Abstract: Application of carbon isotopes in natural gas by "taking the carbon isotopic ratio in ethane as a criteria to differentiate the properties of organic gas precursors and taking that in methane as a criteria to study the migration of gas in the lateral-diffusion mode" is proposed in this paper based on experimental and field data. Two kinds of natural gas originating sapropelic type of organic matter in the carbonate source-rocks in the Ordovician and from coal-bearing formation in the Carboniferous-permian respectively are differentiated mainly according to carbon isotopic ratio in ethane. The gas from the Ordovician has characteristics such as more negative carbon isotopic ratio in ethane, lower content of C²⁺ hydrocarbons and higher concentration of hydrogen sulphide relative to the gas from the Carboniferous-permian. Qualitative identification and quantitative calculation show that the natural gas in the Ordovician reservoirs of the Central gas field is a two-component gas mixture which comes mainly from the Ordovician source rock while those from the Carboniferous is in a small quantity only. The gas which comes from the Carboniferous in the Ordovician reservoirs is found in the eastern part of the field and is associated with the erosive gulleys on the top of the Ordovician. The natural gas in the Ordovician of the central-eastern part of the basin is divided into four migration fields by using the data of carbon isotopic ratio in methane. This paper also discusses the effect of the palaeo-geomorphology,palaeo-Karst and the present structural feature on the distribution of natural gas of different origin and its migration

Key words: Shang Gan Ning region, basin, natural gas, origin, migration mode, ethane, methane, isotope, gas reservoir

摘要： 根据实验和实际资料提出了“以乙烷碳同位素为主区分生气母质,以甲烷碳同位素为主研究侧向─扩散性运移”的指标使用原则.总结出陕甘宁盆地中部气田奥陶系腐泥碳酸盐来源气具乙烷碳同位素相对偏负─乙烷等重烃含量低─硫化氢高的组合特征,石炭-二叠系煤系来源气呈相反组合.在定性判别和定量计算基础上,指出该气出奥陶系储集气是以奥陶系来源为主、兼有石炭系供给的二无混合气.奥陶系储层的石炭系源气主要分布在气田东部,与奥陶系顶古侵蚀沟关系密切.利用甲烷碳同位素资料将盆地中东部奥陶系天然气划分出四个运移场.同时讨论了古地貌、占岩溶、今构造面貌等对气源展布和天然气运移的控制作用.

关键词: 陕甘宁地区, 盆地, 天然气, 成因, 运移方式, 乙烷, 甲烷同住素, 气藏