BURNHAM, ALAN K., Lawrence Livermore National Laboratory, Livermore, CA
Over the past decade, maturation modeling has ascended from TTI to chemical kinetics. Progress has been so fast that before some geologists have adapted to the Arrhenius equation, elaborate chemical reaction networks are being published for predicting details of oil and gas composition. But are chemical kinetics reliable? Done poorly, they can be misleading and counterproductive. Geochemists argue about the relative merits of hydrous and open-system pyrolysis kinetics, and there are no convincing theoretical arguments that either should extrapolate to geologic conditions. We aren't even sure how geologic oil potential relates quantitatively to Rock-Eval hydrogen index.
This paper addresses the abilities and limitations of chemical kinetic maturation modeling. It has been shown empirically that open-system kinetics predict the rate of oil formation in both hydrous pyrolysis and the natural environment (at least semi-quantitatively). However, ambiguity arises when trying to distinguish between expelled oil and bitumen, the role of pressure is controversial, and comparisons with nature are complicated by uncertain paleothermal histories and migration. Oil is undoubtedly stable for geologic times at temperatures greater than 150 degrees C in the absence of thermochemical oxidation, but chemical kinetic expressions for oil cracking are less certain than for oil generation. The stabilizing effect of pressure and the relative importance of thermal destruct on and migration on the oil floor are still controversial. Kinetic models of vitrinite reflectance are better than TTI but have uncertainties related to vitrinite type, pressure dependence, and interaction with bitumen from other macerals.
AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)