Magnetic
Characterization of Folded Aeolian Sandstones
Callot, Jean-Paul1, William Sassi1, Martin L.E.
Guiton1, Narjes Kallel1, Philippe
Robion2, Jean-Marc Daniel1 (1) Institut Français du
Pétrole, Rueil Malmaison, France (2) Université de Cergy Pontoise, Cergy-Pontoise, France
A large sampling
for magnetic survey of the Weber Sandstone was undertaken in Split Mountain
(Utah, USA). The data set is composed of
391 cores from 31 locations. Although the bulk susceptibilities are comprised
in the range -5 to 5 microSI with variable degree of
anisotropy
, the fabrics are generally triaxial and
rather well clustered. Magnetic fabrics of sedimentary origin show a magnetic
foliation plane parallel to the cross bed lamina of the sand dunes, without
defined lineation. Most of the measured sites display a low intensity tectonic
fabric. The main ferromagnetic carrier is either pur
Fe-magnetite, or maghemite at very low amount (~ppb).
Hysteresis cycles indicate that magnetite is most
probably in the superparamagnetic state. The fracture
network at Split Mountain is composed of a regional N120
set and a N050 secondary set, which is related to the Laramide
inversion in the early Tertiary. A tentative history of layer parallel
shortening can be retrieved from (1) the fracture network, (2) the distribution
of diagenetic cementation, (3) the microtectonic faults and (4) the magnetic susceptibility.
First, the Laramide fracture network and AMS signal
are recorded before any reactivation of pre-existing structures. Strain
orientations, deduced from the fracture network and the AMS fabrics, are
compatible with the regional Laramide stress field.
Then the pre-cretaceous sedimentary pile records a stress reorientation within
the inverted graben, illustrated by late but pre- to synfolding striated planes. Finally the fold is formed.
Implications of the results for the fractured reservoir mechanical modelling are discussed, particularly for improving the
proposed geologic history of strain record.
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