The role of glaciation and its implications for monitoring ... · “The North Sea Basin is the most important CO. 2 . storage region for the whole of Europe” Stuart Haszeldine,

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The Glaciation of the North Sea Basin: implications for Carbon Capture and Storage Sites

Tom Bradwell

School of Natural Sciences, University of Stirling, UK and British Geological Survey, Edinburgh, UK


CO2 storage

Basic criteria for site selection • Adequate storage capacity

• Adequate CO2 injectivity

• Security of storage

• Minimal environmental impact • Cost

Apologies...


Effects of glaciation on storage reservoirs

• Geological properties of overburden

• Sediment characterisation • Discontinuities & heterogeneities • Major fluid flow pathways

• Geomechanical response to glacial loading-unloading cycles

• Stress fields & neotectonics • Rock/sediment mechanics • Porosity & permeability changes

• Hydromechanics

• Hydraulic pressurisation • Freshwater ingress

• Diagenesis & lithification

• Glacigenic (Pleistocene) reservoirs

Seal integrity


“The North Sea Basin is the most important CO2 storage region for the whole of Europe” Stuart Haszeldine, 2008

FAR

UK

NOR

DAN

GER

NED

BEL

FRA

North Sea Basin

Setting

NW Europe

NSB


Carbon Capture & Storage

Potential sites in North Sea Basin

Sleipner

www.npd.no

SCCS, 2014


North Sea Basin

Cenozoic Geology

Seismic data courtesy of Schlumberger WesternGeco

Courtesy of Sam Holloway

Almost 1000 m of Quaternary (glacial/interglacial) overburden sediments in central NSB

Ottesen & Dowdeswell, 2014


North Sea Basin

Cenozoic Geology

Seismic data courtesy of Schlumberger WesternGeco

Courtesy of Sam Holloway

Almost 1000 m of Quaternary (glacial/interglacial) overburden sediments in central NSB

Graham et al., 2011


North Sea Basin

Quaternary history

Record of oxygen isotopes from deep-sea sediments (Shackleton et al., 1990).

Oxygen isotope fluctuations from Greenland Ice Core record (Wolff et al., 2010).

Quaternary stratigraphy, seismic character and inferred glaciation extent (Graham et al., 2011).


Geological properties

Glacial sediments

Glacial sediments are very diverse and heterogeneous…!

Overconsolidated, low porosity, clay-silt mtx diamicton, with subhorizontal fissures subglacial till

Poorly consolidated, high porosity gravels & sands, low-angle bedding glaciofluvial outwash

Finely laminated, clay/silt/sand, variable porosity glaciolacustrine rhythmites



Glacial sediments

• Characterization can be difficult • Detailed 3D descriptions needed • In situ engineering properties (over time?) • Depositional process & genesis

Thick bedded, high porosity outwash sands & gravels dominant in temperate (high discharge) systems

Fractured and dislocated bedrock, forming openwork or high permeability authochthonous breccia (glaciotectonite)

Locally highly heterogeneous; sub- pro- & supraglacial



Glaciotectonic structures

• Deformation and discontinuities very common; can be pervasive • Faulting and folding – from micro (~mm) to macro (~km) scale

Phillips et al., 2010



Glaciotectonic structures

• Strong glaciotectonic deformation common around dynamic ice sheet margins south NSB • Shear stress concentrated along weaker strata

• Freezing on and ?thrust-stacking of porous, well-consolidated pre-Quat. Strata

• Thrust depth varies; <50 to 500 m (extreme).

• Structures disrupt seal integrity; high permeability?

Large-scale glaciotectonic complex, ~400 m deep; 1.5 km long; SE Danish sector. L.T. Andersen (2012, PhD thesis)

Glaciotenic thrust belt complex, SW Alberta. (Kellett, 2007)



Fluid migration pathways

• Faults / fracture zones

• Tunnel valleys and channels

• ‘Chimneys’ and pockmarks

• Other vertical porous bodies (e.g. sand wedges)


Potential fluid migration pathways

Tunnel valleys

Seven separate generations recognised in NSB

Stewart & Lonergan, 2011

Graham et al., 2011



Tunnel valleys

open TV Fill

closed TV Fill

• Often comprise coarse-grained, high porosity, laterally extensive units

• Undulating erosional base

• Intersecting & branching plan morphology

• Can be buried (closed) or open (on seabed)

• Highly variable sediment-ological composition

• Over-pressurised pore fluids?

