has become a focus point for the future exploration and exploitation of shale hydrocarbon in China. Low-temperature nitrogen adsorption (LTNA), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and other experiments were combined to characterize the pore structure and fluid division in deep-marine shale of the southern Sichuan Basin in this study. The results suggest that the deep-marine shale had a relatively developed nanopore network, especially with honeycomb organic pores. These organic pores were largely macropores with good connectivity in three-dimensional space and constituted the major reservoir space of the deep-marine shale gas. Microfractures were predominantly clay-mineral-related fractures, and the development degree of microfractures connected with organic pores was low, which contributed to the preservation of organic pores. Within the deep-marine shale interval, the pore volumes of Section 1 and Section 3 were higher. Pore volume was predominantly contributed by pores above 10 nm, where macropores accounted for a large proportion. Based on a combination of high-speed centrifugation and gradient temperature drying, the pore fluid of deep-marine shale reservoirs was quantitatively classified into four types: clay-bound fluid, capillary-bound fluid, free-flowing fluid, and closed-pore fluid. The clay-bound fluid existed in pores of less than 4.25 nm, which cannot be exploited. Quantitative division of the shale pore system could be realized by using the pore space differences of different types of fluids.

Copyright of Petroleum Geology & Recovery Efficiency is the property of Petroleum Geology & Recovery Efficiency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Pore and its structural characteristics are key parameters affecting shale gas reservoir development. Accurate quantitative characterization of shale pore and its structural characteristics is of great significance for evaluating shale reservoir state. In this study, 15 shallow marine shale samples were collected in Well Y108. X-ray diffraction results indicate that brittle minerals are the most common components in shale. In this paper, various pore types are classified and characterized by scanning electron microscope images. The total porosity of shale measured by the mercury intrusion method is between 3.2% and 6.5%. In addition, a petrophysical model is established to calculate matrix porosity and fracture. The results of this model are consistent with the measured porosity. Three key parameters (VTOC > VBri > VClay) were obtained. The low-pressure N2/CO2 adsorption experiment allows for the analysis of pore volume, specific surface area, and pore size. Finally, it was determined that the primary pore types and primary shale gas reservoir space in shallow marine shale are mesopores and micropores. The impact of shale constituents on pores and their structural properties is also covered in this work. The results indicate that the enrichment of total organic carbon and brittle minerals is conducive to the development of shallow marine shale pore-fracture system. Additionally, there is a positive linear relationship between matrix porosity, pore volume, specific surface area, average pore diameter, and surface porosity. [ABSTRACT FROM AUTHOR]

The Shale pore structure of the Wufeng-Longmaxi formation in the Jiaoshiba area of southeastern Sichuan is complex, and the clarification of controlling factors is of guiding significance for the effective development of shale gas. Shale pore types were analyzed by organic carbon content, thermal evolution degree, whole-rock "X" ray diffraction, argon ion polishing electron microscope observation, and low-temperature nitrogen adsorption experiment. Based on the fractal geometry theory of low-temperature nitrogen adsorption, the fractal characteristics of shale pores were studied, and the effects of mineral composition, pore structure, organic matter content, and maturity on the fractal dimension were discussed. Shale pores are mainly organic, dissolution, and intergranular pores, the internal characteristics are mainly ink bottle-shaped and slit type. Shale pores have dual fractal characteristics, and the fractal dimension is between 2.5389 and 2.9126. D1 and D2 represent pore surface fractal dimension and structure fractal dimension with relative pressure less than 0.45 and greater than 0.45, D1 is between 2.5389 and 2.7199, and D2 is between 2.7343 and 2.9126. Quartz mineral, organic matter content, and maturity positively correlate with D1 and D2. Quartz and organic matter are the carriers of pore development. The maturity of organic matter controls the development of micropores and mesopores of organic matter. Micropores have a greater influence on pore structure parameters. Carbonate minerals and feldspar mainly provide macropores and increase the pore size, which is negatively correlated with D1 and D2. The clay mineral content is also negatively correlated with D1 and D2, directly related to the compaction leading to the simplification of pore morphology and the decrease of micropores. [ABSTRACT FROM AUTHOR]

