Xianming Xiao

The water adsorption behavior of marine overmature kerogen is a theoretical foundation for the shale gas exploration and development and CO2 geological sequestration. However, this issue remains poorly understood, especially regarding graphitization effects. This study investigates water vapor adsorption (WVA) behavior for a suite of Lower Cambrian kerogen samples with a maturity range of 3.23–3.98% EqRo (equivalent vitrinite reflectance) under different temperatures and relative humidity (RH) conditions through thermodynamic and kinetic models. The results indicate that, over the maturity range investigated in this study, the WVA capacity of kerogen is closely related to the maturity-governed structural evolution. With increasing maturity, the adsorption capacity first decreases progressively, then increases markedly, with the turning point of 3.44–3.55% EqRo, corresponding to the graphitization threshold. Thermodynamic analysis reveals that the affinity of kerogen to water vapor and the spontaneity of the WVA process significantly enhance as maturity across this EqRo value. Furthermore, the driving force for secondary adsorption undergoes a fundamental transition from “entropy reduction control” to “enthalpy-driven dominance”. This variation may be associated with the attenuation of residual polar sites in kerogen, the development of graphitic structures, and the restructuring of the pore network. The double-exponential model exhibits the best compatibility with variations in RH, temperature, and maturity. The kinetic behavior of WVA in response to maturity also exhibits a similar inflection as well as RH dependence. At low RH, it is predominantly governed by the evolution of surface chemical properties, whereas at high RH, it is controlled by the evolution of pore structure. This work elucidates the enhancing effect of graphitization on adsorption behavior from both thermodynamic driving forces and kinetic mass-transfer pathways.

Shale gas production from the Longmaxi Formation in the Sichuan Basin, southern China, faces rapid decline and challenges in maintaining stable output. The upper part of the Long-1 Member is considered a potential replacement interval, yet detailed investigations into its reservoir heterogeneity characteristics and gas content controlling mechanisms remain lacking. This study conducted geochemical analysis, connate water content measurement, porosity testing, FE-SEM imaging, and low-pressure N₂/CO₂ adsorption experiments on core samples from the upper and lower Long-1 Member in the Jiaoshiba gas field. Multifractal characteristics were analyzed, and their influence on gas content was compared and discussed. The results show that the upper interval is jointly influenced by organic matter pores, clay mineral interlayer pores, and carbonate cementation, with water likely occurring as clay-bound water, exhibiting higher structural heterogeneity at the non-micropore scale. In contrast, the lower interval is affected by carbonate mineral dissolution, with dissolution pores and microfractures developed, and water likely occurring as water clusters within organic matter pores, showing stronger heterogeneity at the micropore scale. Furthermore, in the upper interval, higher pore uniformity and continuity correspond to higher gas content, whereas in the lower interval, higher micropore heterogeneity and more pronounced dominant pore size characteristics correspond to higher gas content. These results reveal significant differences in the heterogeneity characteristics of the upper and lower intervals of the Long-1 Member at different pore size scales and their distinct responses to shale gas enrichment, providing a reference for detailed comparison in three-dimensional shale interval development and for the evaluation of lower-quality shale intervals.

