Fracture Characterisation in Crystalline Basement: Building an Analogue for the Clair Basement Based on the Lewisian Gneiss Complex of NW Scotland

Fractured basement reservoirs, like all unconventional hydrocarbon plays, seem set to be an increasingly attractive prospect for energy companies as conventional oil and gas reservoirs are preferentially exploited and depleted. Basement is often vaguely defined but here we use the definition of Landes et al (1960) that it is any metamorphic or igneous rock (regardless of age) which is unconformably overlain by a sedimentary sequence. Motivation for this study was provided by the Clair Oil field, located 75km west of Shetland. The primary Clair reservoir is situated within Devonian and Carboniferous fluvial/lacustrine sediments that overlie and onlap a topographic basement-cored high, the Rona Ridge of Late Archaean to Early Proterozoic granodiorite/diorite/granitic gneisses and pegmatites. Well tests from the basement and from the overlying sediments suggest that there must be fluid pathways through the basement (Falt et al., 1992, Coney et al., 1993) connecting sedimentary packages across the main ridge structure, and that fracture systems within the basement may also provide significant storage space for hydrocarbons. Thus a key role for fractures cutting basement gneisses is becoming increasingly recognised as the field is being developed.

In this study, we present 3D fracture network characteristics of basement rocks of NW Scotland. These have been analysed and quantitatively compared to assess their suitability as an analogue for the fracture networks within the Clair basement. Onshore datasets (outcrop, terrestrial laser scans & NEXTMap DEM) from mainland NW Scotland exhibit prominent NE-SW and/or NW-SE fault and fracture trends (Fig. 1). Fracture spacing distributions from the mainland LGC are well described by power-law relationships over at least five orders of magnitude. Similar power-law relationships are also indicated from the Clair basement datasets however the limited numbers and scale range of structures observed means that these relationships are much less well constrained.

The Outer Hebrides show different primary fracture orientations, with those on Lewis and Harris displaying strong NNW-SSE and ENE-WSW trending fractures (Fig. 2). Relative age dating of these fractures using the Permo-Triassic Stornoway Formation demonstrates they are of similar age to important open fractures at the Clair Field, with multiple phases of faulting in the basement of the Outer Hebrides producing very similar fault rocks to those seen at Clair. Large seismic-scale Mesozoic faults in the Lewisian of Lewis and Harris are accessible. An additional finding here is that in directions perpendicular to the fault plane fracture spacing is described by power law spanning at least four orders of magnitude, whereas parallel to the fault plane, fractures exhibit a random fracture spacing distribution attributable to jointing and a background level of fracturing present in the gneiss

This work has shown that the LGC in the mainland and Hebrides is a suitable analogue for the Clair basement ridge, however there are important differences to account for when building an analogue model. The Clair basement seismic dataset exhibit comparable NE-SW & NW-SE trending faults to mainland Scotland, however basement core samples exhibit a strongly aligned NNE-SSW fracture trend that is not clearly represented in the onshore datasets. Faults and fractures on Lewis and Harris have different orientations to Clair (and the mainland) but are of similar age and contain similar fault rocks to the offshore field. The mainland fault rocks are not similar to Clair and are likely to be of differing ages. The onshore data provide a range of model types that can be used in sensitivity models to ultimately assess which onshore dataset provides the best geological and statistical analogue for the Clair basement.

The observations, analysis and discussions that have taken place during the course of these projects and associated industry fieldtrips have been used by the industry sponsors (Clair Joint Venture) to better understand the Clair basement fracture network potential and improve the geological models for the Clair Field as a whole. This in turn fed-in to the Clair co-ventures economic re-assessment of the Clair field. The newly constrained geological model and the improved economic assessment mean that the Clair Ridge project (Clair Field Development - Phase 2) has recently been sanctioned for development resulting in an investment of approximately 4.5 billion into the UK economy.

Figure 1. Onshore datasets from the Lewisian Gneiss Complex from Scottish mainland. a) Linements picked on NEXTMap data, b) Outcrop-scale fracture dataset locations, c) Lidar (Terrestrial laser scan) derived 3D fracture networks.
Figure 2. a). Fracture patterns on Lewis and Harris and b) study locations.

References
Landes, K.K., Amoruso, J.J., Charlesworth, L.J., Heany, F. Lesperancep, J. 1960. Petroleum Resources in Basement Rocks, AAPG Bulletin 44, 1682-1691.
Falt, U., Guerin, G., Retail, P., Evans, M. 1992. Clair Discovery: Evaluation of Natural Fracturation in a Horizontal Well Drilled in the Basement and Producing From Overlying Sediments. Proc. European Petroleum Conf., Cannes, France, SPE 25016.
Coney, D., Fyee, T.B., Retail, P. Smith, P.J. 1993. Clair Appraisal: The Benefits of a Cooperative Approach. In: Proc. 4th Conference Petroleum Geology of Northwest Europe, pp.1409-1420.