辽河盆地天然气中重烃异常富集重碳同位素的成因探讨

近年来大量的研究结果表明天然气碳、氢同位素组成特征是判识天然气成因类型、进行气源对比、估算天然气的成熟度和确定天然气是否被次生改造等的有效地球化学手段。通常认为甲烷的碳同位素组成主要受源岩的母质类型和热演化程度的影响，乙烷、丙烷等重烃碳同位素组成主要决定于源岩有机母质的碳同位素组成，同时也明显地受热演化程度的影响。在辽河盆地发现一类碳同位素组成异常的天然气。这类天然气是分布于辽河盆地东部凹陷桃园地区和黄金带地区的部分浅层天然气，其甲烷的碳同位素组成δ13C₁值为-44‰～-40‰，乙烷δ13C₂值为-13‰～-6.6‰，丙烷δ13C₃值-6.1‰～+3.3‰，该类天然气的乙、丙烷异常富集重碳同位素，到目前为止在天然气藏中还是首次发现。根据地球化学资料和地质背景分析认为这天然气应是未成熟阶段的生物气或低成熟阶段的生物-热催化过度带气经过细菌氧化后形成的。

Discussion about the Ethane and Propane Intensely Richin Heavy Carbon Isotopein Natural Gases from Liaohe Basin

Stable isotope geochemistry provides some insight into the origin, correlation and secondary reformation of natural gases. Generally, the carbon isotope composition of methane is controlled by maturity and organic matter type of source rock, and that of C₂₊ gaseous components of natural gases is depended on the carbon isotopic composition of organic matter in source rock and also influenced by maturity. The bacterial gas has- 55‰ of 1 and - 30‰ of 2. The thermogenic gases, with W13 C₁ from - 50‰～- 25‰, W13 C₂ from - 45‰～ - 22‰ and W13 C₃ from - 38‰～- 21‰, are rich in heavy isotope as the maturity increasing. The abiogenic gas, with W13 C₁- 25‰, has a carbon isotope reverse distribution of C₁ to C₄. The measurement of chemical components and carbon isotopic composition of natural gases from Liaohe basin shows that the natural gases form this basin can be divided into three groups. The group I gases, with W13 C₁ f rom- 59‰～ - 50‰ ,W13 C₂ from -48‰～-36‰, W13 C₃ from- 40‰～-24‰ and very low concentration of C₂₊ (< 1.0%), are bacterial gases. The group Ⅱ gases, with W13 C₁ from -49‰～-38‰, W13 C₂ from -31 to-26‰, W13 C₃ from -29‰ to-26‰ and concentration of C₂₊>5%, are typical thermogenic gases. The group III gases, with W13 C₁ from-40‰～-40‰ ,W13 C₂ from-13.0‰～-6.6‰, W13 C₃ from - 6.1‰～+3.3% and relative low content of C₂₊ (< 2%), are enigmatic. The depth of reservoirs of group III gases is relative shallow, ranging from 1270 m to 1330 m. The group III gases are considered to be secondary reformed by bacterial oxidation, according to geochemical characters and geological background.