Abstract:

This paper is concerned with a study of about 100 crude oil samples fromvarious major basins in China,which was made through“the whole hydro-carbon chromatography”.In combination with the occurrence of crude oil,the distribution of isoprenoid alkanes and its relation with the geologic environmentare discussed here.The authors consider that the conspicuous difference in total concentration of isoprenoids and pristane-phytane ratios(Pr/Ph)in crude oil may indicatethe variability of the continental lake-basin depositional envisonments of sourcerocks.According to the range of the Pr/Ph ratios,the crude oil can be sta-tistically classified into the following three types:phytane-predominant type(0.20—0.80),pristane-phytane balanced type(0.8—2.80)and pristane-pre-dominant type(2.80—4.00).The source rocks of these three types of oil weredeposited under strong reduction,reduction and weak oxidation-reduction environments,respectively.Being mainly dependant on the depositonal envi-ronments of source rocks,the pristane-phytane ratio can serve as good in-dicator for oil-to-oil and oil-to-source rock correlation. The increase in terrestrial phyto-constituents of primary organic matter andthe rise in maturity of source rocks may lead to an increase in Pr/Ph ra-tios for source rocks and crude oils in various degrees.It seems,however,that the influence of these two factors is too small to change thoroughly thepristane-phytane ratios determined by related depositional environments ofsource rocks.Nevertheless,in the case that the crude oil was subjected tosecondary oxidation,the bacterial degradation would lead to considerable chan-ges in the primary distribution of the isoprenoid alkanes.