Geoscientist Online

Simon Passey* describes a recent multidisciplinary conference where oil industry met volcanology to explore the potential for reservoirs within continental flood basalts.

Geoscientist 18.4 April 2008

In 2004, and to a great deal of surprise, Chevron with partners Statoil, OMV and Dong made an oil and gas discovery within a Palaeogene volcanic sequence in the Faroe-Shetland Basin. The reservoir sandstones are contained within a ~100 m thick interbedded sequence of basaltic lava flows, hyaloclastites and volcaniclastic and siliciclastic sandstones.

This “Rosebank” discovery in Well 213/27-1z in the UK area of the basin has opened up new potential plays within the volcanic-dominated Faroese area, where previously only sub-volcanic plays have been considered.

With the forthcoming third licensing round in the Faroese area, oil industry geologists are taking a renewed interest in volcanic rocks and their interaction with sedimentation. Consequently, an industry-funded conference was organised to advance its understanding of volcanic processes within passive volcanic margins and more generally within large basaltic provinces. The Jóannes Rasmussen Conference, (named in honour of the founding father of Faroese geology) focused on various aspects pertaining to the evolution of basaltic provinces, many of which were exemplified in the two accompanying field excursions.

Continental flood basalt (CFB) provinces occur throughout the geologic past - for from the Siberian Traps at the Permo-Triassic Boundary to the Columbia River Basalt Group in the Mid-Miocene. Traditional studies in CFB provinces have primarily concentrated on the geochemistry and palaeomagnetics of the prevailing lava flows, with little if any attention being given to the minor interlava lithologies (volcaniclastics, palaeosols and sedimentary units) that commonly occur between the flows. However, with the impetus of hydrocarbons behind them, investigations have moved towards understanding the complex internal and external architectures of these flows, their eruption environments and the genesis of the interlava lithologies.

Studies of a number of volcanic-affected basins have shown that volcaniclastic rocks can (contrary to received wisdom) actually have good porosity and permeability suitable for reservoirs – as in the Liaohe Basin, China. Indeed, oil has been produced from basaltic tuffs in the South Cuba Basin, and other commercial reservoirs have been discovered in association with volcanic rocks from a variety of settings.

Jóannes Rasmussen Conference

The Faroe Islands in the NE Atlantic Ocean are one of the best areas in Europe, if not the world, to study a completely accessible sequence through a ~6.5km thick CFB province, the Faroe Islands Basalt Group (FIBG). Geologists have been interested in the islands from early in the 17th Century - not so much for its basalts but for the thin, <1m thick, coal seams intercalated between two basalt-dominated formations. Later, Faroese basalt took centre stage and even featured in Neptunists v. Plutonists debates at the end of the 18th Century. The first geological survey of the islands began in the 1820s and even the renowned Geikie brothers, James and Archibald, made contributions at the end of the century.

Archibald’s work and others' paved the way for Jóannes Rasmussen, considered the forefather of Faroese geology. His methodical mapping, (with his supervisor Arne Noe-Nygaard) from the 1930s-60s led to the publication of the first comprehensive geological map and memoir in 1969. This was conducted at a time when no detailed mapping of CFBs had been performed elsewhere.

The conference was hosted by Jarðfeingi (Faroese Earth and Energy Directorate) at the Nordic House in the heart of Tórshavn between the 27 August and 3 September 2007. Its theme was the Evolution of Basaltic Provinces, and covered traditional aspects such as geochemistry to more recent developments concerning lava flow emplacement.

Weathering, sedimentation and pyroclastic activity

An aspect commonly overlooked in CFB provinces is the weathering and erosion of the volcanic landscape and the transportation and deposition of sediments. Good examples of weathered flow-tops and the development of palaeosols were observed on the field excursion to Suðuroy - dominated by laterally extensive sheet lobes of the Beinisvørð Formation that are commonly overlain by saprolitic boles (palaeosols) that grade downwards through weathered flow tops to fresher basalt beneath.

Martine Gérard (IMPMC, France) gave a keynote talk on the formation of such palaeosols from the Deccan and Karoo Traps. Gérard suggested that the time required to form such units are much less than previously envisaged - in the order of 1-50kyrs for even the thickest saprolites.

Andersen & Boldreel also demonstrated how such units can be delineated in boreholes directly from wireline log measurements, which was of particular interest to the hydrocarbon industry.
Inter-eruption facies, including palaeosols, are developed during periods of volcanic quiescence and within CFB provinces typically consist of “normal” terrestrial sedimentary units. This is no exception for the FIBG, which contains numerous examples of fluvial volcaniclastic sandstones previously thought of as pyroclastic tuffs. Such units were observed during the field excursion around the central and northern islands and included, for example, the channel-confined sandstones at Leynavatn, the maximum ~6m thick Argir Beds and the Equisetum-bearing beds at í Bugum.

