Characterising Fracture Systems on the Isle of Lewis: An Onshore Analogue for the Clair Field

The Clair oil field lies offshore from western Shetland and is estimated to contain over 4 billion bbl. The basement here is composed of Lewisian-like rocks that are overlain by a reservoir of Devonian-Carboniferous sandstones and conglomerates of the Clair Group. Reservoir performance indicates that fractured basement rocks play a significant role in the resource development of the Clair field. Basement within the Clair field lies in the footwalls of major normal faults with Mesozoic age displacements whose characteristics are important to understand in order to gain a more accurate insight into fracture network behavior. Consequently it is important to characterise and understand the fracture networks within the basement and, in so doing, determine a connected volume within the reservoir. The Hebridean Islands lie in blocks that were upfaulted during the Mesozoic in analogous structural situations to the Clair Field. Lineament analysis reveals significant variation between postulated terranes, and comparison between the cover and basement lineaments indicates the presence of older pre-Mesozoic structures that are dominant within the basement. Fieldwork is hence required to determine the impact of Mesozoic faulting within the Lewisian.

On the west coast of Lewis the Permo-Triassic Stornoway Formation is a synrift conglomeratic unit faulted against Lewisian Complex basement and the Outer Hebrides Fault Zone. Analysis of fractures within the Stornoway Formation reveals at least three sets of cross-cutting faults. Associated fault rocks are typically red-brown and associated with carbonate cements and veins, with varying degrees of induration. These same fault rocks and carbonate cements can be used to identify Mesozoic faulting within the Lewisian Complex of the region and it is clear that many of these faults have formed very significant fluid flow conduits synchronous with and subsequent to faulting. The Mesozoic faults, including the main bounding faults of the Stornoway Formation exposed on Lewis, have a strong tendency to reactivate local foliation within the basement. The relationship between pre-existing fractures within the basement is also investigated. Mesozoic reactivation and fluid flow along older fractures plays an important role within the basement.