Kish Bank Research Articles

An Overview of Deep Geothermal Energy and Its Potential on the Island of Ireland

Summary This paper provides a short overview of geothermal energy, including a discussion on the key geological controls on heat distribution in the subsurface, and on the different types of geothermal resource and their potential uses. We then discuss the island of Ireland as an example of the role that geothermal energy can play in decarbonising the heat sector in a region characterised by relatively low-enthalpy (temperature) resources. Significant shallow geothermal potential exists across the island via the deployment of ground source heat pumps. The geology of onshore Ireland provides relatively limited potential for deep hydrothermal aquifers with primary porosity and permeability. Therefore, deep geothermal exploration on the island is likely to be focused on fractured carbonate reservoirs of Carboniferous age, with recorded groundwater temperatures reaching 38°C at 1 km depth, or on lower permeability petrothermal reservoirs developed as Enhanced or Advanced Geothermal Systems. The exception to this occurs within Mesozoic basins in Northern Ireland where porous and permeable Permo-Triassic sandstones are preserved beneath Paleogene basalts. Geothermal potential also exists in equivalent basins immediately offshore Ireland. For example, Triassic sandstones within the Kish Bank Basin, a few kilometres off the coast of Dublin, have estimated reservoir temperatures of 20–120°C across the basin.

The correlation and sequence architecture of the Ormskirk Sandstone Formation in the Triassic Sherwood Sandstone Group of the East Irish Sea Basin, NW England

AbstractThe existing stratigraphic nomenclature applied to the Early and Middle Triassic Sherwood Sandstone Group in NW England has resulted from more than 150 years of geological investigation, but is characterized by a lithostratigraphic system that is insufficiently flexible to allow for variations in lithology and sedimentary facies within a continental depositional system. A revised well correlation based on the detrital mineralogical and chemical composition of the Ormskirk Sandstone Formation in four offshore wells, that is then extended to provide near‐basin‐wide well correlations using a regional shale marker, confirms previously suggested but unproven diachroneity at the top of the Sherwood Sandstone Group. It also reveals the presence of incised valleys filled by stacked amalgamated fluvial channel sandstones and cut into previously deposited aeolian and sandflat sequences as well as older fluvial channel sandstones. The combination of well correlations indicates that the valleys were incised by a fluvial system flowing NW from the Cheshire Basin into the East Irish Sea Basin and then west towards the Peel and Kish Bank basins. The stratal geometry of the upper part of the Sherwood Sandstone Group is suggested to conform to models of climatically mediated alternations of fluvial degradation and aggradation in response to changes in the relationship between sediment flux and stream discharge. This model is supported in the Sherwood Sandstone Group by climatically driven variations in the non‐channelized facies which record upward wetting and drying cycles that can be locally tied to fluvial incision surfaces, and suggest a hierarchy of at least three levels of climatic cyclicity recorded within the sedimentary succession. Copyright © 2006 John Wiley & Sons, Ltd.

The Powerscourt Conglomerate and its Bearing on Neotectonic Activity in Central East Ireland

The Powerscourt Deerpark Cave is a small cave situated on the eastern side of the Wicklow Mountains. The cave has formed within coarse ferruginous conglomerate on the footwall of the Bray Thrust that bounds the western edge of the Calary Plateau. The conglomerate consists of poorly sorted angular-to-well-rounded clasts of phyllite, vein quartz, greywacke and granite weakly cemented by goethite. We propose that the Powerscourt Conglomerate (our new name) is composed of fluvial gravels that became trapped in the easily eroded Bray Thrust during the earlier Cenozoic. The existence of the Powerscourt Conglomerate does not support or undermine the idea of normal faulting and scarp retreat on land, related to topographic subsidence in the nearby Kish Bank Basin during the earlier Cenozoic. The cave is a rare example of pseudokarst development, typical of tropical climates.

