东濮凹陷刘庄地区天然气乙烷碳同位素异常成因分析

Genesis of Carbon Isotope Anomaly in Ethane of the Natural Gas of Liuzhuang, Dongpu Depression

乙烷的碳同位素组成是天然气成因类型判别的重要指标,在天然气成因类型判别和气源岩研究中具有重要作用。通过对东濮凹陷刘庄地区沙河街组天然气组分及碳同位素组成的分析测试,发现该区天然气相较于东濮凹陷其他气田的天然气,其乙烷碳同位素异常偏重。在此基础上,并结合烃源岩特征及地质背景的综合分析表明,刘庄地区天然气为典型的混合成因,主要为来自早期石炭-二叠系烃源岩生成的煤型气,后期伴有少量的沙河街组烃源岩晚期生成的油型气的混合充注。造成乙烷碳同位素值偏高的主要原因是石炭-二叠系烃源岩生成的煤型气,在烃源岩最高成熟阶段瞬间天然气聚集成藏,热演化程度对其乙烷碳同位素组成有一定的影响,其次可能与早期成藏后抬升剥蚀发生微生物氧化作用有关。

The carbon isotope composition of ethane is an important index in identifying the genetic type of natural gas,and plays an important role in studies of identifications of natural gas genetic type and source rocks.By analyzing compositions of natural gas from the Shahejie formation in the Liuzhuang area of the Dongpu depression and their carbon isotopes,this paper found that,compared with other gas fields in the Dongpu depression,natural gas in this area could be characterized by extremely heavy carbon isotope in ethane.On the basis of characteristics of source rocks and history of buried and hydrocarbon generation,this paper comprehensively analyzed sources of natural gas and discussed potential causes of abnormal accumulation of heavy carbon isotopes in ethane.The results showed that the natural gas in the Liuzhuang area is a typical natural gas of mixed origins,mainly coal-type gas generated by the early Carboniferous-Permian hydrocarbon source and mixed filled by a small amount of oil-type gas generated by the late Shahejie formation hydrocarbon source.The key cause of the heavy carbon isotope in ethane is that the natural gas mainly came from the coal-type gas of Carboniferous-Permian.The natural gas accumulated rapidly and became a reservoir during the highest maturity stage of hydrocarbon source rocks,and the high maturity of hydrocarbon source rocks may also play a certain role.In addition,the microbial oxidation during the uplift denudation after the early accumulation may also have some impacts.