Characteristics and occurrence mechanism of coal-measure shale gas reservoir of the Longtan Formation in the southern Sichuan Basin, China

The complex lithological assemblage of coal-measure shale leads to strong heterogeneity in its composition and microstructure. Key scientific issues such as the main controlling factors of pore development and the gas occurrence mechanism remain unclear. To further investigate the reservoir characteristics and the main controlling factors of gas content in the coal-measure shale of the Longtan Formation in the southern Sichuan Basin, samples from Well XM1 and the Yuping outcrop in Xingwen County were selected for organic geochemical analysis, scanning electron microscopy (SEM), nitrogen adsorption, and high-pressure methane adsorption experiments. These analyses aimed to study the hydrocarbon generation potential, reservoir physical properties, and methane adsorption characteristics of the shale. The results indicate that the coal-measure shale reservoirs in this area are characterized by high TOC (content mainly ranging from 2% to 4%, with a portion exceeding 4%), high clay mineral content (ranging from 35% to 90%, with a mean of 68%), high water saturation (mean value of 71.07%), low brittle mineral content (quartz content ranging from 10% to 60%, with a mean of 22%), and low gas content (mean value approximately 2.56 cm3/g). The study reveals that the storage mechanism of the Longtan Formation coal-measure shale is dominated by clay mineral pores. The organic matter is primarily Type III, with underdeveloped organic pores, and the pore system is mainly composed of mesopores. Pore volume and specific surface area show a significant positive correlation with clay mineral content (particularly illite-smectite mixed-layer minerals) but a weak negative correlation with TOC. The Longtan Formation coal-measure shale exhibits strong methane adsorption capacity (Langmuir volume VL ranging from 4.18 to 7.22 cm3/g), and the adsorbed gas content is positively correlated with both TOC and clay mineral content. However, the field-desorbed gas content is generally low and primarily controlled by TOC, confirming that organic matter abundance is the key factor governing gas content. The gas in the Longtan Formation coal-measure shale exists predominantly in adsorbed form, with a relatively low proportion of free gas. Carbonaceous shale and high-TOC shale are identified as favorable lithofacies. This study enhances the understanding of the enrichment patterns of coal-measure shale gas and provides theoretical guidance for its further exploration and development.