Structural controls on the Late Jurassic age shelf system, Ula Trend, Norwegian North Sea

Abstract

The Ula Trend of the Norwegian North Sea is an important producing area sited on the eastern margin of the Central Trough. Two fields, Ula and Gyda (combined reserves of 635 MMBBL), are on stream, and several other petroleum pools have been discovered. They key to success in the area is to predict the presence of Kimmeridgian to Portlandian age shallow-marine shelf sandstone reservoirs. Well and seismic data have shown the reservoir sequences to have a complex distribution controlled largely by the structural topography at the time of deposition. Structural interpretation of the 3D seismic datasets over the Ula and Gyda field areas and dense 2D data over the Ula Trend as a whole have revealed new insights into the geometry of the Jurassic rift margin. A relationship has been established between the Jurassic faults and the underlying salt wall topography to a deeper Early Permian age extensional system. During Late Jurassic extension, the deeper faults cut up-section and involved the Upper Permian, Triassic and Jurassic cover. Original Early Permian dip-slip faults maintained their sense of motion, while a linked oblique-slip set acted in sinistral transtension. The cover section accommodated, the differing styles with both simple extensional structures and more complex transtensional and contractional geometries, respectively. The structural pattern has been further complicated by episodic hangingwall inversion in Cretaceous and Tertiary times and the inversion geometries provide important petroleum traps (e.g. Ula Field). The Jurassic rift margin fault system and the geometry of its hangingwall governed the location of the Kimmeridgian shelf and shelf-break, as well as the entry and dispersal of sediment on the shelf. The structural interpretation has thus allowed a predictive model for shelf geometry and hence reservoir distribution to be established, and this forms the basis for continued exploration in this complex and increasingly mature play fairway.