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Magnetostratigraphic Applications |
1 CNRS-UA 723, Département de Geology, Bât. 504, Université de Paris sud, 91405 Orsay, France
2 Elf Aquitaine Production, DEX/DES Tour ELF, 92078 Paris La Défense, France
3 Elf Aquitaine Production, DEPF-Exploration, 31360 Boussens, France
Shallow water carbonates from the Middle Jurassic of the Paris Basin include oil reservoirs, mainly oolitic and bioclastic limestones. Biostratigraphic markers within these deposits are generally used for regional correlations. Unfortunately, these markers are not available in all cored sections. A pilot study has shown that palaeomagnetism could be used to better constrain the chronostratigraphic framework of the Jurassic shallow water carbonates. Because the magnetization of sediments is independent of facies and location, palaeomagnetic dating may allow higher resolution.
Further magnetostratigraphic results are reported for three additional well dated core sections (53, 18.50 and 36.35 m in length) correlated throughout the Paris Basin. Specimens were subjected to alternating field and stepwise thermal demagnetization. The demagnetization paths show either one, two or three components. Based on the primary magnetization component, normal and reverse polarity intervals occur in the three sections. Rock magnetic experiments performed on representative specimens from both sections indicate that the bulk of the natural remanent magnetization is carried mainly by low coercivity minerals such as magnetite/maghemite and high coercivity minerals exclusively in altered layers of the cores. Single domain maghemite, probably biogenic in origin, has been observed by transmission electron microscopy. Larger detrital irregular titanomagnetite and secondary, framboidal magnetite spheres have been characterized with a scanning electron microprobe. The occurrence of the secondary magnetic spheres is probably related to the transformation of framboidal pyrite into magnetite during hydrocarbon migration. This has presumably resulted in the overprint of a secondary magnetization.
The polarity sequences have been correlated with the magnetostratigraphic record defined for the Bathonian and Callovian. The correlation is fairly good for the Bathonian, but ambiguous for the Callovian. However, we used a characteristic reverse polarity interval occurring in all our cores, including that studied previously, to correlate strata throughout the Paris Basin. The positive correlation within these strata provides a strong potential for the analysis of reservoir geometry and quantitative modelling of the Middle Jurassic shallow water carbonates.
