ABSTRACT: Differences and Similarities in the Geochemistry of Closely Interbedded Coals and Shales

CURRY, DAVID J., Exxon Production Research Company, Houston, TX

Coals and interbedded shales frequently contain the same type of organic matter and can be very similar geochemically. However, differences in depositional and diagenetic conditions between clastic-poor and clastic-rich environments can cause significant differences, especially at the molecular level, between coals and shales from the same organic facies.

These differences and similarities are apparent in Eocene cores from the Taranaki basin of New Zealand. Pristane/phytane ratios are generally higher in the coals than the shales. There are no differences, however, in pristane/n-C17 ratios, n-C27/n-C17 ratios, and CPIs between the coals and shales. The C27 Tm/Ts ratios are usually higher in coals, but other biomarker ratios are generally the same for coals and shales. However, the absolute biomarker concentrations are as much as two to three times higher in the coals than the shales. Significantly, the concentration of rearranged steranes in the low ash coals approaches those in the shales. Differences and similarities in other parameters (e.g., isotopic distributions, pyrolysate compositions) are also evident. These same effects are a so observed in other basins.

These effects are probably the result of differing depositional and early diagenetic environments, including differences in the pH-eH conditions and microbial action. These differences, however, may be diminished with increased maturity. The impact of these similarities and differences in geochemistry on various oil source correlation parameters is illustrated with examples from different basins.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)