Fig. 1 Sketch map showing geographical position of tectonic units and oil and gas fields in the Xihu Sag, East China Sea Shelf basin

The Xihu Sag is located northeast of the ECSSB and belongs to a secondary sag of the western geotectogene. The Xihu Sag is approximately 400 km long from south to north and 100 km wide from east to west; it has a total area of approximately 5.18 × 10⁴km². The sedimentary thickness in the sag is approximately 9–15 km, making it the largest sedimentary sag in the ECSSB; in particular, the sedimentary characteristics indicate a great potential for oil and gas exploration and development as observed in previous studies.

The sedimentary strata drilled in the Xihu Sag were primarily the Paleogene and Neogene strata, during which there were many depositional hiatuses and regional unconformities. The Paleogene Pinghu Formation in the Xihu Sag underwent multiple tectonic movements and was deposited at the stage of tectonic thermal subsidence in the post-rift period. Consequently, the primary sedimentary facies are clastic rocks of the transitional phase. The Pinghu Formation is divided into three members from bottom to top, and each member has obvious sedimentary cycles of fine sediment in the bottom and coarse sediment on the upside. The lithology primarily comprises of yellow fine sandstone, gray siltstone, gray mudstone, and coal, with a relatively fine grain size. The distinct lithostratigraphic characteristics of the Pinghu Formation in the ECSSB were affected by tectonic and sedimentary patterns, and the overall lithology is finer from west to east (Zhang et al., 2006; Zhu et al., 2012; Lin et al., 2017; Cheng et al., 2019; Wang et al., 2021).