Isotope fractionation during the maturation of organic matter

Abstract

The kinetics of formation and isotopic composition of methane generated by pyrolysis at 500°C from coal samples of different ranks, crude oil fractions and some pure organic compounds have been used to study the mechanisms of natural gas formation and also the concurrent changes in organic matter during thermals-induced maturation processes. Carbon isotope fractionation during formation of methane in coal pyrolysis experiments, measured relative to the isotopic compositions of the total accumulated methane, were -1.8 , -8 .8 , -10.4 and -12.1°/(subscript °°) for lignite, sub-bituminous, bituminous and anthracite, respectively, for a typical series of coal samples. The slope of the plot of isotopic composition versus the amounts of generated methane varied from 0.03 for lignite to 1.2 (°/(subscript °°) /ml/g) for anthracite. Thus both the isotope fractionation and slopes determined from experimental curves are proportional to coal rank, providing a new and novel method for determining the degree of coalification. Whereas isotope fractionation (II) shows a nice gradation from lignite to anthractie for ten samples, vitrinite reflectance measurements for two anthracites show similar values of 4 to 5 and values between 0.3 and 1.3 for the other eight consisting of samples of lignite to high rank bituminous coals. This comparison suggests that vitrinite reflectance measurements may not be as definitive as isotope fractionation for determining the degree of coalification. The isotopic compositions of methane produced by pyrolysis of coals are similar to those found for natural coal bed methane. These compositions are isotopically heavier than petroleum-derived methane by about 10°/(subscript °°) and are a reflection of a stronger bonding of methane precursor functional groups in coal than in petroleum, A careful study of the amounts of coal lipids and the methane generation from these lipids indicates that lipid fraction does not contribute significantly to the methane produced in pyrolysis experiments..