Triassic Sherwood Sandstone Group Research Articles

Review of Triassic Sherwood Sandstone Group reservoirs of Ireland and Great Britain and their future role in geoenergy applications

As the world progressively shifts to a low/neutral carbon economy over the course of the twenty-first century, geoscience will continue to play a vital role in the energy sector and beyond. Static and dynamic characterization of the subsurface, a key component of the petroleum industry for decades, will be required to evaluate and unlock a variety of different initiatives including carbon, capture and storage (CCS), hydrogen (or any other gas) storage, thermal energy storage and geothermal energy development. Onshore and offshore Ireland and Great Britain, Lower Triassic sandstones are likely to represent one of the primary geological targets for future geoenergy applications. This paper provides a review of the Triassic Sherwood Sandstone Group depositional system across the British and Irish region of NW Europe including its overall stratigraphic context, palaeogeography, sediment provenance and transport directions, and present-day distribution. In addition to geological outcrop data, there is a wealth of subsurface data available from historical petroleum wells. Looking forward, one important focus of geoscience research will be to extrapolate away from these well-defined control points to predict sandstone facies distribution, diagenetic history and hence overall reservoir quality in new undrilled areas that will be of interest to the geothermal, CCS and gas storage sectors.

Review of groundwater flow and contaminant transport modelling approaches for the Sherwood Sandstone aquifer, UK; insights from analogous successions worldwide

Sandstones are characterized by different hydraulic behaviours and need to be modelled in various ways to represent groundwater flow and contaminant transport. This review shows how sandstone aquifers within the UK Triassic Sherwood Sandstone Group can be represented using three modelling approaches: the Conduit Network, Discrete Fracture Network and Equivalent Porous Medium. The Sherwood Sandstone aquifer is dominated by matrix flow in the Eastern England Shelf, Worcester, Needwood and Staffordshire basins. Here, the aquifers are modelled as Equivalent Porous Media at different spatial scales. Fractures represent the principal flow pathways in the Cheshire Basin. In this basin, Discrete Fracture Network models that account for diffusivity in the matrix can be used where the domain scale is small. The Sherwood Sandstone aquifer across northwestern England shows evidence of intense groundwater alteration and high flow velocities in solutionally enlarged fractures. Turbulent flowing pipe-elements can be inserted in the modelling domain represented by Equivalent Porous Medium at specific sites. The review shows how the Sherwood Sandstone aquifer as well as other siliciclastic deposits across the world need to be represented using a range of modelling approaches, as they behave as matrix or fracture flow aquifers, or in specific cases show a karst-like behaviour. Thematic collection: This article is part of the Hydrogeology of Sandstone collection available at: https://www.lyellcollection.org/cc/hydrogeology-of-sandstone

The forest effect: Biosphere 87Sr/86Sr shifts due to changing land use and the implications for migration studies

This study documents a transect of 87Sr/86Sr values from a variety of plant, soil and rock samples across the ancient woodland of the Sherwood Forest National Nature Reserve (SFNNR) and into adjoining farmland in Britain. All samples were collected from the Triassic Sherwood Sandstone Group. A shift of +0.0037 in 87Sr/86Sr values is observed between the average plant from the biosphere of the ancient forest and that of the farmland. This shift is caused by the leaf litter accumulation in the forest, through time, leading to soil acidity that leaches out the carbonate component of the soil. This results in the forest floor soil reflecting only the silicate minerals from the original Sandstone rock formation. We have named this process “the forest effect”. Rock samples from boreholes of the Sherwood Sandstone Group, as well as water samples from aquifers and mineral waters from previous studies, further indicate that the change in biosphere 87Sr/86Sr is a result of the wooded environment rather than the anthropological addition of lime to farmland.The extent of the forest effect will vary with differing lithologies with the most susceptible terrains being those with mixed carbonate-silicate composition, and it may be sufficient to impact the interpretation of animal and human 87Sr/86Sr in studies of mobility and migration. The model provides an opportunity to understand and assess food procurement strategies and animal management practices in the past, as well as the interaction of humans with their natural environment.