Evidence for Cenozoic tectonic deformation in SE Ireland and near offshore

An integrated study of topography, bathymetry, high‐resolution aeromagnetic data, and structural observations demonstrates significant Cenozoic fault activity in SE Ireland. Tectonically generated knickpoints and reddened fault breccias along topographic escarpments that are underlain by greywacke bedrock and trend oblique to the regional Caledonian strike provide evidence for fault displacement. Near‐offshore faults with similar geometry produce present‐day bathymetric scarps and localized tectonic topography in the inverted Kish Bank Basin. The integration of offshore high resolution aeromagnetic data and structural interpretation of the Kish Bank Basin provides evidence for dextral transtension on NNW trending faults and sinistral transpression on ENE trending faults bounding a lower Paleozoic to Carboniferous basement block. These faults correlate onshore with previously recognized Caledonian faults producing topographic offsets and surface uplift. To the north, offshore structures can be traced onshore and cut exposures of the Caledonian Leinster Granite. Structural analysis of these outcrops indicates post‐Variscan deformation. A major fault on the NW margin of the batholith cuts a major erosion surface developed on Carboniferous carbonate rocks, and nonmarine Miocene deposits are preserved above this surface. Fault kinematics provide evidence of two paleostress systems: (1) NW‐SE σ1, subvertical σ2 and NE‐SW σ3 followed by (2) a clockwise swing of σ1 to NNW‐SSE. Timing of deformation in both stress systems is probably post‐Oligocene age. The mechanism driving this deformation is likely ridge‐push. Much is already known about Cenozoic tectonics and exhumation from offshore basins. This study shows that onshore Ireland has also been affected by significant tectonic activity and exhumation during the Cenozoic.

Environmental Controls on Shipwreck Preservation: The Irish Context

A discussion is presented of environmental controls that affect the preservation of archaeological artefacts in the marine environment. Physical, biological and chemical degradation processes are discussed although it is appreciated that these processes operate in conjunction and often in a site-specific manner. Two Irish case studies are presented of environmental controls on shipwreck preservation: The Kish Bank, Outer Dublin Bay and La Surveillante, Bantry Bay. These illustrate contrasting environmental factors affecting shipwreck preservation.

Hydrocarbon prospectivity in the Irish Sea area: insights from recent exploration of the Central Irish Sea, Peel and Solway basins

Abstract Compared with the prolific success of the Triassic play in the East Irish Sea Basin (EISB) the lack of hydrocarbon discovery in neighbouring Permo-Triassic basins of the Irish Sea has been an enigma. However, recent exploration of the Peel, Solway and Central Irish Sea basins has provided new insights into the geology of these basins and the controls upon hydrocarbon prospectivity in the Irish Sea area. Regional seismic interpretation suggests that 12 of the 15 exploration wells drilled in the basins adjacent to the EISB tested valid structural closures at top Triassic reservoir level. Re-evaluation of the Irish Sea petroleum system reveals that, although effective reservoirs occur in the Lower-Middle Triassic Ormskirk Sandstone Formation, and evaporites in the Middle-Upper Triassic Mercia Mudstone Group provide a regional top seal, the major factor controlling hydrocarbon prospectivity is the limited presence of effective source rocks in the underlying Carboniferous section. A further control upon prospectivity is the timing of hydrocarbon migration, from those areas where Carboniferous source rocks were deposited and preserved. The Namurian basinal marine oil- and gas-prone shales, which form the principal source of hydrocarbons for the Triassic play in the EISB, are restricted to an east-west fairway extending from the EISB into the Kish Bank Basin. Rocks of this age are absent from the Peel and Solway basins as a result of Variscan uplift and erosion. However, palaeogeographical reconstructions based on well and outcrop data suggest that, even if preserved, the depositional environment was not conductive to the formation of marine oil-and gas-prone source rocks. Well and seismic data suggest that rocks of Namurian age were not deposited in the Central Irish Sea area, which remained high during much of Dinantian and Namurian time. Potential source rock development in the Central Irish Sea area is therefore limited to the Westphalian section, which is organically lean and dominated by intertinitic kerogens. Potential hydrocarbon traps in the Central Irish Sea, Peel and Solway basins formed largely as a result of Early Cretaceous tectonism and were subsequently modified by fault reactivation during Tertiary uplift phases. Trap formation appears to postdate the most likely timing of hydrocarbon charge, which this study suggests would have occurred in Late Triassic and Jurassic time. Hydrocarbon entrapment in the EISB may have been favoured by limited Early Cretaceous uplift, coupled with renewed hydrocarbon generation and re-migration during Early Tertiary time. It is concluded that the remaining prospectivity of the Triassic play in the Irish Sea area is likely to be restricted to the proven play fairway within the EISB.

