ABSTRACT
Biomarker analyses of the Aspen Shale (Mowry Shale equivalent), Skully’s Gap, Wyoming, suggest that vital source rock properties (e.g., kerogen hydrogen-to-carbon [H/C] ratio, Rock-Eval Hydrogen Index (HI), and maceral composition) may be indirectly, yet quantitatively, predicted from the biomarker parameters measured in oils. A strong correlation exists between the “Homohopane Index” of Aspen source rock extracts and both the H/C ratio and HI of the associated kerogen (R2 = 0.93 and 0.83, respectively). The percentages of C30 and C28 4-desmethyl steranes, relative to total extracted 4-desmethyl steranes, correlate with the percentage of marine-derived amorphous type II organic matter in the kerogen (R2 = 0.80 and 0.73, respectiely), and with kerogen H/C ratio and HI (R2 = 0.89 and 0.88, respectively, for C30 steranes; R2 = 0.89 and 0.83, respectively, for C28 steranes). The ratio of 4α-methyl-24-ethylcholestanes to 3β-methyl-24-ethylcholestanes in the extracts also correlates with kerogen H/C ratio and HI (R2 = 0.93 and 0.92, respectively). In contrast, the percentage of C29 steranes in the extracts correlates with the percentage of higher-plant-derived vitrinite in the kerogen (R2 = 0.69), and shows a negative correlation with H/C ratio and HI (R2 = 0.81 and 0.66, respectively). These relationships were used to determine indirectly the source rock kerogen properties of several Mowry-derived oils. The correlations between biomarkers and source rock properties observed in the Aspen Shale probably apply to many deltaic and nearshore marine source rocks that were deposited after the evolution of vascular plants. Therefore, prediction of source rock properties from oils in vertically drained basins could prove extremely useful to explorationists seeking to identify the most oil-prone areas in regions such as the U.S. Gulf Coast, the North Sea, the Beaufort Sea, and the Mahakam and Niger deltas. An example from the Beaufort Sea is given in this same issue (McCaffrey et al., 1994).
