Characteristics and main controlling factors of lacustrine source rocks of Yueguifeng Formation in the Jiaojiang Sag, East China Sea Shelf Basin

In the 1990s, oil and gas were discovered after drilling into the Jiaojiang Sag, East China Sea Shelf Basin, revealing the prospects for oil and gas exploration. However, since then, there has been little research on this sag, especially the lack of systematic research on the source rocks, which hinders the exploration process. To clarify the characteristics and development model of the source rocks of the Yueguifeng Formation in the Jiaojiang Sag and provide the basis for deploying further exploration wells, we summarized the tectonic and sedimentary background of the Yueguifeng Formation, and analyzed the characteristics of the source rock using geochemical data including organic petrology, biomarkers, and trace elements. Based on that, the controlling factors of the enrichment of organic matter in the Yueguifeng Formation were discussed from the aspects of paleo-productivity, preservation conditions, and sedimentation rate, and a model showing the formation of the source rock was established. Results show that the source rocks in the Yueguifeng Formation are dark gray lacustrine mudstone deposited in the continental half-graben, with kerogen type II1-II2, and the abundance of organic matter was rated good to very good in the industry standards. Source rocks deposited during the lake expansion are the thickest and have the widest distribution and the best quality. The development of high-quality source rocks requires abundant algal material, semi-closed stable water under slightly brackish conditions, and a moderate sedimentation rate. Source rocks are developed in both hanging wall and dip slope under the background of a graben shaped fault depression, but the types and abundance of the two are different. The depocenter developed good source rocks of middle-deep lacustrine facies, which was mainly contributed by algal organisms and formed in a relatively reduced environment. In contrast, the dip slope developed medium to shallow lacustrine facies, which was mainly contributed by terrestrial plants and formed in a relatively oxidized environment.