Biodegraded Oil and Its High Molecular Weight (C_(35+)) n-alkanes in the Qianmiqiao Region in the Bohai Bay Basin, Northern China

Abstract With a production of 208.2 m³/d, heavy oil was produced by drill stem test (DST) from three shallow reservoirs in Sand Group Nos. I and III of the Neogene Guantao Formation (Ng^I and Ng^III) and the Eogene Dongying Formation (Ed) in an exploratory well Ban‐14‐1 within the Qianmiqiao region, Bohai Bay Basin, northern China. Based on the GC and GC‐MS data of the Ng^I and Ng^III heavy oil samples, all n‐alkanes and most isoprenoid hydrocarbons are lost and the GC baseline appears as an evident “hump”, implying a large quantity of unresolved complex mixture (UCM), which typically revealed a result of heavy biodegradation. However, there still is a complete series of C₁₄‐C₇₃ n‐alkanes in the high‐temperature gas chromatograms (HTGC) of the heavy oil, among which, the abundance of C_30‐ n‐alkanes are drastically reduced. The C₃₅‐C₅₅ high molecular weight (HMW) n‐alkanes are at high abundance and show a normal distribution pattern with major peak at C₄₃ and an obvious odd‐carbon‐number predominance with CPI_37–55 and OEP_45–49 values of 1.17 and 1.16‐1.20, respectively. According to GC‐MS analysis, the heavy oil is characterized by dual source inputs of aquatic microbes and terrestrial higher plants. Various steranes and tricyclic terpanes indicate an algal origin, and hopane‐type triterpanes, C₂₄ tetracyclic terpane and drimane series show the bacterial contribution. With the odd‐carbon‐number preference, HMW n‐alkanes provide significant information not only on higher plant source input and immaturity, but also on the strong resistibility to biodegradation.