Equisetum , or horsetail, is typically found growing within or along the banks of fluviatile systems. Equisetum can survive environmental disturbances such as fire, rapid burial and drought, all particularly prevalent in volcanic areas, where Equisetum is one of the first post-eruption colonisers. Other inter-eruption facies included the extensive, ~9 m thick, coal-bearing Prestfjall Formation on Suðuroy. Small parties were taken into a 200m deep mine adit south of Hvalba where the coal and associated claystones of the formation were examined. Ólavsdóttir highlighted the potential that many of the FIBG interlava units maybe of reservoir quality and substantially thicker and cleaner at the distal end of the volcanic province.

The Prestfjall Formation is overlain by the >30m thick Hvannhagi Formation, consisting of interbedded airfall tuffs and terrestrial mass flow deposits (i.e. syn-eruption) facies. A day was spent in the Hvannhagi valley on Suðuroy examining these deposits and the Stapin Vent that comprises good examples of cow-dung and breadcrust bombs. The significance of adding large quantities of unconsolidated tephra to the landscape, and of subsequent reworking and deposition by sedimentary processes including mass flow events, was discussed during the excursion. A number of presentations emphasised the often-neglected role of mafic pyroclastic activity, particularly in the initial stages, in the development of CFB provinces (Peate et al.; Ross & White). Jolley et al. also discussed the potential link between the volcanism associated with the North Atlantic Igneous Province (NAIP) and the Palaeocene-Eocene Thermal Maximum (PETM), a global warming event that would have resulted in increased rainfall and weathering rates that may have led to increased sedimentation, including within volcanic areas.

Structural evolution

Due to the vast areal extent of CFB provinces it may be difficult (particularly in poorly exposed areas) to assess the influence of the pre-existing structural grain on the development of a province - for example, on the distribution of the lava flows. This is pertinent to the FIBG where the dominant NW-SE configuration of the fjords and sounds have been attributed to fissures and more recently to the so-called "transfer zones" inferred offshore from potential field and seismic data.

Madsen reported that no significant fracture zones or dyke swarms were encountered during the construction of the Vágar and Norðoyar sub-sea tunnels. However, Passey using a detailed sequence-stratigraphic approach, suggested the presence of increased accommodation spaces on the north-eastern sides of some of the fjords and sounds, which were attributed to deep-seated faults. The sub-sea tunnels and a number of fractures were visited during the two field excursions and one of the key observations was the negligible movement associated with the fractures.

In a more regional context, Tony Doré (Statoil, Norway) gave a thought-provoking keynote talk about the development of the North Atlantic volcanic passive margin and its assumed causal relationship to the Iceland hotspot. He went on to suggest that if a mantle plume was responsible for all the magmatic features of the NAIP it would have had to “perform extraordinary acrobatics in both time and space” to explain them all. This was aptly followed up by the talk by Elkins-Tanton on lithospheric thinning through gravitational instabilities at the lithosphere-asthenosphere boundary, resulting from the initiation of large igneous province formation. These talks were the subject of much discussion during the poster sessions and the conference dinner at Hotel Hafnia.

Lava Morphologies

Within the last decade the concept that vast, laterally extensive lava flows in CFB provinces are rapidly emplaced simple a’a flows has been questioned. The keynote talk by Thor Thordarson (University of Edinburgh, UK) focused on the idea that these are actually large-scale compound flows composed of inflated pahoehoe lava emplaced over many months to years. This was reiterated throughout the conference with examples from the Deccan, India (Jay et al.; Self et al.); Columbia River, USA (Thordarson); Chilcotin, Canada (Farrell et al.); Iceland (Thordarson) and from the two field excursions around the Faroe Islands. On Suðuroy, the laterally extensive, typically ~20 m thick, sheet lobes of the Beinisvørð Formation have features consistent with having been emplaced as inflated pahoehoe lobes. First, they generally have sharp planar bases (rather than clinkery basal carpets expected for a’a lava). Second, the sheet lobes can generally be separated into a vesicular basal crust, a massive core and a vesicular upper crust that sometimes exhibits horizontal vesicle zones - all indicative of the endogenous (inflation) emplacement mechanism. Lastly, the upper "clinkery zones" have been re-interpreted as weathering zones, associated with soil development.

Another aspect of the lava flows discussed during the field excursions was the link between morphological features and the environment of eruption. On Suðuroy, the sheet lobes within the upper 200m of the Beinisvørð Formation generally exhibit well-developed columnar jointing. The occurrence of the well-developed columnar jointed flows is associated with the appearance of substantial fluvial and other terrestrial sedimentary deposits suggesting a relatively “wet” and eroded landscape allowing for the ponding of flows. In support of a wet environment is the presence of the only observed multi-tiered lava flow within this upper sequence of the Beinisvørð Formation. This flow can clearly be subdivided into colonnade and entablature tiers, where the wavy columns of the latter are thought to be the result of water ingress through the upper surface of the flow, and modifying the internal isotherms. Ponding of flows was clearly observed at the famous locality at Kulagjógv, E of Froðba, where a flow has inundated a river channel and solidified - resulting in a superb fanning array of columns. Inference of the environmental setting was highlighted by Farrell et al. and Pedersen et al. who presented examples of hyaloclastite facies formed by subaerial lava flows building outwards into valley-confined fluvial systems in Chilcotin, Canada, and into a lake in West Greenland respectively.

