Advmmm ia Organic Geochemistry 1991 Org. Geochem. Vol. 19, Nos 1-3, pp. 89-105, 1992 Printed in Great Britain.All rightsreserved 0146-6380/92 $5.00 + 0.00 Copyright© 1992Pergamon Press Ltd Variation in stable carbon isotope ratios of individual hydrocarbons as a function of artificial maturity MALVIN BJORDY, 1 PETER B. HALL,1 ELI HUSTAD l and JACK A. WILLIAMS 2 ~Geolab Nor A/S, Hornebergveien 5, 7038 Trondheim, Norway and 2Amoco Production Company, 7317 East 59th Street, Tulsa, OK 74145, U.S.A. Abstract--This paper reports isotopic variations in n-alkanes, isoprenoids and terpenoids from different source rock types as a function of maturity by analysis of these compounds in hydrous pyrolysis products and oil. At high pyrolysis temperatures n-alkanes, isoprenoids and terptmoids in the pyrolysis products become depleted in ~2C.The most marked depletion occurs in the terpenoids, particularly in the C~-C3~ hopanes. These compounds are isotopicallyvery light in the immature unheated samples of the lacustrine Green River shale (Tertiary) and a Guatemalan carbonate source rock. There is a marked depletion in ~2C at 330°C and higher pyrolysis temperatures for these two samples. Moderately mature samples of the Toarcian Paris Basin shale and a Jurassic coal from the Hammerfest Basin showed less or no t2C depletion. Fairly uniform values were recorded for the hopanes in pyrolysates of all samples heated at temperatures of 350°C and above. The isotopic composition of n-alkanes and isoprenoids are fairly similar to the hopanes in samples pyrolysed at the higher temperatures. It appears, therefore, that with increasing maturity n-alkanes, isoprenoids and terpenoids become fairly similar in their carbon isotopic composition. The isotopic compositions within the different compound groups in the different source rocks are sufficiently different for this technique to be used in source rock/oil correlation. Key words--isotope GC, stable carbon isotopes, artificial maturity, source, alkanes, triterpanes INTRODUCTION In recent years there have been numerous publications on stable carbon isotope analysis of individual hydrocarbon compounds in oils and rock extracts. This procedure involves coupling a combus- tion furnace to a gas chromatograph and subsequent analysis on a mass spectrometer (Sano et al., 1976; Matthews and Hayes, 1978; Gilmour et al., 1984). More recently, instruments with direct coupling of gas chromatograph via a combustion furnace to a mass spectrometer have been developed and their usefulness demonstrated (e.g. Bjor~y et al., 1990, 1991; Freeman et al., 1990). This technique has made it possible to undertake rapid, direct, stable carbon isotope analysis of hydrocarbons in oils and sediment extracts. Bjorey et al. (1990, 1991) have reported on stable carbon isotope variations of n-alkanes and iso- prenoids in whole oils and the saturated hydrocarbon fractions of oils as a function of maturity and source type. Freeman et al. (1990) have reported on stable carbon isotopic variations of biomarkers (mainly acyclic isoprenoids and bopanoids) in the lacustrine Messel shale deposit and showed that there are wide variations in isotopic composition of individual compounds. They concluded that the triterpanes were derived from different groups of organisms, i.e. com- pounds with less than 30 carbon atoms being derived from methanotropic bacteria, while those of 30 or more carbon atoms were most probably derived from chemoautotrophs. Sofer and Bjorey (1990) observed similar trends for hopanoids from lacustrine Autun shales of Permian age, but not for the lacustrine Green River shales (Tertiary).They also observed that the wide variationsin isotopic composition of hopanoids from extracts of immature shales were not seen in the hopanoids of oils. The lattershowed rather uniform values around -30%e for the C27-C35 hopanes. The published data suggest that the isotopic compositions of individual hopanoids, which show considerable variations with carbon number in immature sediments become fairly uniform with increasing maturity, and isotopic compositions of the hopanes become similar to the n-alkanes in the same sample. To further evaluate this problem, samples of shales with type I, II and III kerogens, a coal with type III kcrogen and a limestone with type II kerogen were artificially matured using the hydrous pyrolysis tech- nique (Lewan et al., 1979). The hydrocarbon prod- ucts wcrc subsequently analysed by the GC-isotope technique. In addition, hydrocarbons of oils which are considered to be mainly sourced by more mature equivalents of the analysed shales (Paris Basin and Green River) were also analysed. The hydrocarbons of the unheated source rock samples were also analysed. The purpose of this study was to determine possible trends in isotopiccomposition of n-alkanes, isoprenoids and hopanoids with increasingmaturity of typical oil and gas source rocks. Clearly, from the limited work already performed there appear to be considerablechanges in isotopiccomposition with increasing maturity. The problem is to explain the 89