Molecular geochemical characteristics of source rock in faulted lacustrine basin: A case study of the Lishu Fault Depression in southern Songliao Basin, China

Terrestrial faulted lacustrine basin is considered as a favorable place for the development of source rocks, especially the fault basins in eastern China. Based on molecular composition study of saturated and aromatic hydrocarbons in the extracts of source rocks of the Yingcheng and Shahezi formations in the Lishu Fault Depression, it is revealed that the extracts of source rocks are provided with low ratio of Pr/Ph, low content of C₂₄ tetracyclic terpanes, high content of tricyclic terpanes and gammacerane, relatively high content of C₂₇ steranes in the sag belt and its periphery; relatively high ratio of Pr/Ph, relatively high content of tetracyclic terpanes and gammacerane, low content of C₂₇ steranes and obvious advantages of C₂₉ steranes in its gentle slope belt; with high ratio of Pr/Ph, high content of C₁₉, C₂₀ tricyclic and C₂₄ tetracyclic terpanes, very low content of gammacerane and C₂₇ steranes, and high content of C₂₉ steranes in the edge of fault depression. According to the organic matter input and sedimentary environment, three types of source rocks have been established, which, respectively, are the mode of mixing organic matter input in deep and semi-deep lake, the advantage of terrigenous input in shallow lake and terrigenous input in shore-shallow lacustrine. The first mode is supposed to generate oil and gas, while the second one tends to generate oil. The distribution characteristics of oil and gas in the Lishu Fault Depression are very likely to be controlled by hydrocarbon generating characteristic of different source rock types.