The Onshore Structural Geology of Foula, Shetland and Implications for the Devonian-Carboniferous Development of the Offshore Clair Basin Thomas Utley
Total Page：16
File Type：pdf, Size：1020Kb
The onshore structural geology of Foula, Shetland and implications for the Devonian-Carboniferous development of the offshore Clair Basin Thomas Utley Bob Holdsworth Ken McCaffrey (Durham) Eddie Dempsey (University of Hull) Richard Jones (GRL) Rob Strachan (Portsmouth University) Graham Blackbourn (Graham Blackbourn Geoconsulting) With support from the Clair Joint Venture Group AAPG/ICE October 2017 The Clair Field Line of section 75km Largest known resource in the UKCS, with an estimated 7-8 Billion BOE in place Fractured Devonian-Carboniferous sandstones (Clair Group) that overlie an up-faulted ridge of fractured Precambrian metamorphic basement Significant proportion of hydrocarbons Top Basement Map are stored within fractures in basement ∂ Courtesy of Shell and overlying Devonian sedimentary rocks A greater understanding of fracture systems is required to produce effectively/efficiently Also need to better understand Devonian- Basement unconformity Cross section through the Clair Field. After Barr et al. 2007 and Ogilvie et al. 2015) Regional Context Clair Basin developed during the Mid-Devonian to Early Carboniferous in a NE/SW trending half graben The Orcadian Basin of Caithness/Orkney has long been used as the classic analogue for the Clair Field, BUT… …alternatives exist in Shetland – and these are transtensional ∂ Regional Map of the Orcadian Basin. With cross section highlighted. After Dichiarante (2016). Regional cross section through FSB and Shetland After Ogilvie (2015) Dewey and Strachan (2003). Seranne (1992). Regional Context Foula is closest onshore analogue ~70km ~70km Relatively poorly studied, despite its proximity Comprises Middle (?) Devonian Line of section sediments and Precambrian basement Correlated regionally both onshore and offshore based on lacustrine facies and palynology ∂ General Geology of Foula Foula part of a long lived NE/SW trending basement structural high: Foula Ridge – a similar setting to Clair (Rona Ridge) 1600m of Middle Devonian clastics, unconformable & faulted against a ~1km wide strip of Precambrian Line of section basement Map of gravity. Positive = red Asymmetric NNE-SSW half graben on the Western Negative = blue margin of Orcadian Basin Stoker et al. (1993). ∂ Basement and Devonian highly fractured and show clear similarities in fracture/fault orientation ∂ Approaches • Photogeology and lineament analysis. • Fieldwork • Sampling and HM provenance analysis(ongoing) • Photogrammetric reconstruction and building of Virtual Outcrop Models (VOMs) Updated geological After Blackbourn map of Foula. After (1985). Blackbourn (1985). Basement Geology Basement may correlate with either Moine or Dalradian of Shetland Amphibolite facies, garnetiferous pelites, psammites, mafics and microgranite sills and dykes ~20m Multiple phases of brittle and ductile deformation-strongly foliated Pervasive pegmatitic/granitic veining increasing with proximity to the Ruscar Head Microgranite in the North ∂ Granite Pre-Middle Devonian as granite clasts in sedimentary breccias ~20m Basement Structures Fold Hinge ~100m Foliation subtly folded, W through to SW N/S orientated E steeply dipping normal faults • With associated epidote alteration, iron/copper mineralisation/staining and quartz mineralisation Low angle detachments • N/S to NNW/SSE with dextral shear sense ∂ Discrete brittle N/S fracture zones/corridors • Vertical/sub vertical, some sinistral oblique and rare dextral strike slip indicators ~1m ~10cm Southern Contact Sheared Unconformity • 3-5m zone of sheared basal conglomerate Bedding sub parallel to SW dipping unconformity surface Unconformity perpendicular faults. Long axis of clasts parallel to fabric Asymmetric boudinage of clasts ~1km Small syn-sedimentary NE dipping normal faults ∂ ~5m Northern Contact Faulted contact: • W dipping normal fault, ~100m throw • FW-foliated basement with abundant granite sheets • HW-Interbedded sand and siltstones of Da Ness Fm 20-30cm folded /mullioned microbreccia below fault Localised folding of both Basement foliation and Devonian • Curvilinear minor folds generally plunging S to SSW ~1km Curviplanar N/S to NNW/SSE faults and fracture corridors:∂ ‘folded geometry’ W E Beta axes of folded bedding, foliation and