A diagenesis model for geomechanical simulations: formulation and implications for pore pressure and development of geological structures

Abstract

Forward basin modelling is routinely used in many geological applications, with the critical limitation that chemical diagenetic reactions are often neglected or poorly represented. Here, a new, temperature‐dependent, kinetic diagenesis model is formulated and implemented within a hydromechanical framework. The model simulates the macroscopic effects of diagenesis on: 1) porosity loss, 2) sediment strength, 3) sediment stiffness and compressibility, 4) change in elastic properties, 5) increase in tensile strength due to cementation and 6) overpressure generation. A brief overview of the main diagenetic reactions relevant to basin modelling is presented and the model calibration procedure is demonstrated using published data for the Kimmeridge Clay Formation. The calibrated model is used to show the implications of diagenesis on prediction of overpressure development and structural deformation. The incorporation of diagenesis in a uniaxial burial model results in an increase in overpressure of up to 9 MPa due to both stress‐independent porosity loss and overpressure generated by disequilibrium compaction caused by a reduction in permeability. Finally, a compressional model is used to show that the incorporation of diagenesis within geomechanical models allows the transition from ductile to brittle behaviour to be captured due to the increase in strength that results in an over‐consolidated stress state. This is illustrated by comparison of the present day structures predicted by a geomechanical‐only model, where a ductile fold forms, and a geomechanical model accounting for diagenesis in which a brittle thrust structure is predicted.