Chalk Reservoir Properties Controlled by Composition, Age, Temperature, Burial Stress and Pore Pressure – Relationship to Rock Physical Properties

Fabricius, Ida Lykke¹, Morten Leth Hjuler² (1) Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark (2) Technical University of Denmark, Kgs. Lyngby, Denmark

Reservoir properties of chalk are controlled by primary sediment composition and subsequent diagenesis. The resulting porosity is mainly reflected in bulk density. Pore size is reflected in density and Biot's coefficient. Permeability and capillary entry pressure are reflected in density, Biot's coefficient and Poisson's ratio.

The primary sediment composition varies as a reflection of proportion of calcareous and siliceous microfossils and nannofossils as well as siliciclastic quartz, clay, and feldspar. As the sediment ages and as temperature rises due to burial, the components equilibrate chemically: in pure chalk, chalk particles grow and fuse creating larger, stiffer and smoother pores. Biogenic silica first dissolves and reprecipitates as fine, permeability-reducing opal-CT, which equilibrates first to fine, dispersed quartz, later to larger quartz crystals. Feldspar may dissolve and reprecipitate as permeability-reducing kaolinite.

As the sediment is buried, vertical stress increases, and in case of rapid burial, pore pressure also increases, causing a regionally varying effective stress –depth relationship. The effective stress causes porosity to be reduced by mechanical compaction, which vanes exponentially with depth. Where burial is slow, this process may in pure chalk be halted by the pore-stiffening process. Where the calcareous component is load-bearing, and clay becomes squeezed between calcareous particles, stylolites and flaser structures arise causing chemical compaction, leading to marked increase in elastic moduli.