The characteristics and developmental control factors of microscopic pore structure in shale gas reservoirs in the Lower Cambrian Qiongzhusi Formation

Abstract

The Early Cambrian Qiongzhusi Formation on the southern margin of the Sichuan Basin (Southern China) has experienced a long geological process. High degree of thermal evolution of the organic matter shows a great impact on the organic evolution, reservoir microscopic pore types and structure characteristics in this formation. In this study, we investigate the major types and structural features of microscopic pores in shale gas reservoirs of the Qiongzhusi FM. Different types of matrix pores are found, over-mature shale gas reservoirs in the Qiongzhusi FM. mainly have well-developed micropores, but their average specific surface area (SSA) and pore volume are smaller than those in the Longmaxi Formation. The degree of thermal evolution poses the greatest impact on microscopic pore structure, which is significantly positively correlated with SSA and pore volume; however, when exceeding this range, SSA and pore volume both drastically decrease with increasing degree of thermal evolution. Compared with those in the Longmaxi FM., microscopic pores of shale gas reservoirs in the Qiongzhusi FM. feature pore- SSA and large surface porosity. The better the development of micropores, the larger the SSA and Vp of shales, which are more conducive to shale gas adsorption and accumulation.