Iso-frame Modeling of Marly Chalk and Calcareous Shale.
Ida Lykke Fabricius1, Manika Prasad2, and Casper Olsen1
Search and Discovery Article #40151 (2005)
Posted May 1, 2005
*Adapted from extended abstract, prepared by the authors for presentation at AAPG International Conference & Exhibition, Cancun, Mexico, October 24-27, 2004.
1E&R, DTU, Bygningstorvet 115, DK-2800 Kgs. Lyngby, Denmark ([email protected])
2SRB Project, Geophysics Department, Stanford University, Stanford, CA 94350, USA ([email protected])
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.
Valdemar field in the North Sea is a low-relief marly chalk structure, sealed from the overlying Chalk by a calcareous shale (Figures 1, 2a, 2b, 2c). We studied core-calibrated 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 increasing 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 (Figure 3a, 3b, 3c). 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 (Figure 4). IF indicates (on a scale from 0 to 1) to which extent 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. Thus, up to 40% clay softens the chalk; more clay stiffens the sediment (Figure 5a, 5b, 5c).
Log and core data were kindly provided by Mærsk Oil and Gas AS. Core and pore fluid data were kindly provided by Geological Survey of Denmark and Greenland. Joint Chalk Research is thanked for financial support.
Fabricius, I.L., 2003, How burial diagenesis of chalk sediments controls sonic velocity and porosity: AAPG Bulletin, v. 87, p. 1755-1778.
Jakobsen, F., J.R. Inseon, L. Kristensen, and L. Stemmerik, 2004, Characterisation and zonation of a marly chalk reservoir: the Lower Cretaceous Valdemar Field of the Danish Central Graben: Petroleum Geoscience, v. 10, p. 21-33.
Nur, A., G. Mavko, J. Dvorkin, and D. Galmudi, 1998, Critical porosity: a key to relating physical properties to porosity in rocks: The Leading Edge, v. 17, p. 357-362.