Comparison of Pyrolysis Products from Different Subbituminous Coals
Shan-Tan Lu, Ed Ruth, Isaac R. Kaplan
Two different immature subbituminous coals (Australia, Gippsland, Latrobe, an oil-prone coal, and Rocky Mountain Upper Cretaceous, a gas-prone coal) were selected to conduct pyrolysis at 300°C from 2 to 1,000 hours. The liquid hydrocarbon, i.e., C12+-saturated and aromatic hydrocarbons generated in the Australia Gippsland Coal (AGC), was up to four times higher than those formed from Rocky Mountain coal (RMC), although the amount of gaseous hydrocarbon and starting H/C ratio is similar for both coals. Furthermore, in the extracts the distribution of n-alkane is similar; both have high OEP in the range of n-C25 to n-C34. Pristane/phytane ratios are 5.01 and 4.51, and ^dgr13C is -25.7 ^pmil and -24.9 ^pmil in RMC and AGC, res ectively. However, the amount of diterpenoid hydrocarbon was unusually high in the original bitumen of the RMS, whereas triterpane was the major component in that of AGC.
Hopane distribution obtained from both the bitumen and pyrolysates of both coals shows that C31 ^agrß 22R homohopane was released at very early stages of diagenesis. It is also possible that the C31 ^agrß homohopane precursor was unbound to kerogen because it predominated in the original bitumen but occurs only to a small extent in the pyrolysates.
Surprisingly, the distribution of biomarkers in the pyrolysates of RMC look more immature than those of AGC, although the vitrinite reflectance values (Ro) of the starting and residual coal kerogen were consistently higher in the RMC than those in the AGC. Nevertheless, the maturation index, e.g., C29 ^agr^agr^agr 20 S/S + R and C31 and C32 ^agrß 22 S/S + R, increased with maturity (increased with the Ro of vitrinite reflectance) and heating time during pyrolysis. This suggests that although biomarkers may give consistent maturation trends in a given basin, these trends can vary significantly with different types of humic coals.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.