Sedimentary characteristics and sedimentary model of marine carbonate rocks of the Ordovician Baota Formation in Weiyuan area, Sichuan Basin

The Upper Ordovician Baota Formation is a new and emerging hot spot for carbonate petroleum exploration in the Sichuan Basin, SW China. For a long time, there have been insufficient studies on the depositional environment of the Baota Formation in the Weiyuan area, which has restricted the progress of petroleum exploration and development in this area and the understanding of the Ordovician paleogeographic pattern in the Sichuan Basin. By integrating outcrop analysis, drilling, logging, seismic, and thin section data, we clarified the sedimentary facies types and distribution patterns of the Baota Formation in the Weiyuan area, and revealed that the paleo-topographic scheme of “high in the NW and low in the SE” was governed by the Central Sichuan Paleouplift in the sedimentary evolution of the Baota Formation. Results show that the sedimentary environment of the Baota Formation in the Weiyuan area is significantly different from those in the southern and eastern (northern) areas of Sichuan Province. Three main sedimentary facies types, i.e, dolomitic platform (dolomite flat), tidal flat-associated reef-shoal complex, and restricted platform, were recognized. The dolomitic platform facies is mainly fine to medium crystalline dolomite. The tidal flat-reef-shoal complex facies is thick-bedded limestone, in which a partial dolomitization belt was developed in windward high terrain area of reef-shoal body, with dolomitic limestone and (partially) dolomitized limestone developed. The restricted platform facies is mainly interbedded micritic and grainy limestone, with common horizontal bedding and bioturbation structures and typical desiccation cracks, indicating restricted water circulation. Through the analysis of the vertical evolution and lateral comparison of sedimentary facies, we believe that the sedimentary scheme of the Baota Formation in the Weiyuan area transitioned southeastward from dolomitic platform and tidal flat-reef-shoal complex to restricted platform. The dolomite of the dolomitic platform facies and the partial dolomitization zone on the windward side of the tidal flat-reef-shoal complex facies are favorable reservoirs (prospective reservoir intervals). This study deepened our understanding of the Ordovician paleogeography in the Sichuan Basin and provided a new theoretical basis for reservoir prediction in this area.