Geochemistry and distribution of biogenic gas in China

Of twenty-nine known biogenic gas fields in China, three with reserves of nearly 100 × 109 m3 are located in Qaidam Basin, western China, and two with reserves of 50 × 109 m3 are located in Yinggehai Basin, southern China. The other fields, concentrated in eastern and south-eastern China, have smaller reserves. The gas geochemical characteristics, including data from more than 300 gas samples, over 200 isotopic analyses and 12 inert gas analyses, reveals the following. Gases are predominated by methane (CH4) (95%), followed by nitrogen (N2; 0–15%) and carbon dioxide (CO2; <3% but commonly <1%). Methane δ13C1 values are very light, usually <−55‰. The ethane δ13C2 values range widely from −60‰ to −20‰ and the δ13CCO2 values are between −39‰ and 5‰. Hydrogen isotope values of CH4 range from −260‰ to −190‰, which indicate that the gases are formed by CO2 reduction. 3He/4He ratios are between n × 10−8 and n × 10−7. 40Ar/36Ar ratios are between 231 and 439 and R/Ra ratios are 0.03. These biogenic gases exhibit geochemical characteristics and occur in geological settings that indicate two major biogenic gas types in China: early, or primary, biogenic gas and secondary biogenic gas that formed through crude oil biodegradation. Primary biogenic gas reserves are large while secondary biogenic gas reserves are smaller but widely distributed. The geological settings of the two types differ significantly. The primary biogenic gases are concentrated in Cenozoic successions characterized by rapid sedimentation, high organic matter content and syndepositional entrapment. Most secondary biogenic gases are associated with biodegraded heavy oil occurrences and these are not confined to any specific sedimentary strata or epoch.