The multistage dissolution characteristics and their influence on mound-shoal complex reservoirs from the Sinian Dengying Formation, southeastern Sichuan Basin, China

The Sinian Dengying Formation is a typical gas-bearing marine carbonate reservoir in the Sichuan Basin with a great exploration potential. Based on outcrop and core observations, petrography, mineralogy, and petrophysical as well as geochemical analyses were performed to reveal the porosity system formed by a sequence of complex dissolution. This porosity system is the best reservoir space in the mound-shoal complex reservoirs of the Sinian Dengying Formation in the southeastern Sichuan Basin. The reservoir rocks mainly comprise two lithologies: granular dolomite and microbial dolomite. The reservoir spaces are dominated by various types of dissolution pores, caves, and fractures. Furthermore, the complex porosity system is characterized by an intense diagenetic control. The important dissolution events that influenced reservoir quality are as follows: (1) meteoric water leaching that occurred in micritic grains during the penecontemporaneous diagenetic period, forming intragranular dissolution pores and mold pores; (2) regional exposure karstification during a long subaerial diagenetic environment, which was afforded by tectonic uplift during the supergene period, showing features typical of karst fracture-cave systems formed by nonfabric-selective dissolution; (3) organic acid dissolution related to hydrocarbon generation occurred during the medium-deep burial period, showing typical characteristics of dissolution in recrystallized dolomite and mainly developing intercrystalline dissolution pores, intergranular dissolution pores, and dissolution expanded fractures; (4) hydrothermal alteration occurred in deep burial environments, which is characterized by the dissolution and expansion of pre-existing reservoir spaces, with saddle dolomite, quartz, and other hydrothermal minerals developed in partial caves. The superposition of meteoric water leaching and regional exposure karstification enhances the total porosity of the reservoirs. Furthermore, the organic acid dissolution and hydrothermal alteration change the quality and distribution of the reservoirs. This study aims to improve the understanding of dissolution in mound-shoal complexes and its effects on the reservoir development.