Division of the Diagenesis Stages of the Lower Permian Maokou Formation in Hechuan-Tongnan Area

The high-quality reservoirs of the Lower Permian Maokou Formation in the Hechuan Tongnan Block are mainly dolomite reservoirs, mainly composed of (residual grain) fine crystalline dolomite and medium crystalline dolomite. The type of reservoir space is mainly dissolution fractures and caves, and diagenesis is extremely critical in the formation process of the reservoir. In order to solve the problem of unclear effects of different diagenesis on reservoir reconstruction in the Lower Permian Maokou Formation, combined with mineralogy, chronology, and elemental geochemistry analysis, the types and evolution sequences of diagenesis are established, and combined with pore evolution characteristics analysis, the impact of different diagenesis on reservoir space is determined. Based on comprehensive analysis of rock slices, cathodoluminescence, and homogenization temperature of inclusions, seven types of diagenesis have been identified, and a diagenetic sequence has been established for stage I calcite filled fractures, stage I quasi syngenetic shallow burial dolomitization, stage II dolomite filled corrosion fractures, asphalt filling, dedolomitization, stage III calcite filled fractures, and stage III dolomite filled structural fractures. Based on core characteristics, cast thin sections, normal trace elements, inclusion homogenization temperature, and component analysis techniques, the Maokou compositional diagenesis stage is divided into three stages: syngenetic, early diagenetic, and intermediate diagenetic. Five pore evolution stages are established, including stage one of karstification pore formation, stage two infiltration and backflow filling pores, stage three early dolomitization pore preservation, and stage four tectonic superimposed fluid transformation to increase porosity in the reservoir stage five filling reduce porosity. The study and stage division of diagenesis are of great significance for providing geological basis for oil and gas exploration in the Hechuan-Tongnan area.