Use of stable carbon isotope compositions of pyrolytically derived methane as maturity indices for carbonaceous materials

Nineteen coals of varying ranks and twelve organic-solvent-extracted shales of different ages and maturities were subjected to a long-term, closed system, isothermal (5000C) pyrolysis procedure until no additional methane or carbon oxide gases were generated. Three significant trends were observed: (1) For all of the samples, the stable carbon isotope difference between the total accumulated methane and parent carbon was linearly correlated with yield of methane with r = 0.88. (2) For a series of thirteen model compounds, two lipid-free shales and twelve coals, the H/C ratio of each was linearly correlated with the yield of methane with r = 0.93. (3) For a series of eleven coals samples, the O/C ratio of each was linearly correlated with the yield of carbon oxides with r = 0.97. The experimental relationships described above indicate that a simple pyrolysis procedure and measurements of the amounts of generated CH4, CO2 and CO relative to the carbon content of the sample and the stable isotopic compositions of the methane and parent organic carbon give (a) an index of the maturity and (b) values which are directly related to the H/C and O/C ratios of the carbonaceous materials. (author)