Characteristics of carbon and hydrogen isotopic compositions of light hydrocarbons

摘要

Light hydrocarbons named in the present paper refer to the natural gas-associated light oil and condensate. 46 light oil and condensate samples from 11 oil-bearing basins of China were collected and their carbon and hydrogen isotopic compositions were analysed in terms of their total hydrocarbons, saturated hydrocarbons and a part of aromatic fractions, and gas-source materials and their sedimentary environments were discussed based on the above-mentioned data and the geological background of each area. From the view of carbon and hydrogen isotopic composition of total hydrocarbons and saturated hydrocarbons, it is revealed that the condensate related to coal-bearing strata is enriched in ~(13)C and D while that related to the source material of type Ⅰ-Ⅱ is enriched in ~(12)C. In general, the isotopic composition of carbon is mainly attributed to the inheriting effect of their source materials, whereas that of hydrogen principally reflects the correlationship between hydrogen isotopes and the sedimentary environment as well as the salinity of water medium. The author proposed, based on the data available, that the light hydrocarbons with δD value of less than -150 per thousand might be considered as generated from the fresh to brackish water environment and those with δD value of higher than -150 per thousand from the brackish to marine environment. It can be concluded that the features of organic materials are controlled by sedimentary fades and the different types of organic materials characterized by different compositions of stable isotopes. Additionally, the data of Jurassic condensate of Australia are listed, and its hydrocarbon-generating environment is deduced as fresh-water swamp based on the rich H and ~(13)C.