Discussion and significance of evidence for Paleocene strata by drilling in Yingcuixuan area of Xihu Sag

The Xihu Sag of the East China Sea Shelf Basin is characterized by thick Cenozoic sediments. Extensive research has been conducted on the geological conditions and hydrocarbon accumulation of the Eocene, Oligocene, and Miocene strata, with limited studies on the Paleocene strata. Recent studies indicate that the Paleocene strata in the Xihu Sag have significant hydrocarbon generation potential and constitute an important source rock system, which plays a key role in oil and gas generation and accumulation in the Xihu Sag. To clarify the development characteristics of the Paleocene strata and their implications for oil and gas accumulation, this study analyzed data from well J-1 in the Yingcuixuan area, located in the northern slope segment of the Xihu Sag. Four lines of evidence supported that well J-1 has penetrated the Paleocene strata: ⑴ regional seismic correlation suggested that the deep layers of well J-1 exhibited medium-to-high frequency, medium-to-weak amplitude, and medium-to-low continuity reflections, with distinct stratigraphic folding. The strata below the T40 seismic reflection interface showed truncation features, which are typical seismic facies characteristics of the top of the Paleocene strata. ⑵ Lithological assemblage comparison revealed that the lower section of well J-1 contained marker layers of reddish-brown mudstone, indicating a Paleocene lake-delta depositional environment. ⑶ Palynological comparison showed that the Paleocene sporopollen characteristics in the study area were similar to those in the Changjiang Sag, both lacking marine foraminifera. ⑷ During the Paleocene, a large fault-depression structure formed, controlled by basement faults. The downthrow side of a fault provided favorable geological conditions for the formation of thick Paleocene strata. By analyzing the geochemical characteristics of the Paleocene source rocks in well J-1 and comparing them with geochemical indicators from other sags in the East China Sea Shelf Basin, it was concluded that the Paleocene strata in the Xihu Sag host medium-to-good source rocks with significant hydrocarbon generation potential. This study provides valuable guidance for oil and gas exploration deployment in the Xihu Sag. The findings suggest that the area near the depression in the Yingcuixuan area is favorable for large-scale oil and gas exploration due to the development of thick Paleocene source rocks (dark mudstone).