A reaction kinetic approach to the temperature-time history of sedimentary basins

Three biological marker reactions have been studied in order to determine the temperature — time history of a sedimentary sequence. Two of these reactions are configurational isomerization reactions, at C-20 in a C29-sterane and at C-22 in C31 and C32 hopane hydrocarbons. In the third reaction two C29 C-ring monoaromatic steroid hydrocarbons convert to a C28 triaromatic one. The progress of these reactions is different because of their different rate constants. Based on temperature and age data obtained from field measurements and on concentration measurements of reactants and products in core samples of a Pannonian borehole, we calculated the rate parameters: pre-exponential factors, enthalpies and entropies of activation. It is obvious, that at least two different reactions are necessary to characterize the maturity of any system. The aromatization seems to be a rather complicated reaction, and we believe its use to be premature. Fortunately, two isomerizations work well and are suitable for elucidation of thermal history in different basins if the rate constants are universally valid. The pre-exponential factors and activation enthalpies are 2.4·10−3 s−1 and 91.6 kJ mol−1, 3.5·10−2 s−1 and 87.8 kJ mol−1 for isomerization of steranes and hopanes, respectively. It was demonstrated, that by measuring the extent of the two-mentioned or other specially-chosen reactions in a lot of layers, the determination of the temperature-time history will be possible within a given temperature-time range.