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The Irish government is looking off its west coast at the Rockall and Porcupine basins for a recently announced new exploration licensing round, scheduled to close in May 2011.

State-of-the-art 2D regional reconnaissance seismic data such as this seismic depth section (30km) across the southern Porcupine Basin have shed new light on the hydrocarbon potential of an area that has remained an exploration backwater for many years. Since rift-ing occurred in the Jurassic, the basin has remained underfi lled and a prominent 2 km deep depression is present at the seabed. Eight kilometers of post rift sediment fi ll is present in the central axis. Rifted continental crust appears to extend across the whole basin (Vertical exaggeration 2.5).

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ION-1023_NEAtlantic_GeoExpro.indd 1 10/14/10 12:42:01 PM

LOOKING OFFSHORE: Ireland’s Wild Western Frontiers

Imaging below BasaltsThe new seismic data shown here was shot and processed by GXT in 2007 and 2008. The seismic line acquisition and Pre-Stack Depth Migration (PSDM) processing are all designed to provide optimum seismic imaging below thick (>1 km) Paleogene basalts, which cover much of the Northeast Atlantic margin. This was achieved through using a long receiver offset (10.2 km) and large source size (7440 cu. in.) with 253 barm peak output. In addition, the airgun was towed at depth (17.5m), as was the geophone streamer (18m) and bubble interference was kept to a minimum. The air gun and streamer deep-tow provided a maximum energy input at 20-30 Hz frequency. This low frequency energy penetrates through the basalts more effectively. Another signifi cant data quality benefi t of deep-tow is the reduced noise caused by near surface current and wave action. This acquisition produced more coherent refl ections below the basalts which usually have a high seismic attenuation factor. Data was recorded to 18 seconds TWT, to image the complete crustal structure as seen on line 1060 (foldout line pages 36 - 38). All the PSDM seismic data was migrated using velocities derived from iterative tomographic velocity modeling. The velocity-modeled depths are within 5% of the depths from several calibration wells in the Faroes-Shetland Basin located offshore north of Scotland. Data processors and interpreters collaborated throughout the data processing work fl ow by using interim processing results for interpretation reviews and for consultation on structure and velocities, e.g. brute stacks, preliminary migrations, potential fi elds modeling results and fast track geologic checks. The results are improved imaging of the mid and deep crustal levels in both the fi nal Pre-stack Time Migrated (PSTM) and PSDM volumes.

IAN DAVISON, GEO INTERNATIONAL LTD.; MENNO G. DINKELMAN AND WOUTER KOOL, ION-GX TECHNOLOGY (GXT)

� e Rockall Trough has been the center of political and geologi-cal controversy for the last two hundred years. � e area is named after the remote rocky island of Rockall and has been variously claimed by Iceland, Denmark, Ireland, and the United Kingdom. It was � rst claimed by the British in 1955 during the cold war to help prevent the Soviets spying on guided missile testing from the Outer Hebrides. Since then, various adventurers have placed � ags or lived for short periods on the rock. Greenpeace occupied it for 42 days in a solar-powered capsule in 1997 and renamed the rock as Waveland in a protest against oil exploration west of Britain. � e ownership remains in dispute and is providing yet another headache for the United Nations Convention on the Law of the Sea. � e geological controversy stems from the area having been described as both a continental and oceanic-� oored basin. In addition, the age of the main rifting phases remains unknown. Seismic imaging traditionally has been poor because of the presence of numerous intrusive sills and shallow extrusive basalts that mask the underlying strata. In the summer of 2007, as part of the regional NEAtlanticSPANTM program, ION GX Technology (GXT) shot more than 2,000 km of seismic data across the Rockall Trough and adjacent areas in an attempt to resolve the deep geological structure of this enigmatic area and evaluate the area’s hydrocarbon potential. � e Irish government has recently initiated a trans-Atlantic research program, using state of the art technology, building plate reconstruction models that accurately depict the geometry of the basin at the time of “break-up”. Studying palaeographic reconstructions will help to better understand the Atlantic shelf sedimentary sequences and possible derivation of clastics from the North American Plate.

