Central Irish Sea Research Articles

The influence of thermal history on hydrocarbon prospectivity in the Central Irish Sea Basin

Abstract Thermal history reconstruction studies of four hydrocarbon exploration wells located in the Central Irish Sea Basin (CISB) reveal three major regional episodes of heating and cooling. Units throughout the pre-Quaternary section intersected in wells 42/12-1, 42/16-1 and 42/17-1 began to cool from their maximum post-depositional palaeotemperatures in Early Cretaceous time, between 120 and 115 Ma. Cooling from subsequent palaeotemperature peaks began in Late Cretaceous-Early Tertiary (70–55 Ma) and Late Tertiary (25–0 Ma) time. Results from well 42/21-1 are dominated by the two more recent episodes, and show no evidence of the Early Cretaceous episode. This is thought to reflect a different structural setting of this well, within a North Celtic Sea-Cardigan Bay trend. Palaeotemperature profiles suggest that heating in each episode was due largely to deeper burial, with subsequent cooling caused mainly by uplift and erosion. A maximum of c. 3 km of additional Late Triassic to Early Cretaceous section is required to explain the observed Early Cretaceous palaeotemperatures. Appropriate values for the Late Cretaceous-Early Tertiary and Late Tertiary episoders are c. 2 km and c. 1 km, respectively. All of these cooling episodes correlate closely with similar episodes recognized from previous studies in surrounding regions, from onshore Ireland, Scotland, South Wales and northern, eastern, central and SW England, and each appears to be of truly regional extent. Exploration risk in the CISB generation can be significantly reduced through recognition of the major palaeothermal episodes that have affected the region, and the variation in the magnitude of their effects across the region. The challenge for future exploration in the region is to identify regions where the main phase of hydrocarbon generation post-dated structuring.

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.

TECTONIC EVOLUTION OF THE SOUTHERN ‐ CENTRAL IRISH SEA BASIN

Interpretation of 2D seismic reflection data combined with correlation of five wells in the southern part of the Central Irish Sea Basin show a NE‐SW trending graben, whose bounding faults are considered to be reactivated lineaments of Precambrian age. The basin‐fill comprises mainly Carboniferous and Triassic successions, with localised and thin occurrences of Lower Jurassic and Tertiary rocks, all of which are unconformably overlain by Quaternary sediments. Due to poor data quality, the structural evolution of the area during the Late Palaeozoic is poorly understood. During the Triassic, the basin was subjected to thermal subsidence with a phase of minor uplift in the Anisian. The major phase of extension in the basin took place during the ?Middle ‐ Late Jurassic and had a NW‐SE orientation. Subsequent Late Jurassic sinistral shear along the NE‐SW trending basin bounding fault is suggested to have taken place, giving rise to a series of north‐south intra‐basinal faults. The present‐day structure of the basin is a broad anticline, inherited from a Cretaceous ‐ Early Tertiary compressional phase. During the Palaeocene the area was subjected to regional uplift, followed by minor extension along the NW boundary fault during the Eocene ‐ Oligocene. A Late Tertiary phase of transpression is postulated to have occurred, which inverted north‐south trending faults and folded the base‐Tertiary unconformity.

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.

Rare records of Euchaeta species (Crustacea: Copepoda) in the Irish Sea

Specimens of Euchaeta norvegica and Euchaeta hebes (Crustacea: Copepoda) were collected west of the Isle of Man, Irish Sea, between February and April 1993. There are no previous records of these species in the plankton of the Irish Sea. It is thought that they were carried into the central southern Irish Sea by incursions of water through the St George's and North Channels.

Variability in Stage I egg production and settlement of plaice Pleuronectes platessa, on the west side of the Isle of Man, Irish Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 189:241-250 (1999) - doi:10.3354/meps189241 Variability in Stage I egg production and settlement of plaice Pleuronectes platessa on the west side of the Isle of Man, Irish Sea Richard D. M. Nash*, Audrey J. Geffen School of Biological Sciences, University of Liverpool, Port Erin Marine Laboratory, Port Erin, Isle of Man IM9 6JA, United Kingdom *E-mail: rdmnash@liv.ac.uk ABSTRACT: The variability in Stage I egg production and subsequent settlement of plaice Pleuronectes platessa L. on the west side of the Isle of Man, Irish Sea, was investigated over the period 1992 to 1998. This small spawning ground contributed approximately 0.6% of the total plaice egg production in the northern Irish Sea for 1995. The timing of peak egg production varied between years from approximately the middle of February to the end of March. The timing of peak egg production was related to the mean autumnal water temperature, with earlier spawning in warmer years. Egg production on the west side of the Isle of Man in 1995 was earlier than in the eastern Irish Sea, probably reflecting the slightly warmer water conditions in the central Irish Sea. The estimated Stage I egg production varied considerably between years, differing by a factor of 2.8 over all years. There was a positive relationship between the annual egg production and the numbers of individuals settling in Port Erin Bay. Interannual variation in abundance was greater at settlement than in either Stage I eggs or the size of the juvenile population in July. Higher inter-annual variability in the pelagic phase is dampened during the nursery ground phase. KEY WORDS: Irish Sea · Plaice · Egg production · Settlement · Variability Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 189. Publication date: November 26, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Evidence for eutrophication of the Irish Sea over four decades

