Recognition of cyclicity in the petrophysical properties of a Maastrichtian pelagic chalk oil field reservoir from the Danish North Sea

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.