Porosity and permeability variability across a chalk reservoir in the Danish North Sea: Quantitative impacts of depositional and diagenetic processes

Abstract

Abstract The Upper Cretaceous to Lower Paleocene Chalk Group in the North Sea comprises important reservoir targets for hydrocarbon and energy storage. The quantitative impacts of mineralogy, chalk diagenesis and oil saturation on petrophysics are often based on general trends that cannot be directly applied to reservoir engineering. The present study aims at identifying quantitatively the individual effects of mineralogy, compaction and cementation, and oil saturation on porosity and permeability by means of detailed samplings and mineralogical and petrophysical analyses from base to top of the Kraka reservoir. Special attention is brought to the visual inspection of the rock fabric and its variations across the reservoir in order to better constrain interpretations and establish relationships within an otherwise scattered dataset. Porosity and permeability variations are categorised into two trends, a mineralogical and diagenetic trend, which can both form tight rock. For chalk containing >7% quartz and > 4% clay, porosity decreases linearly from 35% to 21% and permeability from 0.5 mD to 0.07 mD with increasing quartz content. The amount of quartz rather than clay appears as an effective tool to estimate the petrophysical properties of impure chalk. Below 7% quartz and 4% clay, compaction and cementation decrease porosity and permeability in two steps along transects from oil- to water-bearing intervals. A first step of predominantly mechanical compaction reducing porosity and permeability from >41% to 37% and from 3 to 3.5 mD to 1.5–2.5 mD, respectively, is followed by a phase of chemical compaction and cementation that further deteriorates petrophysical properties. In Ekofisk Fm., stylolitised chalk and deposits located adjacent to chert horizons display singular petrophysical properties that do not fit the observed trends. The outliers likely result from phases of local chemical compaction and early cementation associated with chert formation. Porosity-permeability functions and uncertainty analyses are also proposed for the different controlling factors and at different scales of observation. The proposed petrophysical relationships are instrumental in building more realistic reservoir models and can assist geoscientists in interpreting and upscaling the large amount of geological data collected in chalk fields.