Abstract:
To investigate the unknown coexistence relationships and controlling factors for coal measure gas in the Upper Permian of the South Sichuan area, we methodically studied the characteristics of the reservoir lithological assemblage, the mode of formation and the factors controlling the formation of coal gas by using the Markov chain method, the analysis of logging and recording wells, and the experimental test. The results show that the Upper Permian coal measure in the South Sichuan area is well developed, with remarkable characteristics of coal, mud and sand interbedding, which has favorable geological conditions for the co-generation of coalbed methane, shale gas and dense sandstone gas. The study area mainly develops four types of lithological syngenetic assemblages, namely coal-mud-coal, sand-mud-sand, mud-sand-coal and sand-mud-coal. There are three main types of coal gas co-production and storage modes: mode a is coal shale gas-coal tight gas-coal bed methane co-production combination, which is commonly found in the top and middle of Longtan Formation, with weak potential for gas formation; mode b is multi-dimensional coal gas co-production combination, which is developed in the middle of Longtan Formation, with general potential for gas formation, but with the characteristics of multi-dimensional gas collection; mode c is coal shale gas-coal bed methane co-production combination, which is located in the lower part of Longtan Formation, with the highest gas content and excellent preservation conditions. Mode c is a coal shale gas-coal bed methane syngenetic assemblage, located in the lower part of Longtan Formation, with the highest gas content and superior preservation conditions. Coal gas formation and storage is controlled by the combination of sedimentary environment, petrogenesis, hydrogeological conditions and tectonic background, among which the sedimentary environment is the most important controlling factor, controlling the development of biogenic storage and cover assemblage, while the tectonic background provides the conditions for enclosure, petrogenesis affects the physical properties of the reservoir, and the hydrogeological conditions assist in the preservat of coal gas through the provision of confinement conditions.