科研之友
微信
新浪微博
Facebook
分享链接
摘要
A suite of oils and associated gases from the Liaohe Basin NE China, which have similar maturity and source rock origin but with varying degrees of biodegradation, have been analyzed for the molecular compositions of oil and gas as well as for the isotopic values. Isotopic compositions of the gas samples show wide variation (delta C-13(1) from -55.4 parts per thousand to -36.8 parts per thousand, delta C-13(2) from -39.4 parts per thousand to -28.5 parts per thousand, delta C-13(3) from -31.9 parts per thousand to -16.4 parts per thousand and delta(13)n-C-4 from -26.6 parts per thousand to -20.4 parts per thousand). Most gas samples display clear geochemical evidence of alteration by biodegradation, with relatively high dryness (C-1/C1-4 > 0.97), high ratios of isobutane/n-butane (0.4-1.8) and C-2/C-3 (0.3-42.3). The carbon isotopic compositions of wet gas components (C-3 and C-4) in the biodegraded gases are heavier than these in non-biodegraded gases with Delta delta C-13 (C-3 - C-2) of up to 13.4 parts per thousand, but methane and ethane isotope signatures are variably heavier or lighter than those in non-biodegraded gases. Carbon dioxide contents and isotopic values provide evidence for gas origins. Gases with isotopically heavy carbon dioxide (delta C-13 > +2 parts per thousand), and isotopically light methane (delta C-13 < -50 parts per thousand) indicate that secondary microbial methane was generated from CO2 reduction under anaerobic conditions. Gases with isotopically heavy methane (> -45 parts per thousand), together with heavy carbon dioxide (> + 8 parts per thousand), probably indicate a high degree of CO2 conversion to methane under later stages of secondary microbial methane generation. We speculate that the presence of isotopically heavy methane (> -45 parts per thousand) and isotopically light carbon dioxide (< -8 parts per thousand) may be indicative of anaerobic methane oxidation but much additional work is needed to evaluate that. It seems likely that most of the variations in isotopic signatures of gases related to biodegraded oil can be described by various sequential biodegradation, carbon dioxide reduction and gas mixing processes. Methanogenesis through carbon dioxide reduction does not necessarily produce isotopically depleted methane. The methane becomes richer in C-13 when a large fraction of carbon dioxide is reduced to methane, resulting in newly generated methane that is sometimes heavier than the original thermogenic methane. Anaerobic methane oxidation may exist in the present case study. Mixing of newly generated methane with thermogenic methane already in place in the reservoir, coupled with anaerobic methane oxidation, can result in very complicated isotopic signatures. Care should be taken when gas isotopic values were applied for source and maturity assessment.
- 出版日期2014-3
- 单位中国地质大学（北京）