Petrographic and diagenetic investigation of the distal Triassic ‘Budleighensis’ fluvial system in the Solway and Carlisle Basins for potential CO 2 storage

Failure to find hydrocarbon prospects in the Solway Basin region has resulted in a lack of research into the local Sherwood Sandstone Group petrography, reservoir quality and depositional history compared to the analogous southern reservoirs in the EISB which will be utilized for carbon storage. A detailed petrographic study is presented which aims to understand if the Solway Firth could have similar utility. The Permo–Triassic Sherwood Sandstone Group is believed to be deposited in depocentres connected during the Early Triassic by the extensive ‘Budleighensis’ fluvial system. Here, the Solway and Carlisle basins are proposed as terminal sites for this endorheic system, with the Lower Triassic Annan Sandstone Formation ascribed to the distal region of a fluvial distributary zone and the overlying Kirklinton Sandstone Formation thought to mark a transition to a basinal zone, depositing aeolian sandstones and locally associated playa lake facies. Fluid inclusion, stable isotope burial history modelling and field observations have been used to assess the relative timing and importance of different diagenetic cements. Early diagenetic cements include grain-rimming haematite and patchy calcite cement, especially in the Annan Sandstone Formation. Later burial diagenesis sees further calcite cement, quartz overgrowths and, restricted to the Kirklinton Sandstone Formation, ferroan dolomite. Porosity and permeability show significant differences between fluvial Annan and aeolian Kirklinton facies associations. Despite the finer grain size, a reservoir with excellent porosity and permeability as well as no hydrocarbon charging or legacy hydrocarbon extraction is persevered, suggesting the Solway Basin could be a secure CO 2 storage site. This article is part of the Energy Geoscience Series available at https://www.lyellcollection.org/cc/energy-geoscience-series Supplementary material: An overview of the primary and secondary data collected and utilised in this study, as well as raw data values are available at https://doi.org/10.6084/m9.figshare.c.5906677

Geology and petroleum prospectivity of the Larne and Portpatrick basins, North Channel, offshore SW Scotland and Northern Ireland

The Larne and Portpatrick basins, located in the North Channel between SW Scotland and Northern Ireland, have been the target of a small programme of petroleum exploration activities since 1971. A total of five hydrocarbon exploration wells have been drilled within the two basins, although as of yet no commercial discoveries have been made. The presence of hydrocarbon shows alongside the discovery of two good-quality reservoir–seal couplets within Triassic and underlying Permian strata has encouraged exploration within the region. The focus of this study is to evaluate the geology and hydrocarbon prospectivity of the Portpatrick Basin and the offshore section of the Larne Basin. This is achieved through the use of seismic reflection data, and gravity and aeromagnetic data, alongside sedimentological, petrophysical and additional available datasets from both onshore and offshore wells, boreholes and previously published studies. The primary reservoir interval, the Lower–Middle Triassic Sherwood Sandstone Group (c. 600–900 m gross thickness), is distributed across both basins and shows good to excellent porosity (10–25%) and permeability (10–1000 mD) within the Larne Basin. The Middle–Late Triassic Mercia Mudstone Group should provide an excellent top seal where present due to the presence of thick regionally extensive halite deposits, although differential erosion has removed this seal from the margins of the Larne and Portpatrick basins. The Carboniferous, which has been postulated to contain organic-rich source-rock horizons, as inferred from their presence in adjacent basins, has not yet been penetrated within the depocentre of either basin. There is, therefore, some degree of uncertainty regarding the quality and distribution of a potential source rock. The interpretation of seismic reflection profiles presented here, alongside the occurrence of hydrocarbon shows, indicates the presence of organic-rich pre-Permian sedimentary rocks within both basins. 1D petroleum system modelling of the Larne-2 borehole shows that the timing of hydrocarbon generation and migration within the basins is a significant risk, with many traps post-dating the primary hydrocarbon charge. Well-failure analysis has revealed that trap breach associated with kilometre-scale uplift events, and the drilling of wells off-structure due to a lack of good-quality subsurface data, have contributed to the lack of discoveries. While the Larne and Portpatrick basins have many elements required for a working petroleum system, along with supporting hydrocarbon shows, the high risks coupled with the small scale of potential discoveries makes the Portpatrick Basin and offshore section of the Larne Basin poorly prospective for oil and gas discovery. Thematic collection: This article is part of the Under-explored plays and frontier basins of the UK continental shelf collection available at: http://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

