Sequence lithofacies paleogeography and reservoir potential of the Maokou Formation in Sichuan Basin

Based on multidisciplinary analyses of outcrops, core description and log data, two types of sequence boundary are identified, including a partially exposed unconformity surface and a drowned unconformity surface. The Maokou Formation in the Sichuan Basin is divided into three third-order sequences, each lasting 3.3 Ma on average. In this study, system tracts are used as mapping unit to reconstruct systematically the lithofacies paleogeography map within the sequence framework, and to describe paleogeographic features. The results indicate that the Sichuan Basin is dominated by shallow, open marine carbonate platform deposits during the Middle Permian Maokou stage, in which a range of sedimentary facies developed, such as shallow water open platform, deeper water open platform, platform margin bank, and platform front slope facies. Physical property analysis indicates that shallow calcsparite granular limestone has good reservoir qualities, especially after the reservoir rock is reworked by dolomitization and epigenetic karstification. Thus, dolomite reservoir, paleokarst reservoir and shallow bank reservoir make up the main reservoirs of the Maokou Formation. The study on reservoir distribution patterns within the sequence framework indicates that HST reservoirs are relatively well developed, especially the shallow bank sedimentary regions of SQ2-HST in the western-southwestern Sichuan Basin and the paleokarst development area of SQ3 are the most favorable.