Noel J. Murphy, Peter F. Croker
Department of Energy
Dublin
The Erris and Slyne troughs are underexplored Mesozoic sedimentary basins off Ireland's northwest coast (Fig. 1).
The Irish Minister for Energy announced on Apr. 19, 1991, a frontier acreage licensing round of 128 blocks covering 29,000 sq km in these basins and the adjacent Rockall trough (OGJ, Apr. 29, 1991, p. 77). Closing date for the round is June 30, 1993, set to allow two seasons for the acquisition of new geophysical and geological data over the area.
Ireland has recently announced a new petroleum taxation regime (OGJ, May 4, 1992, p. 50). Revised licensing terms, which will acknowledge the specific circumstances of frontier acreage, will be announced shortly.
REGIONAL SETTING
The Erris and Slyne troughs are on the continental shelf edge northwest of Ireland. Each are deep and elongate Mesozoic sedimentary basins about 150 km long and 20-30 km wide.
Water depths range from less than 200 m to more than 2,000 m. These basins form part of an extensive northeast-southwest trending belt of rifted sedimentary basins to the west of Ireland and Britain (Fig. 2).
Most of the basins, including the Erris and Slyne troughs, are strongly asymmetric, and basin-bounding faults and transfer zones can frequently be interpreted as reactivated Caledonian fractures.
The region as a whole experienced several phases of rifting activity beginning in the Permo-Triassic and ending in the early Paleocene just before sea-floor spreading along the Reykjanes ridge.
Basins in the Hebridean and Orkney areas appear to have been gradually abandoned as the locus of rifting migrated westward towards the Rockall-Faeroe basin system. As a result, basins adjacent to the Rockall trough, such as the Erris and Slyne troughs, tend to contain a more complete Mesozoic stratigraphic record.
Localized structural inversion and widespread basic igneous activity occurred during the early Tertiary.
EXPLORATION HISTORY
Three wells have been drilled in the frontier round area so far.
Amoco drilled wells 19/5-1 and 12/13-1A in the Erris trough in 1978 and 1979, respectively, and Elf drilled well 27/13-1 in the Slyne trough in late 1981-early 1982. Texaco drilled a well in the neighboring Donegal basin to the northeast in 1978. Only limited exploratory drilling has taken place in the Rockall trough for which information is not yet available.
Seismic coverage is uneven, and much of the data are unmigrated and of poor quality. A art from a few recent well-tie lines and surveys acquired in 1991 (currently being interpreted), the most recent seismic data in the Slyne trough are from 1984 and in the Erris trough from 1978.
Some 49 seismic reflection surveys have been shot over the area since 1969. The area is also covered by two aeromagnetic surveys, and there is a limited amount of gravity, magnetic and deep seismic refraction and reflection data available.
All well data from the area and all seismic data acquired before 1986 are now released by the Department of Energy.
EXPLORATION RESULTS
Two of the three wells drilled in the Erris-Slyne area recorded hydrocarbon indications.
Minor amounts of C1-C4 gases were logged while drilling Carboniferous section in Erris well 19/5-1, and geochemical analysis suggested the presence of unaltered free oil.
Oil shows were recorded from Middle Jurassic sandstones in Slyne well 27/13-1 but were unsuccessfully tested. Fluorescence and C1-C3 gases were also observed while drilling thick Lower Jurassic claystones in that well.
There are no major obstacles to good seismic data quality in most of the area, and it is certain that the application of present-day acquisition and processing techniques will result in significant improvements.
STRUCTURE
The principal structural elements within the area (Fig. 3) are based largely on earlier seismic interpretations (Northwest Irish Offshore Basins Report, Department of Energy, 1982) and on the interpretation of deep seismic reflection profiles (WIRE) shot by the British Institutions' Reflection Profiling Syndicate. 1
Three seismic profiles (Figs. 4, 5, 6) have been selected to illustrate basin structure.
The Erris trough is an asymmetric westerly-tilted basin, fault-bounded against a Paleozoic platform to the east and apparently separated from adjacent basins, north and south, by reactivated Caledonian faults.
