Geochemical Behaviors And Origin Of Reservoir Bitumen In Eastern Si-chuan Basin

The biggest gas field has been found in northeast area of Sichuan basin up to date. Puguang gas field is one of them. Reservoir solid bitumens were discovered in Basin. The reservoir in Puguang gas pool generally contains solid bitumen and its content is 0.09%—2. 50%. Of all the types of reservoir rocks, the content of bitumen is the most abundant in the oolitic dolomite.Geochemical characteristics of reservoir bitumens were analyzed and the extracts correlations between reservoir bitumen and source rocks were demonstrated by application of biological markers.Systematic geochemical analyses show that the reservoir bitumen in Feixianguan and Changxing Formations in Puguang gas pool are residuals of the original oil in fossil oil pools after they have been thermally cracked into gas. The organic elementary compositions of the bitumen are characterized by poor hydrogen and rich oxygen and sulfur, suggesting that it is of pyrobitumen as a result of high maturity and TSR (Thermo—chemical Sulfate Reduction). The bitumen has ¹³C values ranging from—25. 60 to—27. 78‰and fits isotopically with kerogen in the Upper Permian source rocks(—26.50‰—29.20‰) , indicating a genetic relationship between them. The n—alkanes with a complete range of carbon number are observed in the saturated hydrocarbons of the bitumen and no evidences of severe biodegradation occur. They exhibit a phytane predominance with Pr / Ph ratios less than 1.0; prdominance of C₂₇ compounds occur in the steranes with detectable C₃₀ and C₂₆ steranes; and the terpanes are characterized by abundant tricyclic terpanes and gammacerane. All these indicate that aquatic organism dominated marine source rocks have been deposited in a reducing environment. As a result of aromatization of organic matter during the thermal cracking of original bitumen into gas, the aromatic hydrocarbons in the bitumen are characterized by abnormally abundant tetra and pentacyclic compounds, and predominated by parent aromatic compounds without alkyl chain, such as fluoranthene, pyrene, chrysene, benzofluoranthene etc. The distribution pattern and MPI1 index of aromatic hydrocarbons reveal a high thermal evolution for the bitumen with a corresponding R₀ value of over 2.0%.