Sedimentary control on diagenetic paths of dolomite reservoirs in a volcanic setting: A case study of the Permian Chihsia Formation in the Sichuan Basin, China

Abstract

The formation mechanisms of multi-stage dolomites and hydrocarbon reservoirs associated with volcanic activity are complex. This study investigated the Permian Chihsia Formation in the Sichuan Basin, China, which is the location of recent discoveries of giant natural gas reserves. We undertook a petrological and geochemical study, including trace element and C–O–Sr isotopic analysis. The quality of the dolomite reservoirs associated with volcanic activity was controlled by different diagenetic pathways associated with the sediment types and fault systems. Outcrop, core, and thin-section observations identified three types of replacement dolomite: very finely to finely crystalline dolomite (Rd1), medium crystalline patchy dolomite (Rd2), and coarsely to medium crystalline dolomite (Rd3). The tidal flat sediments only experienced limited seepage reflux dolomitization, which resulted in the formation of the dense very finely to finely crystalline dolomite (Rd1). The bioclastic grain shoal sediments deposited in a high-energy shallow-water environment contain abundant intergranular and dissolution pores, and vugs, which allowed convection of seawater and dolomitization during the initial phases of volcanic activity of the late early Permian. Subsequently, the bioclastic grain shoal sediments were mostly replaced by coarsely to medium crystalline dolomite (Rd3), which formed the highest-quality reservoirs in the study area. In addition, the incompletely dolomitized medium crystalline patchy dolomite (Rd2) was formed due to the low original porosity of micritic, peloidal, grain shoal sediments during a transgression in the middle and late depositional stages. During the peak of volcanism at the end-Guadalupian, deep faults allowed seawater to mix with ascending hydrothermal fluids, which resulted in the precipitation of dolomite cement in pores near the fault fracture zones, which blocked the pores. Therefore, these high-energy shoal sediments deposited in the slope break zone far from strike-slip faults have a relatively high porosity. Our results provide new insights into the origin of dolomite reservoirs formed in association with a volcanic setting, and are applicable to deep dolomite reservoirs worldwide.