Origin of sulfur rich oils and H2S in Tertiary lacustrine sections of the Jinxian Sag,Bohai Bay Basin,China |
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| Institution: | 1. Department of Geology, National Iranian South Oil Company (NISOC), Iran;2. Department of Geology, Faculty of Earth Sciences, S. Chamran University of Ahvaz, Iran;3. Department of Geology, Faculty of Sciences, Bu-Ali University of Hamedan, Iran |
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| Abstract: | Very high S oils (up to 14.7%) with H2S contents of up to 92% in the associated gas have been found in the Tertiary in the Jinxian Sag, Bohai Bay Basin, PR China. Several oil samples were analyzed for C and S stable isotopes and biomarkers to try to understand the origin of these unusual oil samples.The high S oils occur in relatively shallow reservoirs in the northern part of the Jinxian Sag in anhydrite-rich reservoirs, and are characteristic of oils derived from S-rich source rocks deposited in an enclosed and productive stratified hypersaline water body. In contrast, low S oils (as low as 0.03%) in the southern part of the Jinxian Sag occur in Tertiary lacustrine reservoirs with minimal anhydrite. These southern oils were probably derived from less S-rich source rocks deposited under a relatively open and freshwater to brackish lake environment that had larger amounts of higher plant inputs.The extremely high S oil samples (>10%) underwent biodegradation of normal alkanes resulting in a degree of concentration of S in the residual petroleum, although isoprenoid alkanes remain showing that biodegradation was not extreme. Interestingly, the high S oils occur in H2S-rich reservoirs (H2S up to 92% by volume) where the H2S was derived from bacterial SO4 reduction, most likely in the source rock prior to migration. Three oils in the Jinxian Sag have δ34S values from +0.3‰ to +16.2‰ and the oil with the highest S content shows the lightest δ34S value. This δ34S value for that oil is close to the δ34S value for H2S (～0‰). It is possible that H2S was incorporated into functionalized compounds within the residual petroleum during biodegradation at depth in the reservoir thus accounting for the very high concentrations of S in petroleum. |
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