松辽盆地大庆长垣伴生气中二氧化碳成因讨论

松辽盆地大庆长垣伴生气中δ¹³C_CO2值为+3.65‰～+11.81‰,重于碳酸盐岩受热分解特征值（0±3‰）,是国内发现最重的δ¹³C_CO2值。通过对大庆长垣伴生气及松辽盆地其他地区浅层气中二氧化碳及与其共生甲烷碳同位素判别和数值模拟计算认为,造成长垣伴生气中的δ¹³C_CO2值偏重的原因是CO₂被细菌还原;CO2被细菌还原为CH₄的过程中,在转化相同比例CO₂的情况下,高分馏条件比低分馏条件下剩余CO₂的δ¹³C值变重要快;要使剩余CO₂的δ¹³C值表现出现今特征值,单一有机质热降解来源的CO₂需要被细菌还原40%,碳酸盐岩热分解的CO₂则只需6%;由于来自有机质热降解的二氧化碳对气藏组分产生较大影响,因此大庆长垣伴生气中CO₂应主要来自碳酸盐岩热分解。

Discussion about the CO2 Origin in Associated Gas from Daqing Placanticline, Songliao Basin

The isotopically heavier carbon for CO₂ in the associated gases from the Daqing Placanticline is observed in China, and the δ¹³C_CO2 values rang from +3.65‰ to +11.81‰, more positive than those from thermal decomposition of carbonate with δ¹³C_CO2= 0±3‰. According to the identification of the carbon isotopic composition of CO2 and symbiotic CH4 in the associated gas and shallow gases in the Songliao basin, the heavier δ¹³C_CO2 is caused by bacteria reduction. Meanwhile, based on numerical modeling, the δ¹³C_CO2 value changes fast for residual gas from the higher fractionation than from the lower during the bacteria reduction from CO₂ to CH₄. If the reduced CO₂ has the similar δ¹³C values to the present gas, the proportions of the reduced CO₂ from the thermal degradation of organic matter and carbonate decomposition are up to 40% and 6%, respectively. Obvious change of gas composition in the associated gas in the Daqing Placanticline would happen if the CO₂ compound were derived from organic matter. Therefore, the thermal decomposition of carbonate is the main source of CO2 in the associated gas of the Daqing Placanticline.