Establishing credible reaction-kinetics distributions to fit and explain multi-heating rate s2 pyrolysis peaks of kerogens and shales

Abstract

Extracting reaction-kinetic distributions, in terms of activation energies (E) and pre-exponential factors (A), from the S2 peak data generated by pyrolysis tests conducted at three or more distinct heating ramps, is a well-established technique. These reaction-kinetics distributions are of paramount importance in establishing the timing and degree of petroleum generation from shales undergoing a range of burial and thermal histories. A commonly adopted approach is to determine and define reaction kinetics using a derivative of the Arrhenius equation configured in terms of a fixed/constant A value. Although the fixed-A approach can obtain good fits to multi-rate pyrolysis data, here it is shown that a formulation of the Arrhenius equation that involves reactions with a range of E and A values provides equally good fits to the multi-rate pyrolysis data. Moreover, the kinetic distributions with variable E-A provide more credible reaction kinetics consistent with those established for a range of kerogen types known for decades. To establish accurate fits to multi-rate pyrolysis S2 peak data at 1◦C intervals from 250 to 700◦C an optimizer is applied to the preferred Arrhenius equation formulation to derive reaction increments and transformation fractions to a range of reaction kinetics (E-A pairs). The methodology applied involves two steps: Step 1 finds the single E-A pair that best matches the S2 peak temperatures (three or more for multi-rate pyrolysis data); step 2 uses the E-A pair from step 1 as its modal focus and fits the full S2 peak shape using a distribution of 11 distinct reaction. This approach can replicate the fixed-A approach but is best applied using reactions with variable E-A values. The results of applying this method to multi-rate pyrolysis data for ten published kerogens and shales show credible kinetic distributions spread along the established E-A trend for kerogen/shales.