碳同位素组成异常的天然气成因探讨——以辽河坳陷东部凹陷为例

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、确定天然气成熟度等的有效地球化学手段.研究认为,甲烷的碳同位素组成主要受源岩母质类型和热演化的影响,乙烷、丙烷等重烃的碳同位素组成主要取决于源岩有机质的碳同位素组成,同时也明显受热演化程度的影响.在辽河坳陷发现一类碳同位素组成异常的天然气,分布于辽河坳陷东部凹陷南部地区,其甲烷的碳同位素δ13C₁值为-44‰～-40‰,乙烷δ13C₂值为-13‰～-6.6‰,丙烷δ13C₃值为-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,该天然气应该属于无机气和有机气的混合气体. 

ORIGIN OF NATURAL GAS WITH ABNORMAL CARBON ISOTOPE COMPOSITION-A CASE STUDY OF THE EAST DEPRESSION,THE LIAOHE BASIN

Carbon and hydrogen isotope composition of natural gas are effective geochemical measures to distinguish gas origin and type,provide insight of the correlation of different gas sources, and determine gas maturity. Study shows that, composition of carbon isotope of methane is mainly influenced by type of organic matters and thermal evolution. Composition of carbon isotope in heavy hydrocarbons, such as (ethane) and propane, is mainly determined by carbon isotope of organic matters of source rock. At the same time, it is apparently influenced by the degree of thermal evolution. Gas with abnormal carbon isotope composition has been found in the Liaohe Basin. This type of gas is distributed in the southern part of East Depression in the Liaohe Basin. The δ13C₁ value of carbon isotope of methane is -44‰ to -40‰, the (δ13C₂) value of ethane is -13‰ to -6.6‰, and the δ13C₃ value of propane is -6.1‰ to +3.3‰. Ethane and propane of this type of gas are abnormally rich in heavy carbon isotope. This is the first time that this special type of gas is found. Geochemical data and geological setting analysis have proved that,this type of gas should fall into the category of mixed gas.