MATURATION EFFECTS ON ABSOLUTE BIOMARKER CONCENTRATION IN A SUITE OF COALS AND ASSOCIATED VITRINITE CONCENTRATES

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. Geoche mical differences among the samples are due to rank, with vitrinite re flectances ranging from 0.53 to 1.76%. The sample suite affords an exc ellent opportunity to evaluate the effects of coal rank on a rang of c ommonly used molecular indicators of thermal maturity. Absolute concen trations of molecular constituents, rather than peak ratios, were dete rmined based on addition of internal standards (5 beta cholane and d(1 0)-anthracene) to the extractable organic matter prior to separation b y liquid chromatography. Absolute concentrations of C-29 alpha alpha a lpha 20R and 20S sterane isomers provide clues to the processes involv ed in the apparent ''sterane epimerization'' reaction. The 20R isomer is initially present in the free state in much higher concentrations t han the 20S isomer. With increasing thermal evolution of the kerogen, both isomers are liberated from the kerogen with increasing proportion ality of 20S, resulting in a net increase in 20S/(20R + 20S) ratio. Th ere appears to be little evidence for epimerization of molecules in th e free state. Kinetic modeling of sterane epimer evolution therefore r equires a more elaborate reaction scheme involving both formative and degradative reactions, as opposed to a simple A-to-B conversion. The a mount of 1-MDBT remains constant over a wide maturity range, signifyin g equal rates of formation and destruction of this compound relative t o the total EOM. The amount of 4-MDBT shows a general increase with ma turity, the most pronounced change occurring at maturities above about 1.3% R(m). The ratio of 4/1 MDBT (''MDR parameter'') therefore appear s to be particularly useful for maturity assessment at elevated levels (e.g. light oil/condensate range). Some source-dependent molecular pa rameters, such as pristane/phytane, C-27/C-29 sterane, 4 beta(H)-eudes mane/8 beta(H)-drimane and C-30 hopane/C-23 cheilanthane, were found t o change as a function of coal rank. The most pronounced changes occur at higher levels of maturity (R(m) > 1.1 %), where these molecular so urce 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 af fect 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 st udies involving non-marine coaly systems.