The Irish Sea is showing early signs of eutrophication extending offshore, beyond localized inshore effects, according to an unusually long time series of measurements taken in the central Irish Sea. Background levels of dissolved inorganic forms of nitrogen and phosphorus have risen substantially over the last 30 to 40 years. This rise has coincided with a significant rise in phytoplankton biomass, measured as chlorophyll a, during the late spring bloom. Contrary to trends in other coastal seas, the increase in N and P was not accompanied by a decline in silicate; in fact, a small but significant increase in Si was noted in autumn and winter. This may be related to the well‐mixed conditions over much of the Irish Sea. However, the common assumption that there are no anthropogenic sources of dissolved Si may not be valid in this area and requires further consideration.

Source Rock Burial History and Seal Effectiveness: Key Facets to Understanding Hydrocarbon Exploration Potential in the East and Central Irish Sea Basins

The timing of hydrocarbon generation from Carboniferous source rocks and the lack of evapo rites within the Triassic Mercia Mudstone Group seal sequence have contributed significantly to the lack of exploration success in the Triassic Sherwood Sandstone play of the Central Irish Sea Basin. Apatite fission-track analysis (AFTA®), combined with vitrinite reflectance data from the Central Irish Sea Basin and adjacent areas, indicates that maximum burial and heating of the Carboniferous section was achieved by the Early Cretaceous. The long residence period for trapped hydrocarbons generated in the Early Cretaceous, combined with the ineffectiveness of the Mercia Mudstone Group seal caused by the absence of annealing halites and the presence of thief sandstones, led to hydrocarbon loss through trap breaching during subsequent tectonic events. In contrast, maximum burial of Carboniferous source rocks in the central East Irish Sea hydrocarbon province was achieved in the Late Cretaceous-early Tertiary, giving a shorter residence period for generated hydrocarbons. The combination of a short residence period and the presence of a halite-bearing Mercia Mudstone seal sequence significantly improved hydrocarbon retention within the Sherwood Sandstone reservoir. If traps were episodically breached during the late Tertiary, they were recharged by the inversion process, which caused gas expansion, spill from charged traps, and lateral remigration of hydrocarbons updip. The inversion-related pressure drop also caused retrograde condensate dropout from gas accumulations to provide a supply of late condensate for preferential spill and remigration.

PCBs and organochlorine pesticides in shag ( Phalacrocorax aristotelis) eggs from the central Irish Sea: a preliminary study

PCBs and organochlorine pesticides in shag ( Phalacrocorax aristotelis) eggs from the central Irish Sea: a preliminary study

Seventy years' observations of changes in distribution and abundance of zooplankton and intertidal organisms in the western English Channel in relation to rising sea temperature

1. 1. Extensive changes in marine communities in southwest Britain and the western English Channel have been recorded during the past 70 years. 2. 2. Over the same period there was a climatic warming from the early 1920s, then a cooling to the early 1980s, with recent resumption of warming; the change in annual mean temperature was approximately ±0.5°C. 3. 3. Marked changes occurred in plankton community structure; the distribution of both plankton and intertidal organisms was affected, with latitudinal shifts of up to 120 miles; there were increases or decreases of 2–3 orders of magnitude in abundance. 4. 4. Warm water species increased in abundance and extended their range during periods of warming, while cold-water species declined or retreated; the reverse occurred during the period of cooling. 5. 5. Climate change can influence marine communities by a combination of: direct effect on the organisms; effects mediated by biotic interactions; and indirectly through ocean currents. 6. 6. From climate models that indicate rises of mean temperature of 2°C in the next 50 years, and from the observed changes, we can expect future latitudinal shifts of 200–400 miles in distribution of plankton, fish and benthos, with extensive restructuring of planktonic, pelagic and benthic communities. 7. 7. Species common now in the Bay of Biscay will become common in the English Channel; those presently restricted to the western English Channel could colonise the central Irish Sea; changes in community structure could lead to lower abundances of infaunal benthos and fish. 8. 8. To fully prove the effects of global warming, future changes in the marine biota must exceed those recorded in the 1950s and 1960s.

