Lithofacies Assemblages, Source-Reservoir Characteristics, and Gas Enrichment Mechanisms of the Permian Longtan Formation Shale in Central Hunan Province, South China

The marine-continental transitional shale of the Upper Permian Longtan Formation is widely distributed in Hunan and shows significant exploration potential. Frequent changes in lithofacies have however notably influenced the shale gas enrichment. The strata of the Longtan Formation in the Shaoyang Depression, central Hunan, were taken as the study object for this project. Three lithofacies assemblages were identified: shale interbedded with sandstone layer (SAL), sandstone interbedded with shale layer (ASL) and laminated shale layer (LSL). The SAL shale shows significant variability in hydrocarbon generation potential, which leads to shale gas characterized by ‘hydrocarbon generation in high total organic carbon (TOC) shale, retention in low TOC shale and accumulation in sandstone’. The ASL shale, influenced by the redox conditions of the depositional environment, shows a lower concentration of organic matter. This results in an enrichment model of ‘hydrocarbon generation and accumulation in shale, with sealing by sandstone’. The laminar structure of LSL shale causes both quartz and clay minerals to control the reservoir. Shale gas is characterized by ‘hydrocarbon generation in mud laminae, retention and accumulation in silty laminae, with multiple intra-source migration paths’. In the marine-continental transitional shale gas system, the enrichment intervals of different types of shale gas reservoirs exhibit significant variability.