curviplanar faults all sub-parallel Fold Hinge ~0.5m = Paleocurrent indicators for each Devonian Stratigraphy formation Locally derived basal conglomerate/breccia Fluvial/alluvial environment with frequent flooding and development of lacustrine facies. Lacustrine Blobersburn Fm a regional marker Soft-sediment deformation widespread in lower sequences Palynology: L. Eifilian to Givetian ∂ Changing temporal palaeocurrents during basin development Updated geological map of Foula. After Blackbourn (1985). Devonian Structures Broad open folds • Gently open S to SSW plunging syncline • Smaller scale broadly open folds S N • Data missing from W limb due to inaccessibility Fold Hinge Remote sensing/VOMs to supplement the data Down plunge projection of folds show them Western limb Eastern Limb opening progressively upwards: • Interlimb angle increases up stratigraphy ~250m • Also seen on the Walls Peninsula, Shetland. ∂ • Possibly other examples in other parts of Orcadian Basin?? ~75m E 168° W Fold Projection 162° Formation 143° Tops-Foula Seranne (1992). Sandstone 129° Devonian Structures Major structures: • N/S trending normal faults • Conjugate sets of def. bands/ fractures, increase in frequency with proximity to meso scale faults • No. of faults increases with proximity to the East Minor structures: • N/S and E/W discrete fracture corridors • Sub-vertical, limited normal offsets ∂ ~25m • En-echelon • No clear cross cutting/abutting relationships • Distributed small offsets • Overall polymodal fracturing No mineralisation and rare sinistral oblique slickenlines ~50m Rare dextral reactivation of some N/S faults ~1m Basin Development ∂ Faults and folds suggest syn-sedimentary sinistral transtensional tectonics • Thinning of Soberlie Fm. from 400m in the HW to 35m in the FW and 650m to 220m in the Sneug Fm • Changing palaeocurrent in response to active faults and generation of folds channelising flow Summary Main structures: • N/S dip slip to oblique slip normal faults and fracture corridors in basement and cover Bedding • N/S to E/W mostly strike slip faults and discrete fracture corridors with limited offset • Little mineralisation or fracture fills • SSW to SW plunging folds in basement and cover, which open upwards • Faulting and folding synchronous Foliation∂ Syn-sedimentary faulting/folding related to constrictional strain during transtension • Fold hinges consistent with sinistral transtension along WBF/GGF in Middle Devonian • Fold geometries open progressively upwards Local folding • Related examples in Norway, Orkney and Shetland (Dewey and Strachan, 2003 and Astin, 1990) • Laboratory examples (Venkat-Ramani & Tikoff, 2002) Venkat-Ramani & Tikoff (2002) Conclusions 1600m of gently folded Middle Devonian clastic sediments overlying ?? Precambrian basement which accumulated in a half graben on W margin of the Orcadian Basin. Dewey and Strachan (2003). Basin fill shows synformal growth folding consistent with constrictional extension during regional sinistral transtensional Devonian basin development Similar to structures observed in Shetland, Orkney/Caithness & Inner Moray Firth ∂ Folds formed during early basin development & therefore present as potential traps before hydrocarbon generation/ or for subsequent reactivation- implications for other petroleum systems elsewhere? = Fold Hinges Diversity of basement/cover relationships = possible drilling hazards, and seismic interpretation/processing challenges Is the Clair Basin transtentional? What evidence is there for growth Burial history plot. Orkney area Seranne (1992). folding in Clair Group? After Astin (1990) Acknowledgments: ∂ Jim Henderson for assistance in the field and with logistics whilst in Shetland. Calum Fraser (www.instagram.com/calimski) for capturing drone imagery. Jim and Richard Scott and the crew of LK241 for the use of the boat. Magnus Holbourn and the residents of Foula. Graham Blackbourn for permission to use information from his unpublished report on the geology of Foula. Photogrammetric point clouds generated using Agisoft Photoscan and interpreted using the Virtual Reality Geological Studio, courtesy of David Hodgetts, Manchester University. BP for providing imagery and the Clair JVG for continued interest into research at Durham. Aerial Images courtesy of Canmore. Foula panaromic photo reproduced from John Pedley. panphotos.co.uk Offshore shapefiles from Oil and Gas Authority, Open Data (OGA), 2016. .