Lithofacies paleogeography of the Ordovician and its petroleum exploration potential in the Middle-Upper Yangtze Area, South China

Based on the drilling, logging and field analysis, this paper discusses the lithofacies paleogeography of the Ordovician and its petroleum potential in the middle-upper Yangtze area, South China. Results show that Ordovician of the Middle-Upper Yangtze region can be divided into Tongzi Formation, Honghuayuan Formation, Meitan Formation, Shizipu Formation, Pagoda Formation, Linxiang Formation and Wufeng Formation from bottom to top. During the early Ordovician Tongzi period and Honghuayuan period (Tremadocian stage), a carbonate rimmed platform developed in the study area with lots of grain shoals in Guangyuan-Weiyuan areas and Lichuan-Tongzi areas of Southeastern Sichuan basin. To the Meitan period (Floian and Dapingian and early Darriwilian stage), a mixed carbonate platform with clastic sedimentary rock deposition developed in study area. In middle- late Ordovician Shizipu and Linxiang-Pagoda period (late Darriwilian and Hirnantian-Sandbian stage), a carbonate ramp developed in Middle-Upper Yangtze region. At the end of the Ordovician (Hirnantian stage), Wufeng Formation deposited in a retention basin due to the restriction of peripheral uplift and paleo-land. Two sets of reservoir-source assemblages developed in the Ordovician, with three sets of source rocks developed in the study area. First, the Lower Cambrian Qiongzhusi Formation acted as the main source rock, and the hydrocarbon migrated upward to the Ordovician reservoir along the fault. Second, the Wufeng-Longmaxi Formation acted as source rock, and hydrocarbon migrated to the Lower Ordovician along the karst crust and the fault. Third, the Lower Ordovician Meitan-Shizipu Formation acted as source rock, hydrocarbon can migrate upward to the Upper Ordovician reservoir directly, which deserves exploration attention.