Permeability of rock discontinuities and faults in the Triassic Sherwood Sandstone Group (UK): insights for management of fluvio-aeolian aquifers worldwide

Fluvio-aeolian sedimentary successions host groundwater aquifers at shallow depths (<~0.15 km), which overlie geothermal and shale-gas reservoirs, and nuclear waste repositories at intermediate depths (~0.15–2.0 km). Additionally, such deposits represent petroleum reservoirs at greater depths (~2.0–4.0 km). The need to improve conceptual understanding of the hydraulic behaviour of fluvial-aeolian sandstone successions over a large depth interval (~0–4 km) is important for socio-economic reasons. Thus, the hydraulic properties of the Triassic Sherwood Sandstone aquifer in the UK have been reviewed and compared to similar fluvio-aeolian successions. The ratio between well-scale and core-plug-scale permeability (Kwell-test/Kcore-plug) acts as a proxy for the relative importance of fracture versus intergranular flow. This ratio (which typically varies from ~2 to 100) indicates significant contribution of fractures to flow at relatively shallow depths (<~0.15 km). Here, permeability development is controlled by dissolution of calcite-dolomite in correspondence of fractures. The observed ratio (Kwell-test/Kcore-plug) decreases with depth, approaching unity, indicating that intergranular flow dominates at ~1 km depth. At depths ≥ ~1 km, dissolution of carbonate cement by rock alteration due to groundwater flow is absent and fractures are closed. Aeolian and fluvial deposits behave differently in proximity to normal faults in the Sherwood Sandstone aquifer. Deformation bands in aeolian dune deposits strongly compartmentalize this aquifer. The hydro-structural properties of fluvio-aeolian deposits are also controlled by mineralogy in fault zones. A relative abundance of quartz vs. feldspar and clays in aeolian sandstones favours development of low-permeability deformation bands.

Palaeoenvironment of braided fluvial systems in different tectonic realms of the Triassic Sherwood Sandstone Group, UK

Fluvial successions comprising the fills of sedimentary basins occur in a variety of tectonic realms related to extensional, compressional and strike-slip settings, as well as on slowly subsiding, passive basin margins. A major rifting phase affected NW Europe during the Triassic and resulted in the generation of numerous sedimentary basins. In the UK, much of the fill of these basins is represented by fluvial and aeolian successions of the Sherwood Sandstone Group. Additionally, regions that experienced slow rates of Mesozoic subsidence unrelated to Triassic rifting also acted as sites of accumulation of the Sherwood Sandstone Group, one well-exposed example being the eastern England Shelf. The fluvial depositional architecture of deposits of the Sherwood Sandstone Group of the eastern England Shelf (a shelf-edge basin) is compared with similar fluvial deposits of the St Bees Sandstone Formation, eastern Irish Sea Basin (a half-graben). The two studied successions represent the preserved deposits of braided fluvial systems that were influenced by common allogenic factors (climate, sediment source, delivery style); differences in preserved sedimentary style principally reflect their different tectonics settings. Analysis of lithofacies and architectural elements demonstrates that both studied successions are characterized by amalgamated channel-fill elements that are recorded predominantly by downstream-accreting sandy barforms. The different tectonic settings in which the two braided-fluvial systems accumulated exerted a dominant control on preserved sedimentary style and long-term preservation potential. On the eastern England Shelf, the vertical stacking of pebbly units and the general absence of fine-grained units reflect a slow rate of sediment accommodation generation (18–19.4m/Myr). In this shelf-edge basin, successive fluvial cycles repeatedly reworked the uppermost parts of earlier fluvial deposits such that only the lowermost channel lags tend to be preserved. By contrast, in the eastern Irish Sea Basin of west Cumbria, the rate of sediment accommodation generation was substantially greater (119m/Myr) such that space was available to preserve complete fluvial cycles, including silty drape units that cap the channelized deposits.

