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Early Hydrocarbon Generation Mechanisms and Characterization of Sulfur-Rich Lacustrine Source Rocks in Offshore Bohai Bay Basin

Abstract

Bohai Bay Basin is a Cenozoic intracratonic rifted basin. There are four subbasins in offshore Bohai Bay Basin. Each subbasin has several depressions and each depression can be further divided into several sags. Shallowly buried marginal sags around the basin or as the conjunction part of two depressions may be isolate from others during rifted stage when source rock deposits and they are becoming the exploration frontier. Unique kerogen type, biomarker characteristics and geothermal history are identified for such sags. East-east sag of Huanghekou depression has a maximum burial depth of only 4000m (13123 ft). This sag is shallowly buried but petroliferous. Besides two discoveries, oil-oil correlation indicates that reservoirs in the surrounding uplifts also have hydrocarbon migrated from the sag. Oils are not subjected to biodegradation but have high content of non-paraffin and asphaltene. Sulfur content can reach up to 2.9%, relatively high compared to other oils with sulfur content generally lower than 0.5% in offshore Bohai Bay Basin. Biomarkers are characterized by n-C17 dominated, low C19/C23 tricyclic terpane and C24 tetracyclic terpane/C26 tricyclic terpane ratios and high abundant 4-methyl steranes, indicating dinoflagellates such as Bohaidina and Parabohaidina as major organisms input. Phytane/pristine<1, high abundant gammacerane, C35/C34 homohopanes>1 and low abundant diasteranes indicate high salinity and reducing environment. Oils from deeper layers with better preservation conditions have higher density and viscosity with higher sulfur content, more obvious phytane, gammacerane and C35 homohopanes preference. Two excellent source rocks, the upper Eocene third member and the lower Oligocene first member of Shahejie formation exist and proved to be the effective source rocks. Unique biomarker characteristics indicate isolated evaporative high salinity paleowater lacustrine during source rocks deposition. Sulfer rich kerogen formed in such environment has low activation energy and generates oil with high non-paraffin and asphaltene content in the early mature stage. High heat flow background speeds up hydrocarbon generation. Kerogen maceral contituents analysis, holoclastic rock analysis, major and trace element analysis, vitrinite reflectance, Rock-Eval pyrolysis, sealed gold tube pyrolysis are used to study source rock sedimentation and hydrocarbon generation for such marginal sags.