Fine dissection of microscopic pore structure variations between the Niutitang Formation and the Wufeng-Longmaxi Formation will help to improve the understanding of the underlying geological theory of shale gas in northern Guizhou Province. The stratigraphic, geochemical, physical, and tectonic properties of the two formations vary greatly, resulting in differential development of the microscopic pore structure among reservoirs and, as a result, major variances in gas concentration. To explore the mechanism of differential pore evolution, experimental techniques and instruments such as gas adsorption, liquid intrusion, SEM, XRD, and organic geochemical tests were utilized. The results indicate that the Wufeng-Longmaxi Formation is in a high-maturity stage, while the Niutitang Formation is in an over-mature stage. The latter has a higher TOC content. Both petrographic phases are siliceous shale petrographic phases, and the former has more developed dissolution pores with better pore volume, throat radius, and macropore pore diameters than the latter, as well as organic matter pores, intergranular pores, and microfracture structural parameters, whereas the specific surface area is the opposite. The differences in reservoir pore formation between the two formations were analyzed, and the results showed that the petrographic type, thermal evolution, and tectonic preservation conditions were the primary controlling elements of differential shale gas reservoir formation. A differential reservoir-forming model of the Wufeng-Longmaxi Formation and the Niutitang Formation was constructed, providing a geological and theoretical basis for shale gas geological exploration in northern Guizhou Province. [ABSTRACT FROM AUTHOR]

Biogenic quartz in the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation (Wufeng– Longmaxi) shale layers in the Sichuan Basin and its periphery is qualitatively analyzed and quantitatively characterized by organic petrologic, mineralogic, and geochemical methods to find out the coupling effect between organic matter and quartz. (1) There are two types of biogenic quartz in the shale layers: Type I quartz is submicron quartz appearing in clusters around the organic matter. Type II quartz is in nano-scale grain size and floats in spherical shape on organic matter, with grains in point-to-point or surface-to-surface contact; this type of quartz is mainly biologic origin and slightly affected by hydrothermal activity in local parts. (2) The reservoirs in the Wufeng–Longmaxi formations is consistent in distribution with biogenic silica content in them, and mainly concentrated at the bottom of the Wufeng–Longmaxi formations, and is thinner in the Changning and Weiyuan regions, while thicker in the Fuling region. (3) The biogenic quartz in the Wufeng–Longmaxi shale worked through the entire evolution process of hydrocarbon generation. The presence of biogenic quartz can enhance the development of organic matter pores and microcracks, and can effectively preserve the organic matter pores and residual intergranular pores, forming “biological silicon intergranular pores, organic pores and micro-fractures”. This would benefit later hydraulic fracturing and result in high production/stable production of well. The coupling effect between biogenic quartz development and organic matter evolution and hydrocarbon generation is a critical factor for high-quality shale reservoir development.

Investigations into the connectivity and complexity of pore systems in shales are essential for understanding the flow of shale gas and the capacities of the associated reservoirs. In the present study, eight shale samples from the Wufeng-Longmaxi (WF-LMX) Formation that were collected from Well Yucan-6 in the southeast of the Sichuan Basin were analyzed for microstructural, pore network, and pore connectivity characteristics. The measurement results of low-pressure nitrogen adsorption illustrated that all shale samples contain micropores, mesopores, and macropores. Micropores and mesopores account for a high proportion of the total pores, and the dominant pore throat size is in the range of 2–6 nm. High-pressure mercury injection tests reveal that the porosity, total pore volume, and total specific surface area of pores for samples from the WF Formation are higher than those for samples from the LMX Formation. In spontaneous absorption experiments, the slopes of the absorption curves of n-decane (oil-wetting) and deionized water (water-wetting) in the WF and LMX Formations varied from 0.254 to 0.428 and from 0.258 to 0.317, respectively. These results indicate that shales in both formations exhibit mixed wettability characteristics, but lipophilic pores are better connected relative to hydrophilic pores. The total organic carbon and silica contents are the main factors controlling the pore connectivity in these shales, while the effects of other minerals are not significant. The findings of this work can improve our understanding of the pore structure characteristics of black shale. [ABSTRACT FROM AUTHOR]

