Ireland's Wild Western Frontiers

TOPIC EXPLORATION

Bathymetric map and 3D relief of the offshore western Ireland. The Rockall and Porcupine basins open southwards into the LOOKING OFFSHORE: Atlantic Ocean. The traces of the long regional seismic lines used in this evaluation are shown in on the map. Line 1060 is highlighted.

Ireland’s Wild Western Photo: Deb Bertossa Frontiers © ION Geophysical

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 geological controversy for the last two hundred years. e area is named Imaging below Basalts after the remote rocky island of Rockall and has been variously The new seismic data shown here was shot and claimed by Iceland, Denmark, Ireland, and the United Kingdom. processed by GXT in 2007 and 2008. The seismic It was rst claimed by the British in 1955 during the cold war to line acquisition and Pre-Stack Depth Migration (PSDM) help prevent the Soviets spying on guided missile testing from processing are all designed to provide optimum seismic the Outer Hebrides. Since then, various adventurers have placed imaging below thick (>1 km) Paleogene basalts, ags or lived for short periods on the rock. Greenpeace occupied which cover much of the Northeast Atlantic margin. The Irish government is looking off its west coast at the Rockall and Porcupine basins for a recently announced new exploration licensing round, it for 42 days in a solar-powered capsule in 1997 and renamed This was achieved through using a long receiver scheduled to close in May 2011. the rock as Waveland in a protest against oil exploration west of offset (10.2 km) and large source size (7440 cu. in.) Britain. e ownership remains in dispute and is providing yet with 253 barm peak output. In addition, the airgun another headache for the United Nations Convention on the was towed at depth (17.5m), as was the geophone Law of the Sea. streamer (18m) and bubble interference was kept to With proven hydrocarbon accumulations and very large structural closures, the Rockall e geological controversy stems from the area having been a minimum. The air gun and streamer deep-tow provided a described as both a continental and oceanic- oored basin. In and Porcupine basins off Ireland’s west coast could be put on the front burner for the maximum energy input at 20-30 Hz frequency. This addition, the age of the main rifting phases remains unknown. low frequency energy penetrates through the basalts Seismic imaging traditionally has been poor because of the next licensing round. more effectively. Another signifi cant data quality presence of numerous intrusive sills and shallow extrusive benefi t of deep-tow is the reduced noise caused by basalts that mask the underlying strata. In the summer of 2007, TM near surface current and wave action. as part of the regional NEAtlanticSPAN program, ION GX This acquisition produced more coherent refl ections Technology (GXT) shot more than 2,000 km of seismic data below the basalts which usually have a high seismic across the Rockall Trough and adjacent areas in an attempt to attenuation factor. Data was recorded to 18 seconds resolve the deep geological structure of this enigmatic area and TWT, to image the complete crustal structure as seen evaluate the area’s hydrocarbon potential. on line 1060 (foldout line pages 36 - 38). All the e Irish government has recently initiated a trans-Atlantic PSDM seismic data was migrated using velocities research program, using state of the art technology, building derived from iterative tomographic velocity modeling. plate reconstruction models that accurately depict the geometry The velocity-modeled depths are within 5% of the Green: Top Cretaceous/Base Tertiary of the basin at the time of “break-up”. Studying palaeographic depths from several calibration wells in the Faroes- Light Blue: Base Upper Cretaceous reconstructions will help to better understand the Atlantic shelf Shetland Basin located offshore north of Scotland. Dark Blue: Base Cretaceous sedimentary sequences and possible derivation of clastics from Data processors and interpreters collaborated In rotated fault blocks below base throughout the data processing work fl ow by using Cretaceous: Jurassic to Top mid Jurassic the North American Plate. sedimentary packages interim processing results for interpretation reviews State-of-the-art 2D regional reconnaissance seismic data such as this and for consultation on structure and velocities, e.g. seismic depth section (30km) across the southern Porcupine Basin brute stacks, preliminary migrations, potential fi elds have shed new light on the hydrocarbon potential of an area that modeling results and fast track geologic checks. The has remained an exploration backwater for many years. Since rift- results are improved imaging of the mid and deep ing occurred in the Jurassic, the basin has remained underfi lled and crustal levels in both the fi nal Pre-stack Time Migrated 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 (PSTM) and PSDM volumes.

