Pore Characteristics and Methane Adsorption Capacity of Different Lithofacies of the Wufeng Formation–Longmaxi Formation Shales, Southern Sichuan Basin

Abstract

The significantly different geological processes experienced by the shales in the southern Sichuan Basin provide a means for comparing their pore characteristics and their supercritical methane adsorption capacities. In this study, we analyzed the Wufeng Formation (O3w)–Longmaxi Formation (S1l) shales in the southern Sichuan Basin using multiple analytical methods, including mineralogy, field emission scanning electron microscopy (FE-SEM), mercury intrusion capillary pressure (MICP), low-pressure gas adsorption (LPGA), and high-pressure methane adsorption. Four types of shale lithofacies were identified including clay-rich siliceous shale (S-3), argillaceous/siliceous mixed shale (M-2), silica-rich argillaceous shale (CM-1), and mixed argillaceous shale (CM-2). With increasing burial depth, the pore system gradually shifted from organic matter (OM) pores to intergranular (interP) pores between the OM clumps and the brittle minerals. The results of the MICP and LPGA were not significantly affected by burial depth, but were closely related to the total organic carbon (TOC) content and lithofacies. The surface areas and total pore volumes (V p’s) varied from 5.44 to 11.73 m2/g with an average of 9.89 m2/g and from 0.88 to 2.04 cm3/100 g, respectively. Patterns in the V p and surface area values of the S-3 and M-2 samples exhibited four-modal features, while the CM samples were bimodal. The excess adsorption capacity reached its maximum value when the pressure was about 9–11 MPa, and then it decreased as the pressure increased. The mesopores provided the main V p (average 43.1%), and the micropores provided the main surface area (average 74.38%). The adsorption positions of the gas molecules can be provided by micropores and mesopores, and even by microcracks and macropores, and as the TOC content of the shale increases, the contribution of the micropores to adsorption becomes more significant.

Establishing a New Piecewise Method for Understanding and Rectifying Mass Balance Miscalculation of Gas Adsorption on Coal and Shale

“…The multiparameter fitting methods, ,,,, such as the modified three parameter Langmuir model ,, and the multiparameter Dubinin–Radushkevich (DR) model, , are accustomed to calculating the adsorbed phase density and further calibrating the Gibbs adsorption isotherm. In contrast to the piecewise method, the multiparameter fitting method still can only obtain a constant density of adsorbed phase.…”

Deformation Differences in Complex Structural Areas in the Southern Sichuan Basin and Its Influence on Shale Gas Preservation: A Case Study of Changning and Luzhou Areas

“…The buried depth of shale in the Luzhou area is generally more than 3,500 m, the thickness of high-quality shale is between 20 and 80 m, the sedimentary environment is the deep-water shelf, and the lithology of high-quality shale is mainly organic siliceous shale and graptolite shale. The reservoir space includes primary matrix pores, hydrocarbon generation pores, and microfractures (Casini et al, 2016;Feng et al, 2020;Wang et al, 2020). The porosity of the high-quality shale section is between 2.35% and 7.84%, with developed foliation and good transverse permeability.…”

Factors Affecting Shale Gas Chemistry and Stable Isotope and Noble Gas Isotope Composition and Distribution: A Case Study of Lower Silurian Longmaxi Shale Gas, Sichuan Basin

“…WY and CN are in the Emeishan large igneous province region (Figure 3). The thermal evolution of source rocks in the WY and CN areas was strongly affected by the ELIP [62][63][64].…”