Isotopic composition characteristics and identification of immature and low-mature oils

Isotopic composition characteristics and the significance of immature and low-mature oils are first systematically discussed. The carbon isotopes of the whole oil can be divided into two groups, one has delta C-13 main peak values ranging from -30 parts per thousand to -29 and the other from -27 parts per thousand to -25 parts per thousand, they are related to lacustrine and salt-lake facies or swamp facies, respectively. The carbon isotopic fractionation among different group components is relatively small, usually less than 2 parts per thousand and the biggest difference in fractionation often occurs between saturated and aromatic fractions. Their deltaD values vary between -180 parts per thousand, and -130 parts per thousand. The main peak of their deltaD values concentrates between -170 parts per thousand and -150 parts per thousand, suggesting a domination of lacustrine facies. However, the secondary peak ranges from -160 parts per thousand to -150 parts per thousand, showing a requent salinization of paleo-water bodies. The average delta C-13 values of the methane vary between -50 parts per thousand and -52 parts per thousand, about 10 parts per thousand lighter than those of mature oils. There is a relatively good correlationship between immature and low-mature oils and their source rocks in carbon isotopic compositions of group fractions and monohydrocarbons; moreover, compared with the source rocks of mature oils, that of immature oils is often relatively depleted in C-13, which is one of the characteristics of immature oils, differing from those of mature oils.