Within the trough, large fault-block structures are suggested by existing seismic data (Fig. 5). To the west, the Erris trough is partly separated from the Rockall trough by the southwesterly plunging Erris ridge (Figs. 4, 5).
The ridge, which is considered to have a Paleozoic core, is covered by Cretaceous-Tertiary strata and is fault-bounded to the west. Structural definition of its eastern flank is less clear.
The Slyne trough, bounded east and west by uplifted Paleozoic platforms, comprises two half-grabens of opposite polarity separated by a centrally located transfer zone.
The main basin-bounding fault lies to the west in the southern half-graben, while a fault-structured dip slope is developed towards the east (Fig. 6).
The northern half-graben is essentially a mirror image of the southern half-graben. Structural definition deteriorates towards both ends of the Slyne trough.
DEVELOPMENT, FILL
Fault-confined Permo-Triassic red beds with suspected unconformities pass upwards into overstepping fully marine Lower Jurassic sediments in the Erris and Slyne troughs (Fig. 7).
While the Slyne well only tagged the Triassic, seismic evidence indicates a relatively complete early Mesozoic section in that area.
Further rifting during the Middle-Upper Jurassic is indicated by increased rates of subsidence, terrigenous influx and fault activity in the Slyne and is expected to have occurred in the Erris trough, also.
Major end-Jurassic uplift and erosion, which produced a marked angular unconformity (Fig. 4), was probably responsible for the absence of post-Hettangian Jurassic section in the Erris wells, though in the case of 19/5-1, intra-Tertiary erosion may have also contributed. Off-structure, in deeper parts of the basin a potentially thick and relatively complete Jurassic section is indicated by seismic. End-Jurassic and/or intra-Tertiary erosion is also recognized in the Slyne trough (Fig. 6).
Renewed tectonic activity during the early Lower Cretaceous, indicated by faulting in the northern Erris trough, largely ceased by Hauterivian/Barremian times, coincident with the development of a further unconformity.
Thermal subsidence prevailed throughout the remainder of Cretaceous and early part of Tertiary, although intermittent faulting did continue along the western side of the Erris ridge and locally within the Erris and Slyne troughs.
A major westward tilting of the Erris trough occurred during the early Tertiary, apparently driven by the thermal subsidence in the Rockall trough to the west. Tertiary basin inversion, with an associated unconformity (? near-base Oligocene), is also recognized.
While the effects of this inversion are confined to the eastern parts of the Erris trough, the response seems to have been more widespread in the Slyne trough with the result that Cretaceous and Tertiary sections are largely absent there.
Geophysical data acquired in the Rockall trough indicate a thinned continental crust. 2
A block-faulted syn-rift sequence has been recognized, 3 possibly of Middle Jurassic-Lower Cretaceous age by analogy with the Faeroe basin. This is unconformably overlain by a Cretaceous-Tertiary thermal subsidence sequence that can be correlated on seismic data to the Erris trough.
The presence of localized deeply-buried Permo-Triassic basins has also been postulated. 4
STRATIGRAPHY
Drilling in the Erris and Slyne troughs has demonstrated thick Carboniferous sequences unconformably overlain by an extensive stratigraphic record ranging from Permian through to Tertiary and Quaternary deposits (Fig. 7).
Erris well 19/5-1 drilled about 5,500 ft of Carboniferous (Visean-Westphalian 'B'/'C') shallow marine and fluviatile sediments including carbonates, sandstones, carbonaceous shales, and coals.
Similar facies occur in the Donegal (well 13/3-1) and Porcupine basins and are expected to underlie much of the Erris and Slyne troughs.
In well 19/5-1 the Carboniferous is unconformably overlain by a thin Upper Permian (Zechstein equivalent) section comprising hypersaline marine mudstones, dolomites, and anhydrites, while red bed sandstones and shales of possible Lower Permian age are preserved in well 12/13-1A.
