The kinetics of in-reservoir oil destruction and gas formation: constraints from experimental and empirical data, and from thermodynamics

Experimental kinetic data on the reactions of pure chemicals, destruction o f heavy hydrocarbons, and gas formation have been combined with thermodynam ic theory and empirical data on oil and gas occurrences to constrain the ra nge of plausible activation energies and frequency factors for oil destruct ion and gas formation in nature. It is assumed explicitly here that the kin etics of oil destruction and gas formation can be adequately described usin g a set of parallel first-order reactions. At geologic temperatures and pre ssures the mean activation energy for oil destruction and gas generation is about 59 kcal/mol (246.9 kJ/mol), with a frequency factor of about 10(14.2 5) s(-1) (1.78.10(14) s(-1)). A narrow distribution of activation energies [sigma=1.5 kcal/mol (6.3 kJ/mol)] for destruction of oil seems intuitively more reasonable than a single activation energy, and also seems to fit empi rical data on high-temperature occurrences of condensate slightly better. N o large or systematic variation in cracking rates or kinetics is apparent f or different oil types. Using these recommended kinetic parameters, the max imum temperature at which oil can be preserved as a separate phase varies f rom about 170 degrees C at geologically very slow heating rates to slightly over 200 degrees C at geologically extremely fast heating rates. Using thi s model, oil destruction occurs at slightly higher temperatures than those predicted by older kinetic models, but at considerably lower temperatures t han those suggested by some recent studies. Differences in predicted levels of cracking obtained from the various models in use today can affect explo ration decisions. (C) 2000 Elsevier Science Ltd. All rights reserved.