Geochemical Characteristics and Depositional Environment of Coal-Measure Hydrocarbon Source Rocks in the Northern Tectonic Belt, Kuqa Depression

Abstract

A total of 21 samples were selected from a total of Jurassic coal-measure source rocks in the northern structural belt of the Kuqa Depression, in the Tarim Basin. By using a carbon-sulfur content analyzer, Rock Eval 7 rock pyrology instrument and gas chromatography–mass spectrometry (GC-MS), the abundance, type, depositional environment, and source of organic matter are confirmed. The factors that control the development of coal-measure hydrocarbon source rocks are analyzed. The pyrolysis parameters of rocks, such as total organic carbon (TOC), hydrocarbon generating potential (S1 + S2), hydrogen index (HI) and the highest pyrolysis peak temperature (Tmax), show that good to excellent coal-measure source rocks constitute the majority, and the types of organic matter are predominantly type III and occasionally type II. Low S/C ratio, high Pr/Ph value, and high C29 regular sterane levels suggest that the environment in which the coal-measure source rocks were deposited was oxidative, and the majority of the organic matter comes from higher terrestrial plants. In addition, the cross plot of isoprenoids with n-alkanes and the triangle diagrams of regular sterane also show that the primary source of organic matters in coal-measure source rocks is terrestrial higher plants. Because the shallow and turbulent water body is not easily stratified, the gammacerane content is low, which reflects the characteristics of low salinity. Combined with the cross plots of isoprenoids, it might be demonstrated that the freshwater environment is where the coal-measure source rocks were deposited. Comprehensive analysis shows that the oxidational and freshwater depositional environment is favorable to coal-measure source rock development.