Microbial alteration of natural gas in Xinglongtai field of the Bohai Bay Basin, China

Xinglongtai field has been an important petroleum-producing area of Liaohe Depression for 30 years. Oil exploration and production were the focus of this field, but the gas was ignored. This study examined twenty five gas samples with the purpose of determining the gas genetic types and their geochemical characteristics. Molecular components, stable carbon isotopic compositions and light hydrocarbons were also measured, and they proved that microbial activity has attacked some of the gas components which resulted in unusual carbon isotopic distributions. Propane seems to be selectively attacked during the initial stage of microbial alteration, with abnormally lower content compared to that of butane as well as anomalously heavy carbon isotope. As a consequence, the carbon isotopic distribution among the gas components is partially reverse, as δ¹³C₁<δ¹³C₂<δ¹³C₃>δ¹³C₄. Besides, n-alkanes of C₃₊ gas components are preferentially attacked during the process of microbial alteration. This is manifested that n-alkanes are more enriched in ¹³C than corresponding iso-alkanes. As a result, the concentrations of n-alkanes become very low, which may be misleading in indentifying the gas genetic types. As to four gas samples, light hydrocarbon compositions display evidence for microbial alteration. The sequence of hexane isomers varies obviously with high content of 2,3-DMC₄, which indicates that they have been in the fourth level of extensively bacterial alteration. So the geochemical characteristics can be affected by microbial alteration, and recognition of microbial alteration in gas accumulations is very important for interpreting the natural gas genetic types.