Impact of mineralogy and diagenesis on porosity and permeability of chalk in the Ekofisk formation (Kraka field, Danish

"Decrease in permeability and porosity with increase in clay and quartz content is a relationship used to explain, among other factors, changes in petrophysical properties across tight and porous chalk reservoirs. However, superimposed depositional and diagenetic processes recorded in the rock fabric alter the signal and correlations remain difficult to demonstrate. Using petrophysical measurements and core, XRD, XRF, and BSEM analyses, the present study aims at investigating the influence of depositional and diagenetic processes on porosity and permeability with an interest in the role of mineralogy. Two trends are observed. Samples with less than 5% quartz and 4% clay display a wide range of permeability and porosity values, respectively. Petrophysical properties are controlled by compaction and cementation, which are more pronounced towards the base of the reservoir. Locally, calcite cementation associated with the formation of silicified condensed surfaces decreases porosity below 30%, whereas permeability remains above 1 mD. For samples with more than 5% quartz and 4% clay, permeability reduces with an increase in nano-quartz content, whereas clay content influences porosity. Together with mineralogical data, visual inspection of the rock fabric enables to quantitatively assess the effective impact of mineralogy on petrophysical properties, which would be, otherwise, challenging to extract".