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Seismic Prediction of Dolomized Reef and Shoal Reservoirs in Permian-Triassic Changxing and Feixianguan Formations, Eastern Sichuan Basins, China


This study shows how high-resolution reef- and shoal-carbonate facies can be interpreted and thin dolomized reservoirs can be predicted by integrating outcrop, core, wireline log and 3D seismic data in a seismic-sedimentologic workflow in Permian-Triassic Changxing and Feixianguan Formations in Longgang field, Sichuan, China. Outcrop and well data revealed the major depositional facies of the carbonate sequences, including evaporite platform, open platform, platform-margin reef or oolite shoal, slope, and basin. A high-quality 3D seismic data set provided crucial spatial coverage for study of lithology, diagenesis, geomorphology, and depositional history. Development of dolomized reservoirs are inferred to closely relate to high-frequency sequence frameworks and can be tied to exposure of platform margin during sea-level falls. To reconstruct a high-frequency sequence framework, we generated seismic-stratal slices among time-equivalent sequence boundaries and maximum-flooding surfaces. With each selected stratal slice representing a high-frequency sequence and being tied to facies analysis in sparse wells, seismic-geomorphologic patterns can be interpreted in terms of depositional facies and elements. Viewing of stratal slices in the order of geologic time revealed migration of facies along paleoshore lines, which were controlled by high-frequency sea-level cycles. A seismic-lithologic analysis was performed by using core-lab data, wireline logs, and seismic models, which correlated multiple thin-bedded dolostone reservoir units to amplitude and other seismic attributes. Sweet spots in domolized reef and shoal carbonates can be predicted in the facies and attribute maps with variable reliability.