Organic geochemical characteristics and generating potential of source rocks from the Lower-Middle Jurassic coal-bearing strata in the East Junggar Basin, NW China

The Junggar Basin located in NW China is a large, intracontinental superimposed basin. Samples were collected from the Lower-Middle Jurassic coal-bearing strata of the eastern Junggar Basin and include mudstones, carbargilites and coal. The origin of organic matter, thermal maturity, paleoenvironment of sediment deposition and hydrocarbon potential of drill core samples was investigated. Organic geochemical analysis performed on 65 source rock samples indicates a wide range of TOC values from 0.81 wt % to 81.06 wt %. This study combined sedimentology, biomarker, organic petrography, and stable carbon isotopic composition of kerogen and extracts analysis demonstrating the organic matter of the source rocks is derived from terrestrial higher plants with a minor contribution of aquatic algae and microbes. Rock-Eval data, elemental composition of kerogen and odd over even normal alkane distribution reveal immaturity to low maturity with a humic - type organic matter. The source rocks were deposited in a freshwater oxic to sub-oxic environment indicated by high Pr/Ph ratios, a predominance of C29 steranes, low tricyclic terpanes and a low gammacerane index. High dia/regular C29 sterane ratios and moderate to high C29/C30 hopane ratios found in most samples are related to the presence of clay minerals. Based on the following parameters: H/C vs. O/C, TOC, S1+S2, hydrogen index, saturated hydrocarbons, aromatic hydrocarbons and Chloroform A, we concluded that the samples collected from organic-rich coal-bearing strata may yield not only gas, but also have the capability to generate and release liquid hydrocarbons.