Triassic sections penetrated in the Erris trough comprise fluviatile or locally aeolian sandstones (Sherwood equivalent) overlain by red anhydritic playa-lake mudstones with sandstone interbeds, sometimes composed of volcaniclastic material (Mercia mudstone equivalent).
Late Triassic-Lower Jurassic (Rhaetian-Hettangian) shallow marine or locally brackish to fresh water 5 carbonates and claystones are present in all three Erris-Slyne wells. The remainder of the Lower Jurassic, only preserved in the Slyne well, is dominated by marine claystones, frequently rich in organic matter.
Middle-Upper Jurassic sediments in the Slyne well comprise shallow marine to coastal claystones, carbonates, and sandstones of Aalenian-?Bathonian age, followed by a thick, poorly dated interval mostly comprising marginal marine to nonmarine claystones and sandstones.
An Upper Jurassic age is indicated for this interval based on wire line correlation with the North Porcupine basin through a middle-basal Upper Jurassic (Callovian) age has also been suggested from sequence stratigraphic correlations with the Hebridean basins. 6
A fairly complete Cretaceous section is preserved in the Erris trough. In well 12/13-1A, shallow to marginal marine claystones and sandstones of late Ryazanian-Valanginian age are overlain by thick late Valanginian-Hauterivian sandstones ("Hauterivian sandstone") of interpreted submarine fan origin.
The remainder of the Lower Cretaceous is dominated by marine claystones giving way to argillaceous limestone at Albian level. A typical Upper Cretaceous section comprising micritic limestones and chalks, also present in that well, is overlain by Paleocene-Eocene marine carbonates and claystones.
The nature of younger Tertiary sediments is not well constrained due to the lack of data.
RESERVOIRS
Excellent reservoir quality sandstones have been proven by drilling in the Slyne and Erris troughs, while further potential is indicated by regional geological comparisons.
PALEOZOIC
High energy fluviatile Devonian sandstones similar to those occurring onshore Ireland and further north in the Orcadian basin and in Clair field (West Shetland basin) are likely to be present within the area.
Shallow marine and fluviatile sandstones penetrated in the Carboniferous by Erris well 19/5-1 (e.g. Namurian sandstone: net 200 ft, average porosity 22%) also constitute valid reservoir objectives in the Erris and Slyne troughs.
MESOZOIC
Both Erris wells penetrated thick Triassic Sherwood equivalent sandstones of mostly fluviatile origin representing excellent reservoir potential (19/5-1: net 550 ft, porosity 20-30%; 12/13-1A: net 330 ft, porosity 15-30%).
Comparable high energy facies have been encountered in the North Porcupine basin and can also be anticipated in the Slyne trough. Middle-Upper Jurassic fluviatile to coastal sandstones, similar to the oil-bearing reservoirs of Connemara field in the Porcupine basin, were present in Slyne well 27/13-1 (e.g. Middle Jurassic sandstone: net about 200 ft, average porosity 18%).
The occurrence of submarine fans in the eastern Rockall trough can also be predicted based on analogies with the Faeroe basin. The massive Hauterivian submarine fan sandstones recorded in Erris well 12/13-1A (net 400 ft plus, porosity 14-35%) provide excellent reservoir potential for the Erris trough and possibly also for the Rockall trough based on similar analogies.
Fluvio-deltaic sandstones documented in the Albo-Aptian of the Porcupine basin, 7 are also likely to be developed locally in both the Erris and Rockall troughs.
CENOZOIC
Late Paleocene-Eocene deltaic and/or submarine fan sandstones, similar to those encountered in the Porcupine and Faeroe-Shetland basins, should provide further reservoir potential in the Erris and Rockall troughs.
SOURCE ROCKS
Rich gas- and oil-prone source rocks have been proven in the Upper Carboniferous and Lower Jurassic, respectively.
In addition, good to rich oil- or oil and gas-prone Upper Jurassic-lowermost Cretaceous source rocks can be expected to be present over parts of the area by analogy with other basins in the region.
