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

The Baota Formation of the Ordovician is a new and emerging hot spot for carbonate petroleum exploration in the Sichuan Basin. 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. Through a comprehensive analysis of outcrop, drilling and logging, seismic and thin section data, this study has clarified the distribution characteristics, sedimentary facies types, and distribution patterns of the Baota Formation in the Weiyuan area, and revealed the control of the "northwest high, southeast low" topographic pattern, governed by the Central Sichuan Paleouplift, on the sedimentary evolution of the Baota Formation in the study area. The research results show that the depositional environment of the Baota Formation in the Weiyuan area is significantly different from that in the southern and eastern (northern) Sichuan areas. Three sedimentary facies types, namely dolomitic platform (dolomite flat), tidal flat-associated reef-shoal complex, and restricted platform, are mainly developed in the study area. The dolomitic platform facies is dominated by fine to medium crystalline dolomite; the tidal flat-reef-shoal complex facies is characterized by thick layers of limestone, and a partial dolomitization zone in the form of a band is developed on the high ground along the windward side of the reef-shoal body, with dolomitic limestone and (partially) dolomitized limestone developed; the restricted platform facies is characterized by 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, it is clear that the Baota Formation in the Weiyuan area transitions from the northwest to the southeast from dolomitic platform and tidal flat-reef-shoal complex to restricted platform, and 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 further deepens the understanding of the Ordovician paleogeographic pattern in the Sichuan Basin and can provide a new theoretical basis for reservoir prediction in this area.