Survey well HY2 reveals the reservoir characteristics of the Wufeng-Longmaxi shale and its gas-bearing properties in the Dahegou anticline, Hefeng area. Through analysis of the organic geochemical character, X-ray diffraction whole-rock mineralogy, SEM, isothermal adsorption, and live desorption experimental data, the Wufeng-Longmaxi shale is characterized. It has a high organic carbon content, an advanced thermal maturity, mainly clastic brittle minerals, and an ultralow porosity and permeability. By comparing data from well HY2 with those from well LD1, which is located in the Lianghekou syncline, Laifeng-Xianfeng area, the shale quality of the two wells is generally consistent. For the shale in well LD1, the average organic carbon content is 1.58%, average vitrinite reflectance is 2.94%, average porosity is 1.0%, average clay mineral content is 36.0%, and average quartz + feldspar content is 54.19%, while for the shale in well HY2, the average organic carbon content is 4.43%, average vitrinite reflectance is 2.65%, average porosity is 1.23%, average clay mineral content is 29.0%, and average quartz + feldspar content is 64.20%. The isothermal adsorption experimental results show that the average Langmuir volume of Wufeng-Longmaxi shale in well HY2 is 2.40m3/t, but the total gas content of well HY2 is low, with a maximum gas content of only 0.08 m3/t, while the average total gas content of well LD1 is 1.72m3/t. On the basis of fine description of well rock core, it is found that the vertical fractures developed in the upper part of the Linxiang Formation underlying the Wufeng-Longmaxi shale at well HY2 which were filled with calcite, and the cross-section of the core shows extensive microcracking. This result indicates that the Dahegou anticline was damaged to a certain extent after shale gas accumulation due to tectonic movement. Therefore, the destruction of preservation conditions in geological history is the main reason for the low gas content of well HY2. [ABSTRACT FROM AUTHOR]

China’s most successful horizon for shale-gas exploration and development is the Upper Ordovician Wufeng Formation through the Lower Silurian Longmaxi Formation in its Upper Yangtze Region. In this study, the Wufeng–Longmaxi Formation black shales in the Upper Yangtze Region are analysed to determine their microstructural characteristics, total organic carbon (TOC) content, and well log characteristics and to identify information on the parameters of Earth’s orbit from its gamma-ray (GR) data series. Then, paleoenvironmental modes are established over a long time scale. On this basis, the Earth’s orbital control on organic matter enrichment in the black shales is examined. The black shales of the Wufeng Formation and the lower Longmaxi Formation are rich in biogenic siliceous fossils and framboidal pyrite. Their TOC content, GR value, and uranium (U) content increase gradually from the bottom of the Wufeng Formation to the Guanyinqiao Member at its top, peak at the Guanyinqiao Member, and then decrease gradually up to the Longmaxi Formation. Approximately six and seven long eccentricity cycles can be identified from the GR curves of the Wufeng Formation and the lower Longmaxi Formation, respectively. During the long eccentricity minima, corresponding to Earth’s cold period, the black shales have a relatively high level of enrichment of organic matter. This study can provide an important reference for investigating the mechanism by which Earth’s orbits control the climate and sedimentary environment, as well as the mechanism of organic matter enrichment.