Hydrocarbon potential of the Kish Bank Basin: integration within a regional model for the Greater Irish Sea Basin

Abstract The Kish Bank Basin lies in the western Irish Sea c. 20 km east of Dublin. It is one of a number of remnants of a larger Permo-Triassic basin system that may have extended across the whole of the Irish Sea. It has a geological history similar to that of the East Irish Sea Basin, initially developing by the reactivation of Caledonian faults that controlled subsequent deposition during Dinantian and Namurian time, with Westphalian deposition in a sag-basin that overstepped the adjacent basement highs. Variscan dextral transpression resulted in the formation of the Codling and Bray faults, and Permian to Jurassic extension formed a set of north-south-trending faults. Liassic outliers are preserved in the hanging walls of the basin margin faults. Early Cretaceous uplift was followed by chalk deposition. Tertiary movements reactivated older faults, isolating the Kish Bank Basin, and producing 9 km of dextral strike-slip along the Codling Fault Zone. The main reservoir in the hydrocarbon play is provided by the Sherwood Sandstone Group, as successfully exploited in the East Irish Sea. Three wells have been drilled to test this reservoir. These encountered high-quality Sherwood Sandstone reservoirs beneath the good potential seal of the Mercia Mudstone Group (which included thick halites). Source rock potential is from either the Westphalian Coal Measures, as penetrated in well 33/22-1, or from inferred Dinantian to Namurian basinal shales. There is good evidence of an active source system, with oil shows in wells 33/17-1 and 33/22-1, data from geochemical analysis of sea-bed cores, a ‘Seepfinder’ survey, sea-bed mounds and seismic evidence of shallow gas. The main risks of the play are the migration pathway and the timing of trap formation with respect to migration. Migration favours the eastern side of the basin, and many of the tilted fault blocks that formed during Permian to Jurassic time have been modified by Early Cretaceous inversion and by Tertiary strike-slip compression. All of the structures that have been drilled to date have been either formed or modified after the time of peak hydrocarbon generation and migration.

INTERPRETATION OF VITRINITE REFLECTANCE PROFILES IN THE CENTRAL IRISH SEA AREA: IMPLICATIONS FOR THE TIMING OF ORGANIC MATURATION

The Central Irish Sea area, from Kish Bank to St. George's Channel and Cardigan Bay, consists of a series of Late Palaeozoic to Cenozoic extensional and transtensional basins which have experienced a multiphase inversion history. Potential hydrocarbon source rocks of Carboniferous and Jurassic ages have been recognised in this area. In the Kish Bank and Central Irish Sea Basins, maturation modelling of these source rocks is hampered by the severely truncated rock record and by the relative paucity of vitrinite throughout much of the preserved post‐Palaeozoic (Triassic) section.Vitrinite reflectance data from six exploration wells have been used to quantify the peak palaeotemperatures attained by the rocks in this area and to estimate the magnitude of net exhumation at these locations. An apparent palaeogeothermal gradient of ˜26°C/km is recorded by the Jurassic sediments in well 42/21‐1, whereas significantly higher palaeogeothermal gradients of 74–78°C/km are interpreted for the Westphalian/Stephanian sediments in the area. At least two periods of rock exhumation have occurred; during the Late Carboniferous‐Late Permian, and again sometime between the latest Jurassic and early Tertiary. Estimates of net exhumation vary from ˜350 m at well 42/12‐1 to ˜1,900 m at well 42/17–1. Our interpretation suggests that the higher palaeogeothermal gradients recorded by the Westphalian/Stephanian sections reflect elevated heat flows during Stephanian to Early Permian times.

