Abstract 3D seismic and well data from the Ormen Lange Field, Mid Norway have been used to analyse the development of a system of polygonal faults affecting the Late Cretaceous-early Paleocene reservoir. These faults have the typical properties of polygonal fault systems recognized elsewhere in mainly fine-grained successions. They grew by upward propagation from the thick, shale-prone interval of the Late Cretaceous in the Møre Basin and were reactivated during the deposition of the Balder Formation. They have throws ranging from a few metres to 80 m, are typically 1–3 km in length and have highly irregular throw distributions along strike, mainly as a result of complex fault intersection geometries. The Ormen Lange Field is the first described example of polygonal faults that completely transect a major sandstone reservoir interval. The presence of these faults has important implications for the likely production behaviour of the field. Fault seal analysis shows that they are unlikely to form juxtaposition seals, except locally, but that they may have a significant risk for clay smear seals, particularly in the lower reservoir unit.

Abstract Several master faults in the North Sea basin tend to flatten to give low dips at depth, and in this sense form detachments in the rift system. Such low angle faults are identified in the western flank of the Viking Graben (Tampen Spur area), where they occur as both intra- and supra-basement detachments. Interference between detachments and steeper faults results in ramp–flatramp–ramp geometries. In the eastern part of the Gullfaks fault block, a supra-basement detachment is probably associated with anomalously high late Jurassic extension in the Gullfaks Field area. The low-angle Gullfaks detachment also helps explain the presence of sets of parallel east-dipping faults (domino systems), a common feature in the collapsed hanging wall to low-angle detachments. Similar detachments probably exist beneath the Gullfaks Sør block and SE of the Visund fault block. All of these are interpreted as late Jurassic collapse structures directly related to active late Jurassic extensional tectonics. Strong indications of intra-basement detachments are also found in the Tampen Spur area. These detachments are formed by major normal faults that flatten in the basement, as seen beneath the Visund fault block. This geometry may to some extent be related to fault rotation during repeated phases of extension in the Palaeozoic-Early Mesozoic period. However, abrupt flattening of some of the faults in the basement indicates that the master faults follow some of the many pre-existing mechanically weak zones in the basement, primarily low-angle Devonian extensional shear zones or Caledonian thrusts.

Abstract An upper Jurassic, wedge-shaped syn-rift succession, comprising the Heather and Draupne Formations, is present in the hangingwall trough of the Snorre Fault Block. The succession is bounded to the west by the Statfjord East Fault, whereas it onlaps the Snorre Fault Block to the east. It consists of a two-fold coarsening-upward sequence from shale to sandstone of shallow marine/shoreline origin. Active fault block rotation and subsidence in the Snorre-H area commenced in the Mid-Bathonian and lasted through the Ryazanian. The Heather Formation was deposited during the early rift stage (Mid-Bathonian-Early Oxfordian; 3° cumulative tilt), whereas the Draupne Formation (Late Oxfordian-Ryazanian; 9° cumulative tilt) accumulated during the main and late rift stages. The lower part of the Heather Formation was likely deposited across a submerged tilted fault block terrain, with a predominant extra-basinal sediment supply. Deposition of the upper Heather Formation, however, was governed by gradually emerging footwall islands, albeit yet without significant local erosion. As a result of increased fault/block rotation during deposition of the Draupne Formation Shale, Sequences I-II (late Early Oxfordian-early Mid-Volgian) footwall islands became firmly established, providing a predominant local sediment source to the Snorre-H sub-basin. Clay and silt were supplied from erosion of the Heather Formation on the Snorre-H hangingwall, with subordinate input of sand from the Statfjord East footwall. Subsequent deposition of the Draupne Formation, Sequences III-V (late Mid-Volgian-Ryazanian), was governed by significant relief on the footwall islands, causing deep erosion into the Brent Group on the Snorre-H hangingwall dip-slope and leading to progradation of the Upper Draupne Sandstone shoreline complex across the Snorre-H area. Deposition of the Draupne Formation and temporal shoreline position were likely partly controlled by northwards fault-tip propagation of the Statfjord East Fault. Various syn-rift play models and depositional reservoir facies are present within the Snorre-H hangingwall basin. They include dip-slope shallow marine/shoreline sands, basin floor gravity transported sands and likely footwall talus sands enveloped in organic rich shales of the Draupne Formation. The distribution of reservoir facies is intimately linked to exposure and erosion of the middle Jurassic Brent Group below the syn-rift unconformity.