Thermal stability of alkylaromatics in natural systems: Kinetics of thermal decomposition of dodecylbenzene

The aim of this work is to determine the apparent rate, constants for dodecylbenzene (DDB) thermal cracking in laboratory conditions in order to evaluate alkylaromatic stability in geological conditions. Pyrolysis experiments were carried out under Argon:atmosphere, in anhydrous closed system (gold bags) during times,ranging from 1 to 72 h under isothermal conditions (325-425 degreesC). The global rate constants were determined based on the reactant. conversions obtained at various temperatures. For all temperatures investigated, bulk. decomposition of the DDB obeys first-order kinetics and the resulting apparent activation energy, derived from an Arrhenius diagram is found at 53.3.kcal/mol and the corresponding frequency factor A at 1.3. x 10(13) s(-1). When these kinetic parameters are. used for predicting global rate constants at lower temperatures, results show that the DDB should decompose below 166 degreesC assuming a constant thermal history at 1.25 degreesC/my. This means that, even within source rocks, the monoalkyl aromatics are likely to decompose during the, late stage of kerogen cracking. Consequently, depending on the timing of petroleum expulsion, the I expelled fluid may be depleted in alkyl aromatics due to their partial secondary cracking before the migration threshold is reached. The major products observed during DDB cracking are on-one hand, a light C-7-C-14 fraction dominated by decane (n-C-10) and undecene/undecane (alpha-C-11/n-C-11) toluene and ethyl benzene and on the other hand, heavy C14+ aromatic fraction. For DDB conversion lower than 88%, hydrocarbon gas generation was negligible and no insoluble residue was observed.