Altered basement rocks on the Utsira High and its surroundings, Norwegian North Sea

Abstract

As part of the recent discoveries on the Utsira High (Edvard Grieg and Johan Sverdrup fields), altered and fractured basement rocks were for the first time shown to act as a reservoir and possible migration paths for commercial hydrocarbon deposits on the Norwegian Continental Shelf. Altered basement rocks are underlying the main Upper Jurassic reservoir rocks in Johan Sverdrup and the main Cretaceous, Jurassic and Triassic reservoir rocks in Edvard Grieg. In the present study, eighteen basement cores from the Utsira High have been classified and investigated for signs of alteration, including subaerial weathering. The results show highly variable basement composition, including metasandstones, phyllites, granites, granodiorites and gabbroic rocks. In core view most of the basement rocks show signs of a medium to high degree of fracturing. Alteration has taken place in most of the cores, ranging from slight discoloration to disintegration along fractures, and to total fragmentation of the rock. The fragmentation of the rock is commonly associated with the dissolution of primary minerals and precipitation of secondary clays in the newly formed pore space. The upwards increasing disintegration and increasing amount of clay observed in the basement rocks from two of the wells (16/3-4 and 16/1-15) indicate that subaerial weathering was the main alteration agent.

The Influence of Structural Inheritance and Multiphase Extension on Rift Development, the NorthernNorth Sea

“…The Utsira High forms a prominent intrabasin high within the northern North Sea that appears only weakly faulted throughout RP1 and RP2 (Figures and ). Multiple basement well penetrations across the Utsira High suggest that, at least in the upper few meters, crystalline basement is granitic (Fazlikhani et al, ; Lundmark et al, ; Riber et al, ; Slagstad et al, ). Granitic bodies typically have large density and rigidity contrasts with surrounding lithologies, and as such are often thought to resist extensional stresses and localize strain around their margins (Bott et al, ; Critchley, ; de Castro et al, ; Howell et al, ).…”

“…Crystalline basement is penetrated by numerous wells across the northern North Sea and has been interpreted in terms of the tectonic units identified onshore in Norway and Scotland (Lenhart et al, ; Lundmark et al, ; Riber et al, ; Slagstad et al, ). Well penetrations across the Utsira High, a long‐lived structural high in the center of the northern North Sea rift, indicate that, at least in the top few meters penetrated by the wells, crystalline basement is dominantly granitic (e.g., wells 16/1‐15, 16/5‐1, 16/6‐1; Figure ; Lundmark et al, ; Riber et al, ; Slagstad et al, ). Slagstad et al () and Lundmark et al () present U‐Pb ages suggesting that the granitic basement of the Utsira High formed part of a volcanic arc terrane incorporated into the Caledonian orogeny.…”

Deep structure of the northern North Sea and southwestern Norway based on 3D density and magnetic modelling

“…Unfortunately, we cannot properly distinguish between the metasedimentary and crystalline rocks of the modelled block, suggesting that the greater part of it consists of crystalline rocks which can be lithologically represented by granite or granitic gneiss. The crystalline basement of the south-eastern flank of the Viking Graben is fractured and porous within the Utsira High (Riber et al, 2015(Riber et al, , 2016, allowing us to suggest that crystalline rocks of the low-density crustal block within the northeastern flank of the Viking Graben can also be fractured. The latter suggestion is supported by the results of 2D modelling of coupled groundwater flow and heat transfer through the middle-upper-crustal rocks of SW Norway (Maystrenko et al, 2015a).…”

Complex Bedrock Fracture Patterns: A Multipronged Approach to Resolve Their Evolution in Space and Time

“…Finally, discoveries of exploitable hydrocarbon reserves in heavily fractured and weathered basement blocks (e.g., Lie et al, 2016;Lothe et al, 2017;Petford & McCaffrey, 2003;Riber et al, 2015) will generate enhanced geophysical imaging of such basement domains. To correctly interpret the geophysical data, however, an integrated understanding of the geological history, especially the brittle deformation history recorded by the individual bedrock domains, remains indispensable.…”

“…Similar fracture networks in corresponding basement units of the North Sea offshore domain are presently largely buried underneath Permian and/or younger sedimentary cover sequences but are readily imaged by high‐resolution geophysical techniques (Lie et al, ). Recent discoveries of major hydrocarbon reserves accumulated in heavily fractured and weathered basement blocks offshore SW Norway (Riber et al, , and references therein) have triggered significant interest in understanding the complex brittle tectonic evolution of basement domains in general and, in addition, the brittle evolution history of SW Norway.…”