Much of the discussion concerning the mechanisms of emplacement and environments of eruption of CFBs was an eye-opener for many from the hydrocarbon industry. The industry has generally tended to interpret parallel bedded lava sequences on seismic data as rapidly emplaced a’a lava flows. Now the possibility exists for the complex and drawn out interaction between the emplacement of the different lava flow facies and synchronous sedimentation in, for example, the Faroe-Shetland Basin.

 

Geochemical evolution and dating

Geochemistry has been used as a major tool in understanding the evolution of - and to correlate between - disparate basaltic provinces ever since such areas were first investigated. Indeed, Saunders & Reichow (University of Leicester, UK) gave an interesting keynote talk comparing the Siberian Traps to the NAIP and speculated that the two provinces may have been initiated by the very plume situated beneath Iceland today. Similarly, Søager & Holm refined the geochemical correlation between the FIBG and the volcanic province of East Greenland by suggesting the presence of lava flows from the uppermost part of the FIBG that are geochemically similar to flows from the Geikie Plateau Formation, East Greenland. Differences between the geochemistries and petrologies of the lava flows from the Enni Formation were highlighted during the field excursion to the north-eastern islands. The olivine-bearing basalts have been shown to have a low-TiO2 (<1.5%), MORB-like composition and these flows are easy to distinguish in the landscape because they have a brown weathering attribute. This contrasts with the high-TiO2 plagioclase-phyric basalts that have a pale (grey) weathering attribute that is accentuated by the presence of whitish-grey lichen.

In their poster, Halton et al. a highlighted the difficulty of accurately dating seemingly fresh basalts using the 40Ar/39Ar dating technique due to a number of problems that can cause disturbance to the 40Ar/39Ar system. Consequently, different approaches maybe needed to constrain the durations and ages of basaltic areas. Based on the geochemical correlation between the FIBG and East Greenland, Tegner & Larsen suggested that the Malinstindur and Enni formations of the FIBG were, most likely, emplaced within 300,000 years. This figure was obtained by the novel approach of constraining the emplacement of the 5-6 km thick flood basalts of East Greenland using the cooling and pressure histories of the coeval Skaergaard intrusion. The duration of the preceding volcanic hiatus in the FIBG was estimated by Mørk using Milankovich cycles to constrain the time-span represented by a ~7.5m thick sedimentary section from the Prestfjall Formation on Suðuroy. This section occurs between the lava flows of the Beinisvørð and Malinstindur formations and Mørk calculated the duration for the volcanic hiatus at this locality to be in the order of ~384,000 years. Relatively short durations for the emplacement of CFBs was a main theme throughout the conference.

Towards the third round…

Volcaniclastic lithologies can make good reservoirs, and their encasing lava flows may act as traps – a fact already borne out by the Rosebank discovery at the feather-edge of the Faroe Islands Basalt Group. At time of writing the third licensing round for the Faroese area is expected to begin in the first quarter of 2008. This conference will have helped exploration managers assess the potential of this volcanic-affected area by highlighting the importance of sedimentation during the development of continental flood basalt provinces.
Acknowledgements

I would like to thank the following people from Jarðfeingi with their help in making this conference such a success: Súsanna Sørensen, Finn Mørk, Gunnleyg Ihlen, Bjørg Svabo, Thomas Varming and Uni Árting. Grateful appreciation also goes to Óluva Ellingsgaard with help in producing the field excursion guides. Thanks also go to Fríðun Hansen and Kamilla Benzon of Greengate Incoming for arranging flights and accommodation for the conference delegates. A big thank you goes to all the participants (keynotes, presenters and attendees) for making this a most relaxed and discussion orientated conference. The conference organisers are also indebted to the following sponsors of the conference: BP, Chevron, ENI, Faroe Petroleum, Statoil and Geysir Petroleum. Finally, thanks go to the Volcanic & Magmatic Studies Group (VMSG) and the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) for supporting the conference.

Further Reading

• The abstract volume is available free of charge as a PDF and the field guides can be purchased for a nominal fee by contacting Jarðfeingi (www.jf.fo)
•  1st Jóannes Rasmussen Conference website: http://jrasmussen.jf.fo/
• LARSEN, L.M., WAAGSTEIN, R., PEDERSEN, A.K. & STOREY, M. 1999. Trans-Atlantic correlation of the Palaeogene volcanic successions in the Faeroe Islands and East Greenland. Journal of the Geological Society, London, 156, 1081-1095.
• PASSEY, S.R. & BELL, B.R. 2007. Morphologies and emplacement mechanisms of the lava flows of the Faroe Islands Basalt Group, Faroe Islands, NE Atlantic Ocean. Bulletin of Volcanology, 70, 139-156.
• SCHUTTER, S.R. 2003. Occurrences of hydrocarbons in and around igneous rocks. In: PETFORD, N. & MCCAFFREY, K.J.W. (eds) Hydrocarbons in Crystalline Rocks. Geological Society, London, Special Publications, 214, 35-68.

Contact Details

*Dr. Simon R. Passey, Jarðfeingi, Brekkutún 1, PO Box 3059, FO-110, Tórshavn, Faroe Islands T: +298 357033; E: [email protected]; W: www.jf.fo