The influence of depositional processes on the porosity of chalk

Abstract

Chalk constitutes challenging low-permeability reservoirs with porosity variations attributable to complex interactions between numerous processes. The influence of depositional processes, and thus the value of depositional models to predict porosity, is subject to continuing debate. In this study, a new approach is applied to investigate the influence of depositional and early post-depositional processes on chalk porosity, based on the 303 m thick Upper Cretaceous chalk succession in the Mona-1 core from the Danish North Sea. The influence of depositional processes on porosity is isolated by a mathematical correction of porosity data. Results confirm that mass-transport deposits are on average more porous than pelagites, whereas turbidites are less porous, given similar composition, burial history, and hydrocarbon migration history. The porosity variation between 12 chalk facies suggests that grain packing of the sediment in the consolidated state caused the facies-dependent porosity variation. Bioturbation caused a relatively tight grain packing compared with deposits that escaped bioturbation. Early plastic shear deformation of tightly packed bioturbated units resulted in dilative behaviour, which increased porosity, whereas more loosely packed units responded contractively, resulting in decreased porosity preservation. A firm understanding of chalk facies and thorough facies analyses are thus considered instrumental in chalk reservoir prediction.