Adsorption of gas is one of the ways that shale gas exists in the transitional shale (TS). The adsorption mechanism and the impact of water saturation on methane adsorption affect the estimated accuracy of the gas in place (GIP). This work constructs a TS molecular model comprising type III kerogen, kaolinite, illite, and quartz by using the molecular simulation (MS) according to the physical properties of the TS components. This is verified by the gas adsorption experiments of the TS and its components. The study establishes the methane adsorption model on the TS surface, the impact of the TS with varying water saturation on methane adsorption, and microscopic mechanisms of the TS methane adsorption. The experimental and simulated results show that it is feasible to construct a TS molecular model based on TS’s realistic porosity and density. Methane molecules in different mineral components of the TS are not characterized by monomolecular layer adsorption, mainly including micropore filling, transitional phase adsorption, and monomolecular layer adsorption. The transitional phase layer includes the strong adsorption layer (SAL), adsorption layer trough (ALT), and weak adsorption layer (WAL). Micropore filling and transitional phase adsorption coexist in the kerogen of the TS, and transitional phase adsorption primarily occurs in the illite of the TS. Kaolinite and quartz of the TS are featured with monomolecular layer adsorption. Moreover, quartz does not increase the pore space under high pressure because of stable silicon-oxygen chemical bonds. The chemical bonds in the kerogen and clay of the TS are relatively plastic, yielding the larger pore space under high pressure. Moreover, when water saturation exceeds 30% in the TS, water molecules cover the TS pore surface, affording the minimal methane adsorption capacity (MAC). In short, the micromechanisms of methane adsorption are determined by the TOC, adsorption model, water saturation, pressure, and isosteric heat of the TS components. The TS model can improve the accuracy of methane adsorption simulation and better understand the micromechanisms of methane adsorption. The establishment of the TS model and the impact of water saturation on methane adsorption also improves the estimated accuracy of the GIP.

Triassic Xujiahe source rocks, the main gas source of shallow tight gas, are the most typical continental coal-bearing source rocks in the Sichuan Basin, South China. However, the organic matter enrichment section cannot be identified easily, leading to limited progress in the exploration of coal-bearing tight gas. This paper reveals the main controlling factors of the organic matter enrichment, reconstructs the evolution process of the Xujiahe palaeosedimentary environment, proposes a dynamic enrichment mechanism of the organic matter, and determines the organic matter enrichment section of the high-quality coal-bearing source rocks by geochemical characteristics of the source rocks, major elements, and trace elements. The results show that the Xujiahe sedimentary environment can be divided into a fluctuating stage of transitional sedimentation, stable stage of transitional sedimentation, fluctuating stage of continental sedimentation, and stable stage of continental sedimentation. The Xujiahe source rocks were featured with high-quality coal-bearing source rocks with high total organic carbon and maturity and good parent material in the stable stage of transitional sedimentation and fluctuating stage of continental sedimentation, in which the water was connected with the Palaeo-Tethys Ocean with abundant terrestrial organisms. The water was shallow in the fluctuating stage of transitional sedimentation with a low sedimentation rate, leading to poor organic matter enrichment. The Palaeo-Tethys Ocean withdrew westward from the Yangtze plate in the late period of the fluctuating stage of continental sedimentation, leading to the absence of algae and dinosteranes and a decrease in biological productivity in the stable stage of continental sedimentation. Therefore, high terrestrial inputs and biological productivity and high sedimentation rate were conducive to the organic matter preservation in the coal-bearing source rocks.

The pore characteristics are studied in the overmatured marine-continental transitional (MCT) shale and simulated shale under different thermal maturity conditions, based on transitional and simulated shale samples in the eastern margin of Ordos Basin. The work uses high-pressure mercury intrusion (MICP), field emission scanning electron microscopy (FESEM), helium-mercury method, X-ray diffraction of whole-rock minerals, and hydrocarbon-generating thermal simulation to quantitatively analyze pore characteristics and main controlling factors of pore development. The results show that the shallow bay and lake facies (SBLF) shale has great exploration potential, while the delta facies (DF) shale has poor exploration potential. The SBLF shale is mainly characterized by pie shale, high quartz and carbonate, low clay, high porosity, and pore volume. The DF shale mainly develops dot shale with low quartz and carbonate content, high clay content, low porosity, and pore volume. Kaolinite has the strongest inhibition on MTC shale pore development. The pore volume of MTC shale decreases first and then increases with maturity. The pie shale is more conducive to the increase of pore volume than the dot shale. The effect of doubled TOC on porosity is greater than that of maturity in the dot shale. The effect of doubled TOC on porosity is less than that of maturity in the pie shale. Organic matter (OM) has the greatest impact on pore development, controlled by the OM content, sedimentary facies, and maturity. OM content, sedimentary facies, and maturity can be used to jointly characterize the MTC shale pore development, providing guidance for multiparameter quantitative characterization of pore development and determining the enrichment area of shale gas.