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Formation and Preservation Mechanism of Deep Carbonate Reservoir Under Tectonic-Fluid Coupling Alterations

Abstract

Affected by complex tectonic-fluid coupling alterations during diagenesis, reef or high-energy shoal facies carbonates could either develop into high-quality hydrocarbon reservoirs due to further dissolution as well as preservation of the original porosity, or on contrast completely lose reservoir capability due to heavy cementation. Therefore, identification of the types of fluid and the dissolution or cementation potential is the focused issue. In this study, the tectonic-fluid environments for the diagenesis of the deep Paleozoic carbonate reservoirs in the Sichan and Tarim Basin in China were determined based on geological and geochemical information, and mechanisms of dissolution, preservation and cementation were revealed, which is significant for oil and gas exploration in the deep carbonate reservoirs. Samples of various carbonate and filling minerals were obtained using a micron-scale micro-sampling technique with aid of cathodoluminescence. Based on the analyses of C, O, Sr and S isotopes, major, trace and rare earth elements and fluid inclusions, three major tectonic-fluid environments affecting the deep carbonate reservoirs were identified, including (1) tectonic uplift-meteoric waters, (2) tectonic subsidence-organic diagenetic and TSR-related fluids, and (3) fault-hydrothermal fluids. In the uplift-meteoric water dominated environment, reservoir spaces were developed predominately hundreds meters below exposed unconformity but were cemented in deeper layers. In the subsidence-TSR dominated environment, the original porosity in the reef or shoal facies belt tended to continuously increase or be well preserved due to increasing CO2 and H2S abundance. In the fault-hydrothermal dominated environment, reservoir spaces tended to develop in much deep strata.