Characterising fracture systems within upfaulted basement highs in the Hebridean Islands: 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 gneiss that is overlain by Devonian-Carboniferous sandstones and conglomerates and a Cretaceous seal. Reservoir performance indicates that fractured basement rocks play a significant role in the resource development of the Clair field. 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. 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 behaviour.
Analysis will be undertaken on the nature and controls of fracture attributes across a range of scales in the crystalline Lewisian basement rocks, and in related Mesozoic cover sequences from Hebridean Islands that are exposed in uplifted footwalls of Mesozoic normal faults. This will enable characterisation of fractures and fracture networks as an analogue for the Clair field. An initial field study of the Stornoway Formation has yielded preliminary results of the structure of the formation and the contact between the Stornoway Formation and Lewisian gneiss basement on the Eye Peninsula.
Data will be used to build fracture networks at km to cm scales using lineament analysis of sources at scales from the regional to the microscopic. The resulting networks and attributes will then be compared both to equivalent fracture networks from the mainland Lewisian and from offshore areas based on interpretations of seismic reflection data and core materials from the Clair field. It will be determined to what extent scaling laws can be used to link across length scales and between 2D and 3D datasets.
The results of these studies will also lead to a new insight into the terranes of the Hebrides. Magnetic and Gravity anomaly maps will also be analysed in order further understand the terrane system and confirm whether the Hebridean Islands are an appropriate analogue of the Clair field.