OUTCROP! CHARACTERISATION! OF! TRANSGRESSIVE! SANDSTONE! RESERVOIRS:! QUANTITATIVE!COMPARISON!OF!OUTCROP!ANALOGUES!

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Dorothy Townsend
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1 OUTCROP CHARACTERISATION OF TRANSGRESSIVE SANDSTONE RESERVOIRS: QUANTITATIVECOMPARISONOFOUTCROPANALOGUES OLIVER D. JORDAN 1, PETER J. SIXSMITH 2, GARY J. HAMPSON, SANJEEV GUPTA & HOWARDD.JOHNSON DepartmentofEarthScienceandEngineering,ImperialCollegeLondon,SouthKensington Campus,LondonSW72AZ,UK. 1 presentaddress:statoilhydro,bergen,norway. 2 presentaddress:chevrontexaco,sanramon,california,usa. Ourrecentlycompletedstudyofoutcropreservoiranalogueshashighlightedtheimportance of shoreline trajectory in controlling the stratigraphic architecture of transgressive shallow" marine deposits. Shoreline trajectory plays such a major role because it controls the preservationpotentialofbothshorefaceandback"barrierdeposits,whichcanberemovedby erosion due to waves and tides during shoreline retreat. Here we analyse net"transgressive successions showing variable shoreline trajectories to investigate variations in sandbody dimensions,distributionsandarchitecture. TheCretaceousCliffhouseandHostaSandstonesexhibitarangeoftransgressiveshoreline trajectoriesatoutcropinthesanjuanbasin,southwesternus.threedetaileddatasetshave beencollected,andfaciesarchitectureandsandbodydistributionshavebeencorrelated withinthenet"transgressivesandstonecomplexoverdipextentsofc.10kmineachdataset. Theunitexhibitsacomplexarrangementof(1)stackedwave"dominatedshoreface sandstones,(2)tide"dominatedsheetandchannelsandstones,(3)lagoonalsandstones, siltstonesandmudstones,and(4)coastalplainsandstonesandsiltstones.thesefaciesare separatedintodistinctstratalpackagesbyahierarchyoferosionalboundingsurfaces(wave andtidalravinementsurfaces)whichcontrolsandbodyconnectivityandmayalsoactas barriersorbafflestoflow.thewaveandtidalravinementsurfacesarecontemporaneous,and preservationofunderlyingstrataisdependantontherelationshipbetweenthe stepup and the stepback oftheshorelinethroughtime.waveravinementsurfacesmarkedbylarge landwardfaciesdislocationsareassociatedwithgreaterpreservationpotential,andnear" completeregressivesuccessionsarepreservedbeneaththem.waveravinementsurfaceswith smallerfaciesdislocationsexhibitgreatertruncationoftheunderlyingsuccession(e.g. foreshoreanduppershorefacefaciesareabsent).thegeometryanddistributionofthese surfacesthereforedeterminesreservoirthickness,dipextent,anddegreeof compartmentalisationbyshaletongues. Theresultsofouroutcrop"basedworkprovideinsightsintoreservoircharacterand architectureinthemiddle"to"upperjurassictransgressivereservoirplaysofthecentraland northernnorthsea,includingthetarbertformation(brentgroup)andfulmarformation (HumberGroup).

Outcrop characterisation of transgressive sandstone reservoirs: quantitative comparison of outcrop analogues Oliver Jordan 1,2, Peter Sixsmith 1,3, Gary Hampson 1, Sanjeev Gupta 1 & Howard Johnson 1

2 Outcrop characterisation of transgressive sandstone reservoirs: quantitative comparison of outcrop analogues Oliver Jordan 1,2, Peter Sixsmith 1,3, Gary Hampson 1, Sanjeev Gupta 1 & Howard Johnson 1 1 Department of Earth Science and Engineering, Imperial College London 2 StatoilHydro Research Labs, Bergen, Norway 3 Chevron Energy Technology Company, San Ramon g.j.hampson@imperial.ac.uk Outcrop datasets (after Molenaar et al. 2002) 1

facies architecture: Hosta Sandstone Sedimentary facies: - wave-dominated shoreface and tide-dominated backbarrier deposits - interfinger with coastal plain (up-dip) and offshore shales (down-dip)

3 facies architecture: Hosta Sandstone Sedimentary facies: - wave-dominated shoreface and tide-dominated backbarrier deposits - interfinger with coastal plain (up-dip) and offshore shales (down-dip) Complex internal stratigraphy: - partitioning of wave-dominated and tide-dominated deposits - facies architecture defined by distribution of wave ravinement surfaces, controlled by shoreline trajectory Outcrop datasets: stratigraphic surfaces 2

facies architecture: Hosta Sandstone Overall long-term transgressive shoreline trajectory: 0.36 - Low-angle (0-0.

