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AAPG Bulletin

Abstract

AAPG Bulletin, V. 95, No. 5 (May 2011), P. 729747.

Copyright copy2011. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/09271010052

Deformation structures in organic-rich shales

Helge Loseth,1 Lars Wensaas,2 Marita Gading3

1Arkitekt Ebbells Veg 10, NO-7053 Ranheim, Norway; heloe@statoil.com
2Arkitekt Ebbells Veg 10, NO-7053 Ranheim, Norway; lawe@statoil.com
3Arkitekt Ebbells Veg 10, NO-7053 Ranheim, Norway; mgad@statoil.com

ABSTRACT

Seismic images of Upper Jurassic organic-rich siliciclastic rocks are studied along the 2000-km (1243-mi) Norwegian margin. These rocks are considered the main source rock for most of the large oil and gas fields in the North Sea and Norwegian Sea. We report characteristic seismic expressions of thin-skinned gravitational gliding structures that are strata bound to the organic-rich formations. The most characteristic structures are listric faults that offset and rotate the upper part and sole out near the base of the organic-rich zone. These may be present in large areas (10,000 km2 [3861 mi2]), but are usually restricted to tilted areas. The strike of the faults was perpendicular to the downdip direction of the movement of mass, and fault directions can therefore be used as paleodip indicators. Several types of contraction structures are observed, and all are formed at a maximum of a few hundred meters of burial. Although they are not limited to organic-rich shales, such strata-bound structures may help identify organic-rich intervals in basins where their presence is unknown.

We suggest that adding organic material to clay leads to reduced permeability. Compaction-related vertical fluid flow may cause fluid overpressure to build up at the base of a less permeable organic-rich layer during early burial. This high fluid pressure zone becomes a low-friction decollement surface on which overlying sediments slide to form characteristic thin-skinned deformation structures.

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