DANMARKS OG GRØNLANDS GEOLOGISKE UNDERSØGELSE RAPPORT 2011/87 Palaeogene volcaniclastic sedimentary units interbedded with basalt lava flows - a 3-D photogeological study of cliff sections from the Faroe Islands Henrik Vosgerau, Simon R. Passey, Kristian Svennevig, Max Nykjær Strunck, David Jolley & Lotte Melchior Larsen GEOLOGICAL SURVEY OF DENMARK AND GREENLAND MINISTRY OF CLIMATE AND ENERGY UAbstract U 3 1.U U IntroductionU U 5 2.U U StudyU area U 6 3.U U GeologicalU setting and stratigraphyU 7 4.U U TerminologyU U 9 5.U U MethodsU U 10 6.U U DigitalizedU cliff sections U 11 6.1U U FugloyU U ...................................................................................................................11 6.2U U SvínoyU U ...................................................................................................................13 6.3U U KolturU U .....................................................................................................................15 6.4U U HesturU U ....................................................................................................................16 6.5U U SandoyU U ..................................................................................................................17 6.6U U SkúgvoyU U ................................................................................................................18 6.7U U StóraU DímunU ..........................................................................................................22 7.U U LargeU scale variations – from island to island U 24 8.U U SummaryU and discussion U 28 8.1U U LaterallyU widespread interlava bedsU .....................................................................28 8.2U U MajorU channel fillsU .................................................................................................30 8.3U U ConnectivityU between interlava bedsU ....................................................................31 8.4U U CompartmentalisationU of interlava bedsU ...............................................................31 ReferencesU U 33 Enclosure 1. Geological cross-section, Fugloy Enclosure 2. Geological cross-section, Svínoy Enclosure 3. Geological cross-section, Hestur Enclosure 4. Geological cross-section, Sandoy Enclosure 5. Geological cross-sections, Skúgvoy Enclosure 6. Geological cross-section, Stóra Dímun Enclosure 7. Geological cross-section, from island to island CD-ROM GIS database G E U S 2 0BAbstract A 3-D photogeological study have been undertaken on selected cliff sections on the Faroe Islands in order to study lateral variations of volcaniclastic sedimentary rocks interbedded between basalt lava flows within the Palaeogene Enni Formation. The high-resolution pho- tographs were taken from helicopter of non-accessible, steep to near-vertical cliff sections providing almost complete vertical exposures of the volcanic successions and allowing the mapping of lateral variations over distances of many kilometres. The 3-D photogeology study documents several features of interlava beds which may form important input for the estimation of reservoir size, geometry, connectivity and compartmentalisation in high- quality intra-volcanic sandstones in the subsurface. Relevant observations grouped into main themes include: Lateral extent and variations in geometry and thickness: • The extent of the interlava beds in general exceeds the length of the studied cliff sections which are 1.7 - 7 km long. Local major channels, up to 45 m deep and 180 m wide, filled in with volcaniclastic conglomerates and lava flows form an exception. • The mapped interlava beds are 1 - 6 m thick. Laterally they have a fairly consistent thickness due to a non-erosive nature of overlying lava flows and a general draping nature of the volcaniclastic sediments which only marginally levels out the relief of the underlying lava flows. • The relief of the lower boundary either reflects the original topography left behind by the solidified lava flows or was governed by differential weathering and erosion into the basalt prior to the deposition of the volcaniclastic sediments. Connectivity between interlava beds: • Lava flows, up to 15 m thick, may wedge out over distances of 1 – 3 km, leading to the merging of the interlava beds they separate. • A major, multiple channel fill complex in one section cut through two interlava beds which may therefore, be hydraulically connected via a channel infill consisting of volcaniclastic conglomerates. Compartmentalisation of individual lava beds: • Interlava beds locally onlap and wedge-out against a rising relief of the underlying basalt implying that basalt on basalt contacts occur where the interlava beds have wedged out. • Intervening lava flows may form hydraulic