Characteristics and Genetic Mechanism of Dolomite Reservoirs:the case study of the Third Member of the Feixianguan Formation in the Puguang Gas Field, Sichuan, China

This study focuses on the dolomite reservoirs of the Third Member of the Feixianguan Formation in the Puguang Gas Field. Based on outcrop, core, and thin-section observations, combined with cathodoluminescence, scanning electron microscopy, and geochemical analyses, we systematically investigate the dolomite types, petrophysical properties, and pore systems of the reservoirs. Furthermore, the genetic mechanisms of dolomite reservoirs in the study area are discussed.The results indicate that dolomite is widely developed in the study area, primarily consisting of grain dolomite and crystalline dolomite. The main reservoir lithologies include grain dolomite, fine-crystalline dolomite, and silt-crystalline dolomite, with medium-crystalline dolomite being the least abundant. The reservoir space comprise intercrystalline pores, intergranular dissolution pores, oversized dissolution pores, intragranular pores, intragranular dissolution pores, and moldic pores. C-O isotopes exhibit positive δ¹³C and negative δ¹⁸O values, with Z-values around 130, confirming a saline marine origin. Major and trace elements reflect variations in seawater salinity and dolomitization intensity. High FeO and MnO contents, along with cathodoluminescence characteristics, indicate a reducing environment during early diagenesis. Rare earth element patterns show overall low concentrations with positive Ce anomalies, further supporting a predominantly reducing environment, though redox conditions vary slightly among different lithologies.Oolitic shoals and grainstone shoals constitute the material foundation for reservoir development, while dolomitization and dissolution are key factors in reservoir formation. The reservoir quality is jointly controlled by sedimentary environment, dolomitization, and dissolution processes. This study enhances the understanding of the genetic mechanisms of dolomite reservoirs in the Third Member of the Feixianguan Formation in the Puguang Gas Field, providing theoretical support for future exploration and development.