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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California