A new magnetic logging tool has been designed and developed for precise and reliable in-situ magnetic measurements in the sedimentary formations encountered when drilling for oil.

The tool consists of two probes, measuring respectively the in-hole effect of the Earth's magnetic field and the magnetic susceptibility of the rocks, and is operated as a standard logging tool.

A proper combination of these magnetic measurements provides the determination of Natural Remanent Magnetization polarity, from which the polarity of the Earth's magnetic field at the time of sedimentation is deduced.

A magnetostratigraphic sequence is obtained for each well, and appears to be similar to the Magnetostratigraphic Reference Scale.

Application of this method to wells in the Paris basin (France), gives good results for correlations and absolute age determination.

Further comparison of results obtained from magnetic measurements and from sequence stratigraphy applied to conventional logs, shows that sequence boundaries can be diachronous in wells 80 km apart. Although the potential of magnetic measurements is still under investigation, the first encouraging results clearly show that the new dating method presented in this paper can be very helpful in petroleum geology.

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