The gas bearing and reservoir characteristics of finely divided lithofacies and dominant lithofacies: an example of Longmaxi Formation shale in northern Yunnan-Guizhou

The Yunnan-Guizhou-North region of the Sichuan Basin in Southern China is a favorable research area for southern marine stratigraphy. The classification and evaluation of shale lithofacies are essential for shale gas exploration and development. This study uses XRD analysis, thin section identification, geochemical analysis, gas content testing, scanning electron microscopy (SEM), high-pressure adsorption, and low-temperature N 2 adsorption. And machine learning methods to predict the shale lithofacies of uncored well sections systematically analyze the shale lithofacies of the Longmaxi Formation in the study area. The study showed that (1) the Longmaxi Formation in the study area mainly develops organic-rich mixed shale (ORM), organic-rich calcareous/siliceous mixed shale (ORM-1), organic-rich argillaceous/siliceous mixed shale (ORM-2), organic-fair argillaceous/siliceous mixed shale (OFM-2), organic-poor mixed shale (OPM), organic-poor argillaceous/siliceous mixed shale (OPM-2), organic-poor clay-rich siliceous shale (OPS-3). (2) ORM, ORM-1, and ORM-2 have high organic matter content and brittleness index (BI), pore-specific surface area, strong adsorption capacity, and generally higher gas content than 2 m 3 /t, with good gas content. The specific surface area of OFM-2, OPM, OPM-2, and OPS-3 is much smaller than the previous three organic matter-rich shale lithofacies, and the total pore volume is also lower. In contrast, their average pore size is higher than the previous three. The average pore size of OPM-2 is the largest, which is favorable to the aggregation of free gas, but its gas content is shallow. (3) The ORM, ORM-1, and ORM-2 shale lithofacies have high organic matter abundance, diverse storage space types, excellent pore structure parameters, high brittle mineral content, and high gas content, and are the shale facies with the advantages of integrated production and storage conditions in the Longmaxi Formation in the study area. The ORM-1 shale is the best shale reservoir facies, followed by the ORM and ORM-2 shale facies, and the combination of ORM and ORM-2 interbedded shale lithofacies is favorable for shale gas enrichment.