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Guidelines for Kinetic Input to Basin and Petroleum System Models

Abstract

Accurate kinetic parameters for the thermal decomposition of source-rock kerogen to oil and gas are needed for reliable computerized basin and petroleum system models (BPSM). This paper provides guidelines for the use of kinetic parameters in BPSM based on data for 81 worldwide source rocks containing types I, II, IIS, II/III, and III kerogen plus calibrated model results for several exploration wells, such as the Aurora-1 well, North Slope Alaska. (1) Kerogen type as defined by Rock-Eval pyrolysis hydrogen index of thermally immature source rock is not linked to kinetic response. For example, the kinetics for type II kerogen from one basin may be unlike those in another. (2) Kinetic parameters measured on thermally immature equivalents of the source rock in the study area are recommended. Use default kerogen kinetics with caution when appropriate samples are unavailable. (3) Descriptions of depositional environment are generally insufficient to define kerogen type or kinetic response. For example, lacustrine source rock from one basin can contain various kerogen types, each having different kinetic parameters. (4) Kerogen kinetics can vary laterally and vertically in source rock. If possible, confirm kinetic variations by measurements. (5) Hydrous and single-ramp programmed pyrolysis kinetics are not recommended because they may not adequately assess the discrete activation energy distribution of the source rock kerogen. Multiple-ramp kinetics are recommended where both the activation energy (Ea) and frequency factor (A) are optimized by the kinetic software. (6) Kinetic uncertainty can be described by the 1-2-3 rule. Because of the Arrhenius compensation law, a 1°C error in the measurement of Ea is compensated by a twofold adjustment of the frequency factor in order to maintain the same calculated laboratory pyrolysis reaction rate. When such erroneous kinetics are extrapolated to geologic time, the corresponding error in predicted temperature is ~3°C. Assuming a universal Ea of 1 × 1014/sec rather than optimizing both Ea and A can result in temperature errors of 20°C or more when extrapolated to geologic time. (7) Easy%Ro may be less accurate than Basin%Ro for calibration of BPSM. Basin%Ro more accurately replicates the dogleg in vitrinite reflectance versus depth that is commonly observed at depths corresponding to ~0.7 to 1.0% Ro, where hydrogen index decreases due to kerogen transformation after being approximately uniform at lower maturity.