The Lower Cambrian shale has been considered an important target for shale gas exploration in South China. However, systematic research on the differences in pore structure and their controlling factors in shales from different sedimentary facies is still lacking, which hinders the accurate prediction of favorable areas. In this study, pore types, structural parameters, heterogeneity and connectivity of the Lower Cambrian shale samples from the shallow shelf, deep shelf and slope were qualitatively and quantitatively characterized by using field emission scanning electron microscope (FE-SEM), low-pressure CO₂ adsorption (LPCA), low-pressure N₂ adsorption (LPNA) and multifractal theory, in order to reveal the reservoir characteristics and differences among these sedimentary facies. The results show that organic pores dominate the pore space in shales from different sedimentary facies, but their morphology and abundance are controlled by the depositional environment. The shallow shelf shales are characterized by scattered organic pores and abundant intragranular pores. Deep shelf shales feature uniformly distributed bubble-like organic pores and have the highest non-micropore volume. Slope shales contain the most developed micropores, with organic pores mainly occurring within organic matter (OM). Total organic carbon (TOC) is the primary factor controlling pore development. It affects pore connectivity and heterogeneity by regulating the development of pore space. Micropore connectivity increases with TOC content. In contrast, non-micropore connectivity first increases and then decreases as TOC increases. The influence of mineral components on shale pore structure varies with sedimentary facies. Clay minerals promote micropore development in shallow and deep shelf shales by adsorbing abundant OM. Carbonate minerals inhibit micropore development through diluting TOC content and cementing pore. However, dissolution of carbonate minerals can contribute to non-micropore space in shallow shelf shales. Deep shelf and slope shales with TOC between 4% and 6% have optimal pore volume, high connectivity, and low heterogeneity, and are the main intervals for forming high-quality shale reservoirs. This study clarifies the pore structure characteristics and controlling factors of shale reservoirs in different sedimentary facies, providing a theoretical basis for predicting and exploring shale gas sweet spots in the Lower Cambrian of South China.

The Lower Cambrian shale gas resources in southern China have huge potential. However, the reservoirs generally contain a relatively high content of nonhydrocarbon gases, which has a significant impact on the exploration and development of shale gas. The source of nonhydrocarbon gases in the Lower Cambrian shales has become a research hotspot. Based on recently published data, this paper systematically summarizes the geological and distribution characteristics, gas-bearing properties, and genesis mechanism of Lower Cambrian shale gas in southern China. The Lower Cambrian shales in southern China are widely distributed in the Yangtze Platform. The overall total organic carbon (TOC) content is relatively high. The kerogen type is mainly type I, and the equivalent vitrinite reflectance (EqRo) value ranges from 2.5 to 6.0%. The mineral components of the Lower Cambrian shales are mainly quartz, clay minerals, and carbonate minerals, and the main lithofacies types are mainly siliceous shale and mixed (siliceous-clay) shale. The gas-in-place (GIP) content of the Lower Cambrian shales varies greatly in different regions. Currently, shale gas, mainly composed of CH₄, has only been discovered in the southwestern Sichuan, western Hubei, northeastern Chongqing, and northern Guizhou areas, while in other Yangtze regions, the GIP content of shales is very low or almost nonexistent. The distribution of δ¹³C₁, δ¹³C₂, and δ¹³C₃ in the Lower Cambrian shale gas shows a significant inversion. Among the nonhydrocarbon gases, N₂ mainly originates from atmospheric and/or pyrolysis, CO₂ is mainly of organic origin, while He exhibits typical shell-source characteristics, which may be from the ancient basement and/or the radioactive decay accumulation of U and Th in the shale. On this basis, an evolution model of the Lower Cambrian shale gas was constructed. Hydrocarbon generation in the Lower Cambrian shales can continue until EqRo = 3.5%, after which N₂ is mainly produced. CO₂ is mainly formed in the low mature stage, and the He content shows continuous generation throughout the shale evolution process.

Current models of sedimentary environments and organic matter (OM) enrichment for the Lower Cambrian black shales in the Yangtze Platform have not yet incorporated its northern carbonate platform margin where the related research is lacked. This study focuses on the SNZ1 well in the northern carbonate platform margin, utilizing total organic carbon (TOC) content and major and trace element data to reveal the main controlling factors of OM enrichment during the Early Cambrian. The results show that the shale stratum is tentatively ascribed to the Lower Cambrian Stage 3 and that, during its deposition, the redox transitioned from anoxic to suboxic–oxic conditions, the hydrodynamic conditions weakened initially and then strengthened, the primary productivity first increased and then decreased, the paleoclimate shifted from arid–cold to warm–humid conditions, and the terrigenous clastic input gradually diminished. Overall, the OM enrichment is primarily controlled by preservation conditions. By systematically analyzing the data from the intraplatform basin to the deep-sea basin across the Yangtze Block, a model of the sedimentary environments and OM enrichment during the Early Cambrian was suggested. Additionally, this study highlights the intrinsic link between the expansion of oxygenated surface water and the Cambrian explosion. These results provide critical insights for shale gas exploration in this region.