Origins of Organic Matter, Paleoenvironment, and Hydrocarbon Potential of the Carboniferous Source Rocks from Shibei Sag, Junggar Basin, NW China

Abstract

The organic-rich sediments of the Carboniferous in the Shibei Sag, Junggar Basin, China are potential source rocks with good hydrocarbon generation conditions for oil and gas exploration. Thus, it is meaningful to analyze their geochemical characteristics, including the hydrocarbon generation potential, genetic types of kerogen, thermal maturity, the origin of organic matter (OM), and sedimentary environment. The results reveal that the Carboniferous source rocks have fair to excellent hydrocarbon generation potential. They are mainly gas-prone with type III-IIkerogens. Random vitrinite reflectance (Rr) of the studied samples ranges from 0.63 to 1.04%, suggesting a primarily mature stage, which is also supported by the maturity-related hopane and sterane ratios. Stable carbon isotopic compositions, organic petrological macerals, and biomarker characteristics all indicate a predominant higher plant input. Abundant bicyclic sesquiterpenoids and diterpenoid biomarkers in the studied samples suggest the conifer inputs. In particular, high amounts of phyllocladanes, retene, and kauranes suggest the contribution of gymnosperms, pre-gymnosperms, pteridophytes, or bryophytes in swamps. From the early Carboniferous to the late Carboniferous, the environment became more oxic, the water level dropped, and higher plants flourished. Based on the abovementioned results, the Carboniferous source rocks were likely to be deposited in a transitional environment under lacustrine-fluvial conditions. Specifically, source rocks from the Batamayineishan Formation were deposited in a peat environment during the interval of the volcano eruptions. Considering the quantity and maturity of OM and their large thickness, the Carboniferous sediments have a good petroleum exploration potential.