Hydrocarbon prospectivity of the Central Irish Sea Basin with reference to Block 42/12, offshore Ireland

Abstract The hydrocarbon prospectivity of the offshore Central Irish Sea Basin has been assessed with particular reference to Block 42/12, which was formerly operated by Hydrocarbons Ireland Ltd (British Gas Exploration & Production). Data from the two wells drilled in 42/12, and other wells and boreholes in the area, have been integrated with seismic, gravity and magnetic data. Onshore outcrop and sub-surface information were also utilized. Interpretation of these data shows that the proven basin-fill consists of several megasequences bounded by major unconformities. At least three separate phases of extension are recorded, in the Carboniferous, Triassic and Jurassic. These were followed by post-rift, regional thermal subsidence, but the stratigraphy is further punctuated with evidence of compression and transpression associated with the Variscan, Cimmerian and Alpine inversion events. An assessment of hydrocarbon prospectivity indicates that the primary reservoir interval occurs in the syn-rift Lower Triassic Sherwood Sandstone Group. This is sealed by the overlying Middle-Upper Triassic Mercia Mudstone Group consisting of shales with major halite intervals. Westphalian coal-bearing facies, proven in 42/12-1 and 42/17-1, are potential gas-prone source rocks, which are late to overmature for oil generation to early mature for gas generation. Unproven Namurian marine source rocks may occur in the main basin depocentre. Burial history modelling suggests that source rocks probably generated hydrocarbons by the Early Cretaceous, with possible further generation prior to the main Tertiary inversion. Tilted fault-block and rollover anticline trapping structures were probably formed during Permian to Jurassic rifting, pre-dating the main phases of hydrocarbon expulsion. Exploration to date has centred on the southern margins of the basin. Future exploration of the deeper and potentially prospective parts of the basin in the north should resolve uncertainties concerning the possible presence of Namurian source rocks and Permian reservoir and seal facies, and the nature of a post-Triassic section identified on seismic data. The Central Irish Sea Basin is an under-explored Late Palaeozoic to Mesozoic basin which is stratigraphically analogous to the prolific East Irish Sea hydrocarbon province to the northeast. This paper presents the results and interpretations of past exploration activity and reviews these in terms of their impact on the tectono-stratigraphic evolution and future hydrocarbon prospectivity of the basin.

Food availability for sprat larvae in the Irish Sea

The distribution of larvae of sprat (Sprattus sprattus) is described in relation to feeding conditions in the Irish Sea. Larvae were most abundant towards the coastal margins. Relatively few were taken in the mixed water of the central Irish Sea. Preferred food organisms were also more abundant in coastal regions. Both sprat eggs and larvae, and microplankton, were more dispersed down the water column when it was fully mixed than when stratified. Characterization of food availability for larvae, as peak (modal) abundance down the water-column, is shown to be related to the degree of stratification.

Co-occurrence of copepods and dissolved free amino acids in shelf sea waters

The vertical distribution of chlorophylla, copepods, dissolved free amino acid concentration and the fixation of14C by phytoplankton were monitored in the springs of 1983, 1987 and 1988 in the Ushant front region, shelf edge of the Celtic Sea and central Irish Sea, respectively. In each area, two stations characterized by mixed and stratified water conditions were compared. Vertical distributions of amino acids coincided with the distribution of copepods. A positive and significant correlation was found between the abudance of copepods and the concentration of amino acids dissolved in seawater. A negative and significant correlation was found between chlorophylla and the concentration of amino acids. Enrichment of amino acids (≥ 20 to 500 nM l−1 at specific depths) due to aspartic and glutamic acids, glutamine and ornithine, was assumed to reflect copepod feeding activity and faecal production. At these depths, the natural concentration and diversity of amino acids, including aspartic acid, glutamic acid, asparagine, serine, histidine, glutamine, arginine, threonine, glycine, alanine, tyrosine, valine, phenylalanine, ornithine and lysine, were high enough and in the correct proportions for triggering feeding and swimming and swarming behavior of copepods, as well as their remote detection of food at the micro- and meso-scales (1 to 10 m). This accumulation of amino acids also constitutes a potential additional source of organic nitrogen for bacteria and phytoplankton.

