Characteristics and formation mechanisms of dolomites in the Middle Permian Maokou Formation in southeastern Sichuan Basin

Breakthroughs in oil and gas exploration have been made recently in dolomite reservoirs of the Middle Permian Maokou Formation in southeastern Sichuan Basin. However, reservoir heterogeneity in the Maokou Formation is high, which makes it difficult to identify high-quality reservoirs without a clear understanding of the dolomitization mechanisms. Based on sedimentary facies of the region and hydrothermal events associated with the Emeishan Large Igneous Province, dolomites in the Maokou Formation are characterized and their formation mechanisms are summarized through lithological and geochemical analyses. In the Maokou Formation fine-medium-crystalline dolomite is the main type, followed by micritic dolomite and dolomite cements. In the Qijiang area stratified, fine-medium-crystalline dolomite is mainly found. The euhedral to subhedral dolomite crystals appear cloudy at the center and bright around the edges, with obvious graining lines; under cathodoluminescence (CL) microscope they appear brown-red at the interior and dark-red toward the exterior. The δ18OV-PDB, δ13CV-PDB and 87Sr/86Sr ratio values of finemedium- crystalline dolomite indicate it was formed by seepage-reflux dolomitization at medium salinity during shallow burial and later underwent recrystallization and seawater-metasomatism. The Tailai area mainly occur porphyritic- micritic and fine-medium-crystalline dolomites, along with dolomite cements found in rock fractures. Dolomites are compact with no obvious grain structures, while dolomite cements are mainly saddle dolomite with wavy extinction, with multiphase ring-and-ribbon patterns visible under CL microscope. Isotopic data indicate dolomites in this area were formed during shallow burial under seawater environment, and late-stage thermal fluids flowing over clastic rocks directly contributed to the precipitation of dolomite cements and calcite. In general, the key reservoir-controlling factors of the Maokou Formation in southeastern Sichuan Basin are epigenetic dissolution, dolomitization during shallow burial, metasomatism and karstification; and seepage-reflux dolomitization during shallow burial is the main formation mechanism of dolomites. © 2023 Science Frontiers editorial department. All rights reserved.