The Occurrence Of Secondary Biogenic Gas In West Slope Of The Songliao Basin

Most oil on the earth is affected by biodegradation, which is dominated control to form heavy oil and oil sand. Methane, rather than carbon dioxide is the another end product of biodegradation due to methanogenesis. This type of methane shows typical biogenic gas signature and was commonly named as secondary biogenic gas, which widely occurs in eastern China petroliferous basins and northwest slope of the Junggar Basin. The resource for such gas type is huge, but the mechanism for biogenic gas derived from oil biodegradation is not fully investigated.The present study focuses on oil biodegradation and related gas system characterization on oil samples collected from 16 production wells and gas samples collected from 20 production wells in northern part of the Songliao Basin. Gas chromatography-mass spectrometry (GC-MS) was performed for oil geochemical analysis, while chemical compositions and stable isotopes were applied for gas analysis. Detailed geochemical composition was applied to illustrate the origin of oil and degree of biodegradation. Most oil samples except Chouyou54-4 show normal non-biodegraded characteristics. They are derived from single petroleum system with similar source origin and depositional environment. However, slight biodegradation influence does occur in these normal oils on the base of their absolute concentrations, mainly in samples Pul98-16, Puful78-49 and La 2. The biodegradation level is 1-2 on Peters and Moldowan (1993) biodegradation scale. Chouyou54-4 was suffered moderate biodegradation influence, which reaches level 4 on Peters and Moldowan (1993) biodegradation scale.Gas chemical and isotopic analysis show that gases from Alaxin, Aonan and Sizhan gas fields are dry and dominated by methane, with dryness coefficient close to 1.0. Propane content is rare and the small amount of propane if exist has heavy carbon isotopic value, indicating the nature of biodegradation residue. Very light methane carbon isotopic values, coupled with unusually heavy carbon dioxide isotopic values, reflect their biogenic origin. Both gas chemical and isotopic compositions suggest the occurrence of secondary biogenic gas in Alaxin, Aonan and Sizhan gas fields.Preliminary study indicates that oil biodegradation is mainly under anaerobic conditions. Carbon dioxide formed by oil biodegradation is intermediate products and methane is the final end product due to carbon dioxide reduction by methanogen. This phenomenon commonly occurs in biodegraded system, however, convincing evidence of the methanogenesis and signature for secondary biogenic gas identification are rarely available in literature. The reason for this is partially due to the nature of complex oil-gas system. Proportion of carbon dioxide reduced to methane varies significantly in different basins. The new formed methane and carbon dioxide will mix with preexist gas in the system. Therefore, the whole biodegradation system is much more complicated than what we can imagine. Meanwhile, the potential for secondary biogenic gas is probably much larger than our previous expectation.