Analysis of rock lenses in extensional faults
pp. 361-372

Lenses in the cores of extensional faults represent a major uncertainty in fault seal predictions since they may influence flow paths in reservoirs. We investigate the dimensions of lenses in extensional faults, addressing 1) the position of the lenses relative to, or within, the fault core, 2) the influence of lithology, and 3) the mechanism active in the generation and development of the fault lens. Rock lenses have been examined and measured in well-exposed extensional faults in three areas and in analogue experiments. The rocks hosting the lenses included in the study include nearly unconsolidated sandstone (Bornholm, Denmark), limestone interbedded with shales (Kilve, West England), and gneiss (Fr√łya, Central Norway). The remaining dataset is compiled from faults in analogue (plaster-of-Paris) experiments. We determine the geometry of the lenses by normalizing length (measured in the dip-direction) and width (measured in the strike direction) to maximum thickness (c:a and b:a-ratios). It is found that several parameters affect lens shape: (i) Lithology affects the shape and the minimum/ maximum sizes of the lenses, (ii) Primary (1st order) and secondary (2nd and higher orders) fault lenses commonly have different c:a-ratios, and finally, (iii) particularly for the most competent rocks, lenses in faults with several metres of vertical displacement tend to be relatively thicker than lenses occurring in faults with less displacement, probably reflecting a higher number of high order lenses in such zones. The statistical average c:a-ratio for all lenses included in the study is 12,5:1. Keywords: Fault architecture; fault geometry; fault-core lens

Merethe Lindanger: Department of Geoscience, University of Bergen, Norway,
now at ResLab Integration AS, Kokstad, Norway. Roy H. Gabrielsen: Centre of Integrated Petroleum Research, (CIPR), University of Bergen, Norway,
now at department of Geosciences, University of Oslo, Norway.
Alvar Braathen: Centre of Integrated Petroleum Research, (CIPR), University of Bergen, Norway,
now at the University Studies in Svalbard (UNIS), Norway.