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The deep Longmaxi Formation shale in the Zigong area is widely distributed and has a huge resource potential, which has enabled it to become one of the most important replacement areas for shale gas exploration in China. This article investigates the key shale gas evaluation well, Well Z1, in the Zigong area using field emission-scanning electron microscopy, mercury intrusion capillary pressure, low-pressure N2/CO2 adsorption, and fractal theory to determine the pore systems of the different lithofacies of the deep O3w–S1l shale. The mineral composition and total organic carbon (TOC) content (1.79%– 8.16%/3.48%) are highly heterogeneous, and five types of shale lithofacies including mixed shale, mixed siliceous, clay-rich siliceous, argillaceous/siliceous shale, and mixed carbonate shale were identified. The deep shale mainly develops interpores and microcracks associated with the organic matter (OM)–mineral (carbonate, pyrite, and clay minerals) aggregates and a small number of mud pores. The surface areas and total pore volume vary from 16.29 to 41.76 m2/g and from 1.19 to 3.49 cm3/100 g, with averages of 25.03 m2/g and 1.99 cm3/100 g, respectively. The pore distribution exhibits significant multimodal and fractal characteristics. The distribution morphology, peak values, and fractal dimensions vary with lithofacies. The mesopores provide the main pore volume (average of 53.99%), and the micropores provide the main surface area (average of 73.95%). The TOC content and biogenic quartz are beneficial to the development of micro- and mesopores. In the Zigong area, mineral aggregates composed of OM and brittle minerals (quartz, feldspar, and carbonate) are more favorable for pore development than the clay mineral–brittle mineral aggregates.

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.

This paper focuses on Longmaxi shale gas geochemistry and carbon isotopic reversal in Changning and Fuling gas fields through comparative study of shale gas composition and carbon and hydrogen isotopes in North America and Changning and Fuling gas fields. Longmaxi shale gas in Changning and Fuling gas fields exhibits the features of dry gas. Specifically, the average methane (CH 4) content is 98.72 and 98.17%, respectively. The humidity is less than 0.5%. Nonhydrocarbon gases include a small amount of CO 2 and N 2. Extremely heavy d 13 C 1 value, average d 13 C 2 value of À33.3 and À34.6% for Changning and Fuling, and sapropelic organic matter indicate the properties of petroliferous dry gas. Carbon isotopic reversal, i.e. d 13 C 1 >d 13 C 2 >d 13 C 3 , may be caused by combined secondary effects at high maturity and high geotemperature. The reversal may also be related to ethane Rayleigh fractionation and late methane generation by water and transition metals reaction. Geologic setting in these two gas fields may have an impact on carbon isotopes distribution.