Interpretation of nearly 5000 km of seismic data, and facies analysis of approximately 900 m of core from 11 wells, has produced structural and sedimentological models of the Triassic Sherwood Sandstone in the East Irish Sea Basin. Sedimentary facies recorded include moderate to low-sinuosity braided fluvial channel, aeolian dune and sandsheet, unconfined fluvial sheetflood and non-evaporitic playa deposits. The establishment of facies associations and correlation of these with wireline logs has enabled extrapolation of sedimentary models into uncored intervals and between wells. The identification of differing seismic character associated with the various facies has made possible the mapping of these diagnostic seismic packages using a seismo-stratigraphic approach. The resulting distribution of seismically derived facies accords with proposed structural and sedimentological models, predicts the position and course of fluvial channels operating in the basin during late Sherwood Sandstone time, and shows that these are largely controlled by fault activity. Detailed facies analysis of sandflat sequences reveals a systematic variation in the proportion of sheetflood and playa to aeolian deposits. It is suggested that this corresponds to a climatic control with sheetflood- and playa-dominated sequences representing relatively wet episodes with a high water table, and aeolian-dominated sequences representing relatively dry episodes with a lowered water table. Correlation of these climatic events may introduce a chronostratigraphic element to the analysis of these continental ‘red-bed’ sequences.
The Corrib GasFieldi nt he SlyneB asinl iesabout 70 kmw est ofC o. Mayo,offshoreI reland.The basinisanarrow Triassic/Jurassic half-grabenandisoneofaseriesofstructurallylinked basins thatarepresent alongthe west coast ofI relanda ndthe BritishI sles. The fieldwasdiscovered in1 996 byw ell 18/20-1, encounteringa61 mgascolumn inlow-porosity Triassic fluvialsandstonesatasubsea depthof3539.6 m. Using extremely poor 2Dseismic data, itw aso riginally mapped asatilted fault block, witha nassociated rollover structureto the east. The dataquality wasp articularly poor dueto the near-surface Palaeogenevolcanicst hat contributeto energydispersion andextrememultiplegeneration.A3Dseismic surveywasacquired in1997 and resulted inamajor improvement indataquality,a llowingi mproved structuralinterpretation thats howed the Corrib Fieldto be afaulted anticline.The subsequent appraisalwell,18/20-2z,drilled approximately 1kmfrom the subsurface location of18/20-1,penetrated a185 mgascolumn infairtogoodquality sandstone.Ittested dry gasatastabilized rateof62.87 £ 10 6 SCF/don a2 00 choke.Reprocessingofthe seismic data, alongwiththe successful drillingofthe followingwell 18/25-1,confirmed the fieldto be arelatively simpleanticlinaltrap,with acomplexfaulted overburdenthatisstructurally detached from the reservoirbythe Mercia Halite.The source rock isassumed to be the WestphalianCoalMeasures,similartothatencountered bywell 27/5-1. The dry gasis consistent withaType-III humic source rock.The reservoirisbiostratigraphically barren,consistingoffluvialred bed sandstone, andi sassumed to be ofT riassic SherwoodS andstoneG roup equivalent age.The unitis approximately 400 mt hick, consistingofahigh net-to-gross sequence oflow-sinuosity braided fluvialchannel sandstoneswithsubordinatesand-flatandplayamudstonedeposits. Onamacro scale, itisaremarkably uniform sandstonewithonly subtlefaciesvariations,but on amicro scalethereisasignificant variation ingrainsizeand cementation,which affects reservoirp roductivity. Mineralogically,therea red istinctdifferencesfrom the SherwoodS andstoneG roup ofthe East IrishS ea, indicatingthatt he SlyneE arly Triassic systemw as depositionallysimilarto,but distinctandseparatefrom,thattothe east ofIreland.Dipmeterdataindicated that the rivers ystemflowed from the SW to NE, essentiallyalongthe direction ofthe present-daySlyneB asin, suggestingthatitwaslargely sourced from the southinthe Variscanhinterland, but withevidence ofsediment input from localhighssuch asthe ConnemaraMassif.
The Lower Triassic Sherwood Sandstone Group constitutes the principal reservoir target in the East Irish Sea Basin. Interpretation of sedimentary facies allows recognition of major low sinuosity braided fluvial channel, minor ephemeral fluvial channel, aeolian dune and sandsheet, unconfined fluvial sheetflood and playa lake deposits that are discussed in terms of facies associations. Reservoir quality is highly variable, with porosity and permeability data revealing significant differences between fluvial, sheetflood and aeolian facies associations. The latter, in particular, preserve very high reservoir quality reflecting an open grain fabric and paucity of blocky cements. Variations in porosity between the facies associations relate primarily to the distribution of quartz cement which is significantly more abundant in fluvial channel sandstones than other facies. Authigenic illite, while severely limiting permeabilities in some reservoir zones, is not facies specific and does not contribute to the observed variations. It is suggested that the disparity in the abundance of quartz cement between facies associations relates to two distinct grain types that correlate broadly with fluvial and aeolian associations, respectively, and probably derive from different provenances. The evolution of the basin, in controlling the distribution of the major facies, influenced the distribution of the two grain populations and, thus, also the reservoir quality of the sandstones.
The 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.
The identification of early evaporite sulphate cements with a marine isotopic signature within the Ormskirk Sandstone Formation supports previous models of diachroneity at the contact of the Sherwood Sandstone Group and the Mercia Mudstone Group in the East Irish Sea Basin. Basin-wide correlations suggest that, at the onset of transgression, a range of sandflat and aeolian dune environments co-existed on the margins of the basin with marginal marine environments toward the basin centre. One of the major implications of this model is that the high quality aeolian sandstone reservoirs encountered near the top of the Ormskirk Sandstone in the southern parts of the East Irish Sea Basin have no reservoir correlatives in large areas of the basin but can be predicted on the margins.Poor biostratigraphical control combined with historically confused stratigraphical nomenclature within the Sherwood Sandstone Group (SSG) have obstructed the recognition of sedimentological and palaeoenvironmental relationships within the East Irish Sea Basin (EISB) and its margins. Despite an increased understanding of the sedimentary environments within the SSG over the past two decades, little attention has been given to relating sedimentological and stratigraphical relationships on a regional scale. This paper offers a sedimentological model for the top of the Ormskirk Sandstone Formation (OSF) which assumes, in very broad terms, the gradual encroachment of marine conditions southwards across the basin at the onset of Mercia Mudstone Group (MMG) deposition. The key to the model was the discovery of early marine evaporite cements within cored sediments from British Gas well 110/8a-5 (Fig. 1), which effectively invoke marginal marine conditions penecontemporaneous with deposition at the top of the OSE This concept is taken further by integrating these observations with broad-scale facies relationships across large parts of the EISB.
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