How to get relations between bedrocks and δ13C values of methane and ethane in natural gases?

Basing on the hypothesis that δ13C values of different carbon positions within biomolecules are thermodynamically controlled a model was derived to evaluate δ13C values of methane and ethane in the process of thermal genesis from organic matter in bedrocks. To get information about genesis and postgenetic history of a certain natural gas it is necessary to measure both, δ13C of methane (C1) and of ethane (C2), too. The isotopic composition of ethane will be nearly unchanged after genesis. In contrast to this the isotopic composition of methane can be changed drastically by molecular diffusion. Therefore δ13C of ethane is a well defined indicator of the genesis conditions. (author)