The Wufeng–Longmaxi Formation in northern Guizhou Province is the dominant shale gas exploration and development strata in China. This study investigates the Shixi and Daozhen synclines, which are located in the basin margin transition zone. This region experienced complex tectonic stages and superimposed transformations, resulting in heterogeneous preservation conditions and shale reservoir pressure evolution in different tectonic units of the Wufeng–Longmaxi Formation. Based on fluid inclusion analysis, the types of fluid inclusion and the fluid filling stages of the Wufeng–Longmaxi shale reservoir were determined by laser Raman analysis and homogenization temperature and salinity tests. It was found that the fluid inclusion density and paleopressure in each filling stage were then restored. The results confirm that: ① Both the Shixi syncline and Daozhen syncline Wufeng–Longmaxi shale reservoirs experienced two fluid filling phases, in the early Yanshanian and late Yanshanian–Himalayan, respectively. In the course of tectonic evolution, the Wufeng–Longmaxi Formation in the Shixi and Daozhen synclines experienced pressure relief to some extent during the two aforementioned key fluid charging periods but remained in overpressurized states overall; ② The Wufeng–Longmaxi Formations of the Shixi and Daozhen synclines are characterized by high-density methane inclusions; ③ Based on the differences in the structural preservation conditions, the pressure evolution mechanisms of the Wufeng–Longmaxi Formation Shales during the important tectonic evolution period in northern Guizhou Province were revealed. Furthermore, differential enrichment and accumulation models of shale gas were established and named “facing thrust structural sealing type (Shixi syncline)” and “reverse fault lateral blocking type (Daozhen syncline)”.

The brittleness of shale reservoirs is of great significance for shale gas development. The brittleness index is the most intuitive parameter reflecting reservoir brittleness. The key exploration and development strata of shale gas in Pengshui area of southeastern Chongqing are Wufeng Formation of Upper Ordovician and Longmaxi Formation of Lower Silurian, and the high-quality shale reservoir is Wufeng–Long 1 section. The results show that the high brittleness grade of shale corresponds to low Poisson's ratio and high Young's modulus, but the relative distribution index of brittle minerals also affects brittleness. Through the analysis and calculation of brittle minerals and clay minerals in reservoir, the brittle index is calculated using rock physical parameters and mineral composition, and the advantages and disadvantages of the two are compared. Then a new formula for calculating brittleness index is obtained by combining rock physical parameters and mineral composition. Finally, the brittleness of high-quality shale reservoir in southeast of Chongqing area is evaluated comprehensively. Compared with the measured brittleness, the new formula has good effect. The average brittleness index of the Wufeng–Long 1 is more than 70% in Nanchuan area, China. The lateral distribution is stable and the fracturing conditions are good. [ABSTRACT FROM AUTHOR]

To investigate the geological characteristics and exploration potential of shale gas in the southern Sichuan Basin, we analyze the coupling relationship between the hydrocarbon generation and storage conditions of the Longmaxi Formation and discuss the preservation conditions from the lateral and vertical migration mechanisms of shale gas. According to the results, the organic-rich shale at the bottom was formed in a strongly euxinic and anoxic reducing environment, the oxygen content increased in the upper water body in which the Longmaxi Formation was deposited, and the water body became oxidized. The organic matter type in the shale is dominantly type I kerogen and a small amount of type II1 kerogen. The organic matter content is more than 3.0% and is in the high-to postmature stage. The enrichment of siliceous organisms increases the organic matter and enhances the brittleness of shale, resulting in “superior hydrocarbons and a favorable reservoir”. Pyrolysis of organic matter promotes the formation of organic matter pores and dissolution pores, resulting in a coupled “source-reservoir” accumulation control system. The high vertical formation pressure guarantees the sealing of shale and restrains the lateral escape of shale gas. The high-angle intersection of the highly filled fractures and the current crustal stress can effectively enhance the fracture sealing and inhibit the vertical escape of shale gas, forming a three-dimensional effective closure. Hence, the area featuring a short tectonic uplift time, small amplitudes, large-scale underdeveloped fractures, and a high formation pressure coefficient is a favorable area for shale gas exploration, according to the analysis of three-dimensional preservation conditions.

