Hydrogen and carbon isotopic composition of coals and kerogens
Abstract
Hydrogen and carbon bearing constituents depleted in 13C and D with respect to their parent material are released during coalification. The residue should therefore be continuously enriched in both isotopes with increasing maturity. Published pyrolysis experiments have shown methane to be the only compound evolved in significant quantities during heating that also fractionates to a large degree in carbon and hydrogen. Theoretical considerations predict fractionations of more than 5‰ in δ13C and approximately 200‰ in δD for high maturity coals and kerogens. Our investigations of type kerogens have shown minimal carbon isotope fractionation and 10 to 25‰ fractionation in δD. We have thus concluded that at most only approximately 10% of the hydrogen reservoir could have been converted to methane in a direct single-step reaction. Investigations of a contact metamorphosed Tertiary lignite have yielded similar results. This confirms the trend found in the type kerogens. Large hydrogen isotopic variations were found in coals occurring world-wide (-170 to -90‰). These must be interpreted as primary, source controlled variations; as (i) there is no correlation between maturity and δD values, and (ii) the Gondwana coals (-170 < δD < -135) are distinctly different from the American and European coals (-140 < δD < -90). We conclude that methane can only be formed in a single-step reaction from kerogen in significant quantities during late stage metagenesis and metamorphism and that the major portion of hydrocarbons released during catagenesis and early metagenesis is released as a liquid phase from which methane is cracked.
- Publication:
-
Physics and Chemistry of the Earth
- Pub Date:
- 1980
- DOI:
- 10.1016/0079-1946(79)90152-6
- Bibcode:
- 1980PCE....12..711R