THE IRISH SEA REGION: WHY THE GENERAL LACK OF EXPLORATION SUCCESS?

With the exception of the prolific East Irish Sea Basin, there have been no hydrocarbon discoveries to date in the other basins in the Irish Sea region. Excellent reservoirs, viable seals and structural traps have been identified in all the basins, suggesting that other factors have prevented the accumulation or preservation of oil and gas. The lack of exploration success in the Larne ‐ Lough Neagh, Solway and Peel, Kish Bank and Central Irish Sea Basins is attributed to a combination of poor preservation of regional source rocks, unfavourable timing of hydrocarbon migration and trap formation and, locally, a lack of seal integrity. Inversion episodes during latest Carboniferous, Late Jurassic‐Early Cretaceous and Early Tertiary times were critical in preventing the optimum conditions necessary for the generation and preservation of oil and gas in these basins.

The Kish Bank Basin, offshore Ireland

The Kish Bank Basin, a half-graben lying offshore only a few kilometres east and southeast of Dublin in the Irish Sea, is elongated northeast-southwest and measures approximately 48 × 32 km. The basin is bounded to the northwest and north by the Dalkey-Lambay fault system and to the southwest by the Bray Fault. A major dextral strike-slip fault, the northwest-trending Codling Fault, bisects the basin. Three wells have been drilled in the area: Amoco 33/22-1(1977), Shell 33/21-1(1979) and Fina 33/17-1(1986). The Amoco well penetrated a Carboniferous section on the southeastern rim of the basin ( Jenner 1981 ), while the other two holes drilled the Permo-Triassic basin-fill sequence, but without reaching the base of the Permian. The Permo-Triassic succession is similar to that in the East Irish Sea Basin and includes thick Triassic salt sequences. SEM examination of sandstone samples from potential reservoir sections in the 33/21-1 well indicates good porosity in the cleaner sandstones. This is interpreted as largely secondary, following dissolution of early cements. Seismic studies suggest that up to 2.7 km of Jurassic (probably Liassic) strata are present against the western bounding faults. Maturation studies have been carried out on material from two of the wells. The Silesian section penetrated by 33/22-1 ranges in age from Westphalian B to Westphalian D. It is of ‘coal measures’ facies and includes numerous seams of medium to high volatile bituminous coal. Much of the Westphalian section is mature with regard to gas generation, with vitrinite reflectance (Rm) values ranging from c. 0.8% to 1.3%. Maturation levels in the Permo-Triassic section in 33/21-1 range from c. 1.3%–1.6% Rm, suggesting that any underlying Westphalian rocks in deeper parts of the basin would be mature to marginally post-mature with respect to dry gas generation. The burial history of the basin, and the distribution of source rocks within it, are discussed.

The geology of the Kish Bank Basin

A gravity low immediately E of Dublin is interpreted as an Upper Palaeozoic to Cenozoic basin on the basis of seismic reflection profiles and direct sampling evidence. Maximum thickness of the sediment fill may exceed 4 km. It is considered to consist predominantlyof Carboniferous, undivided Permo-Triassic, and remnant proved Liassic. Thin Tertiary units, which unconformably overlie the main fill, are linked to sediments of similar character found in the Central Irish Sea Basin. Thick Quaternary deposits complete the sedimentary sequence. The basin is structurally a half graben located between two narrow, fault-bounded basement ridges, the Mid Irish Sea Uplift to the S and the Lambay Ridge to the N. The whole structure is bisected by a major NNW-SSE trending fault zone which with the margin faults has created two trapdoor-like structures. It is thought that the principal phases of movement which were instrumental in preserving the sediments occurred during the Triassic and late Jurassic period.

