Sedimentary structures of microbial carbonates in the fourth member of the Middle Triassic Leikoupo Formation, western Sichuan Basin, China

Abstract

In recent years, abundant natural gas have been found in microbial carbonates in the fourth member of the Leikoupo Formation in western Sichuan Basin. In this study, from the observation of 626 microbial thin sections, four types of microbial carbonates are classified based on the differences of meso-structures. Among them, thrombolites and stromatolites are subdivided into eight types based on the differences of micro-structures. Six types of microbial micro-structure association (MSA) are identified, and are mainly developed in microbial mounds. The energy of sedimentary environment and hydrodynamic conditions of them from low to high is MSA-5, MSA-1, MSA-6, MSA-3, MSA-4 and MSA-2. Because of the arid climate in the Annie Period, a restricted platform are developed in T2l43 in western Sichuan Basin, and the sedimentary faces are lagoon (gypsiferous lagoon or salt lake in evaporate conditions), microbial mounds, shoals, inner platform shoals and open sea from the east to the west. Microbial micro-structure not only affects the pore evolution of microbial carbonate reservoirs, but also affects the diagenesis of microbial carbonate reservoirs of the Lei-4.