Magnetic Characterization of Folded Aeolian Sandstones

Jean-Paul Callot¹, William Sassi¹, Martin L.E. Guiton¹, Narjes Kallel¹, Philippe Robion², and Jean-Marc Daniel^1
1 Institut Français du Pétrole, Rueil Malmaison, France
² Université de Cergy Pontoise, Cergy-Pontoise, France

As part of the microstructural and fracture characterisation of the Weber Sandstone, a large sampling survey was undertaken in Split Mountain at the Dinosaur National Monument (Utah, USA). The data set is composed of 391 cores from 31 locations, principally within the upper Weber. Although the bulk magnetic susceptibility values are comprised in the range -5 to 5 mSI with variable degree of anisotropy (P' parameter up to 1.6, but on average less than 1.2), 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. Apart from few sedimentary fabrics, most of the measured sites display a low intensity tectonic fabric. The fracture network at Split Mountain is mainly composed of a regional fracture set trending N120 and a N050 secondary set, which is related to the Laramide inversion stress field in the early Tertiary (Sassi et al., 2003). A tentative history of layer parallel shortening record can be retrieved from the comparison of (1) the secondary fracture set orientation, (2) the distribution of diagenetic cementation, (3) the microtectonic faults and (4) the magnetic susceptibility mapping. First, the Laramide fracture network and AMS signal are recorded, before any reactivation of pre-existing structures. Paleostress orientations, deduced from the fracture network and the AMS fabrics as well, are compatible with the regional Laramide stress field. Then the pre-cretaceous sedimentary pile records a paleo-stress reorientation within the inverted graben domain, as shown by late but pre-folding striated planes. Finally the macroscopic 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.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005