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ABSTRACT: Characters of high-sulfur lacustrine source rocks of sour-gas and heavy-oil in the Jinxian Sub-basin of the Bohaiwan Basin, China

He, Sheng1, Mike Middleton2, Kejing Zhao3, and Wenbin Gu3
(1) Curtin University of Technology, Perth, WA 6845, Australia; China University of Geosciences, Wuhan, China
(2) Curtin University of Technology, Perth, Australia (3) Huabei Petroleum Corporation, Renqiu, China

Lacustrine source rocks of the Middle-late Eocene in the Jinxian Sub-basin, Bohaiwan Basin, eastern China are characterized by 100-550 m of high-sulfur and gypsum-bearing shales in a 200-1100 m evaporate sequence. The sequence is predominated by interbedded dark shales, carbonate and sulfate rocks. Total sulfur content from 33 samples of the shales ranges from 1.69-8.19%, among them total organic sulfur (TOS) is 0.01-0.73%. The inorganic sulfur derived from SO42-, FeS2 and S0. Total organic carbon (TOC) of the shales is 0.2-1.4% and has a linear relationship with TOS. Within the sub-basin the dark shales accumulated under strong reducing conditions, as indicated by Pr/Ph: 0.11-0.57 and OEP: 0.51-0.93 from the bitumen. The organic matter of the shales has a high proportion of humic material, and kerogen is primarily Type III according to optical observations, elemental analyses and Rock-Eval data. Burial depths of the shales in the sub-basin are about 2000-3500 m with vitrinite reflectance (Ro) ranging from 0.3-1.0%.

Sour-gas (H2S 92%) and heavy-oil (densities 0.9-1.09*103kg/m3 and sulfur 3.5-10wt%) originated from the source rocks based on geology and geochemical correlation. The pyrolysis of the shale samples with Ro 0.3-0.4% and Tmax 407-422 oC indicates that H2S of thermal genesis is not important. The result shows two peaks of oil-generation correspond to Ro 0.6% and 1.5%. The study suggests that the generation of sour-gas and heavy-oil could be related to the anaerobic action and decomposition of sulfur-rich organic matter in the lacustrine source rock sequence during the diagenesis and early catagenesis.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia