Iso-Frame Modeling of Marly Chalk and Calcareous Shale

Ida L. Fabricius, Department of Environment and Resources, Technical University of Denmark, Technical University of Denmark Bygningstorvet building 115, Lyngby, 2800, Denmark, phone: 004545252162, ilf@er.dtu.dk, Manika Prasad, Geophysics Department / Rock Physics Laboratory, Stanford University, 397 Panama Mall, Stanford, CA 94305-2215, and Casper Olsen, Department of Environment and Resources, Technical University of Denmark, Technical University of Denmark Bygningsstorvet building 115, Lyngby, ME 2800, Denmark.

We show iso-frame model calculations for chalks and compare them to sediment models developed mainly for granular media. The Iso-frame model helps to explain the effects of clay on velocity variations in carbonate sediments.

The Valdemar field in the North Sea is a low relief marly chalk structure, sealed from the overlying Chalk by a calcareous shale. We studied core-callibrated well log data representing a range in clay content from pure chalk to pure shale over a depth interval of 200 m.

For the pure chalk the acoustic velocity varies widely: from 2 to 4 km/s, whereas the range gradually narrows with incresing clay content to 2 to 3 km/s for intervals with more than 60% clay. The velocity variation is largely a reflection of the porosity: in pure chalk porosity varies between 15% and nearly 50%, whereas the range narrows to 15%-35% in clay rich intervals. In order to assess the influence of clay on velocity we thus need a porosity-independent measure.

We calculated iso-frame (IF) values based on modified upper Hashin-Shtrikman bounds. IF indicates (on a scale from 0 to 1) to which extend the solids are part of the load-supporting frame of the sediment. For the pure chalk, IF varies from 0.4 to 0.7. IF decreases with increasing clay content to a low of 0.1 - 0.5 at 40% clay and from there increases with increasing clay-content to 0.6 to 1.0 at 90% clay. Up to 40% clay thus softens the chalk, more clay stiffens the sediment.

New Approaches in Carbonate Stratigraphy and Reservoir Modeling
2004 AAPG International Conference and Exhibition Technical Program