Abstract: Variations in the Geochemistry of Closely Interbedded Oil-Prone Coals and Shales

David J. Curry

Paralic and deltaic environments frequently contain closely interbedded oil-prone coals and carbonaceous shales of similar organic facies. However, differences in depositional and diagenetic conditions can result in significant variations in geochemistry between these coals and shales. These variations are observed in sediments from a range of ages and areas, including Australia, New Zealand, and Indonesia. For example, pristane/phytane and pristane/n-C₁₇ ratios are frequently higher in coals than in associated shales, although n-alkane distributions are similar. The C₂₇ Ts/Tm ratios are frequently much lower in coals, although most other biomarker ratios are generally the same for coals and shales. However, absolute biomarker concentrations, particularly of C27 Tm hopane, can be two to four times higher in coals. Significantly, many coals contain high concentrations of diasteranes (equivalent to those observed in shales). Differences in other parameters such as the C₃₀ diahopane/C₃₀ hopane ratios and pyrolysate compositions are also evident. Data indicate that rocks begin to become more coal like at TOCs of approximately 25%.

The variations in these parameters are probably the result of variability in early depositional and diagenetic conditions, such as eH, pH, and microbial action. In addition, these parameters are probably influenced by the effects of an organic vs. an inorganic matrix. The impact of this variability must be considered when conducting oil-source correlation studies and when assessing the relative contributions of coals and shales to the generation of oil in a basin.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994