Study on Carbon and Hydrogen Isotopes Composition of Crude Oils

Key words: 
• No rmal oil /
• light oil (condensate) /
• carbon and hydrogen isotopes /
• depositional environment of organic sources

Abstract: In this paper 3 85 crude oil samples from 18 oil-bearing areas of China have been analysed fo r carbon and hydrogen isotopic compositions and part of their saturated and aromatic hydrocarbons have been analy sed fo r carbon isotopic compositions.Based on the analy tical data, the geochemical characteris tics of carbon and hydro- gen isotopes have been s tudied for tw o different types of oils, i.e., normal oil and light oil (condensate)and the hydrocarbon generating organic sources and depositional environments have been discussed combined wi th local geological background.The author propose that the δ¹³C values of light oil ( condensate)( δ¹³C-32.5 ‰～ - 24.3‰)are higher than that of normal oil ( δ¹³C-34.4 ‰～ -24.6‰).The variation of carbon isotopic composi tio ns of aromatic hydrocarbons of crude oils w as obviously affected by the inheritance effect of o rganic Continued on page 144 sources, consequently the δ¹³C values of aromat ic hy drocarbons could be used to identify crude oils generated from different organic sources. Lig ht oils (co ndensates) related t o marine facies have δD values higher t han - 15 0 ‰, w hereas that related to no nmarine facies have δD values(-21 0 ‰～-1 05 ‰) which basicly cover the δD dist ribution range of marine lig ht oils (condensates).Wit h t he depositional environments changes from fresh w ater to slig htly salt w ater to brackish w ater to marine salt water, δD values of light oils (condensates) become obviously heavier, indicating that δD values mainly related to the depositional environment.