Abstract: Predicting Variations in Source Rock Quality using Biomarkers in Oils and Bitumens from the Mowry Shale, Wyoming

J. M. Moldowan, M. A. McCaffrey, P. A. Lipton, S. C. Teerman

Steranes provide powerful correlation, maturation and biodegradation parameters for the petroleum geochemist. We have examined distributions of C₂₇, C₂₈, C₂₉, and C₃₀ steranes using gas chromatography-mass spectrometry-mass spectrometry (GC-MS-MS). Among the C₃₀ steranes, 24-n-propylcholestanes are commonly used to indicate marine algal source input. The ratio of 24-n-propylcholestanes to C₂₇-C₃₀ steranes can be used to indicate source rock quality in deltaic and shallow to deep marine rock

sequences. For example, this ratio correlates with % microscopically identified oil prone Type II kerogen in Lower Cretaceous (Mowry or Aspen Shale) sequences from Wyoming. The ratio C₂₈/C₂₇-C₃₀ steranes shows the same trend. In contrast, the ratio C₂₉/C₂₇-C₃₀ steranes correlates with % microscopically identified vitrinite macerals in the same rock sequence. These relationships hold equally for the related diasteranes. The same measurements made on oils that are sourced from this Lower Cretaceous sequence provide information about the quality of the source rocks from which they were generated. Thus, a basin-wide map of source rock quality can be prepared based on biomarker measurements on oils. An application of these c ncepts will be discussed, where source-rock quality has been mapped based on biomarker analyses of oils related to a deltaic source in a vertically drained basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994