Maturation effects on absolute biomarker concentration in a suite of coals and associated vitrinite concentrates

Abstract

Extensive bulk chemical and petrographic characterization of 37 coals and vitrinite concentrates of the Lower Kittanning seam (Carboniferous) from western Pennsylvania and eastern Ohio shows that the coals are compositionally similar and represent a uniform organic facies. Geochemical differences among the samples are due to rank, with vitrinite reflectances ranging from 0.53 to 1.76%. The sample suite affords an excellent opportunity to evaluate the effects of coal rank on a range of commonly used molecular indicators of thermal maturity. Absolute concentrations of molecular constituents, rather than peak ratios, were determined based on addition of internal standards (5β cholane and d 10-anthracene) to the extractable organic matter prior to separation by liquid chromatography. Absolute concentrations of C 29 ααα 20 R and 20 S sterane isomers provide clues to the processes involved in the apparent “sterane epimerization” reaction. The 20 R isomer is initially present in the free state in much higher concentrations than the 20 S isomer. With increasing thermal evolution of the kerogen, both isomers are liberated from the kerogen with increasing proportionality of 20 S, resulting in a net increase in 20S (20R + 20S) ratio. There appears to be little evidence for epimerization of molecules in the free state. Kinetic modeling of sterane epimer evolution therefore requires a more elaborate reaction scheme involving both formative and degradative reactions, as opposed to a simple A-to-B conversion. The amount of 1-MDBT remains constant over a wide maturity range, signifying equal rates of formation and destruction of this compound relative to the total EOM. The amount of 4-MDBT shows a general increase with maturity, the most pronounced change occurring at maturities above about 1.3% R m. The ratio of 4 1 MDBT (“MDR parameter”) therefore appears to be particularly useful for maturity assessment at elevated levels (e.g. light oil/condensate range). Some source-dependent molecular parameters, such as pristane/phytane, C 27 C 29 sterane, 4β(H)-eudesmane/8β(H)-drimane and C 30 hopane C 23 cheilanthane , were found to change as a function of coal rank. The most pronounced changes occur at higher levels of maturity ( R m > 1.1%), where these molecular source indicators take on a more “marine” or “deep-water lacustrine” signature, even though there is no change in depositional environment of the coaly facies. At lower levels of maturity corresponding to the typical oil window, the changes are less pronounced, but may still affect attempts to correlate a crude oil with an immature sample of its suspected coaly source rock. The maturity dependence of these “source” parameters should be taken into account in petroleum geochemical studies involving non-marine coaly systems.