Geochemical Characterization of Released Bound Biomarkers by Catalytic Hydropyrolysis of Ordovician Source Rock from Tarim Basin, China

Abstract

The Salgan and Yingan shales in the Keping area are Middle and Upper Ordovician hydrocarbon source rocks in the Tarim Basin, north-western China. Complete distribution of n-alkanes and isoprenoids can be obtained from the source rocks, while general terpanes (m/z 191) and steranes (m/z 217) were totally absent. Therefore, kerogens in these rocks were further treated by catalytic hydropyrolysis (HyPy) to obtain more biomarker information. As an efficient technique, Hypy successfully released bound terpanes and steranes. Furthermore, some maturity parameters were calculated. Based on the values of these parameters, the effective R o value of bound biomarkers in Salgan and Yingan shale was deduced to be in the range of 0.7-0.75% and 0.65-0.73%, respectively. Considering the artificial heating effect in HyPy technique, relatively lower values would be expected to the maturity of the bound biomarkers. Therefore, the incorporation of bound biomarkers into kerogen should occur even earlier, before the R o value reached 0.65%-that is, prior to oil generation window. For both the Salgan and Yingan shales, neither high maturity nor magmatic hydrothermal effects during the Permian were responsible for the loss of free biomarkers. The simplex organic matter input dominated by acritarchs might be the possible reason for the absence of free biomarkers.