The kinetics of specific organic reactions in the zone of catagenesis

Abstract

Recently it has been shown that the measured extent of reactions in sedimentary biological marker compounds during catagenesis (for example, steroid aromatization, sterane isomerization) can assist in providing information about the extent of maturation of sedimentary organic matter before and during oil generation and also the thermal history of sediments. In the present study, aromatization of a C 27 C-ring aromatic steroid and isomerization at the chiral centres of an isoprenoid alkane, 6( R ),10( S )-pristane, have been brought about under laboratory conditions, elemental sulphur being used as the source of radicals. Precise rate laws have been determined and rate coefficients measured at different temperatures. The pre-exponential factors and activation energies were found to be 6.7 x 10 12 s -1 and 145 kJ mol -1 for the aromatization, and 2.1 x 10 7 s -1 and 120 kJ mol -1 for the configurational isomerization of pristane. These values reflect the relative behaviour of the two types of reactions in sediments, the aromatization being the more temperature-dependent reaction.