CHARACTERISTICS OF CARBON AND HYDROGEN ISOTOPIC COMPOSITIONS OF LIGHT-HYDROCARBONS

Light hydrocarbons named in the present paper refer to the natural gas -associated light oil 46 light oil and condensate samples from 11 oil- bearing basins of China were collected and their carbon and hydrogen i sotopic compositions were analysed in terms of their total hydrocarbon s, saturated hydrocarbons and a part of aromatic fractions, and gas-so urce materials and their sedimentary environments were discussed based on the above-mentioned data and the geological background of each are a. From the view of carbon and hydrogen isotopic composition of total hydrocarbons and saturated hydrocarbons, it is revealed that the conde nsate related to coal-bearing strata is enriched in C-13 and D while t hat related to the source material of type I-II is enriched in C-12. I n general, the isotopic composition of carbon is mainly attributed to the inheriting effect of their source materials, whereas that of hydro gen principally reflects the correlationship between hydrogen isotopes and the sedimentary environment as well as the salinity of water medi um. The author proposed, based on the data available, that the light h ydrocarbons with delta D value of less than -150 parts per thousand mi ght be considered as generated from the Fresh to brackish water enviro nment and those with delta D value of higher than -150 parts per thous and from the brackish to marine environment. It can be concluded that the features of organic materials are controlled by sedimentary facies and the different types of organic materials characterized by differe nt compositions of stable isotopes. Additionally, the data of Jurassic condensate of Australia are listed, and its hydrocarbon-generating en vironment is deduced as fresh-water swamp based on the rich H and C-13 .