Engineering properties of chalk related to diagenetic variations of Upper Cretaceous onshore and offshore chalk in the North Sea area

Predicting properties of reservoir chalk involve testing and modelling of easily obtainable outcrop chalk. However, variations in onshore and offshore chalk properties will expectedly occur due to different diagenetic history. Understanding diagenetic mechanisms and their consequences are thus a key issue when reservoir chalk characterization is based on outcrop chalk. In chalk of the Valhall field high pore pressure and hydrocarbon presence have counteracted recrystallization and cementation and preserved high porosity despite a burial depth of c. 3 km. As a consequence interparticle connections are poorly developed and Valhall chalk is expectedly geomechanically weak. Similar high porosity and weak consolidation were observed in Danish outcrop chalk which expectedly will match the Valhall samples geomechanically. In chalk of the Dan, South Arne and Ekofisk fields recrystallization and cementation features are significantly better developed compared to the Valhall field. Calcite redistribution has strengthened particle contacts and reduced porosity, assumingly providing a geomechanically stronger chalk. Similar low porosity and well-developed recrystallization and cementation features are found in English chalk which from a geomechanical point of view may share characteristics with chalk of the Dan, South Arne and Ekofisk fields. Matrix permeability is related not only to porosity but also to specific surface area of chalk which is controlled partly by calcite recrystallization partly by type and amount of non-carbonate minerals. In onshore chalk significant mineralogical differences are reflected in varying specific surface area. In contrast, specific surface area of Upper Cretaceous reservoir chalk show little variation indicating that care must be taken when choosing an outcrop chalk as substitute for reservoir chalk during flooding experiments. (C) 2009 Elsevier B.V. All rights reserved.