A KINETIC CALCULATION METHOD OF HOMOHOPANOID MATURATION - APPLICATIONS IN THE RECONSTRUCTION OF BURIAL HISTORIES OF SEDIMENTARY BASINS

Hydrous pyrolysis, performed at relatively low temperatures, of an imm ature marl yielded artificial maturation data on changes in homohopane isomer compositions. At higher temperatures enhanced amounts of 17 al pha;21 beta(H)- and 17 beta,21 alpha-hopanes over 17 beta,21 beta(H)-h opanes are observed. The amounts of 22S epimers increase relative to t hose of their 22R counterparts with increasing temperature. These expe rimental data, as well as sedimentary data on homohopane concentration s, have been reproduced using a kinetic calculation method, based on t he thermodynamic stabilities determined by molecular mechanics calcula tion, using a full isomerization scheme rather than a single direct ch iral isomerization step. We need to caution, however, that agreement b etween the predictions from a theoretical scheme and quantitative meas urements is not an unequivocal proof for the chosen kinetic model. Goo d reproduction of the maturation data was obtained even for those obta ined at the highest pyrolysis temperatures, at which significant destr uction of hopane isomers was apparent. These results indicate that the kinetic calculation method can be used to model diagenetic and catage netic reactions affecting biomarker ratios in sediments. Further compu ter simulations indicate that hopane-hopene interconversion reactions probably only have a minor influence on hopane isomer ratios. Copyrigh t (C) 1997 Elsevier Science Ltd.