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Characterising Fractured Basement Using Pseudo-Wells in the Lewisian Complex, NW Mainland Scotland: Implications for Petroleum Potential in the Clair Field

Martin, Jennifer C.1; Holdsworth, Robert 1; McCaffrey, Ken 1; Conway, Andy 2
1 Department of Earth Sciences, Durham University, Durham, United Kingdom.
2 ConocoPhillips (U.K.) Limited (Part of the Clair Partnership (BP Exploration Trading Company Ltd, ConocoPhillips (U.K.) Ltd, Enterprise Oil Ltd, Chevron North Sea Ltd & Hess Ltd)), Aberdeen, United Kingdom.

Crystalline basement rocks are increasingly a target for hydrocarbon exploration in the development of existing fields. The Clair field lies 75km west of Shetland in the Faroe-Shetland basin with reservoirs in Devonian and Carboniferous sediments and within the underlying crystalline basement rocks. The basement sits as a topographic high known as the Rona Ridge, and is believed to be a rotated footwall block of a NE-SW trending extensional fault with significant Devonian and Mesozoic displacements.

To help understand the Clair basement structure, at regional and local scales, the Lewisian Gneiss Complex (LGC) of northwest Scotland is being tested as a suitable analogue. Analysis of the Clair basement in drillcore suggests that it has affinities with the LGC in terms of age, lithologies and fracturing style. The L. Archaean - E. Proterozoic LGC comprises tonalite-trondjhemite-granodiorite gneisses; mafic & ultramafic dykes and metavolcanic & metasedimentary sequences that were accreted as a series of terranes during the Precambrian. The LGC has three regional fracture trends (from oldest to youngest); 1) steeply-dipping NW-SE Paleoproterozoic faults. 2) N-S trending hematite stained Neoproterozoic (1.2 Ga.) ‘ladder fractures’ associated with the deposition of the overlying Stoer Group sediments. 3) NE-SW trending younger (likely Mesozoic?) faults. Each fault set is associated with characteristic fault rock and mineral assemblages.

The present project focuses on creating pseudo-wells from 1D line samples collected from outcrops in the LGC that can then be directly compared to well data from the Clair basement. Pseudo-wells provide information on fracture orientation, spacing, aperture, fill and mode allowing 1D characterisation of the fracture sets seen in the LGC.

Initial results indicate that there are strong NE-SW and NW-SE fracture trends present within the offshore and onshore ‘well’ datasets. Spatial analysis of these 1D samples provides similar results for both onshore and offshore domains. Fracture sets show power-law relationships for spacing, thus suggesting that scale-invariant properties of the fracture sets measured at outcrop or from basement wells can be used as an estimation for the fracture sets seen at regional scales. This, along with the similarities seen in fracture fill, mode and aperture suggests that the 1D fracture sets recorded in the onshore LGC provide a useful analogue for the fracture sets found within the Clair basement.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009