Type and distribution of Mid-Permian Maokou Formation karst reservoirs in southern Sichuan Basin, SW China

Abstract

Based on the analysis of the responses of conventional logs such as natural gamma (GR), density (DEN), acoustic interval transit time (AC), compensated neutron (CNL), dual lateral resistivity (Rlld, Rlls), and caliper log (CAL), combined with drilling data, cores, thin section and productivity of 65 wells, the reservoirs in the Mid-Permian Maokou Formation of southern Sichuan Basin were divided into four types, fractured-vuggy, pore-vuggy, fractured and fractured-cavity. The main reservoirs in high productivity wells are fractured-vuggy and pore-vuggy. The reservoirs of Maokou Formation are generally thin, and can be divided into the upper reservoir segment (layer a of the second member to the third member of Maokou Formation, P2m2a-P2m3) and the lower segment (layer b of the second member of Maokou Formation, P2m2b). The two reservoir segments are mainly controlled by two grain beaches during the sedimentation of P2m2a-P2m3 and P2m2b, the vertical zonation of karst, and the fractures. The upper reservoir segment is generally better than the lower one in development degree and single well productivity, and is much thicker than the lower one. It is thicker in the Yibin-Zigong-Weiyuan-Dazu area, the southwestern area of Chongqing and the southeastern area of Luzhou, while the lower segment is thicker in the Neijiang-Zigong-Luzhou area and the Dazu-Luzhou area. The areas with big reservoir thickness at tectonic slope or syncline parts are the favorable exploration areas. Key Words: southern Sichuan Basin, Mid-Permian, Maokou Formation, karst reservoir, cavity, fracture, grain beach, logging response