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.