4 facies architecture: Hosta Sandstone Overall long-term transgressive shoreline trajectory: Low-angle (0-0.2 ) medium-term transgressive shoreline trajectories: - poor preservation of tide-dominated backbarrier sandstones - thin (<3 m) wave-dominated shoreface sandstones, laterally stacked - High-angle ( ) medium-term transgressive shoreline trajectories: - thick accumulations of tide-dominated backbarrier sandstones - thick (3-8 m) wave-dominated shoreface sandstones, vertically stacked facies architecture: Cliff House Sandstone Complex internal stratigraphy: - variable thickness of net-transgressive shallow marine sandstone (0-120 m) - thicker sections characterised by multiple stacked shallow-marine units, separated by intercalations of offshore and/or lagoonal and coastal-plain shales - partitioning of wave-dominated shoreface and tide-dominated back-barrier deposits within each shallow-marine unit - fluvial influence: wave-dominated mouth bars and estuarine deposits - upper Cliff House Sandstone: Chaco Canyon - lower Cliff House Sandstone: Mesa Verde National Park 3

5 facies architecture: upper Cliff House Sst Complex internal stratigraphy: - partitioning of wave-dominated and tide-dominated sandstones - facies architecture defined by distribution of tidal ravinement surfaces, wave ravinement surfaces and flooding surfaces - gross architecture is controlled by shoreline trajectory - stratigraphy is more layer cake than in the Hosta Sandstone; wavedominated sandstones have greater dip extent and are more fully preserved facies architecture: upper Cliff House Sst Shoreline trajectories observed at three-scales: - vertical stacking of multiple thick (3-10 m) wave-dominated shoreface sandstone tongues (cf. transgressively-truncated parasequences) - within each tongue, short-term trajectories are estimated to be low (<<0.1 ) - between successive tongues, medium-term trajectories vary from (net-regressive) and (net-transgressive) - overall long-term transgressive shoreline trajectory:

6 facies architecture: lower Cliff House Sst Sedimentary facies: - wave-dominated shoreface and tide-dominated backbarrier deposits - interfinger with lagoonal and coastal plain heteroliths (up-dip) and offshore shales (down-dip) Stratigraphic partitioning similar to the upper Cliff House Sandstone facies architecture: lower Cliff House Sst Overall long-term transgressive shoreline trajectory: vertical stacking of multiple thick (3-20 m) wave-dominated shoreface sandstones separated down-dip by thin, extensive offshore-shale tongues 5

Summary of outcrop analogues A range of facies architectures are documented from the three outcrop examples, all associated with thick (>50 m) net-transgressive sandstones.

7 Summary of outcrop analogues A range of facies architectures are documented from the three outcrop examples, all associated with thick (>50 m) net-transgressive sandstones. Medium and long term shoreline trajectories controls: - preserved thickness of shoreface sandstone tongues - vertical and lateral stacking of shoreface sandstone tongues - preservation, vertical and lateral stacking of back-barrier sandstones Degree of direct fluvial sediment input controls: - dip extent of individual shoreface sandstone tongues (cf. short-term shoreline trajectory) - local facies character of wave-dominated sandstones (e.g. mouth bars vs. lower shoreface; along-strike variability) - facies character and geometry of marginal-marine sandstones ( clean tidal inlet sandstones forming shoreline-parallel belt vs. dirty estuarine sandstones forming shoreline-normal belt) Mid-to-late Jurassic transgression (after Partington et al. 1993) 6

8 Tarbert Formation, East Shetland Basin Tarbert Formation, East Shetland Basin Transect 1 Transect 2 - N to S retreat (axial) - hangingwall depocentre - NE to SW retreat (transverse) - stable platform 7

9 Tarbert Formation, East Shetland Basin high-energy, wavedominated shoreface low-energy, bioturbated shoreface fan delta Tarbert Formation, East Shetland Basin 8

10 Application to subsurface examples Shoreline trajectories can be measured in dense subsurface datasets: - requires robust understanding of underlying sedimentology, facies models - requires careful use of (geometrical) assumptions - provides quantitative framework to analyse reservoir distribution and character Application to mid-to-late Jurassic transgressive plays in North Sea rift: - shoreline type and facies character controlled by tectono-stratigraphic evolution of rift - two classes of sandstone tongue: shoreface (sub-regional extent, marine sediment supply) and deltaic-to-estuarine (localised extent, direct fluvial sediment supply) - shoreface tongues have variable dip extent, small thickness, and low-tomoderate long-term transgressive trajectories (limited vertical stacking) - deltaic-to-estuarine tongues have small dip extent, variable thickness, and moderate-to-very high long-term transgressive trajectories (pronounced vertical stacking) 9

Depositional Sequences Sequences

Depositional Sequences Sequences Depositional Sequences Transgressive and Regressive packages can be bound by unconformities Because sediment can only be preserved during net aggradation and progradation All other times there is either