Facies evolution and its controlling factors of the Pinghu Formation in the Kongqueting area of Xihu Depression, the East China Sea

The East China Sea Shelf Basin contains a huge amount of petroleum resources and the Pinghu Formation is one of the major exploration targets. However, the Kongqueting area has rarely been researched, and few studies have been made to sedimentary facies evolution and its controlling factors. Based on drilling and 3D seismic data, microfacies of the Pinghu Formation are carefully described in this paper and the controls of global sea level fluctuation, regional tectonics, paleoclimate, and paleogeography over the facies distribution patterns discussed. The Lower Pinghu Formation is dominated by tidal flat facies, and the braided river delta facies only appeared in some places near uplifts. By contrast, the Middle and Upper Pinghu Formations are dominated by deltaic deposits of braided rivers. Relative sea level falling, as a joint result of global sea level change and regional tectonics, resulted in the progradation of braided river delta towards offshore. Meanwhile, the high concentration of CO₂ in the atmosphere during the Middle and Later Pinghu periods caused a sharp increase of erosion. Enormous sediments provided by the source areas were transported into the Xihu Depression, which accelerated the progradation of braided river delta into the sea. In addition, the paleogeographic framework during the Middle to Lower Pinghu Periods were characterized by many fault-controlled sags, which limited the expansion of braided river delta. These sags were filled by sediments in Middle Pinghu Period, and turned to tidal flat in Later Pinghu Period, which also helped the expansion of deltaic deposits of the Upper Pinghu Formation.