Characteristics and diagenetic evolution of dolomite reservoirs in the Middle Permian Qixia Formation, southwestern Sichuan Basin, China

Great breakthroughs have been recently made in exploring oil and gas in the Middle Permian Qixia Formation, the southwestern Sichuan Basin, China. These discoveries in dolomitic reservoirs revealed abundant hydrocarbon reserves, agreeing on traditional predictions associated with dolomite prospect, which also left the controversy about the nature of dolomitization. Therefore, it is particularly important to research dolomite reservoirs characteristics and their genesis. In this study, comprehensive analyses, including pore-casted thin-section observation, scanning electron microscopy, physical property analysis, mercury injection measurement, fluid inclusion test, and isotopic measurement, are carried out to characterize Qixia Formation dolomites that are sampled from five outcrop sections and nine wells, followed by an investigation into controlling factors of these characteristics and establishment of pore evolution models. Granular dolomites, especially fine-grained and coarser dolomites, are demonstrated to be the best reservoir rocks, with their reservoir capacity positively related to dolomite grain sizes. There are mainly four types of pore-throat structures, namely large pores with wide throats, medium pores with medium throats, small pores with narrow throats, and fracture pore-throat structure. Dolomite reservoirs in the study area have experienced complex diagenetic changes, including the meteoric water dissolution in the penecontemporaneous stage, dolomitization in the shallow-burial stage, and hydrothermal- and formation water-related dissolution during the middle-deep burial stage. High-quality reservoirs are mainly distributed in the shoal facies deposits, especially where both dolomitization and multi-stage dissolution have occurred.