Comprehensive evaluation of Cretaceous source rock maturity in medium and small fault depressions in southern Songliao Basin: a case study of Zhangwu and Changtu fault depressions

A comprehensive evaluation of the maturity of the source rocks in the Cretaceous Shahai and Jiufotang formations in the Zhangwu and Changtu fault depressions of the southern Songliao Basin was made using several indicators, including vitrinite reflectance, hydrocarbon conversion rate, OEP, sterane isomerization and hopanoid distribution. The dynamic evolution process of source rocks with burial depth was described. The hydrocarbon generation threshold is different between the southern and northern fault depressions, resulting in the various maturity stages of source rocks. The hydrocarbon generation threshold of source rocks in the Zhangwu Fault Depression in the south is about 1 000 m. Generally, the source rocks in the Shahai Formation are in the immature and low maturity stages. The source rocks in the upper Jiufotang Formation are in the low maturity stage, while those in the lower part are in the mature stage. The hydrocarbon generation threshold of source rocks in the Changtu Fault Depression in the north is about 1 800 m. Generally, the source rocks in the lower Shahai Formation and the Jiufotang Formation are in the low maturity and mature stages. The middle and lower parts of the Jiufotang Formation in the southern fault depressions developed effective source rocks. The Jiufotang Formation in the northern fault depressions developed effective source rocks, and the middle and lower parts of the Shahai Formation also contributed.