With proven hydrocarbon accumulations and very large structural closures, the Rockall

and Porcupine basins off Ireland’s west coast could be put on the front burner for the

next licensing round.

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Bathymetric map and 3D relief of the offshore western Ireland.The Rockall and Porcupine basins open southwards into theAtlantic Ocean. The traces of the long regional seismic lines

used in this evaluation are shown in on the map. Line1060 is highlighted.

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alGreen: Top Cretaceous/Base TertiaryLight Blue: Base Upper CretaceousDark Blue: Base CretaceousIn rotated fault blocks below base Cretaceous: Jurassic to Top mid Jurassic sedimentary packages

unexplored rockall basinTh e area remains largely unexplored with its remote location, harsh weather and wave conditions. No wells have been drilled in the main deep central part of the Rockall Basin, and the few wells drilled thus far are located on the perched eastern margin rift shoulders or in the northern sub-basin. Most recently, Shell drilled the Dooish well 12/2-1 and sidetrack -1z in 2003, proving a gas conden-sate sandstone reservoir of probable early Permian age. A second deep-water well, 12/2-2, was drilled on the West Dooish Prospect in 2008, but the results remain con-fi dential. Th e presence of Jurassic rocks is perceived to be crucial to successful oil exploration in this basin. Th ey occur in surrounding basins in the Slyne-Erris Trough to the east, and the Goban Spur and Fasnet Basin to the south. Th ere is no direct evidence to confi rm or disprove their presence in the central Rockall Basin. Th e sediment fi ll averages 4-6 km throughout the main basin which is thick enough to ensure hydrocarbon maturation from any Early or Late Jurassic source rocks (refer to seismic foldout line 1060). A large part of the basin was aff ected by volcanism during the Paleogene period (54-59 million years ago) and several major seamounts are present in northern Rockall, as are extensive lava fl ows and intrusive sills. Th ese volcanics have caused major problems

Rockall Island measures just 33m in diameter and is 19m high and the highest oceanic wave ever recorded at 29m crashed over it in 2000. The rough sea conditions in this area have helped it remain a wild frontier. This photo was taken by the RAF in 1943.

West-East seismic depth section [30 km] across the northern Porcupine Basin, showing a mid-basin central ridge - the Dunquin prospect - that is interpreted to be a more than 1.5 km high volcanic buildup of early Cretaceous age which overlies rotated Jurassic fault blocks; the central volcanic high is thought to be topped by carbonate build-ups. The volcanic ridge is probably comprised of mostly submarine basalt fl ows that erupted over a relatively short period of time around 120 Ma ago. The overlying late Aptian to Albian sequence consists of marine shelf claystones and sandstones which grade into deep marine shales in the southern part of the basin. The large angle of onlap of the Cretaceous strata onto the eastern and western basin margins suggests a deep basin hole was always present and which has never fi lled in. The red event at the deep crustal level is the Moho from gravity modeling.

Green: Top Cretaceous/Base TertiaryLight Blue: Base Upper CretaceousDark Blue: Base CretaceousRotated fault blocks below base Cretaceous: Jurassic to Top mid Jurassic sedimentary packages.

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for seismic imaging and have impeded ex-ploration eff orts for deeper targets so far. However, in the southern part of the basin the volcanics are absent. Large inversion fold traps have been imaged at Cretaceous levels in the southern part. Th ese folds probably developed during Oligocene-Miocene times when regional basin inversion took place in the Faroes-Shetland Basin farther north. Cretaceous and Early Paloecene sand-stones would be the most attractive reservoir targets. Sands may well have been derived from Greenland, similar to the inferred sediment provenance in other North Atlantic margin basins such as the outer Faroes-Shet-land and Vøring basins. Th e seismic data indicate these large fold closures are viable exploration targets, although a more detailed seismic grid will be required to confi rm this.

