Origin of sulfur rich oils and H 2S in Tertiary lacustrine sections of the Jinxian Sag, Bohai Bay Basin, China

Abstract

Very high S oils (up to 14.7%) with H 2S 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 H 2S-rich reservoirs (H 2S up to 92% by volume) where the H 2S was derived from bacterial SO 4 reduction, most likely in the source rock prior to migration. Three oils in the Jinxian Sag have δ 34S values from +0.3‰ to +16.2‰ and the oil with the highest S content shows the lightest δ 34S value. This δ 34S value for that oil is close to the δ 34S value for H 2S (∼0‰). It is possible that H 2S 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.