High abundant sulfur-containing steroids were identified and detected in saturate hydrocarbon fractions of heavy oil with a high sulfur content in the Jinxian Sag, Bohai Bay Basin, North China. These sulfur-containing steroids were structurally merged into the D-ring of steroid nucleus with thiophene ring and/or combined into the C-22 in the side-chain. Based on the previous reports of sulfur-containing steroids with methylthio-steroids and intra-molecular form, four formation mechanisms of sulfur-containing steroids and diagenetic pathway of steroids under S-rich conditions were proposed in this paper according to the double bond positions in the sterene compounds. Hydrogenation and sulfurization both occurred in the diagenetic processes of olefinic bond in the side-chain of steroids: abiogenic chemical hydrogenation of H 2 S and HS -leads to the formation of regular steranes; a successful sulfurization process leads to the formation of the side-chain sulfur-containing steroids whereas unsuccessful cyclization and/or sulfurization result in the generation of short-chain steranes. This kind of mechanism of hydrogenation/sulfurization of side-chain olefinic bond provides a potential genesis clue for the occurrence of high abundance of short-chain steranes (higher than the common regular steroids, phytane and n-alkanes) in S-rich heavy oils and source rocks in the Jinxian Sag, Bohai Bay Basin, North China. sulfur-containing steroids, short-chain steranes, sulfurization, hydrogenation, C-S bond, H 2 S, Jinxian Sag Citation:Lu H, Wang Q T, Jiang L X, et al. Identification and formation of sulfur-containing steroids in sulfur-rich heavy oils in the Jinxian Sag, Bohai Bay

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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.