Influence of Permian basaltic volcanic activity on coal-bearing shale reservoirs, Southern Guizhou Depression

To investigate the influences of volcanic activities on shale reservoirs, a case study was carried out on the Permian Mount Emei basalt and coal-bearing shale of Longtan Formation in the Southern Guizhou Depression. Based on the analytical results of total organic carbon content, XRD, vitrinite reflectance and cryogenic nitrogen and carbon dioxide adsorptions, the effects of volcanic activities on the hydrocarbon generation, mineral composition and reservoir microstructure of coal-bearing shale were discussed. The results showed that volcanic activities had obvious effects on the hydrocarbon generation, mineral composition and pore structure of coal-bearing shale. They significantly promoted the evolution and maturation of organic matters in shale, which quickly entered the over-mature stage (Ro% increased from 2.0% to 2.88%), and accelerated the hydrocarbon generation and expulsion efficiency. With the decrease of the distance to the basalt, the contents of clastic minerals and carbonate minerals increased, the content of clay minerals decreased, and the contents of illite and chlorite changed regularly, which indicated that the evolution of shale diagenesis had also been affected to a certain extent. The heat brought by volcanic activities had an important influence on the pores of reservoirs. As it got closer to basalt, the proportion of micropores increased while the content of mesopores and macropores decreased. It was speculated that the thermal effects of volcanic activities promoted the formation of organic pores and also affected the development of inorganic pores.