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(Begin page 2003)
AAPG Bulletin, V.
Recognition of cyclicity in the petrophysical
properties of a Maastrichtian pelagic chalk oil field reservoir
from the Danish North Sea
1Department of Geology, Chalmers University of Technology, Sven Hultinsgata 8, S-412 96 Gothenburg, Sweden; Geological Survey of Denmark and Greenland (GEUS), Thoravej 8, DK-2400 Copenhagen NV, Denmark; email: [email protected]
Morten Stage has been employed in the Reservoir Geology Department at the Geological Survey of Denmark and Greenland (GEUS) since 1996. He received his M.Sc. degree in physics at Odense University, Denmark in 1995. His major research interests are in the borderland between physics and geology with an emphasis on signal analysis of geological data and magnetic properties of sediments. This article on cycles in the North Sea area is part of his recently completed Ph.D. project.
This work is supported by the Nordic Energy Research program and the Geological Survey of Denmark and Greenland (GEUS) and is published with the permission of GEUS. I thank P. Schioler for doing the biostratigraphic work in this project and acknowledge K. L. Rasmussen for providing access to the Kappabridge susceptibility meter. I thank the core laboratory at GEUS for their assistance with sampling the drill core. I also thank P. Frykman, N. Springer, and N. Stentoft for reading and correcting the manuscript.
Subdivision of the Maastrichtian chalk from the Danish North Sea has proven difficult because of its homogenous composition, which is caused by a fairly stable depositional environment; nevertheless, the chalk petrophysical properties show cyclic behavior that potentially can be used for subdivision.
Petrophysical properties of a 23 m-long succession of Maastrichtian chalk from the Dan field were examined for cyclicity by spectral analysis. Magnetic susceptibility values measured on plug samples, together with the neutron porosity and natural gamma radiation well-log data, were analyzed both as complete data sets and in subsets to survey the general existence of cycles in the chalk.
All three properties contain consistent cycles at approximately 2 m wavelength, as well as additional frequencies where the relative dominance of the different frequencies differs. Cross-spectral analyses of subsets of the petrophysical parameters provide substantial evidence for the coexistence of the approximately 2 m cycles in the Maastrichtian chalk. Furthermore, the magnetic susceptibility carries spectral evidence for a higher frequency because most power spectra of the subsets from the magnetic susceptibility have a spectral peak in the 1.8-2.0 cycles/m interval (i.e., approximately 0.5 m cycle). The cycles can be interpreted as resulting from the eccentricity (94.5 k.y.) and the precession (22.5 k.y.) of the Milankovitch cycles, assuming a sedimentation rate of 2.6 cm/k.y.
The existence and the understanding of cycles at a submeter scale yield possibilities for an advantageously precise subdivision of chalk reservoirs and correlation between wells.
(Begin page 2004)
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