The application of Stokes' wave theory under changing sea levels in the Irish Sea

Although recent models of wave-dominated coastal systems have incorporated the Stokes wave equations, neither the flow predictions nor the shorewards limits of the theory have been adequately tested with field or laboratory experiments. Results are reported from 315 laboratory measurements of peak onshore and offshore flow speeds under a variety of wave conditions. Use of the complete data set indicates that predictions of nearbed flow velocity by Stokes' second-order theory are not accurate over the entire range of depths tested, and the importance of defining a shallow-water limit is discussed. The results suggest that the ratio of the first two harmonics of the nearbed flows is a useful criterion for constraining the depth range of the corrected wave theory. The results also indicate that flow velocity attenuates more rapidly with depth than is predicted by theory, and an empirical correction is derived. These analyses are applied to the wave-dominated seabed off the west coast of the Isle of Man in the central Irish Sea and show that the corrected theory is applicable to within 80 m of the shoreline at high water on a typical spring tidal cycle.

Glaciomarine sediments of the Isle of Man as a key to late Pleistocene stratigraphic investigations in the Irish Sea Basin

The northern part of the Isle of Man in the central Irish Sea Basin is identified as a raised marine foreland containing the largest subaerial exposure of last glaciation marine and glaciomarine sediments yet recognized on the British continental shelf. A large push ridge that crosses the foreland identifies a major ice limit. The ridge divides the foreland stratigraphy into a coarsening-upward marine sequence of pelagic muds with dropstones, sand, and gravel to the south from offlapping stratified and massive sandy diamict assemblages overlying glaciotectonized marine sediments to the north. Coarse-grained diamicts are argued to form part of a push-ridge-subaqueous-outwash depositional system deposited when processes of suspension deposition, density underflow, ice rafting, highly variable traction-current activity, and sediment gravity flow were operative adjacent to a grounded marine ice margin. The identification of large offshore submarine banks aligned parallel to the Bride moraine suggests that construction of the push-ridge-subaqueous-outwash depositional system may have been repeated during ice recession. This and published descriptions of other “Irish Sea tills” indicate that analogous structures and sediments are present offshore and around the Irish Sea Basin.

The determination of technetium-99 in seawater and marine algae

Technetium-99 is preconcentrated from acidified seawater containing a trace of bromine by adsorbing it as the pertechnetate ion on the anion exchanger Duolite A101D. After elution with 4 M nitric acid, it is purified from other radionuclides by scavenging with hydrous iron(III) oxide and extracting from 5 M sodium hydroxide medium into methyl ethyl ketone. The organic phase is then evaporated and technetium is electrodeposited from oxalic acid medium onto a bronze disc and counted with a low background β-counter. Overall recovery is > 90% and the precision is ±0.2 pCi l -1 at a technetium level of 0.6–1.0 pCi l -1. Concentrations of the radionuclide in the central Irish Sea were found to lie in the range 0.4–2 pCi l -1. The same analytical scheme can be used for determining the element in nitric acid digests of marine algae.

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.

The Geological Structure of Part of the Central Irish Sea

Summary Gravity measurements in the eastern part of the Central Irish Sea show two gravity ‘lows’ with a Caledonoid direction. These ‘lows’ are interpreted as sedimentary basins with a fill of possible Carboniferous and Triassic age.

Geophysical Results from Caernarvon and Tremadoc Bays

DURING March–April 1968 and December 1968, a marine magnetic and continuous reflexion (Sparker) survey was carried out simultaneously in the central Irish Sea between 3° 15′–5° 50′ longitude and 52° 40′–53° 35′ latitude. There were enough magnetic data to draw contours at intervals of 25 gamma, after removing the regional, secular and diurnal effects (Fig. 1). In its broad features the map seems consistent with the aeromagnetic map of Great Britain1 and ties on well to the marine magnetic survey of Tremadoc Bay2. Compared with the aeromagnetic map, Fig. 1 emphasizes anomalies of shallow origin. A linear belt of such anomalies, 0.5–1.5 nautical miles wide, stands out to the north-west of Anglesey and extends north-west from the vicinity of Holy head Harbour across the Irish Sea. It consists of highly variable anomalies, with the chief negative anomaly varying between 500 and 150 gamma in amplitude and 180 to 235 m in half-width. The main part of the anomaly can be explained if a steep dyke-like body with a magnetization similar to that of the Tertiary lavas and dykes of Skye3 and Antrim4 is assumed to be running along it. Such a Tertiary body has to be about 70 m wide and − 70 m O.D. near the harbour, deepening north-westwards. The shape of the anomaly suggests that the body may be made up of a group of dykes running parallel in this narrow belt. A similar linear anomaly runs parallel to the south-west coast of Holyhead Isle and may also extend to the southeast parallel to the Anglesey coast. Fig. 1 also shows a broad linear “high” in the extreme north-west of the area, which trends north-east–south-west. This feature has been recorded farther west, off the coast of Ireland, south-east of Dublin Bay5.

Caledonian Orogeny of the Central Irish Sea Region

Caledonian Orogeny of the Central Irish Sea Region