Definition and distribution prediction of effective source rocks in Lishui Sag, East China Sea Basin

Lishui Sag, a sag with great exploration potential in the East China Sea Basin, has a low degree of exploration and few drilling results in recent years, so it is urgent to comprehensively evaluate its source rocks. Comprehensive evaluation of source rocks in Lishui Sag was carried out based on the Rock-Eval pyrolysis experiments. The results show that the kerogen type of the source rocks in the Sag is dominated by type Ⅲ, with a small amount of type Ⅱ₁ and Ⅱ₂ kerogen. Source rocks of Yueguifeng Formation are in mature to over mature stage.Most of the source rocks of Lingfeng Formation are in mature stage. While source rocks in Mingyuefeng Formation are dominantly in low maturity. According to the relationship between total organic carbon (TOC) content and pyrolysis parameter S₁, the base limits TOC values of effective source rocks were determined to be 0.7% in Yueguifeng Formation, 1.0% in Lingfeng Formation and 0.9% in Mingyuefeng Formation. Biomarkers show that the source rocks in Lishui Sag are characterized by a mixed origin of organic matter in a transitional environment. Prediction of effective source rocks shows that the effective source rocks in Lishui Sag is locally distributed, and the northeast of West Sub-sag in Lishui Sag is the concentrated distribution area of effective source rocks. The hydrocarbon generation process of source rocks took place in the Paleocene, with a relatively short time (about 4.5 Ma). Source rocks of Yueguifeng Formation and Lingfeng Formation have great gas potential. The exploration of gas reservoirs in the northeast of the West sub-sag of Lishui Sag should be strengthened.