Diagenetic Variations Between Upper Cretaceous Outcrop and Deeply Buried Reservoir
Chalks of the
Hjuler, Morten
Leth1, Ida Fabricius1 (1)
In the central
Predicting geomechanical,
petrophysical, petrochemical and petrographical
properties of reservoir chalks often involve describing, testing or modeling of
the easily obtainable outcrop chalks. However, as diagenetic
alterations of onshore and offshore chalks are not identical, accurate
descriptions of reservoir chalks using outcrop chalks as models depend heavily
on our ability to understand and describe diagenetic
mechanisms. This study describes and compares petrophysical
and petrographical differences between various Upper
Cretaceous outcrop and reservoir chalks.
Few differences were detected between
reservoir and outcrop chalk regarding porosity, permeability, carbonate
content, grain density and specific surface area. However BSE, SEM and isotope
analysis show significant cementation and reshaping of particles (overgrowth)
together with strengthening of particle contacts in the deeply buried reservoir
chalks. In contrast outcrop chalks are moderately affected with less developed
cementation features, looser inter-particle connections and particle shapes
remaining closer to original morphology.
The non-carbonate mineralogy of outcrop
chalks is dominated by quartz, occasionally opal-CT and clinoptilolite,
and the clay mineral smectite. In offshore chalks
quartz still dominates, opal-CT has recrystallized in
to submicron-sized quartz crystallites and smectite
has been replaced by kaolinite.
Higher temperatures and pressures as a
consequence of much deeper burial for longer periods of time explain the fact
that reservoir chalks have suffered more extensive diagenetic
alterations than outcrop chalk.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California