Factors affecting Rock-Eval derived kinetic parameters

Abstract

Absolute temperatures were measured inside a Rock-Eval crucible in order to determine the true starting and ending temperatures, heating rates, and oven gradient. Using this experimental data enabled computation of kinetic parameters which compare favorably (within 2 kcal/mol) to those generated in other laboratories. Kinetic parameters were determined on a variety of Monterey formation samples as well as other potential oil and gas source rocks. Differences within a given organic facies and across kerogen types range from 1 to 9 kcal/mol. Specific differences not readily apparent from kerogen typing plots are apparent in pyrolysis peak shape analysis which is essentially a gross kinetic measurement. A kerogen typing plot was constructed using a mathematical estimate of pyrolysis peak shapes. This enabled discernment of compositional differences within a given kerogen type. Changes in kinetic parameters due to maturation were illustrated by artificially maturing an immature Monterey sample from a starting T max value of 395° to 433°C. The shift in the principal activation energy is 4.5 kcal/mol with a corresponding shift in the frequency factor. Kinetic parameters from the various source rocks detailed in this paper were used to describe the onset of generation in an arbitrary burial and thermal history model. The onset of generation is found to vary over 5300 ft depending on the type of organic matter modeled although differences in processes and maturity are indicated in some cases. An experimental error of 2 kcal/mol in kinetic calculations resulted in an error of 768 ft when locating the onset of generation for Green River shale. The effect of a distribution of activation energies and associated frequency factor on the onset of generation versus a single, principal activation energy from that distribution is shown to alter the onset of generation by approximately 500 ft.