The role of peri‐glacial active layer development in determining soil‐regolith thickness across a Triassic sandstone outcrop in the UK

ABSTRACTThis paper examines the weathering processes that have combined to produce the distribution of soil‐regolith (SR) thickness across the Triassic Sherwood Sandstone Group outcrop (750 km2) in Nottinghamshire, UK. Archive borehole logs (n = 282) taken across the outcrop showed that SR thickness had mean and median depths of ~1·8 and 1·5 m, respectively. Cores were taken from a forested site to depths ~3 m for geochemical analysis. At this site the SR thickness was ~1·7 m. Analysis of the loss of elements, compared to bedrock using mass balance calculations (τ) showed that all the calcite and gypsum cement had been removed to depths of &gt;3 m. Thus the major difference between the SR and the underlying saprolite was that the former exists as loose sand as opposed to a semi‐durable rock. Scanning electron microscopy (SEM) analysis of core samples suggested that the non‐durable rock or saprolite had greater cementation of clay particles. We propose that the mechanism through which the clay cement (and other interlocking grain bonds) was eased apart was through freeze–thaw processes associated with the summer ‘active layer development (ALD)’ during the last glacial activity in the UK. We tested this theory by developing a Monte Carlo simulation based on a simplified version of the Stefan equation. Current Arctic datasets of air and ground temperatures were obtained to provide reasonable starting conditions for input variables. These were combined with known data for thermal conductivity, bulk density and moisture content of the Sherwood Sandstone regolith. Model predictions (n = 1000) of the distribution of SR thickness accurately reflect the observed distribution thickness from the borehole logs. This is strong evidence that freeze–thaw and ‘ALD’ processes are major factors in determining the thickness of SR across this outcrop. British Geological Survey © NERC 2012

The spatial variation of weathering and soil depth across a Triassic sandstone outcrop

AbstractIn this study, we used an archive of borehole logs from the British Geological Survey to collect information on the spatial structure of weathering that extends from the surface to competent bedrock across the Triassic Sherwood Sandstone Group outcrop (750 km2), in the East Midlands, UK. The borehole logs were used to estimate the thickness of the soil (n = 280) and soil and saprolite (S&amp;S) to competent rock (n = 500). The weathering profile of the sandstone consisted of soil (median thickness ∼ 1·5 m) overlying a transition zone of compacted and weakly cemented weathered sandstone saprolite over bedrock. Topographic analysis using a NEXTMAP 5 m × 5 m digital elevation model (DEM) revealed no significant relationships between slope properties (relief, flow length, flow accumulation or slope angle) and soil or S&amp;S thickness. A weak, but statistically significant correlation was found between the thickness of the soil and S&amp;S (rs = 0·25, p &lt; 0·001, n = 192). The variation in soil thickness may be related to changes in current and historic and land‐use, variation in sandstone properties and the influence of glacial/peri‐glacial processes. The thickness of the saprolite was more variable towards the southern part of the study area, where it increased to a maximum 40 m. We hypothesize and provide evidence that the greater weathering thickness is related to the occurrence of increased faulting in this part of the study region, allowing increased access to meteoric waters. A possible source of increased water supply is meltwater from Quaternary ice sheets; the overburden of ice may have increased sub‐glacial pore water pressure, with the fractures and faults acting as a drainage system for the removal of dissolved weathering products. British Geological Survey © NERC 2010