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ION-1023_NEAtlantic_GeoExpro.indd 1 10/14/10 12:42:01 PM t o p i c EXPLORATION

unexplored rockall basin Th 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 condensate 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 confi 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, Rockall Island measures just 33m in diameter and is 19m high and the highest oceanic wave ever as are extensive lava fl ows and intrusive sills. recorded at 29m crashed over it in 2000. The rough sea conditions in this area have helped it remain Th ese volcanics have caused major problems a wild frontier. This photo was taken by the RAF in 1943.

Green: Top Cretaceous/Base Tertiary Light Blue: Base Upper Cretaceous Dark Blue: Base Cretaceous Rotated fault blocks below base Cretaceous: Jurassic to Top mid Jurassic sedimentary packages. © ION Geophysical 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.

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for seismic imaging and have impeded exploration 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 © ION Geophysical 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 sandstones 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 Large inversion anticline imaged at Cretaceous level in the southern Rockall Basin. Seismic imaging is seismic grid will be required to confi rm this. good in this part of the basin as there are no volcanics present. proven sources in porcupine basin To date, exploration in the Porcupine Basin Vertical gas chimneys and deep water carbon- Jurassic source rocks are proven in the Porcu- has found several non-commercial fi elds. In ate mounds are present above seabed seeps pine Trough, and non-commercial hydrocar- the 1970’s, BP discovered two oil accumula- indicating that the southern part of the basin bons have been discovered. Unfortunately, the tions at Connemara (200 MMbo in place) is capable of generating hydrocarbons. remote location and early lack of commercial and in 1981, Phillips discovered the 100-200 Providence Resources and Exxon currently success have limited the interest in this basin. MMbc Spanish Point gas and condensate hold license blocks over this ridge known as Th e Rockall Trough is a huge basin with suf- fi eld. GXT acquired more than 1,600 km of the Dunquin prospect. Th ey were attracted fi cient thickness of sediment to generate oil reconnaissance seismic across the Porcupine by the large closure where carbonate reser- from source rocks in the bottom third of the area in 2007. voirs have been interpreted on the crest of the rift. Th e presence of many large intrusive Th is Basin is a sediment-starved Jurassic structure. sills is a signifi cant risk to exploration, due rift which opens southward into a bathymet- While the remote location and harsh con- to possible overcooking of source rocks, but ric low reaching up to 4 km water depth at its ditions found in Ireland’s western off shore the southern part of the basin appears to have southern end. Th ere are good quality Jurassic basins have limited exploration, this new largely escaped volcanism and large Cenozoic source rocks of Kimmeridgian and Middle data clearly show promising source rocks and folds have been imaged in this area that have Jurassic age. large untested prospects like Dunquin. Th e never been tested. Realistically it may be In the deep central part of the basin these new leasing round should spur companies to some time before more seismic is shot in this rocks are buried to 8 km or more below sea re-examine this area. remote basin and additional wells are drilled, bed where, at present, they are probably gen- but we hope the data shown here will spur erating gas. Perched large rotated fault blocks a first step? companies on to re-examine this basin. Th e occur along the margins of the basin and may We consider the Irish off shore region to be a 2011 Irish Bid Round could be a fi rst step in contain mature sources in the oil window. much neglected area of oil exploration. Good this direction.

geo expro november 2010 43 Map showing the 2011 Irish and UK Offshore Licensing The roughly south-east to north-west trending 420 km long seismic line 1060 shown here crosses the southern Rockall Basin. This 30 km Rounds. The NEAtlanticSPAN TM survey grid that was © ION Geophysical depth section (PSDM) shows up to 6 km of sedimentary ﬁ ll and large rotated fault blocks or half graben that contain potential Jurassic used in this evaluation is shown. Line 1060 shown in 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 Two black is the foldout line shown here km under the center of the Rockall Trough, is derived from gravity modeling and is interpreted to be the Moho. Note the signiﬁ cant Frontier Basins 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 Come to Light the Rockall Trough. Off the western Irish coast, new regional 2D seismic data is providing key information See the complete story starting on page 39 on the petroleum potential of a large and underexplored area.

Yellow: Top L. Eocene Green: Top Cretaceous/base Tertiary Dark Blue: Base Cretaceous Pink and Purples: Various volcanic sill complexes © ION Geophysical

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