Downhole magnetostratigraphy in sediments: Comparison with the paleomagnetism of a core.

Abstract

The magnetostratigraphy of weakly magnetized sediments is determined for the first time from direct downhole measurements. Experimental results were obtained in both scientific and oil‐exploration drillholes penetrating Jurassic sediments of the Paris basin, France. The magnetic polarity of the remanence is deduced from the total magnetic induction, combined with the magnetic susceptibility. The magnetic induction is measured in depth by a Proton Precession Magnetometer with Overhauser effect designed by the Laboratoire d'Electonique de Technologie et Instrumentation (Commissariat à I' Energie Atomique). This method gives a continuous record with an accuracy of 0.1 nT, and an excellent stability. In order to cancel out the transient component of the Earth's magnetic field, the magnetic induction is computed as the difference between the induction measured in the borehole and that obtained at a surface reference point located near the drillhole. The dipole‐dipole susceptibility probe gives a continuous record with an accuracy of a few 10−6. The four total magnetic induction logs recorded in oil exploration holes show a tight correlation, although distant from up to 80 km. This technique is therefore excellent for precise stratigraphic correlation. Data processing allows us to isolate the component of induction due to remanent magnetization and to find the magnetic polarity. A sequence of magnetic polarity is proposed for Kimmeridgian and Oxfordian stages. The variation of the sedimentation rate is analyzed with a very high resolution in time, and compared with the mean sedimentation rate deduced from a lithologic log. Cores were collected in the lowermost part of two holes. The correlation between the magnetostratigraphy deduced from paleomagnetic measurements performed on 240 standard plugs and that obtained by logging is good. A study of the components of magnetization found in the core has shown that the remagnetization occurred mainly during the coring operation itself or during the storage of the core. Consequently, downhole magnetostratigraphy proves to be reliable, besides providing the advantage of a continuous investigation performed over a large volume of rock scrutinized in situ.