Lithostratigraphical subdivision of the Sherwood Sandstone Group (Triassic) of the northeastern part of the Carlisle Basin, Cumbria and Dumfries and Galloway, UK

Synopsis Two formations, the St Bees Sandstone and Kirklinton Sandstone, have been mapped in the past within the Triassic Sherwood Sandstone Group of the eastern part of the Carlisle Basin, Cumbria, and adjacent parts of Dumfries and Galloway, UK. However, previous workers have found considerable difficulty in consistently identifying, defining and mapping the Kirklinton Sandstone Formation. The principal lithological change within the Sherwood Sandstone Group is between mainly fine-grained sandstones that are commonly micaceous and contain numerous mudstone interbeds in the lower and middle parts of the group, and fine- to coarse-grained sandstones with rare or no mica and few mudstone partings at the top of the group. This change occurs within the Kirklinton Sandstone Formation as previously mapped. This paper considers ways of subdividing the group lithostratigraphically and the nomenclature to be adopted. All options have some associated problems, but, in view of the recently determined similarities with the contiguous sandstones offshore, the adoption of the same terminology as that in the adjacent East Irish Sea and Solway Firth basins is suggested, i.e. the St Bees Sandstone Formation below (subdivided where possible into Rottington Sandstone and Calder Sandstone members) and the Ormskirk Sandstone Formation above.

Sedimentary facies and the context of dolocrete in the Lower Triassic Sherwood Sandstone group: Corrib Field west of Ireland

The Lower Triassic Sherwood Sandstone Group (SSG) reservoir in the Corrib Field is predominantly fine- to medium-grained fluvial sandstone with minor aeolian and playa sediments that formed under semi-arid to arid conditions. There is no evidence of roots, burrows or organic-rich soils in the SSG suggesting an environment with sparse vegetation in the catchment and sedimentation from a predominantly braided sand-dominated river. Dolomite is the main porosity-occluding cement (< 25 vol.%) in the Sherwood Sandstone Group reservoir in the Corrib Gas Field, offshore west of Ireland. Dolomite is demonstrably early diagenetic since it is non-ferroan, brightly luminescent, occurs in rocks with high (uncompacted) intergranular volumes and grew before all burial diagenetic cements. Conversely, calcrete is the dominant early diagenetic cement in well 27/5-1 in the SSG, ∼50 km south of Corrib. Dolocrete and/or calcrete are represented throughout the entire SSG interval and typically form under conditions of very low sediment accumulation rates. Dolocrete occurs equally in all facies showing that it is not a result of depositional processes. Core goniometry data in the Corrib Field revealed that the general palaeoflow direction was from south to north. During progressive evaporation of river- and ground-water calcite formed first, followed by dolomite further downstream with gypsum formed last if evaporation continues. This well-established pattern indicates that well 27/5-1 has calcrete and Corrib has dolocrete since they both result from evaporative concentration of ground- and river-water during flow down the very shallow Triassic palaeogradient.

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 &amp; Sons, Ltd.

Carbon isotope stratigraphy using carbonate cements in the Triassic Sherwood Sandstone Group: Corrib Field, west of Ireland

