Geochemistry of source rocks in the Es3 and Es4 members in the Shahejie Formation of the Western Depression,Liaohe Oilfield,China

The Western Depression of the Liaohe Basin is the major exploration area of the Liaohe Oilfield, and its main source rocks consist of the third and fourth members of the Shahejie Formation (Es₃ and Es₄). These source rocks are widely distributed in the depression, with semi-deep lake and fan delta as the main sedimentary facies, brown oil shale and black gray-dark gray mudstone as the main rocks, and a total thickness of 270–1450 m. The kerogens are mainly of the types I and II_A, and partly of the type IIB and least of the type III. The R ^o values range from 0.4%–0.8%, indicating an evolution stage from immature to mature. The maturity of Es₄ source rocks is relatively high, reaching the early mature stage, but their distribution and thickness are lower than those of Es₃. Besides, according to biomarker analysis, it is thought that the source rocks of Es₃ and Es₄ are characterized by mixed input, and most of the source rocks were formed in the brackish water-saline and strongly oxygen-free environment. Furthermore, the Qingshui, Niuxintuo and Chenjia sags are believed to possess greater potential for hydrocarbon generation and expulsion, for they are source rocks with a larger thickness, have higher organic carbon contents, belong to better organic matter types and possess higher maturities.