--> Abstract: Diagenetic Variations Between Upper Cretaceous Outcrop and Deeply Buried Reservoir Chalks of the North Sea Area; #90063 (2007)

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Diagenetic Variations Between Upper Cretaceous Outcrop and Deeply Buried Reservoir Chalks of the North Sea Area

 

Hjuler, Morten Leth1, Ida Fabricius1 (1) Technical University of Denmark, 2800 Kgs. Lyngby, Denmark

 

In the central North Sea Basin hydrocarbon bearing chalks are deeply buried (2-3 km) whereas chalks in the rim areas are cropping out in the surrounding countries.

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chalks quartz still dominates, opal-CT has recrystallized in to submicron-sized quartz crystallites and smectite has been replaced by kaolinite.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