In the Middle and Upper Yangtze region of South China, there are well developed sets of marine shale strata. Currently only Wufeng Longmaxi shale gas has been developed in scale, while the Sinian and Cambrian shale gas are still under exploration. The various shale strata show different characteristics in lithological features, such as lithofacies types and reservoir physical properties, which are due to the influence of tectonic pattern, sedimentary environment, and diagenesis caused by tectonic subsidence. This will affect the later fracturing technology and fracturing effect. The shale reservoirs of Sinian doushantuo, Cambrian Niutitang, Upper Ordovician Wufeng, and the Lower Silurian Longmaxi Formation shale were evaluated and compared with each other with respect to their sedimentary environments, lithofacies, minerals compositions, and micro pore characteristics. The reservoir characteristics of the shale and the main control factors of shale gas enrichment were summarized in this study. [ABSTRACT FROM AUTHOR]

Pore and its structural characteristics are key parameters affecting shale gas reservoir development. Accurate quantitative characterization of shale pore and its structural characteristics is of great significance for evaluating shale reservoir state. In this study, 15 shallow marine shale samples were collected in Well Y108. X-ray diffraction results indicate that brittle minerals are the most common components in shale. In this paper, various pore types are classified and characterized by scanning electron microscope images. The total porosity of shale measured by the mercury intrusion method is between 3.2% and 6.5%. In addition, a petrophysical model is established to calculate matrix porosity and fracture. The results of this model are consistent with the measured porosity. Three key parameters (VTOC > VBri > VClay) were obtained. The low-pressure N2/CO2 adsorption experiment allows for the analysis of pore volume, specific surface area, and pore size. Finally, it was determined that the primary pore types and primary shale gas reservoir space in shallow marine shale are mesopores and micropores. The impact of shale constituents on pores and their structural properties is also covered in this work. The results indicate that the enrichment of total organic carbon and brittle minerals is conducive to the development of shallow marine shale pore-fracture system. Additionally, there is a positive linear relationship between matrix porosity, pore volume, specific surface area, average pore diameter, and surface porosity.

Fine dissection of microscopic pore structure variations between the Niutitang Formation and the Wufeng-Longmaxi Formation will help to improve the understanding of the underlying geological theory of shale gas in northern Guizhou Province. The stratigraphic, geochemical, physical, and tectonic properties of the two formations vary greatly, resulting in differential development of the microscopic pore structure among reservoirs and, as a result, major variances in gas concentration. To explore the mechanism of differential pore evolution, experimental techniques and instruments such as gas adsorption, liquid intrusion, SEM, XRD, and organic geochemical tests were utilized. The results indicate that the Wufeng-Longmaxi Formation is in a high-maturity stage, while the Niutitang Formation is in an over-mature stage. The latter has a higher TOC content. Both petrographic phases are siliceous shale petrographic phases, and the former has more developed dissolution pores with better pore volume, throat radius, and macropore pore diameters than the latter, as well as organic matter pores, intergranular pores, and microfracture structural parameters, whereas the specific surface area is the opposite. The differences in reservoir pore formation between the two formations were analyzed, and the results showed that the petrographic type, thermal evolution, and tectonic preservation conditions were the primary controlling elements of differential shale gas reservoir formation. A differential reservoir-forming model of the Wufeng-Longmaxi Formation and the Niutitang Formation was constructed, providing a geological and theoretical basis for shale gas geological exploration in northern Guizhou Province.

The pore characteristics of shale reservoir determine the shale gas content. This article uses three shale gas wells in Weiyuan area as the research object. Based on scanning electron microscopy and low-temperature liquid nitrogen adsorption experiments, the effect of shale lithofacies on the pore structure of the Wufeng-Longmaxi formation is studied by combining semi-quantitative and quantitative experiments. The results show that the shale of the LM1-LM3 interval in Weiyuan area located at the bottom of the Longmaxi Formation, is composed mainly of siliceous minerals, with high organic content. The LM4-LM8 interval located in the middle of the Longmaxi Formation has high content of clay minerals and carbonate minerals and low organic matter content. The shale pores in the LMl-LM3 interval are mainly provided by organic matter, and the shale pores in the LM4-LM5 interval are mainly provided by organic matter and clay minerals. Carbonate minerals have a certain contribution to the development of shale pores. The organic pores have relatively good homogeneity, which contributes the most to pores below lOnm. The internal pore development of carbonate minerals is highly heterogeneous, and the pore size is mainly distributed between 20 and 100 nm. The pores of clay minerals are highly heterogeneous, and the pore diameters are mainly distributed between 60 and 200 nm, which makes a great contribution to the pores of LM4-LM5 and LM6-LM8 formations. In the process of shale sedimentary diagenesis, fluid substances modify the soluble substances in the rock, providing space for the methane in the shale. [ABSTRACT FROM AUTHOR]

