Compound-speci®c stable carbon isotope analysis as a tool for correlating coal-sourced oils and interbedded shale-sourced oils in coal measures: an example from Turpan basin, north-western China

Molecular geochemical methods have shown that it is often dicult to di€erentiate between coaland interbedded shale-sourced oils, even though coals and interbedded shales may exhibit considerable organic in ̄ux variation (e.g. land plant vs algal organic matter) due to the changes of depositional setting. However, compound-speci®c stable carbon isotopic compositions are sensitive to the source input variations. Typically, speci®c molecules are more depleted in C with increasing content of aqueous biota. This hypothesis is examined and exempli®ed by comparing the stable carbon isotopic ratios of n-alkanes from source rock extracts and related oils of the Turpan basin, northwestern China. Stable carbon isotopic values of n-alkanes extracted from coals and interbedded shales show that dC values of n-alkanes with less than 20 carbon atoms vary only slightly. However, there are dramatic changes in the isotopic compositions of higher molecular weight n-alkanes. Furthermore, n-alkanes from coal extracts are enriched in C relative to that of interbedded shales with excursions up to 2±3%. This comparison enables the di€erentiation of coaland interbedded shale-sourced oils, and provides information useful in assessing the hydrocarbon system of a basin. # 2000 Elsevier Science Ltd. All rights reserved.