VARIATION IN STABLE CARBON ISOTOPE RATIOS OF INDIVIDUAL HYDROCARBONS AS A FUNCTION OF ARTIFICIAL MATURITY

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 terpenoids in the pyrolysis products become depleted in C-12. The most marked depletion occurs in the terpenoids, particularly in the C29-C31 hopanes. These compounds are isotopically very 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 C-12 at 330-degrees-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 C-12 depletion. Fairly uniform values were recorded for the hopanes in pyrolysates of all samples heated at temperatures of 350-degrees-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.