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In this work, a novel thermal–hydraulic–mechanical (THM) coupling model is developed, where the real geological parameters of the reservoir properties are embedded. Accordingly, nine schemes of CO2 injection well (IW) and CH4 production well (PW) are established, aiming to explore the behavior of free gases after CO2 is injected into the depleted Wufeng–Longmaxi shale. The results indicate the free CH4 or CO2 content in the shale fractures/matrix is invariably heterogeneous. The CO2 involvement facilitates the ratio of free CH4/CO2 in the matrix to that in the fractures declines and tends to be stable with time. Different combinations of IW–PWs induce a difference in the ratio of the free CH4 to the free CO2, in the ratio of the free CH4/CO2 in the matrix to that in the fractures, in the content of the recovered free CH4, and in the content of the trapped free CO2. Basically, when the IW locates at the bottom Wufeng–Longmaxi shale, a farther IW–PWs distance allows more CO2 in the free phase to be trapped; furthermore, no matter where the IW is, a shorter IW–PWs distance benefits by getting more CH4 in the free phase recovered from the depleted Wufeng–Longmaxi shale. Hopefully, this work is helpful in gaining knowledge about the shale-based CO2 injection technique. [ABSTRACT FROM AUTHOR]

Based on the accumulative pore volume and specific surface area by pore diameter of shale samples from JY-1 well of Wufeng-Longmaxi Formation and CY-1 well of Niutitang Formation, combined with the geochemical parameters and percentage of mineral components in samples, the paper analyzes the influence of shale components on the pore development differences between the two formations. The results show that TOC is positively correlated with micropores, indicating the organic micropores contribute to the development of shale pores, and organic micropores are more developed in the shale of Wufeng-Longmaxi Formation than that in Niutitang Formation. The positive correlation between quartz-pyrite contents and micropores indicates the primary pores and developed marginal pores contribute to the development of shale pores. The different correlations between quartz and meso- and macropores reveal the different protection degrees of quartz to the two by biogenic quartz content. The positive correlation between pyrite content and macropores reflects that pyrite can protect macropores to a certain extent in terms of pyrite content. The negative correlation between feldspar and pores shows the less obvious support of feldspar on pores due to the influence of compaction and complex structure. The unobvious correlation between carbonate minerals and pores shows the weak rigidity and instability of chemical properties and low content have no significant influence on pore development. Compared with Niutitang Formation, the high content of rigid minerals in the shale of Wufeng-Longmaxi Formation has a more favorable influence on the development of micropores (such as intergranular and marginal pores), mesopores and macropores. The negative correlations between clay minerals and micro- and mesopores indicate the intermineral pores easy to shrink under strong compaction have a negative effect on the pore development of reservoir, reflecting that the deep burial of Niutitang Formation has an unfavorable effect on the development of intermineral pores. [ABSTRACT FROM AUTHOR]