proven sources in porcupine basin To date, exploration in the Porcupine Basin has found several non-commercial fi elds. In the 1970’s, BP discovered two oil accumula-tions at Connemara (200 MMbo in place) and in 1981, Phillips discovered the 100-200 MMbc Spanish Point gas and condensate fi eld. GXT acquired more than 1,600 km of reconnaissance seismic across the Porcupine area in 2007. Th is Basin is a sediment-starved Jurassic rift which opens southward into a bathymet-ric low reaching up to 4 km water depth at its southern end. Th ere are good quality Jurassic source rocks of Kimmeridgian and Middle Jurassic age. In the deep central part of the basin these rocks are buried to 8 km or more below sea bed where, at present, they are probably gen-erating gas. Perched large rotated fault blocks occur along the margins of the basin and may contain mature sources in the oil window.

Large inversion anticline imaged at Cretaceous level in the southern Rockall Basin. Seismic imaging is good in this part of the basin as there are no volcanics present.

Vertical gas chimneys and deep water carbon-ate mounds are present above seabed seeps indicating that the southern part of the basin is capable of generating hydrocarbons. Providence Resources and Exxon currently hold license blocks over this ridge known as the Dunquin prospect. Th ey were attracted by the large closure where carbonate reser-voirs have been interpreted on the crest of the structure. While the remote location and harsh con-ditions found in Ireland’s western off shore basins have limited exploration, this new data clearly show promising source rocks and large untested prospects like Dunquin. Th e new leasing round should spur companies to re-examine this area.

a first step?We consider the Irish off shore region to be a much neglected area of oil exploration. Good

Jurassic source rocks are proven in the Porcu-pine Trough, and non-commercial hydrocar-bons have been discovered. Unfortunately, the remote location and early lack of commercial success have limited the interest in this basin. Th e Rockall Trough is a huge basin with suf-fi cient thickness of sediment to generate oil from source rocks in the bottom third of the rift. Th e presence of many large intrusive sills is a signifi cant risk to exploration, due to possible overcooking of source rocks, but the southern part of the basin appears to have largely escaped volcanism and large Cenozoic folds have been imaged in this area that have never been tested. Realistically it may be some time before more seismic is shot in this remote basin and additional wells are drilled, but we hope the data shown here will spur companies on to re-examine this basin. Th e 2011 Irish Bid Round could be a fi rst step in this direction.

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WGP

WGPGROUP.CO.UK

Exploration and B eyond.. . .

OCEAN BOTTOM SEISMIC • 2D • SOURCE SYSTEMS • PROJECT MANAGEMENT

36 GEO EXPRO NOVEMBER 2010 GEO EXPRO NOVEMBER 2010 38

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Yellow: Top L. EoceneGreen: Top Cretaceous/base TertiaryDark Blue: Base CretaceousPink and Purples: Various volcanic sill complexes

Two Frontier Basins Come to Light

Off the western Irish coast, new regional 2D seismic data is providing key information

on the petroleum potential of a large and underexplored area.

Map showing the 2011 Irish and UK Offshore Licensing Rounds. The NEAtlanticSPAN TM survey grid that was used in this evaluation is shown. Line 1060 shown in black is the foldout line shown here

The roughly south-east to north-west trending 420 km long seismic line 1060 shown here crosses the southern Rockall Basin. This 30 km depth section (PSDM) shows up to 6 km of sedimentary fi ll and large rotated fault blocks or half graben that contain potential Jurassic source rocks. The red line thought to be the top of crystalline basement or continental crust at depths of 23 – 27 km, rising to 16 – 17 km under the center of the Rockall Trough, is derived from gravity modeling and is interpreted to be the Moho. Note the signifi cant thinning and extension of the continental crust under the Rockall Trough to about a 6 km thickness. Results of gravity inversion stud-ies (Welford, J.K., Shannon, P.M., O’Reilly, B, & Hall, J. 2010. Lithospheric density variations and Moho structure of the Irish Atlantic continental margin from unrestrained gravity inversion. Geophys. J. Int., 17 pp.) show similar thinning and major crustal stretching across the Rockall Trough.

See the complete story starting on page 39

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