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 (NgⅠ and NgⅢ) 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 NgⅠ and NgⅢ 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 C30- 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 CPI37-55 and OEP45-49 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.
With wax content of 1.62%, heavy oil has been produced from the sandstone reservoirs of Neogene Guantao Formation (Ng1m). In the GC and GC-MS RIC profiles of its aliphatic fraction, n-alkanes are totally lost, which shows the result of heavy biodegradedation. However, the remaining trace C13-C36 n-alkanes can be still seen from its mlz 85 mass chromatogram. In addition, a complete series of C35-C73 high molecular weight (HMW) n-alkanes was detected by high-temperature gas chromatography (HTGC). The HMW R-alkane series shows a normal distribution pattern, a major peak at nC43, obvious odd-carbon-number predominance, CPI37-55 and OEP45-49 values up to 1.17 and 1.16-1.20 respectively. The present study not only has conformed the strong resistibility of HMW n-alkanes to biodegradation in crude oils as concluded by previous researchers, but also has provided some significant information on source input and maturity for the heavily biodegraded oil in the Qianmiqiao region.
WANG Tieguan (T.-G. WANG), ZHU Dan, ZHANG Zhihuan, LU Hong & YANG ChiyinKey Laboratory of Petroleum Entrapment Mechanism of the Ministry of Education, University of Petroleum, Beijing 102200, China