• Graptolite biozones linked to diverse minerals, lithofacies and paleoenvironment. • Sedimentation on organic matter: 0∼5m/Ma stable, >5m/a causes dilution. • Wufeng-Longmaxi formation's early-late stages' provenance changes shale's minerals. • The restricted sedimentary environment resulted in acidic pore water, limiting carbonate preservation. The shales show obvious heterogeneity during the Late Ordovician-Early Silurian in South Sichuan Basin, China. To elucidate the sedimentary control factors of the heterogeneity of the Wufeng-Longmaxi Formation shales and developmental model, we conducted petrological observations, X-ray diffraction analysis, and geochemical studies of the two wells. The findings indicate 10 graptolite biozones are identified in both two studied wells. Notably, the thickness, lithofacies, mineral composition, and total organic carbon content varied noticeably throughout these marine shales within each graptolite biozone. The WF1–LM1 shales mainly accumulated under anoxic conditions that lacked terrigenous input, which was beneficial for the preservation of organic matter. After the LM1 interval, the input of terrigenous material increased, the diluting effect on organic matter was increasingly enhanced. Significant dilution of organic matter occurred after the LM4 interval, when the apparent sedimentation rate increased to over 15.0 m/Ma (49.21ft/Ma). Additionally, the early-late stage of the Wufeng-Longmaxi Formation has different provenance. Within the WF1–LM1 interval, the terrigenous material was composed primarily of carbonate minerals. Above the LM1 interval, the contents of clay, felsic, and silicate clasts increased. In more anoxic conditions, the limited supply of detrital iron from the source region may have led to more acidic pore water conditions during early diagenesis, which restricted early carbonate cementation and resulted in a more clay-rich lithofacies. [ABSTRACT FROM AUTHOR]

Investigations into the connectivity and complexity of pore systems in shales are essential for understanding the flow of shale gas and the capacities of the associated reservoirs. In the present study, eight shale samples from the Wufeng-Longmaxi (WF-LMX) Formation that were collected from Well Yucan-6 in the southeast of the Sichuan Basin were analyzed for microstructural, pore network, and pore connectivity characteristics. The measurement results of low-pressure nitrogen adsorption illustrated that all shale samples contain micropores, mesopores, and macropores. Micropores and mesopores account for a high proportion of the total pores, and the dominant pore throat size is in the range of 2–6 nm. High-pressure mercury injection tests reveal that the porosity, total pore volume, and total specific surface area of pores for samples from the WF Formation are higher than those for samples from the LMX Formation. In spontaneous absorption experiments, the slopes of the absorption curves of n-decane (oil-wetting) and deionized water (water-wetting) in the WF and LMX Formations varied from 0.254 to 0.428 and from 0.258 to 0.317, respectively. These results indicate that shales in both formations exhibit mixed wettability characteristics, but lipophilic pores are better connected relative to hydrophilic pores. The total organic carbon and silica contents are the main factors controlling the pore connectivity in these shales, while the effects of other minerals are not significant. The findings of this work can improve our understanding of the pore structure characteristics of black shale.

The formation environment and preservation conditions of sedimentary organic matter (OM) play an important role in the accumulation of shale gas. In the present study, inorganic and organic geochemical data were analyzed to determine the origin and preservation environment of sedimentary OM in the Wc-1 well of the Wufeng–Longmaxi (WF–LMX) Formation in northeastern Chongqing, China. In a biomarkers analysis, the numerical characteristics of n -alkanes (n -C17/ n -C31>4.0), tricyclic terpenes (C23TT/C30H>1.0), and steranes (C27/C29St>1.0) suggested that the main origin of OM in the black shale was planktonic algae. High values of P/Ti and BaXS in the paleoproductivity indices suggested that primary productivity in the WF–LMX Formation was relatively high, peaking in the lower LMX Formation. Relative enrichment in U, V, and Mo, and the changing trends in V/(V+Ni) and Ni/Co suggested that the redox conditions for the bottom water, which changed from the WF Formation to the lower and upper LMX Formation, were oxic/dysoxic to anoxic and dysoxic, respectively. The relationship between total organic carbon and the above indexes indicates that different key factors controlled OM enrichment in the WF–LMX Formation. In the WF Formation, oxic bottom water was not conducive to the preservation of sedimentary OM. In the lower LMX Formation, the highest paleoproductivity and anoxic bottom water conditions promoted the enrichment and preservation of sedimentary OM. In the upper LMX Formation, excessive terrigenous inputs and relatively low paleoproductivity limited the enrichment of sedimentary OM. [ABSTRACT FROM AUTHOR]

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