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 simula ted 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) tech nique. Bulk kinetics of oil-to-gas cracking and accompanying compositi onal 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-tem perature shift of gas evolution curves with increasing rate of heating turned out to be more pronounced for the marine than for the non-mari ne 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 approx imate to 2.10(18) min(-1)) and the min gas potential vs. activation en ergy distributions were calculated to be centered around 71-72 kcal/mo l 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 th e case of the marine oils whereas alkene intermediates seem to be invo lved in the case of the non-marine high wax oils. Under geological hea ting conditions (e.g. 5 K/My), the onset of gas generation and peak ga s 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 slight ly less stable with peak gas generation at 215 degrees C and the onset of decomposition reactions predicted at about 170 degrees C. In the a bsence 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 E lsevier Science Ltd.