Characteristics and controlling factors of dolomite karst reservoirs of the Sinian Dengying Formation, central Sichuan Basin, southwestern China

Abstract

The Sichuan Basin is a typical superimposed oil- and gas-bearing basin with abundant hydrocarbon resources in deeply buried marine carbonate rocks. The petrological characteristics, origin, diagenetic evolution and controlling factors of the high-quality Sinian Dengying dolomite reservoirs were investigated based on outcrop and core observations in addition to data from various analyses, including thin section, casting thin section, scanning electron microscopy, cathodoluminescence, carbon and oxygen isotope, major and trace elements, and X-ray diffraction analyses. The main lithologies of the Dengying reservoirs are microbial dolomite, crystalline dolomite, granular dolomite and karst breccia dolomite. The reservoir spaces include microbial pores, interparticle dissolution pores, intraparticle dissolution pores, intercrystalline pores, karst caves and fractures. Three models to explain the origin of Dengying dolomites are proposed: (1) syngenetic microbial dolomitization, (2) penecontemporaneous reflux dolomitization and (3) burial dolomitization. Development of the high-quality Dengying dolomite reservoirs in the central Sichuan paleo-uplift was essentially controlled by two factors: sedimentation and supergene karstification. High-quality reservoirs were developed mainly in algal mound and grain-shoal sediments deposited on the platform margin and intraplatform margin. Affected by the multistage Tongwan movement, long-term weathered crust karstification resulted in the formation of a large number of dissolution-enlarged pores and caves in the Dengying reservoirs. Multistage fractures, especially the unfilled tectonic fractures that formed during the late Yanshanian–Himalayan period, connected the relatively isolated pores and karst caves and effectively improved the permeability of the reservoirs. The results of this study provide a deeper understanding of the origin and preservation mechanism of deeply buried paleocarbonate reservoirs and can also effectively guide future exploration in the Sinian Dengying Formation of the Sichuan Basin.