Diagenesis of an evaporite-related carbonate reservoir in deeply buried Cambrian strata, Tarim Basin, northwest China

Abstract

Many hydrocarbon shows have been reported from potential evaporite-related carbonate reservoirs in the lower to middle Cambrian units in the Tarim Basin. Petrography, facies, and geochemical analyses from outcrop and core samples were integrated to document the effect of diagenetic evolution on these evaporite-related carbonate reservoirs. A simplified depositional model has been developed that reveals a restricted carbonate platform dominated by an evaporatic inner platform lagoon, with shoal and reef facies developed around the platform margins. Grainstones and packstones were deposited at the platform margin and on inner platform shoals, whereas carbonate mudstones and wackestones were deposited in the lagoon associated with evaporites. Early dolomitization and dissolution occurred related to the reflux of evaporated seawater. Breccia-associated fracture porosity caused by a short period of meteoric water-induced dissolution is the predominant type of effective porosity in the anhydrite-bearing dolomudstone reservoir. Thermochemical sulfate reduction (TSR) occurred in the deep subsurface diagenetic environment characterized by high-temperature (100°C–160°C) and high-salinity (16–26 wt. %) aqueous fluid inclusions in TSR calcite that has relatively negative δ13C values (as low as −12‰ Vienna Peedee belemnite) and elevated 87Sr/86Sr values. The porous dolomites contain solution-enlarged pores caused by dissolution of anhydrite and carbonate as a secondary consequence of TSR. This study shows that deeply buried, evaporite-related, ancient carbonate rocks may be effective reservoirs because of a combination of fracture porosity and TSR-induced secondary porosity. Evaporitic Cambrian carbonates buried to over 5000 m (16,500 ft) can thus be considered as targets for future exploration.