Diagenetic systems of Cambrian DEPS in Eastern Sichuan basin and geological significance

Abstract

To determine the diagenesis and evolution of the Cambrian DEPS (dolomite-evaporite paragenetic system) in the eastern Sichuan basin, the isotopic geochemistry characteristics of C, O, and fluid inclusions were studied. Four diagenetic systems were determined in the Cambrian DEPS based upon the hydrological system. These include the pore brine during the penecontemporaneous stage, sealed brine during the early diagenetic stage, compaction hot brine during the middle and late diagenetic stage, and mixed hydrothermal fluids involving deep thermal water during the later tectonic uplift stage. The character and source of fluids of various diagenetic stages and systems have a certain familiarity and inheritance features of development and evolution. However, fluids of various diagenetic stages and systems have different effects on the reservoirs. The diagenetic systems and manner of diagenesis are related to reservoir development closely. These include burial dolomitization and dissolution of sealed brine of the early diagenetic stage, and TSR (thermochemical sulfate reduction) and recrystallization of the mixed hydrothermal fluids involving deep thermal water during the tectonic uplift stage. Analyzing the basin structure indicates that a series of faults caused by the Cambrian detachment layer in the Eastern Sichuan Basin are the dominant factor controlling the spatiotemporal coupling relationship between the diagenetic systems and the reservoir in DEPS. The shoal subfacies control the regional distribution of the reservoir. During the early diagenetic stage, buried dolomitization helped with continued reservoir development, while TSR and recrystallization in the tectonic diagenetic stage improved reservoir quality by enhancing porosity and permeability. The Cambrian DEPS strata and the underlying Qiongzhusi Formation mud shale form a high-quality source-reservoir-caprock assemblage in the eastern Sichuan Basin.