Carbon stable isotopes from carbonate minerals (mainly dolomite) from six wells from the Lower Triassic Sherwood Sandstones of the Corrib Gas Field, Slyne Basin, west of Ireland, allow stratigraphic correlation. The results also provide information on palaeoenvironmental change during the deposition of these continental redbed sedimentary rocks. The Triassic reservoir rocks have been buried to > 4000 m and heated to > 165 °C and now contain methane-rich gas. Although the oxygen isotopic signal has been at least partially reset during burial and heating, a primary carbon isotopic signal appears to have survived diagenesis. The carbon isotope ratio varies from − 3.2‰ to + 2.1‰. All six wells show similar stratigraphic changes when all the carbon isotope data are plotted relative to a major playa horizon. δ 13C increases from about − 3‰ at the base of the Sherwood to about + 2‰ 170 m above the base. δ 13C then decreases to about − 2‰ for the next 70 m and remains steady for the following 50 m. The top 20 m of the Sherwood contains carbonate with a δ 13C values decreasing to about − 3‰. The occurrence of a stratigraphically-correlatable carbon isotope pattern implies that the primary evolution signal has been preserved. The change in δ 13C correlates with indicators of aridity and biological stress such that the highest δ 13C values are in sedimentary rocks deposited in a playa lake (arid times); these rocks contain the greatest quantity of dolomite cement. Conversely, the lowest δ 13C values correspond to sedimentary rocks deposited from well-developed rivers (relatively humid times) from the lowest quantity of dolomite cement. The same carbon isotope evolution has been found in another well in the Slyne basin and in Belgium, suggesting that the palaeoenvironmental isotope signal in the Triassic sedimentary rocks of the Corrib Field may have a regional significance.

Recharge velocity and geochemical evolution for the Permo-Triassic Sherwood Sandstone, Northern Ireland

The Triassic Sherwood Sandstone Group is a major European aquifer system. It is also the principal groundwater source in Northern Ireland. However, key aspects of its regional hydrogeology, such as age distribution and geochemical evolution, remain largely unknown. Here the groundwater geochemistry and isotopic composition were investigated in order to evaluate groundwater recharge and flow processes in a complex regional hydrogeological setting. The dominant geochemical processes, such as dissolution of carbonate cements were determined from the major and trace element chemistry. Stable and radio-isotope measurements were taken as residence time and flow path indicators and, together with physical and geochemical groundwater modelling, revealed groundwater ages of up to 9000 years. The importance of infiltration from overland flow from springs deriving water from the adjacent Cretaceous chalk aquifer and subsequently re-infiltrating into the Sherwood Sandstone was confirmed. In addition, evidence was found of a slow recharge component through low conductivity mudstones that yielded significant groundwater resident times throughout the Lagan Valley. These findings provide improved understanding of groundwater flow processes in Northern Ireland and serve as an example of methods that can be applied to water management elsewhere.

The Corrib gas field, offshore west of Ireland

The Corrib Gas Field in the Slyne Basin lies about 70 km west of Co. Mayo, offshore Ireland. The basin is a narrow Triassic/Jurassic half-graben and is one of a series of structurally linked basins that are present along the west coast of Ireland and the British Isles. The field was discovered in 1996 by well 18/20-1, encountering a 61 m gas column in low-porosity Triassic fluvial sandstones at a subsea depth of 3539.6 m. Using extremely poor 2D seismic data, it was originally mapped as a tilted fault block, with an associated rollover structure to the east. The data quality was particularly poor due to the near-surface Palaeogene volcanics that contribute to energy dispersion and extreme multiple generation. A 3D seismic survey was acquired in 1997 and resulted in a major improvement in data quality, allowing improved structural interpretation that showed the Corrib Field to be a faulted anticline. The subsequent appraisal well, 18/20-2z, drilled approximately 1 km from the subsurface location of 18/20-1, penetrated a 185 m gas column in fair to good quality sandstone. It tested dry gas at a stabilized rate of 62.87 × 106SCF/d on a 2″choke. Reprocessing of the seismic data, along with the successful drilling of the following well 18/25-1, confirmed the field to be a relatively simple anticlinal trap, with a complex faulted overburden that is structurally detached from the reservoir by the Mercia Halite. The source rock is assumed to be the Westphalian Coal Measures, similar to that encountered by well 27/5-1. The dry gas is consistent with a Type-III humic source rock. The reservoir is biostratigraphically barren, consisting of fluvial red bed sandstone, and is assumed to be of Triassic Sherwood Sandstone Group equivalent age. The unit is approximately 400 m thick, consisting of a high net-to-gross sequence of low-sinuosity braided fluvial channel sandstones with subordinate sand-flat and playa mudstone deposits. On a macro scale, it is a remarkably uniform sandstone with only subtle facies variations, but on a micro scale there is a significant variation in grainsize and cementation, which affects reservoir productivity. Mineralogically, there are distinct differences from the Sherwood Sandstone Group of the East Irish Sea, indicating that the Slyne Early Triassic system was depositionally similar to, but distinct and separate from, that to the east of Ireland. Dipmeter data indicated that the river system flowed from the SW to NE, essentially along the direction of the present-day Slyne Basin, suggesting that it was largely sourced from the south in the Variscan hinterland, but with evidence of sediment input from local highs such as the Connemara Massif.

