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Petroleum Generation Kinetics: One- Versus Multiple-Heating Ramp Open-System Pyrolysis Experiments

Abstract

Some recent publications promote one-run, open-system pyrolysis experiments using a single heating rate (ramp) and fixed frequency factor to determine the petroleum generation kinetics of source rock samples because they are faster, less expensive, and presumably yield results similar to those from multiple-ramp experiments. The purpose of this work is to compare the efficiency of various combinations of open-system pyrolysis heating rates to determine the kinetics of petroleum generation. Pyromat II® open-system micropyrolysis experiments were conducted on a large number of drill cuttings samples from a worldwide collection of source rocks at one or more of the following heating rates: 1, 3, 5, 10, 30, and 50oC/min. The resulting pyrolyzate peaks were processed using Kinetics05® software to derive chemical rate models. Although some one-ramp pyrolysis experiments using a fixed frequency factor of 1 × 1014sec indeed yield kinetic results similar to those from multiple-ramp experiments using floating frequency factors, the data illustrate that one-ramp kinetics are generally unreliable. The precision of kinetic results, as measured by calculated temperatures at 10, 30, and 90% transformation ratio, increases with the number of pyrolysis heating ramps in the range 1 to 50oC/min. However, the accuracy of these temperature predictions is unclear. The data show that kinetic results based on three different pyrolysis temperature ramps closely approximate those determined from six runs, provided that that the three temperature ramps span an appropriate range of heating rates. However, temperature ramps of 30 and 50oC/min appear to be too fast to obtain a good kinetic fit because of delayed heat transfer between the thermocouple and the sample. At least three pyrolysis ramps are recommended, such as 1, 3, and 10oC/min or 1, 3, 5, and 10oC/min. Delayed heat transfer between the thermocouple and sample may even affect these comparatively slow heating rate experiments. These conclusions do not address the more fundamental question as to whether discrete kinetic models based on open-system pyrolysis are sufficiently accurate for use in basin simulators.