Over the years estimates of hydrocarbon resources in Irish waters have ranged from 10 Bboe to 50 Bboe, but at present little exploration work exists to substantiate either figure. Indeed, in nearly half a century, almost 200 wells have been drilled around Ireland but there have only been four commercial discoveries. Shell’s problems with the Corrib gas field off western Ireland, when some local landowners refused to allow construction work on the onshore section of pipeline, have been well documented, and that and other environmental issues have been addressed; the project is now due on stream late 2014.
The lack of exploration is not surprising, as many of the Atlantic Margin plays involve operating in water depths of between 500 and 2,000m, in the challenging Atlantic Ocean. These are not hospitable conditions at the best of times and present unique and costly challenges to drilling. However, with an oil price consistently over US$ 100 per barrel, drilling here is now more widely accepted as being economically viable and the situation is changing. Historically, exploration of the Porcupine Basin has focused on Jurassic targets based on similar plays in the northern North Sea but, unlike the North Sea, no wells have been drilled in the basin over the past decade.
Success in different types of reservoirs elsewhere on the Atlantic margin, such as offshore West Africa and South America, have led to fresh ideas that have prompted a re-examination of the whole basin. Couple this with improving technology, sustained high oil prices and competitive terms, and a number of explorers are finding Ireland now offers an attractive opportunity. Supporting this renewed interest, the Petroleum Affairs Division of Ireland’s Department of Natural Resources (PAD) has contracted BGP to acquire a mega-regional 2D long-offset seismic survey of some 18,000 km, covering all the basins west of Ireland. Acquisition should start early April 2013 and be concluded by the end of the year.
The Challenging Porcupine Basin
Recent interest offshore Ireland has focused on the shallow North Celtic Sea Basin, off the south coast, where Providence Resources grabbed the headlines in the final quarter of 2012 declaring it has established a potential of up to 1.6 Bbo in-place at its Barryroe discovery in around 100m of water off the south coast. The company is now planning operations in the more challenging Porcupine Basin.
This basin is a large north–south oriented structure lying 150 km south-west of Ireland that contains up to 10km of Upper Palaeozoic to Cenozoic sediments in water depths ranging from around 350m in the north to 3,500m in the far south. 13 wells have been drilled so far in the key area of Jurassic deposition which covers about 10,000 km2. The PAD consider that a number of major plays can be recognized, with significant potential expected to be located in Jurassic and pre-Jurassic reservoirs in tilted fault blocks developed along the margins of the Porcupine Basin. There is additional potential in post-rift stratigraphic and structural plays in Cretaceous and Tertiary sediments. Further indications of hydrocarbons are the presence of gas chimneys and seeps in areas not yet explored. Potential sources included Upper Jurassic Kimmeridge Clay equivalent mudstones, which are known to be present and mature for hydrocarbon generation in the Porcupine Basin.
Given the size and complexity of the basin it remains underexplored. Flows of hydrocarbons from wells in the basin in the 1980s and 90s, although deemed uneconomic at the time, indicate the presence of a number of working petroleum systems. Few of these wells were drilled on quality seismic data; current thinking is that the basin holds the potential to reveal opportunities that the advent of modern 3D seismic data brings and just about all of the players have revealed structures with massive potential reserves. While this has worked wonders for respective share prices, a sense of reality beckons as a new wave of wildcat drilling is about to commence.
Here Come the Boys!
ExxonMobil has signed a letter of intent to use the Eirik Raude S/S to drill the huge Dunquin gas/condensate prospect during the second quarter of 2013. Located in 1,524m of water, the Dunquin project consists of two main Middle Cretaceous carbonate prospects, Dunquin South and Dunquin North; combined resources are estimated at 1.7 Bboe recoverable, a staggering resource for Europe if proven.
Providence and partner Sosina Exploration were awarded Licensing Option 11/9 as part of the 2011 Irish Atlantic Margin Licensing Round, which contains the southern Porcupine Basin Drombeg prospect in around 2,500m of water, 220 km off West Cork and around 60 km from the Dunquin exploration prospect. According to Providence, preliminary analysis has indicated a recoverable P50 prospective resource potential of 872 MMbo, based on an oil-in-place volume of 2,970 Bbo. However, further technical data, including 3D seismic, will be required in order to better assess the ultimate resource potential. A number of similar Lower Cretaceous seismic anomalies have been identified both laterally offset to, as well as vertically stacked with, the Drombeg prospect, providing further resource growth potential.
In the main Porcupine Basin, Providence is relinquishing operatorship of Frontier Exploration License (FEL) 2/04, which contains the Lower Cretaceous Burren oil discovery, and FEL 4/08 to Chrysaor ahead of a two-well program in around 400m of water which is designed to appraise the 1981 Spanish Point gas condensate discovery, due to spud in the third quarter of 2013. Some 300 km2 of 3D seismic data was acquired in 2009 which firmed up a resource level of up to 510 MMboe with around 200 MMboe recoverable.
Two other Porcupine Basin players, Europa Oil and Gas and Petrel, have identified what they describe as previously unknown and significant prospects. Europa has identified two large prospects in the South Porcupine Basin: Mullen and Kiernan. Mullen, in water depth of about 1,000m, has a P50 resource of 482 MMbo, with plays identified in the Paleocene, Lower Cretaceous and Pre-rift Jurassic. First pass seismic mapping over the Kiernan prospect, which is in deeper waters of about 1,700m, has identified three reservoirs with total mean unrisked indicative resources estimated at 1,612 Bbo and 10 Tcfg. Europa anticipate drilling in the second phase of their frontier exploration license.
Petrel Resources has concluded a technical study of its six option blocks in the Porcupine Basin, revealing that ‘encouraging prospects at the Lower Cretaceous and Tertiary levels’ are evident in both sets of blocks, with those blocks in Quadrant 35 looking ‘particularly promising’.
Other players firming Porcupine Basin plans include San Leon Energy, operating a Frontier Exploration Licence at the northern limit of the North Porcupine Basin, where it has mapped the 180 MMb Benbaun prospect. This is a combined structural–stratigraphic Upper Jurassic prospect analogous to the UK Buzzard Field. Elsewhere, Bluestack has mapped a well-defined Upper Jurassic stratigraphic trap on Licensing Option 11/3 with a PMean of 8.3 Tcf gas in-place.
Two Seas Oil and Gas, meanwhile, have identified two large Paleocene fan prospects on their license area to the west of Spanish Point, which have potential in-place volumes of over 500 MMbo, as well as Jurassic and Cretaceous leads. Another interested party is Antrim Energy, with acreage just north of Dunquin which has promising Cretaceous play fairways.
Excitement is rising in the Atlantic Margin west of Ireland. This year the area will see the shooting of the largest ever seismic survey offshore Ireland and the drilling of one of the most exciting wildcats in a new play. The majority of the companies who were awarded acreage in the area in the recent licence round have worked up their prospects and are now actively looking for partners to progress exploration on their blocks.
Ireland has been one of those places where promise is always just around the corner, tantalizingly close yet so far away. With recoverable reserves for the offshore west of Ireland estimated to be in the region of 10 Bboe, it looks as though 2013 could be the year that the dream finally becomes a reality.
PAD Special Publication 3/06, Petroleum Systems of the Rockall and Porcupine Basins Offshore Ireland, prepared by the PAD and Ternan.