Sinian microbialites represent crucial archives for investigating the co-evolution of life and the environment after the Marinoan glaciation, but their porous facies also form significant hydrocarbon reservoirs worldwide. The giant gas accumulation in the fourth member of the Upper Sinian Dengying Formation (Deng-4 Member, Z 2 dn 4 ) in the Penglai area, Sichuan Basin, is hosted within such a reservoir. Reservoir development, however, is constrained by a complex interplay of tectono-sedimentary factors, generating notable heterogeneity. To elucidate the depositional model and the corresponding reservoir control mechanisms, this study presents a systematic analysis integrating core descriptions, petrographic thin sections, petrophysical measurements, and geophysical surveys. This investigation establishes a ‘step-like carbonate platform’ depositional model for the Deng-4 Member. The model reveals a multi-level, step-like palaeogeomorphic framework. This framework was shaped by the interplay of near north–south trending relict escarpments formed during earlier tectonic movements, and slope breaks created by near east–west trending, syn-sedimentary strike-slip faults. This complex palaeogeomorphology governed the type, evolution, and spatial arrangement of sedimentary facies, which in turn controlled the reservoir development and heterogeneity. This control operated through three principal mechanisms: Firstly, the sedimentary facies determined the origin and assemblages of pore spaces. Microbial mounds developed framework and vuggy porosity, enhanced by later bedding-parallel and honeycomb-like dissolution vugs. In contrast, grain shoals generated a ‘pin-point’ pore system comprising intergranular and intragranular dissolution pores. The inter-mound sea facies, composed of dense micritic dolomite, lacks effective reservoir space. Secondly, the facies framework directed the spatio-temporal differentiation of reservoir properties. The Deng-4 Member constitutes a single shallowing-upward depositional sequence, transitioning from relatively deeper-water grain shoals in the lower sub-member to widespread shallower-water microbial mounds in the upper sub-member. The vertical facies succession forced the locus of premium reservoir quality to migrate from the lower grain shoals to the upper microbial mounds. Thirdly, the sedimentary facies controlled the macroscopic reservoir distribution. The thickest effective reservoir zones correspond spatially to the dominant facies in each interval, matching the grain shoal distribution in the lower sub-member and the extensive microbial mound belts in the upper sub-member. The proposed depositional model provides a robust geological framework for predicting high-quality sweet spots in complex, deep to ultra-deep carbonate reservoirs.
Li et al. (Mon,) studied this question.
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