ICE 2022


The Bohai Bay Basin is a crucial petroliferous basin in eastern China. It accounts for 20% of China's petroleum resources and 40% of China's proven reserves. Its annual crude oil production exceeds 70 million tons. Qikou sag is one of the most petroliferous areas and has been discovered significant proven oil reserves and undiscovered resources, which is a highly-explored area of petroleum exploration in Bohai Bay Basin, China. The most essential oil-producing formations of this sag is the Shahejie formation (Es) and Dongying formation (Ed). The organic matter abundance of source rocks is evaluated by TOC commonly, and the fundamental to hydrocarbon generation in source rocks is the OM. The values of TOC indicates that source rocks in the Es formation are better than that in the Ed formation. Besides, rock pyrolysis parameters are used in evaluating the quality of source rocks. According to the organic matter type classification diagram of HI-Tmax, the organic matter type of source rocks in Es formation are type II1 and type II2 kerogen. While in Ed formation, kerogen composition ranges from type I to type III. The GC and GC−MS analyses were carried out on extracts from source rocks. Low Pr/Ph ratios indicate that source rocks in Qikou sag deposited in a suboxic and weak reducing environments. C29 dominates over C27 and C28 steranes in Es and Ed formations. Tmax values of source rocks reflect early mature-mature character. The low to high 22S/(22R + 22S) homohopane, 20S/(20R + 20S), and ββ/(αα + ββ) sterane ratios and moretane/hopane ratios, indicate early mature-mature character for the samples. Gammacerane is used as an indicator of a salinity-stratified water column. Low Pr/Ph ratios often coexist with high gammacerane index values in the source rock extracts deposited under anoxic saline environments. This is the situation in our study, as suggested by the gammacerane index in Es formation higher than that in Ed formation, while Pr/Ph ratios in Es formation lower than that in Ed formation. In addition, in terms of the trend of the gammacerane index, the paleosalinity of the water body first increased and then decreased. The abundant organic richness, humic kerogen, and medium thermal maturity demonstrate that these rocks are effective oil and gas source rocks. The source rocks in the Es and Ed formations collectively constitute a petroleum generation system and represent the main exploration target for oil and gas discovery for the Qikou Sag.