CONE-SHAPED HORSTS DISCOVERED AMONG NORTH SEA POLYGONAL NORMAL FAULTS: GEOMETRY AND MECHANICS

To our knowledge, this is the first report of map-scale conical faults. They occur in Block F3 of the Dutch sector of the North Sea, within an extensive zone of polygonal normal faults. Polygonal normal faults cover large areas in more than 100 different basins and form potentially important reservoir-scale discontinuities in fluid transmissivity. Polygonal faults, so named because they have a mudcrack-like polygonal map pattern, occur in very fine-grained sediments, either clays, as in the example here, or chalk. The cone-shaped faults, like the neighboring polygonal faults, are strata-bound, terminating upward and downward, with maximum displacements in cross section near the fault centers. The faults were found in a public-domain 3D seismic image using a new 3D seismic processing algorithm that has no bias toward finding planar (or conical) faults. However, even without this processing, the cone shape is evident. The faults are circular in horizontal slices, and have a similar inverted-V shape in vertical profiles at any azimuth through the cone centers. The cone widths are ~1 km at the base, making them map-scale features; maximum throws are ~35 m and dips are 35-38º. Elsewhere, some polygonal faults have been found at or close to the sea floor where fault dips are ~60º. It is inferred that all polygonal faults formed close to the depositional interface and that the low dips of the deeply buried faults, like those reported here, are due to burial compaction. We infer that both polygonal and conical map geometries result from a uniform radial extension stress caused by volumetric contraction, analogous to the formation of mudcracks. For strata to differentiate into horsts and grabens above a planar lower boundary, some amount of differential vertical compaction is required, with more deformation in the grabens. Cone-shaped faults have been observed in laboratory experiments on rocks and ceramics where the fault geometry represents perfect uniaxial flattening. In the subsurface, both the stress and the sediment must be exceptionally homogeneous for conical faults to form. The cone shape seems to require an external control on fault initiation, such as a heterogeneity near the depositional surface, to serve as a trigger for downward propagation, not the random, in-plane propagation and linking inferred for polygonal faults.