Identification of Highly to Over Mature Gas Interval Using Reservoir Bitumen Features: Sichuan Basin of Southwest China

摘要

The identification of oil and gas intervals has been mainly constrained by the interpretation on well logging data. However, this has met great difficulties with petroleum accumulation cases becoming more and more complex. The authors discuss an identification of highly to over mature gas interval using bitumen features, based on a case study in the Sichuan Basin of southwest China. The bitumen in the gas interval is characterized by relatively rich low-weight n-alkanes with a single pre-peak. In addition, pregnane, homopregnane, and tricyclic terpanes are enriched. Thus, the indicated maturity is highly to over mature. The late-arrived highly mature oils were cracked into gas, with little filling of bitumen in reservoir space. By contrast, the bitumen in the interval with little bearing of gas has opposite features. It is relatively enriched in high-weight n-alkanes and extremely low abundance of pregnane, homopregnane, and tricyclic terpanes. Thus the oils in such dry intervals are mostly low-mature early arrived oils, which tightly adsorbed on the carbonate grains. The pore space is almost fully tilled by bitumen, being unfavorable for gas charge and preservation. As these features are all collected from routine analyses, the method can be applicable. The results can provide new fundamental information for regional oil testing and petroleum exploration, thereby reducing economic loss and exploration risk. In addition, the results also have wide implications for the other works with similar aims.