The conversion of oil into gas in petroleum reservoirs .1. Comparative kinetic investigation of gas generation from crude oils of lacustrine, marine and fluviodeltaic origin by programmed-temperature closed-system pyrolysis

The thermal alteration of reservoired petroleum upon burial was simulated comparatively by closed-system programmed-temperature pyrolysis of produced crude oils of lacustrine, fluviodeltaic, marine elastic and marine carbonate origin using the microscale sealed vessel (MSSV) technique. Bulk kinetics of oil-to-gas cracking and accompanying compositional changes were studied at heating rates of 0.1, 0.7 and 5.0 K/min. The oil type related variations of experimental cracking temperatures were small compared to those related to heating rate, but the high-temperature shift of gas evolution curves with increasing rate of heating turned out to be more pronounced for the marine than for the non-marine oils. Accordingly the kinetic frequency factors were derived to be higher for gas generation from the lacustrine and fluviodeltaic oils (A approximate to 4.10(19) min(-1)) than from the marine oils (A approximate to 2.10(18) min(-1)) and the min gas potential vs. activation energy distributions were calculated to be centered around 71-72 kcal/mol for the former and around 67 kcal/mol for the latter. These kinetic parameters and compositional observations give some evidence that gas generation is accompanied by the formation of aromatic compounds in the case of the marine oils whereas alkene intermediates seem to be involved in the case of the non-marine high wax oils. Under geological heating conditions (e.g. 5 K/My), the onset of gas generation and peak gas generation are extrapolated to occur at about 180 degrees C and 225 degrees C for the high wax oils. The marine oils turn out to be slightly less stable with peak gas generation at 215 degrees C and the onset of decomposition reactions predicted at about 170 degrees C. In the absence of reservoir bitumen and minerals severe oil-to-gas cracking is very unlikely to take place at temperatures less than 160 degrees C, whatever the crude oil type or the geological heating rate. (C) 1997 Elsevier Science Ltd.