Pre-Zechstein geological evolution of the south-easternNorth Sea, Norwegian Sector

Abstract

The Palaeozoic strata of the south-eastern North Sea, Norwegian Sector, have not been thoroughly explored by the oil industry. A major pre-Zechstein hydrocarbon producing source rock has not been confirmed in the area, and pre-Zechstein basins have been difficult to image on seismic data because of their depth and salt cover. In 2008, PGS completed a 3D survey at the intersection of Quadrants 3, 4, 9 and 10, just north of the Ringkøbing-Fyn High.Although well data is sparse and age correlation has therefore been difficult, the interpretation of this survey has provided a much improved understanding of the pre-Zechstein geological evolution of the area. Four horizons and three reflector packages were interpreted in the 3D survey, and a number of 2D seismiclines were also interpreted in order to map regional geological features and to attempt to correlate well data to horizons in the 3D survey. The deepest and oldest horizon that was interpreted represents a series of E-W trending tilted fault blocks. This fault block package is truncated by the long-lived Saalian Unconformity, and the age of the fault blocks is therefore uncertain. Their formation is believed to be related to either the breakup of the Old Red SandstoneContinent in the Early Carboniferous or a phase of Late Carboniferousto Early Permian post-Variscan extension. The Lower Rotliegend volcanics caused crustal uplift through thermal expansion in the Early Permian which resulted in the Saalian Unconformity. The topography created by this unconformity was later infilled by Upper Rotliegend sediments. The Ringkøbing-FynHigh became a major positive feature as the Northern Permian Basin subsided after cessation of the Lower Rotliegend volcanism. From the seismic data it is concluded the Ringkøbing-Fyn High played an important part in controlling the depositional patterns of the Upper Rotliegend. The Upper Rotliegendstrata are conformably overlain by the Zechstein Group. The Zechstein transgression and deposition of the Zechstein Group was followed by a phase of E-W extension. This extension resulted in a number of N-S trending normal faults, the largest of which is the regionally mappable Krabbe Fault. Bright spots observed in two horizons within the 3D survey suggest there is potential for an active pre-Rotliegend petroleum system in the area.