济阳坳陷三合村洼陷浅层天然气成因分析

The origin of shallow-buried natural gas in Sanhecun sag,Jiyang depression

厘清济阳坳陷三合村洼陷浅层天然气成因.通过采集浅层天然气及伴生原油样品,在进行天然气组分、碳、氢同位素组成以及伴生原油饱和烃色谱分析的基础上对浅层天然气的成因进行了研究,认为三合村洼陷浅层天然气主要来自微生物对原油的降解,属于典型的次生生物气,微生物还原二氧化碳生成甲烷是次生生物甲烷的主要生成途径.研究结果表明:天然气组分以甲烷为主,干燥系数大(C_/C_1^+>0.931);甲烷碳同位素比值变化大(-50.03‰<δ^(13)C_1<-39.03‰),并与甲烷氢同位素比值呈负相关关系;重烃组分含量低且碳同位素比值普遍较大(-33.62‰<δ^(13)C_2<-19.72‰,-29.27‰<δ^(13)C_3<-7.64‰,-26.28‰<δ^(13)n-C_4<-15.63‰);二氧化碳碳同位素比值表现为异常的正值(2.82‰<δ^(13)CCO_2<17.69‰).微生物对湿气组分的选择性降解以及次生生物甲烷的生成是导致济阳坳陷三合村洼陷天然气组分、同位素组成复杂特征的主要原因.浅层天然气成因的确定对后续勘探浅层气藏附近的降解型稠油油藏具有重要的指示意义.

Based on comprehensive analysis of the natural gas component,carbon and hydrogen isotope composition of the natural gas and associated oil saturated hydrocarbon chromatography of oil samples collected in Sanhecun sag,Jiyang depression,the geochemical characteristics and formation mechanism of shallow gas were studied.It is considered that the shallow gas in Sanhecun sag is typical secondary microbial gas.It is the mixture of newly generated secondary biogenic gas from biodegradation of thermal oil/gas and residual thermal gas.It seems that CO2 reduction with hydrogen dominates the methanogenic process.The results show that the natural gas is basically methane with relatively high dryness（C1/C1^＋0.931）.Carbon isotope value of methane shows a wide variation（δ^13C1from-50.03‰to-39.03‰）and decreases with the increase of the hydrogen isotope value.There is a low content of heavy hydrocarbon and the carbon isotope values are generally higher（δ^13C2from-33.62‰to-19.72‰,δ^13C3from-29.27‰to-7.64‰,δ^13n-C4from-26.28‰to-15.63‰）.Carbon isotope value of CO2 is abnormally positive（δ^13CCO2from 2.82‰to 17.69‰）.The complicated characteristics of gas component and isotope composition of shallow gases can be explained by various sequential biodegradation,carbon dioxide reduction and mixing process.The secondary microbial gas has a very important implication for biodegraded heavy oil and thus the undiscovered heavy oil under the discovered shallow gas will be the target in the future exploration.