The main factors controlling the enrichment and high production of Da'anzhai member continental shale gas in Yuanba area

Abstract：Taking Da'anzhai Member lacustrine shale in Yuanba area of Sichuan basin as the study area and based on the sample analysis of the experimental data and the observation of drilling cores, the authors evaluated the conditions for the formation of continental shale gas in Yuanba area and discussed the main factors controlling the enrichment and high production of the continental shale gas. Da'anzhai Member shale in Yuanba area is characterized by large thickness, high abundance of organic matter, good type of organic matter, moderate thermal evolution degree, good gas-bearing reservoir properties and fracturing performance, suggesting favorable geological conditions for the formation of continental shale gas. In Yuanba area, shale gas content of Da'anzhai Member lacustrine shale is higher, the adsorption capacity is affected by temperature and pressure, TOC, and the thermal evolution degree (Ro) and mineral composition; it is characterized by positive correlation with pressure, TOC, silica and clay minerals, negative correlation with temperature, Ro, and insignificant relationship with carbonate minerals. The influence extent is in order of TOC>Ro>siliceous>clay minerals>carbonate minerals. Free gas is influenced by the development of pores and fractures, and has a positive correlation with porosity. Matrix porosity that can reflect the porosity development degree has positive correlation with TOC, clay mineral content, positive correlation followed by negative correlation with quartz and feldspar (maximum 40%), and negative correlation with the content of carbonate. The development degree of fractures was controlled by lithology, tectonics, sedimentary diagenesis and hydrocarbon generation. The main factors controlling the enrichment and high production of Da'anzhai Member shale gas in Yuanba area included sedimentary facies, lithofacies and the degree of fracture development. The dark shale of the shallow and semi-deep lake constituted the foundation of the enrichment and high production of lacustrine shale gas. The intercalated thin layers (or stripes) of limestone or sandstone with organic-rich shale in the shallow-semi deep lake were favorable for hydraulic fracturing in the later period. The fractures and the "baffle" layer controlled high production of continental shale oil and gas.