Abstract: A large amount of crude oil from the fourth Member of Shahejie Formation (Es4) has been discovered around the
Chezhen Sag, making it an important hydrocarbon-generating layer. Understanding whether the source rocks in the Chezhen Sag also have high hydrocarbon-generating potential is of significant importance for guiding further exploration and development. The systematic study involves selected drill cores from various sub-sags and using technical methods such as Rock-Eval pyrolysis, kerogen microscopic examination, thermal simulation experiments, basin modeling and so on. The analysis reveals the clear difference in the biogenetic origins, sedimentary environment and hydrocarbon generation potential of source rocks from the Es4 in the eastern and western parts of Chezhen Sag. In the western sub-sags (Chexi and Tao’erhe), the parental sources are mainly derived from mixed organic matters and formed in a freshwater-reducing depositional environment. In contrast, in the eastern sub-sags (Dawangbei and Guojizi), the biological sources mainly came from low aquatic organisms, indicating a saline-reducing environment, with Dawangbei Sag showing higher salinity. The source rocks of the Es4 generally range from poor to good, and mainly constitute Type II kerogen with a low-maturity to mature level with regard to hydrocarbon-generating. Thermal simulation experiments show maximum oil and gas generation of 256.4 mg/g. TOC and 671.15 mg/g. TOC. The source rocks of the Es4 in the Chezhen Sag exhibit characteristics of two hydrocarbon generation stages: early and late stages, with thresholds and peaks at approximately 2800 m and 4400 m, respectively. The source rocks of Es4 in the Chezhen Sag are buried at relatively shallow depth, with late hydrocarbon generation, reaching maturity and beginning large-scale oil production in the early Quaternary, and are currently in a major phase of hydrocarbon generation. In summary, the source rocks of the Es4 in the Chezhen Sag show significant heterogeneity, with eastern sags generally having higher quality source rocks than the western sags.

Key words: biogenetic derivation and sedimentary environment, hydrocarbon generation potential, source rock, the fourth
member of Shahejie Formation, Paleogene, Chezhen Sag