The enigma of fine-grained alluvial basin fills: the Permo-Triassic (Cumbrian Coastal and Sherwood Sandstone Groups) of the Solway Basin, NW England and SW Scotland)

The late Permian to Triassic sediments of the Solway Basin consist of a layer-cake succession of mature, predominantly fine-grained red clastics laid down in semi-arid alluvial plain to arid sabkha and saline marginal marine or lacustrine environments. The Cumbrian Coastal Group consists of Basal Clastics and Eden Shales. The Basal Clastics are thin regolith deposits resting unconformably on all-underlying units and are composed of mixtures of angular local gravel and far-transported fine to very fine-grained sands deposited as basal lag. The Eden Shales are predominantly gypsiferous red silty mudstones, with thin very fine-grained sandstone beds, and with thick marine gypsum beds at the base, deposited at a saline lake margin. The overlying Triassic Sherwood Sandstone Group consists of the Annan and Kirklinton Sandstones. The Annan Sandstones are predominantly thick-bedded, multi-storied, fine-grained mature red quartz sandstones in which coarse sand is practically absent despite channels with clay pebbles up to 30 cm in diameter. The overlying, predominantly aeolian, Kirklinton Sandstones consist of festoon cross-bedded and parallel-laminated fine-grained sandstones, almost identical to the Annan Sandstones except that mica and clay are absent. The Stanwix Shales, located above, consist of interbedded red, blue and green mudstones, siltstones, and thin very fine-grained sandstones, with gypsum layers. Although the entire succession can plausibly be interpreted as deposited in a large desert basin opening into a hypersaline marine or lacustrine embayment to the southwest, the uniformly fine-grained nature of the succession is unusual, as is the absence of paleosols, and body and trace fossils. There is almost no coarse sand even in the river channel units, and it seems likely that the basin was not only extremely arid but supplied predominantly by wind rather than water.

The origin and regional distribution of dolomite cement in sandstones from a Triassic dry river system, Corrib Field, offshore west of Ireland

Dolomite cement in the Triassic Sherwood Sandstone Group of the Corrib Field predominantly has a very early diageneticorigin (dolocrete) having formed in evaporated ground and riverwater as it flowed from southwest to northeast in a braided river environment. The mainly non-ferroan dolomite is the dominant carbonate cement in the Corrib Field, whereas calcite is dominant in well data 30 km southwest of Corrib. Petrographic and cathodoluminescence (CL) data show characteristic spherical to prolate nodules of primary dolomite with intercalating clay that is overgrown by non-ferroan. The carbon isotope composition of dolomite fluctuates from −3.8 ‰ to +2.1 ‰ PDB and can be correlated in six wells to a certain stratigraphic datum. Oxygen isotopes vary from −14‰ to −4 ‰ PDB depending on the type of dolomite cement. Burial diagenesis resulted in minor overgrowth of ferroan dolomite but did not involve the introduction of any new carbonate. The distribution of calcrete, dolocrete and gypcrete in the Corrib Field and the Slyne Basin can be broadly predicted on the basis of knowing the palaeocurrent direction and the relative position in the river basin.

