Differential diagenetic evolution of Sinian Dengying Formation reservoirs in Anyue gas field, Central Sichuan Uplift, Sichuan Basin

Large in-situ pyrolysis gas reservoirs have developed in the dolomites of the Sinian Dengying Formation in the Central Sichuan Uplift, Sichuan Basin. Investigating the effects of sedimentation and diagenesis on these reservoirs provides important insights for evaluating and predicting ancient deep carbonate reservoirs. Through hand specimen observation, thin-section identification, cathodoluminescence, and scanning electron microscopy (SEM) of core samples, the study systematically analyzed the lithological variations, diagenetic types, diagenetic sequences, and the constraints imposed by different lithologies on diagenetic processes in the dolomite reservoirs of the fourth member of the Dengying Formation. The results showed that the main lithologies of the Dengying Formation reservoirs in the Central Sichuan Uplift were algal stromatolite dolomite with a skeleton-filling structure, granular dolomite, and mud-fine crystalline dolomite mainly composed of fine-grained dolomite accumulations. The main types of diagenesis followed the sequence of compaction, dissolution, and cementation during the syngenetic-penecontemporaneous and supergene stages, sparry cementation at the burial stage, and late-stage hydrothermal activity. Hydrocarbon charging occurred after sparry cementation, while hydrocarbon pyrolysis coincided with hydrothermal activity. The dolomite reservoirs in the fourth member of the Dengying Formation were algal mound platform deposits formed by typical T-type carbonate factories. The algal stromatolite dolomite and granular dolomite experienced weak compaction but stronger cementation and dissolution during the syngenetic-penecontemporaneous and supergene stages. Sparry cementation significantly reduced porosity, and residual intergranular pores were preserved until the hydrocarbon charging period. With the intrusion of late-stage hydrothermal fluids, these pores were further filled with hydrothermal minerals and pyrobitumen formed through pyrolysis. In contrast, the mud-fine crystalline dolomite underwent strong compaction and weaker cementation during the syngenetic-penecontemporaneous and supergene stages. Sparry cementation further reduced porosity, and residual pores were also preserved until liquid hydrocarbon charging. No obvious signs of hydrothermal intrusion were observed in mud-fine crystalline dolomite, but the high temperature of hydrothermal fluids still promoted liquid hydrocarbon pyrolysis, leading to pyrobitumen formation.