Milankovitch Cycles in Chalks, Danish North Sea, Detected by Use of Magnetic Susceptibility
Subdivision of the Maastrichtian chalk from the Danish North Sea has proven to be difficult because of the homogenous composition and appearance. A high-resolution subdivision of the chalk yields new information and better understanding of the depositional environment along with the possibility of an advantageous correlation. This new knowledge and possibility may be beneficial in reservoir geology where it can be exploited to enhance oil production from existing Danish oil fields.
Petrophysical properties of a 34 m (well MFB-7) and an 11 m (well M-1X) long successions of Maastrichtian chalk from the Dan Field were examined for cyclicity by spectral analysis (i.e. FFT and a new method developed in this study). Magnetic susceptibility plug measurements, together with the neutron porosity and the natural gamma radiation well log data were analysed, and all three properties contain a consistent cyclicity at c. 2 m wavelength. Further-more, the magnetic susceptibility carries spectral evidence for a shorter cyclicity at c. 0.5 m cycle.
The magnetic properties of the insoluble residue of the Maastrichtian chalk are primarily determined by the clay minerals (illite-smectites) and the existence of a detrital component in the insoluble residue suggests, that the cyclic variations in the bulk magnetic susceptibility and consequently of the composition of the chalk, are controlled by changes in the runoff from land into the basin. The cycle lengths in the chalk match Milankovitch cycles and therefore it is suggested that the deposition of the chalk was influenced by Milankovitch-driven climatic changes, which are reflected as variations in the clay mineral content. The cycles are inter-preted to be a result of the eccentricity (94.5 ka) and the precession (22.5 ka) Milankovitch cycles.
The cycles in the neutron porosity logs and the magnetic susceptibility data from the two wells were used to correlate the two cores, and the correlation was corroborated by a biostrati-graphical marker. Consequently, cyclostratigraphy seems to be a useful high-resolution correlation tool in the chalk.