Mechanisms of fabric-selective microbialite karst: a case study of second member of Sinian Dengying Formation, Sichuan Basin

The microbialite karst reservoirs in the second member of Sinian Dengying Formation, Sichuan Basin, are a recent exploration hotspot, and important exploration breakthroughs have been achieved in the Penglai and Daxingchang areas. However, the understanding of dissolution mechanisms and distribution patterns of microbialites remains unclear. Based on profile measurement, thin-section identification, scanning electron microscopy, and energy-dispersive spectroscopy, dissolution experiments were conducted on microbialites with different fabrics to simulate the fabric-selective karst process and investigate the karst mechanisms of microbial dolomite. The experimental results showed that the dissolution rates of microbial dolomite were significantly higher than those of crystalline dolomite, and significant differences were observed among different microbial fabrics, with the average dissolution rate decreasing in the order of thrombolitic dolomite > stromatolitic dolomite > microbial conglomeratic dolomite > microbial arenaceous dolomite > laminated dolomite > micritic dolomite > silt-crystalline dolomite. On this basis, three stages of epigenetic karstification of microbial dolomite were identified. At the early stage, soluble minerals on the rock surface were characterized by preferential dissolution, with dissolution commonly developing along structural fractures. The middle stage was the main dissolution stage, characterized by differential dissolution of microbial fabrics within the rock, mainly occurring along intra-fabric and inter-fabric pores. The late stage involved the slow dissolution of residual matrix grains. The study reveals that differential dissolution based on microbial fabrics is the key to the formation of high-quality karst reservoirs. Thrombolitic dolomite, stromatolitic dolomite, and microbial sandy/gravelly dolomites in mound cores and mound bases are the dominant lithofacies of karst reservoirs, and the second member of Dengying Formation is a typical facies-controlled epigenetic karst reservoir.