Origin of a microbial-dominated carbonate reservoir in the Lower Cambrian Xiaoerbulake Formation, Bachu-Tazhong area, Tarim Basin, NW China

Abstract

Deep and thick Cambrian dolostone reservoirs are important and potential petroleum exploration targets in the Tarim Basin. To improve understanding of the origin of deeply buried microbial-dominated carbonate reservoir of the Lower Cambrian Xiaoerbulake Formation in Bachu-Tazhong area, the influences of microfacies types and carbonate diagenesis on porosity development were carefully investigated using petrographic observations and geochemical analyses from outcrop and core samples. Fifteen microfacies types (MF1-15) were identified in (1) tidal flat, (2) lagoon, (3) ramp-crest shoal, (4) sheltered proximal middle-ramp, (5) microbial mounds-biostromes proximal middle-ramp, and (6) distal middle-ramp environments on a gentle dipping carbonate ramp. A third-order depositional sequence consists of transgressive and highstand systems tract corresponding to the lower and upper member of the Xiaoerbulake Formation respectively. The HST section was characterized by extensively high-energy ramp-crest shoals and microbial mounds-biostromes proximal middle-ramp with moderate–good reservoir quality, while the TST section was composed of distal middle ramp and scattered microbial mounds with poor-non reservoir quality. The vertical cyclic pattern of porosity of microbial-related and dolo-grainstone microfacies types resulted from frequent exposure to meteoric leaching due to high-frequency relative sea level changes, as demonstrated by the diagenetic sequence. Main pore types are intergranular, vuggy pores, and microbial-related framework pores. Dolomitization occurred relatively early in shallow diagenetic environments, which may have improved the resistivity of carbonate rocks against chemical compaction. Preservation of porosity was likely attributed to a high-pressure closed diagenetic system under the middle Cambrian evaporites. The subsequent faulting and uplifting may have locally destroyed the reservoirs and increased the reservoir heterogeneity. This work would provide new sights for better understanding of the microbial-dominated carbonate reservoirs in deeply buried Lower Cambrian strata worldwide.