The South Morecambe Gas Field, Blocks 110/2a, 110/3a, 110/7a and 110/8a, East Irish Sea

AbstractSouth Morecambe Gas Field is situated in the East Irish Sea and produces gas from the Triassic Sherwood Sandstone Group. Exploration of the basin commenced in 1966 and the discovery well, 110/2-1, was drilled in 1974. Appraisal was complete by 1983 and development was carried out in two phases with the object of providing deliverability to help to satisfy the winter peak in demand. First gas was produced in January 1985 and production during the winter can be sustained at 50MMCMD (1750mmscfd). The stratigraphic succession of the East Irish Sea Basin (EISB) consists of Carboniferous (Dinantian to Westphalian) strata unconformably overlain by 15000 to 20000 feet of continental Permo-Triassic strata. The Triassic Sherwood Sandstone Group contains reservoir rocks and the overlying Mercia Mudstone Group evaporites provide a seal. Seismic cover of the area includes 2D and 3D data, the latter providing good images that form the basis of the current structural interpretation. The structural development of the basin commenced with extension in the Permo-Triassic followed by inversions in the late Jurassic and early Tertiary. The reservoir has been zoned using a scheme that recognizes primary depositional facies as the main criterion for correlation. The petrophysical evaluation has introduced new methods of calculating porosity, Sw and net pay. The latest reservoir pressure data has been used in a material balance study and a two tank simulation model, both give GIIP estimates which are in line with earlier estimates. The new petrophysically derived reservoir parameters were also used to make a volumetric estimate of GIIP. Remaining recoverable reserves are at least 3 Tcf.

The North Morecambe Field, Block 110/2a, East Irish Sea

abstractThe North Morecambe Gas Field in the East Irish Sea Basin was discovered by well 110/2-3 in 1976 and contains ultimately recoverable reserves of over one TCF. THe structure is fault closed on three sides and dip closed to the north. Development was by ten conventionally drilled high angle deviated wells, from a not normally manned platform. Gas is exported through a dedicated pipeline to a new terminal at Barrow. The Triassic Sherwood Sandstone Group reservoir is composed of sandstones deposited in a semi-arid, fluvial and aeolian setting. Thin aeolian sandstones dominate flow into the well bore. Platy illite reduces the permeability by two to three orders of magnitude in the lower, illite affected zone of the reservoir, RFT measurements from the first development well proved that the free water level was 25 feet higher than expected, giving a maximum gas column of 975 feet. Re-mapping after drilling has shown that 56% of the GIIP is contained in the high permeability illite-free zone.

Contact Diagenesis: The Effect of an Intrusion on Reservoir Quality in the Triassic Sherwood Sandstone Group, Northern Ireland

ABSTRACT Intrusion of a dolerite sill into clay-bearing, arkosic Triassic Sherwood Group sandstones with moderate porosity (22%) and high but anisotropic permeability (mode 500 md) led to the progressive transformation of precursor diagenetic minerals. In the host rock, locally pore-filling Mg-rich smectite (saponite), quartz, and dolomite, existing in the rock following earlier burial diagenesis, reacted to produce grain-coating flakes of talc at the lowest contact temperatures (130-180°C; based on equilibrium reaction-path and thermal-history calculations). At higher temperatures (200-230°C), talc reacted with calcite to produce an actinolitic amphibole. Close to the intrusion, at temperatures of > 250°C, bundles of acicular needles of actinolite protruding into open pores were produced by a second reaction involving saponite, hematite, and calcite. Framework feldspar minerals (mainly K-feldspars) played little or no part in the reactions because there were no K-Al rich minerals produced by contact diagenesis and because the detrital feldspar grains are essentially unaltered. Thus while equilibrium may have been approached on the intrapore scale ( 10 µm), it was not achieved between grains (e.g., detrital feldspars), pore water, and clays in the pores. Total porosity has been unaffected by the intrusion and consequent mineral reactions, because reactions involved recrystallization of original pore-filling minerals. Despite the contact diagenetic changes in the rock, the permeability of the sandstone remained largely unaffected by intrusion.