Geochemical characterization of secondary microbial gas occurrence in the Songliao Basin, NE China

摘要

A suite of natural gases from the northern Songliao Basin in NE China were characterized for their molecular and carbon isotopic composition. Gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation, with very high dryness (C_1/C_(2+)>100), high C_2/C_3 and i-C_4-C_4 ratios, high nitrogen content and variable carbon dioxide content. Isotopic values show wide range variations (δ~(13)CCH4 from -79.5‰ to -45.0‰, δ~(13)CC2H6 from -53.7‰ to -32.2‰, δ~(13)CC3H8 from -36.5‰ to -20.1‰, δ~(13)CnC4H10 from -32.7‰ to -24.5‰, and δ~(13)CCO2 from -21.6‰ to +10.5‰). A variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence. Secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures. System openness also was considered for shallow biodegraded gas accumulations. Gases from the Daqing Anticline are relatively wet with ~(13)C enriched methane and ~(13)C depleted CO_2, representing typically thermogenic origin. Gases within the Longhupao-Da'an Terrace have variable dryness, ~(13)C enriched methane and variable δ~(13)C of CO_2, suggesting dominant thermogenic origin and minor secondary microbial methane augment. The Puqian-Ao'nan Uplift contains relatively dry gas with ~(13)C depleted methane and ~(13)C enriched CO_2, typical for secondary microbial gas with a minor part of thermogenic methane. Gas accumulations in the Western Slope are very dry with low carbon dioxide concentrations. Some gases contain ~(13)C depleted methane, ethane and propane, indicating low maturity/primary microbial origin. Recognition of varying genetic gas types in the Songliao Basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems.