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Deep-water clastic systems in theUpper Carboniferous (UpperMississippian–LowerPennsylvanian) Shannon Basin,western IrelandOle J. Martinsen, Andrew J. Pulham, Trevor Elliott,Peter Haughton, Colm Pierce, Anthea R. Lacchia,Simon Barker, Arnau Obradors Latre, Ian Kane,Patrick Shannon, and George D. Sevastopulo
ABSTRACT
The Upper Carboniferous Shannon Basin of western Irelandcontains amore-than-2300-m-thick (7540 ft) basin-fill succession,shallowing upward from deep-water to deltaic and incised fluvialdeposits. The deep-water basin floor and slope succession is worldrenowned as an analog for hydrocarbon-bearing deep-water sand-stones on several continental margins such as the basins in Eastand West Africa, South America, the Gulf of Mexico, and notleast offshore northwest Europe.
The Shannon Basin is frequently visited by both academiaand industry for research and training purposes. A series ofbehind-outcrop research boreholes reveals the subsurface ex-pression of the deep-water rocks and is complemented byseismic-scale cliff exposures. The succession is interpreted asa first-order basin-scale linked sedimentary system. This systemcan be analyzed using the principles of source-to-sink analysisand leaves the visitor with a complete picture of the basin fill,enhanced by spectacular sedimentological and stratigraphicdetail.
Petroleum Geological Significance of the ShannonBasin
Since the early 1950s, geologists from academia and industryhave visited the Upper Carboniferous (Upper Mississippian–Lower Pennsylvanian) Shannon Basin of western Ireland for
AUTHORS
Ole J. Martinsen ~ Statoil, P.O. Box 7200,5020 Bergen, Norway; [email protected]
Ole J. Martinsen is chief geologist and vicepresident of geology for Statoil. He haspublished widely in sedimentary geology,sequence stratigraphy, and source-to-sinksubjects and received the Robert R. BergOutstanding Research Award from AAPG in2011.
Andrew J. Pulham ~ Consultant, 5312Gallatin Place, Boulder, Colorado 80303;[email protected]
Andrew J. Pulham received his Ph.D. fromSwansea University in 1983 and worked for BPbefore becoming a geological consultant. Hehas worked for Nautilus running countless fieldschools, particularly in Ireland and the UnitedStates.
Trevor Elliott ~ Deceased
Trevor Elliott was a professor of geology at theUniversity of Liverpool before passing away in2013. His passion was field geology and hetaught countless field geology classes in Irelandfor academia and industry.
Peter Haughton ~ School of GeologicalSciences, University College Dublin, Belfield,Dublin 4, Ireland; [email protected]
Peter Haughton is a professor of geology atUniversity College Dublin and a deep-waterclastic sedimentologist with extensive outcropand subsurface experience. His research hasfocused on sediment gravity flow processesand the flow transformations that ultimatelyimpact on reservoir quality, as well as thedevelopment of sand tracking tools and theirapplication to sediment routing and reservoirpresence and quality issues.
Colm Pierce ~ School of GeologicalSciences, University College Dublin, Belfield,Dublin 4, Ireland; present address: School ofGeosciences, Meston Building, AberdeenUniversity, Aberdeen AB24 3UE, UnitedKingdom; [email protected]
Colm Pierce received both his B.Sc. and Ph.D.at University College Dublin, the latter workingon characterizing the Ross Formation viaoutcropand researchboreholes.He is currently
Copyright ©2017. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received January 16, 2017; final acceptance January 18, 2017.DOI:10.1306/021417DIG17099
AAPG Bulletin, v. 101, no. 4 (April 2017), pp. 433–439 433
research and training purposes (Figures 1, 2; e.g., Hodson, 1953;Gill and Kuenen, 1957; Hodson and Lewarne, 1961; Rider,1974; Gill, 1979; Martinsen, 1989; Pulham, 1989; Collinsonet al., 1991; Chapin et al., 1994; Davies and Elliott, 1996;Elliott, 2000; Martinsen et al., 2000, 2003; Sullivan et al., 2000;Wignall and Best, 2000, 2004; Martinsen and Collinson, 2002;Lien et al., 2003; Strachan and Allsop, 2006; Pyles, 2008;Pyles et al., 2014; Lacchia, 2016). It is one of the most usedoutcrop localities globally for petroleum geological training (cf. alsoElliott and Martinsen, 2012; Stone, 2012). The Shannon Basin(Figure 1) offers a unique, complete continental margin–scaleperspective of a clastic basin-fill succession (Figure 2). It in-cludes examples of a petroleum source rock and deep-waterto shallow-water to fluvial reservoir rock types and structuraltraps, and it allows for in-depth discussions and comparisonswith petroleum systems and plays in active, offshore explorationareas globally.
Tectonics, Eustasy, and Stratigraphy
A series of intracratonic basins developed between Laurentiaand Gondwana in northwestern Europe in the latest Devonianand Dinantian (Leeder and McMahon, 1988). The ShannonBasin is elongate about an east-northeast–west-southwest linepassing through the present Shannon Estuary, a line generallyaccepted to be the Iapetus Suture (Freeman et al., 1988).McKerrow and Soper (1989) showed from British Institu-tions Reflection Profiling Syndicate deep seismic data offshoreIreland that several deep structures dip to the north and comeclose to surface near the Shannon Estuary. These structures areinterpreted as original Caledonian thrusts that were reactivatedas major extensional faults during Devonian and Dinantian ex-tension. The trough-shaped Shannon Basin may compare witha major half graben, and fault-related subsidence may be inferredfrom pronounced thickness variations in the lower basin fill(Collinson et al., 1991). The basin was deformed and invertedduring the middle to late Stephanian (Late Pennsylvanian) byVariscan deformation. The Shannon Basin has an unusual thermalhistory, with vitrinite reflectance values of 6.5%–7.0% (Goodhueand Clayton, 1999). This is attributed to the expulsion of hotfluids from the Munster Basin to the south, because it wasbeing deformed shortly after accumulation of the ShannonBasin fill succession (Fitzgerald et al., 1994).
Sea-level fluctuations in the Late Carboniferous were dom-inated by a glacial-eustatic component related to the Gondwananice sheet and attained scales and periodicities comparable withPleistocene glaciations (Rygel et al., 2008). Condensed sectionslinked to maximum flooding formed during glacial-eustatic high
a post-doctoral researcher with the VirtualOutcropGroup at theUniversity of Aberdeenworking on multiscale-multiresolutionreservoir analog models underpinned byvirtual outcrop data.
Anthea R. Lacchia ~ Irish Centre forResearch in Applied Geosciences (iCRAG),University College Dublin, Belfield, Dublin 4,Ireland; [email protected]
Anthea R. Lacchia investigated the ammonoidbiostratigraphy of the Shannon Basin duringher Ph.D. at Trinity College Dublin. She is nowa post-doctoral research fellow based atUniversity College Dublin, where she is helpingto develop a subsurface training center in WestClare, close to the Shannon Basin outcrops.
Simon Barker ~ Statoil, P.O. Box 7200,5020 Bergen, Norway; [email protected]
Simon Barker is a deep-water clasticsedimentologist working in Statoil’s researchcenter in Bergen, Norway. Before Statoil, heworked for British Gas in the United Kingdomfor eight years in global exploration.
Arnau Obradors Latre ~ School ofGeological Sciences, University CollegeDublin, Belfield, Dublin 4, Ireland; [email protected]
Arnau Obradors Latre received his B.S.degree in geology from the AutonomousUniversity of Barcelona and his M.S. degree ingeology and exploration of sedimentaryreservoirs from the University of Barcelona. Heis currently finishing his Ph.D. in geology atthe University College Dublin focusing on thelateral transitions in the Ross SandstoneFormation, western Ireland.
Ian Kane ~ School of Earth andEnvironmental Sciences, University ofManchester, Williamson Building, OxfordRoad, Manchester M13 9PL, United Kingdom;[email protected]
Ian Kane is presently reader of sedimentologyat the University of Manchester with variousresearch interests in deep-marinesedimentology. He has a Ph.D. from theUniversity of Leeds and completed a 2.5 yearpost-doctoral research position with Statoilbefore working in their Bergen researchcenter for four years and in internationalexploration for one year.
434 Deep-Water Clastic Systems in Upper Carboniferous Shannon Basin, Western Ireland
sea level with distinctive ammonoid bands (Holdsworth andCollinson, 1988). Regionally widespread sequence bound-aries formed during falling sea level and formed incised valleys(Hampson et al., 1997). Climatically, northern Europe wasequatorial during the Late Carboniferous.
The more-than-2300-m-thick (7540 ft) basin-fill succession(Figure 2) contains a shallowing-upward succession startingfrom deep-water shales and siltstones (Clare Shales, ~275 m[900 ft]) through a sand-rich deep-water fan complex (RossFormation, ~495 m [1625 ft]) overlain by a deep-water slopesuccession (Gull Island Formation, ~550 m [1800 ft]), which inturn contains a gradational passage into a deep-water fronteddeltaic succession (Tullig Cyclothem, ~210m [690 ft]). A furtherfour deltaic cyclothems (Kilkee, Doonlicky, and Cyclothems IVand V; ~800 m [2620 ft]) cap the basin-fill succession (Figure 2).The basin-fill succession formed by progradation of a first-orderclastic margin punctuated by several higher-order cycles of sed-iment input and abandonment.
The basin-fi l l succession compares with hydrocarbon-bearing continental-margin and intracratonic successions glob-ally in terms of large scale stratigraphy and reservoir architecture(Chapin et al., 1994; Martinsen et al., 2000; Sullivan et al., 2000;Pyles, 2008). Field logistics and proximity as well as safety andsecurity are well established. The succession is also coeval to activepetroleum systems in northern England as well as in the NorthSea, with time-correlative formations to the Clare Shales pro-viding the source rock. Lately, questions and expectations havearisen as to whether the Clare Shales in itself is an unconven-tional shale-gas petroleum system (Sjurseth, 2011).
The Shannon Basin Borehole Project
A consortium consisting of Statoil andUniversity College,Dublin,drilled 12 behind-outcrop wells targeting the sand-rich RossFormation (Figure 1). These wells penetrate up to 300 m (980 ft)of Ross Sandstone stratigraphy, target various intervals, and col-lectively create a coherent and robust stratigraphic framework forthe Ross Sandstone together with extensive former and recentfield studies (Pierce et al., 2010; Lacchia, 2016; Pierce, 2016).Furthermore, they include a complete suite of core and well logdata, which add to the subsurface application and analogy of theextensive outcrop data in the area.
Source-to-Sink Analysis of the Shannon Basin Fill
An additional asset of the Shannon Basin outcrop successionis that it lends itself to source-to-sink analysis (S2S; Sømmeet al., 2009). At outcrop, this approach has not been used
Patrick Shannon ~ School of GeologicalSciences, University College Dublin, Belfield,Dublin 4, Ireland; [email protected]
Patrick Shannon is an emeritus professor ofgeology in the School of Earth Sciences atUniversity College Dublin. He obtained a B.Sc.and Ph.D. from University College Dublin. Hisresearch interests include basin analysis andpetroleum geology.
George D. Sevastopulo ~ Departmentof Earth Sciences, Trinity College, CollegeGreen, Dublin 2, Ireland; [email protected]
George D. Sevastopulo is professor emeritusat Trinity College, Dublin. His special interesthas been in Carboniferous stratigraphy,Irish geology, and paleontology.
MARTINSEN ET AL. 435
extensively, because it requires reconstruction ofone of the S2S segments from the outcrop data. Inwell-preserved, linked, and complete basin-fill suc-cessions like the Shannon Basin fill, this is possible(Martinsen, 2015).
The Gull Island Formation and the lower part ofthe Tullig Cyclothem measure 700 m (2300 ft) invertical thickness (Figure 2). Using a compactionfactor of 30% for the silt-dominated section leavesa decompacted vertical thickness of the slope suc-cession of 1000 m (3280 ft). Using a sine function fora slope angle of 1°–2° leaves a slope length of 28.6 and57.3 km (17.7 and 35.4 mi), respectively. Input ofthose values to the global S2S statistics of Sømmeet al. (2009) provides dimensions for other key ele-ments of the Shannon Basin S2S system. Comparing
this system and the profile with other sand-pronesubmarine-fan S2S settings (Martinsen, 2015) im-plies a narrow shelf width and a catchment with theability to store and capture larger volumes of sedi-ment than, for example, the California Borderlandsystems (Covault et al., 2011). The interpreted nar-row shelf implies elevated potential for sand supply tothe deep-water areas, particularly during sea-levellowstand, and this can explain the sand-rich nature ofthe Ross submarine-fan complex. The analysis suggestsa Ross fan length of 150–200 km (~90–125 mi) andarea of approximately 10,000 km2 (~3860 mi2). Animportant modifying element is the interpretationthat the Ross submarine-fan complex was depositedwithin the confines of the Shannon basinal trough,aligned parallel to the Shannon Estuary above the
Figure 1. Geological map of the Shannon Basin outcrops around the Shannon Estuary. The map also shows the location of the 12University College Dublin (UCD)-Statoil boreholes and other boreholes in region. The rose diagram shows paleocurrent data collected byColm Pierce and Arnau Obrador Latre for the Ross Sandstone in the area, showing the main paleocurrent directions towards theeast–northeast. The map is compiled from work by Hodson (1953), Rider (1974), Gill (1979), Collinson et al. (1991), Martinsen et al. (2003),and Pierce (2016). The line marks the orientation of the stratigraphic cross section in Figure 2. CE = Clare; GSI = Geological Survey ofIreland; IPP = Irish Petroleum Prospecting Ltd.; KY = Kerry; Limest. = limestone; n = number of measurements; RE = borehole no. 09-CE-UCD-01 was reentered and extended to make borehole no. 09-CE-UCD-01RE.
436 Deep-Water Clastic Systems in Upper Carboniferous Shannon Basin, Western Ireland
Iapetus Suture (see above). The effect of this relativeto deposition on an unconfined basin plain would beto narrow the width of the fan and extend the length,provided sediment volumes were constant.
In conclusion, the ability to perform an outcrop-based S2S analysis of the Shannon Basin fill adds tothe exploration value of visiting these field localities.Although the output values for the basin-fill systemare guiding figures and must honor and be calibratedto local mapping, they provide important scalingrelationships to understand the magnitude of theoriginal depositional system.
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