Magnetic Logging: Detection of the Earth's Magnetic Field Reversals and Application to Borehole Magnetostratigraphy

BOUISSET, PATRICK, and BRUNO LALANNE,* TOTAL CFP, Paris, France, GILLES PAGES,* CEA-LETI, Grenoble, France, and ANTOINE AUGUSTIN, TOTAL CFP, Paris, France

TOTAL, CEP, and CEA (the French Atomic Energy Commission) designed and developed two magnetic logging tools for precise and reliable in-situ magnetic measurements in the low magnetized sedimentary formations encountered when drilling for oil.

The tools, measuring respectively magnetic susceptibility of the rocks, are operated as standard logging tools and logs are recorded during standard logging operations.

A proper combination of these magnetic measurements leads to the determination, every meter, of natural remanent magnetization polarity, from which the direction of the Earth's magnetic field at the time of sedimentation is derived.

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, in Indonesia, and in the North Sea, gives good results for correlations and absolute age determination, as well as facies diachronism.

Further comparison of results obtained from magnetic measurements and from sequence stratigraphy applied to conventional logs, shows that third-order shows that third-order 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.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)