Kerogen’s Potential to Be Converted into Petroleum: Reaction Kinetics and Modelling Thermal Maturity Plus Petroleum Transformation Processes

Abstract

This chapter considers the data measurement, analysis, modelling and interpretation techniques associated with kerogen’s conversion into petroleum fluids, thermal maturity and petroleum generation in organic-rich sediments. The roles of biogenic and thermogenic processes are distinguished. The significant roles in thermogenic generation of petroleum of the variables: temperature, time, burial depth and geothermal gradients are established. Thermal maturity models based on the Arrhenius equation and the merits of a cumulative time-temperature index (\( \sum {TTI_{ARR} } \)) applying representative but simple kerogen kinetics (a single activation energies, E, and pre-exponential factor, A) for calculating specific levels of thermal maturity, correlated with the vitrinite reflectance scale, are described in detail and applied to burial history modelling of shales over geological time scales. For quantifying the cumulative transformation fraction of kerogens into petroleum a range or distribution of kerogen kinetics (i.e., several E and A combinations) is typically required to reflect the mixed kerogen content and multiple first-order chemical reactions involved in the transformation of shale into petroleum. Kerogen kinetics follow a well-defined trend of E-A values and cumulative petroleum transformation fractions using that trend of kinetic values can be modelled and optimized to accurately fit observed pyrolysis S2 peaks for a wide range of shales containing single or multiple kerogen types. Thermally immature shales are more readily modelled (i.e., more accurate fits to their pyrolysis S2 peaks obtained) than thermally mature shales. This supports the view that first-order reaction kinetics following the Arrhenius equation dominates the pyrolysis of immature shales. On the other hand, non-kinetic processes, such as micro-porosity development and connectivity, can significantly influence the pyrograms produced by thermally mature shale samples.