UPPER CARBONIFEROUS
Carbonaceous delta-swamp claystones of Namurian-Westphalian age penetrated by Erris well 19/5-1 (gross 1,500 ft, 2-20% total organic carbon-mostly humic, S2 yields up to 59 mg/g) and recognized elsewhere in the region provide excellent source potential for gas in the Erris and Slyne troughs.
Though immature for gas on the 19/5-1 structure, these source rocks could have generated large quantities of gas in deeper basinal areas of both troughs.
A limited number of coal samples also analyzed for the operator are oil-prone.
LOWER JURASSIC
Marine Toarcian and Sinemurian-Pliensbachian claystones were demonstrated by Slyne well 27/13-1 to be rich oil-prone source rocks (Toarcian: net 250 ft, 4-7% TOC--mostly amorphous, S2 yield 13-40 mg/g; Sinemurian-Pliensbachian: net 100 ft plus, 4% TOC--mostly amorphous, 13 Mg/9 S2 yield).
On-structure, these source rocks are marginally mature to mature for oil generation, but there is ample scope for peak oil-window maturity in deeper basinal areas. Lower Jurassic source rock facies are recognized on a regional scale and are expected throughout the Slyne and western Erris troughs.
U. JURASSIC-L. CRETACEOUS
Kimmeridgian to Tithonian oil-prone source rocks are believed to have sourced Connemara field in the Porcupine basin.
Further north in the Faeroe-Shetland basins, Kimmeridgian to Berriasian claystones are reported to have a good to rich potential for oil and gas 8 and have presumably sourced Clair field as well as the recent Amerada Hess oil discovery on Block 205/26A.
Though not yet proven in the frontier round area, the presence of similar marine source facies is considered likely.
TRAPS
The potential exists for large trapping complexes in the area.
As already noted, faulted features are observed on seismic and are indicative of trap styles at pre-Cretaceous levels.
Most structural traps are likely to have formed by the end of the Jurassic, with some subsequent modification during the Lower Cretaceous and Tertiary.
By contrast, trapping mechanisms for Cretaceous and Tertiary reservoirs are expected to be largely stratigraphic.
SEALS
Major claystone units are present throughout the stratigraphic record and should provide adequate cap rocks and lateral seals.
The main sealing facies recognized include the Zechstein and Mercia mudstone equivalents along with the marine claystones of the Lower Jurassic, Lower Cretaceous, and Lower Tertiary. Further sealing potential is provided by intra-formational shales/claystones present in the Carboniferous and Middle-Upper Jurassic.
COINCIDENCE
It is expected that the timing of hydrocarbon migration and trap development is favorable for hydrocarbon accumulation in the area.
Published 1-D modeling results for Lower Jurassic source rocks in basinal parts of the Slyne trough' have suggested that peak oil migration occurred during the Upper Cretaceous and Lower Tertiary, hence post-dating most of the trap development.
Recent seismic acquisition and geochemical investigations should allow tighter constraints to be placed on source rock maturation histories and the timing of trap development.
HYDROCARBON POTENTIAL
The large area covered by the frontier round is under-explored with only three wells and a patchy coverage of variable quality seismic data.
However, exploration results to date indicate that the elements essential for the accumulation of both oil and gas are present.
Major source rocks, reservoirs and seals are developed, and many play concepts are recognized over a wide stratigraphic range extending from the Paleozoic through to the Cenozoic (see table).
The lack of exploration success is attributed to invalid or breached traps.
The new seismic data acquired during 1991 and planned to be acquired during 1992 together with revised fiscal and licensing terms should enable new and effective exploration work programs to be implemented in the area.
ACKNOWLEDGMENTS
This paper is published with the permission of the Minister for Energy. The authors also wish to express their appreciation to the staff of the Geological Survey's cartographic unit for their contribution and support.
REFERENCES
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- Trueblood, S., Petroleum geology of the Slyne trough and adjacent basins, in Parnell, J. (Ed.), Basins on the Atlantic Seaboard: Petroleum Geology, Sedimentology and Basin Evolution, Special Pub. Geol. Sec., London, Vol. 62, 1992, pp. 315-326.
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