Gravity measurements in the north Irish Sea

This paper presents a Bouguer anomaly map of most of the north Irish Sea, based on under-water gravimeter measurements. The map incorporates earlier surveys in the northern part of the region and new work in the southern part. The new work reveals that the previously discussed Manx–Furness gravity 'low' (now renamed the East Irish Sea 'low') extends southwards to the coast of North Wales. Newly observed detailed profiles across the north-western margins of the 'low' show that it is caused by a partially fault-bounded composite sedimentary basin between the Isle of Man, Lancashire and North Wales, with a local region of basement uplift 30 km southeast of Douglas. High Bouguer anomalies are observed over a wide area between the Isle of Man, Anglesey and Ireland, suggesting a region with relatively shallow basement rocks. A negative gravity anomaly of –4o mgal amplitude occupies an elongated oblong area 40 × 30 km 2 in dimension off the east coast of Ireland near Dublin. The steep gravity gradients and other characteristics of the anomaly suggest that it is caused by a sedimentary basin about 3 to 4 km deep. The high density contrast required to explain the anomaly implies that Permo–Triassic and/or later sediments form the main infill of the basin. This anomaly is called the Kish Bank gravity 'low'.

II. On the causes of the loss of the iron-built sailing-ship 'Glenorchy.’

When the loss of an iron-built vessel has been caused by an error in the direction of her course by dead reckoning, as derived from her course by compass, it is a question of scientific interest whether the error has or has not arisen from an error in the assumed deviation of the compass. By careful consideration of all the circumstances of the case, and by piecing together the generally scanty fragments of information which can be obtained as to the magnetic state of the ship, a probable or certain answer to this question may be given more frequently than might be supposed possible by those who do not know how perfectly definite and well ascertained the laws of the deviation of the compass are, how small is the number of quantities involved which are peculiar to each particular ship, and from what apparently slight indications an approximate estimate of the numerical values of these quantities can be made. The case the circumstances of which I now propose to lay before the Royal Society, is one in which it appears to me that a positive answer to the question can be given. It will, I hope, be found to have some interest as an example of the manner in which such an answer can be elicited from the data. It may have some scientific interest as the first case in which any information as to the magnetic character of an English merchant-ship has been published since the publication of the Third Report of the Liverpool Compass Committee in 1861; and I think it will be found to have much practical interest, as bringing into prominence a particular error of great importance, not as yet, I believe, ascertained or corrected in the usual course of adjustment of compasses in merchant-ships, even by the most experienced and skilful compass-adjusters, but which, ever since the mode of ascertaining and correcting it without heeling the ship was given in the ‘Admiralty Manual for the Deviation of the Compass in 1862, has been ascertained, and when necessary corrected, in the ships of the Royal Navy, viz. the Heeling Error. The case to which I refer is the loss of the ship ‘Glenorchy' of Glasgow, on the Kish Bank, in Dublin Bay, on the 1st of January 1869, on which a court of inquiry was held under the direction of the Board of Trade in pursuance of the Merchant Shipping Act. In examining this case I have had the advantage, by the permission of the Board of Trade, of perusing the evidence taken before the Court of inquiry, and the report of the Court. I have also had the advantage of discussing the nautical as well as the magnetical circumstances of the case with Captain Evans, F. R. S., the highest authority in all that relates to such an inquiry, and who permits me to state his concurrence in the conclusions at which I have arrived; and above all, I have to express my obligations to Mr. William Fleming, compass-adjuster, James Watt Street, Glasgow, for the full particulars with which he has kindly furnished me of the deviations and correction of the compasses of the ‘Glenorchy’—information without which the results of this inquiry would have been in a great measure conjectural.