vertical barriers within interlava beds. However, the observed lava flows in general wedge out laterally within the interlava beds. • Lava filled channels, up to 24 m wide and 15 m deep, in two locations intersect an interlava bed. • Dykes, up to 6 m wide, were rarely observed, but locally intersect the whole vol- canic succession including several interlava beds. • A reverse fault in one section offsets interlava beds by c. 6 m. G E U S 3 The relevant features are richly illustrated on photos and geological cross-sections which have been generated for most of the cliff sections by projecting the mapped high resolution features onto 2-D segments. Furthermore, more than 70 km mapped horizons are stored as 3-D polylines in a GIS database and can be exported as shape files suitable for 3-D modelling using, for example, Petrel reservoir engineering software. G E U S 4 1. Introduction1B This report presents the results of a 3-D photogeological study of volcaniclastic sedimen- tary rocks interbedded between the basalt lava flows of the Enni Formation, Faroe Islands. Exposure of the formation is determined by past glacial and ongoing coastal erosion of the islands. The formation is generally not well exposed away from the coastline being limited to stream cuts, although these are typically accessible. The coastlines on the outside of the archipelago are greatly affected by wave action resulting in non-accessible, steep to near- vertical cliff sections providing almost complete vertical exposures of the formation. The high-resolution photographs taken from helicopter in the present investigation allow for a detailed study of the formation as well as making it possible to map lateral variations over distances of many kilometres. Based on the high-resolution photogrammetric data it has been possible to study the lateral extent, thickness and geometry of the interlava sedimentary units, potential compartmen- talisation of the units by igneous intrusives and extrusives, structural trapping due to faults as well as the nature of the interfingering between the sedimentary units and lava flows. Mapping of these features in the Enni Formation may be highly relevant when reservoir connectivity and actual volumetric and recoverable hydrocarbons in interlava sedimentary units are to be estimated in the subsurface. The widespread and recently documented volcaniclastic Argir Beds, occurring about two- thirds above the base of the Enni Formation, has been a focus for the study. The Argir Beds are up to 6 m thick and interpreted as having been deposited in a complex fluviatile- floodplain environment. Although the reservoir properties of the interlava units in the Enni Formation may be limited by low permeabilities, related to a high content of volcanic mate- rial, the interlava units are still very relevant as an analogue study for high quality, siliciclas- tic reservoir sandstones within lava-dominated successions as the principle governing mechanisms for the geometry, lateral extent, compartmentalisation etc. are the same for both types of sedimentary rocks. The photogeological studies have been undertaken on selected cliff sections of the islands of Fugloy, Svínoy, Koltur, Hestur, Sandoy, Skúgvoy and Stóra Dímun. Giving the results of the 3-D geological study in a paper report will always be a compromise, as the mapped 3-D features should really be observed in the 3-D stereo-plotter in order to provide them full justice. However, the obtained results can still be extracted from the present report where relevant features are described for each island and illustrated on photos and geological cross-sections which have been generated for most of the cliff sections by projecting the mapped high resolution 3-D features onto 2-D segments. In addition, the mapped horizons (3-D polylines) are stored in a GIS database and can be exported as shape files suitable for 3-D modelling using, for example, Petrel reservoir engineering software. G E U S 5 2. Study2B area Digital high resolution photos were originally planned to be taken from helicopter along the steep cliffs of Hestur, Sandoy, Skúgvoy, Stóra Dímun and Lítla Dímun. These were sup- plemented with cliff sections from Nólsoy, Svínoy and Fugloy (Fig. 2.1), as the collecting of photos from helicopter showed out to be very efficient and less time consuming than ex- pected. The cliff sections along the islands originally planned to be studied showed to be of varying quality in respect to the subjects of the present study. The photos of the cliff sections of Sandoy, Skúgvoy, Stóra Dímun, Nólsoy, Svínoy and Fugloy reveals distinct interlava units possible to map over several kilometres, this is not the case for the photographed cliff sec- tions of Lítla Dímun, Koltur and partly Hestur where