Dolomite Reservoir Differences in Characteristics and Controlling Factors of Grainstone Shoal facies in Qixia Formation, Sichuan Basin

Abstract The grain shoal facies dolomites of the Qixia Formation in the Sichuan Basin exhibit extensive distribution, diverse types, and significant variations in reservoir quality. Based on core observations, thin section identifications, and geochemical analysis data, this study systematically compares the characteristics of platform-margin shoal and intra-platform shoal dolomite reservoirs in the Qixia Formation. Results demonstrate that platform-margin shoal dolomites feature greater single-layer thickness (average > 5 m), stronger and more complete dolomitization (dolomite content > 85%), characterized by burial dolomitization, with developed dissolution vugs and intercrystalline pores (average porosity 2.9%). In contrast, intra-platform shoal dolomites show thinner single layers (average < 3 m), relatively lower and incomplete dolomitization (dolomite content 30%-60%), dominated by penecontemporaneous dolomitization, with reservoir spaces mainly composed of residual intergranular pores (average porosity 1.9%), exhibiting inferior physical properties compared to platform-margin shoal facies. Dolomite reservoirs with prolonged and complete dolomitization processes demonstrate better physical properties. The platform-margin shoal facies have higher energy, superior initial physical properties, and stronger fluid mobility compared to intra-platform shoal facies, providing more favorable diagenetic-reservoir forming conditions for multi-stage dolomitization. This holds significant implications for predicting favorable exploration zones of grain shoal facies dolomite reservoirs.