• Terrestrial analogues rare



Tunnel valleys

• Size, high porosity and intersecting nature of NSB tunnel valleys could allow fluid migration

• adverse impact on storage potential of underlying formations

• Over-pressured fluids affect injectivity

Kristensen et al., 2007

Still uncertainty over genesis: rapid vs long-term (?subglacial) glaciofluvial erosion

Kristensen et al., 2007


Fluid escape features

Pockmarks

• Glacial and/non-glacial origin • <1-10 m deep; 1-200 m wide • Evidence of fluid escape at seabed • Common in NSB, esp. in berg-ploughed seabed and fringing moraines

Large pockmark

Pockmark chain

Strange groove

Gales, Bradwell, Stoker, in prep.



Pockmarks

10 m Gas blanking



Pockmarks

10 m

Seabed expression of bedrock fault

reactivation?

seismic ‘chimneys’


Fluid escape feature

Pockmarks

25 ms

Gas blanking

Fluid migration? ‘chimneys’

BGS 1991/3_DTBoomer_line22

Pockmarks Giant pockmark

Example from central NSB


Geomechanical response

Glacial loading-unloading cycles

• Ice sheets exert massive loading force on sediment & rock

• Changes to porosity & permeability;

may never readjust

• Stress propagates to depths >50 km, depending on load

• Experimental work shows increased horizontal stresses even 10ka after ice sheet disappearance

• Impacts on storage & retention potential of subsurface fluids

Klemann & Wolf, 1998


Glacial rebound and neotectonics

Evidence from Fennoscandia

• Recorded seismic events cluster along western margin of NSB

• Tampen Spur high seismicity area associated with zone of dense fracturing ?weakness zone

• Some evidence for neotectonics (esp. in N Scandinavia)

• No direct correlation between uplift rates/pattern and seismicity

• Only 30 yr timeseries; more data needed to test theory

Sleipner

TS

all <6.5

Bungum et al., 2010


Glacial loading and stress fields

Ice sheet models

• Modelled ice sheet geometries used to calculate spatio-temporal rebound and stress patterns

• Wide range of glacio-isostatic & lithospheric loading

scenarios for northern NSB

Wu et al., 1999

Grollimund & Zoback, 2003


Glacial loading

and stress fields

Model results

• Reasonable match between model- and observed stress fields

• Evolution of modelled stress values (at 3000 m depth)

• 1st-order approximation?



Glacial loading

and stress fields

Model results • At maximum ice sheet thickness,

lithospheric flexural stress (forebulge) adds to high vertical loading stresses

• Stress beneath ice sheet centre largely

unchanged • After deglaciation (~15 ka extent) stress

magnitudes return to pre-100ka values

• Permanently increased horizontal stress values around edge of former ice sheet


Brooker & Ireland, 1965


Glacial loading

and stress fields

Conclusions

• Weight of advancing ice over-pressurizes subsurface, raising fluid pressures release?

• Ice loading stress increases (?decreases) fracture permeability • At LGM, weight of ice sheet

stabilizes faults preventing fluid leakage; temporary caprock?

• Leakage promoted during glacial-

interglacial transitions, when stress state change most rapid

• Sites just outside former ice sheet margins most susceptible to fluid leakage


Neuzil, 2012


Glaciation of North Sea Basin

Conflicting models

Ice sheet models for British Isles (Last Glacial Maximum) a. Boulton et al., 1977 b. Boulton et al., 1995 c. Boulton et al., 1991 d. Lambeck et al.,1993

Wide ranging scenarios for Northern NSB !

NB: Lambeck et al. 1993 disproved


Glaciation of NSB

More recent work

• Digital shelf-wide bathymetric datasets key to new insights (e.g. Olex; regional MBES)

• Ice sheet evidence abundant offshore, esp. in northern NSB

• Ice sheet moraines mapped

from seabed geomorphology

C

B

A

B

Tunnel valley

Bradwell et al., 2008



More recent work

Bradwell et al., 2008


Glaciation of NSB

More recent work

• Ice sheet moraines indicate confluence in northern NSB during MIS 2-3

• Tampen Ridge / Witch Ground

Basin acted as suture zone for BIS & FIS ice sheets

C

B

Clark et al., 2012

Ice sheet

suture zone

• Marine cruise planned summer 2015 (Britice-Chrono)

• Understanding ice sheet dynamics, east of Shetland

Bradwell et al., 2008 & in prep.


• New hypotheses re. ice sheet growth/retreat impact on lithospheric loading (e.g. Cavanagh & Haszeldine, 2014)

• Ice sheet growth is slow and steady gradual lithospheric loading in NSB

• Ice sheet retreat is rapid (<< 1ka); NSB breakup by RSL/flotation

• Instantaneous removal of loading pressures (grounded floating)

• Exceeds geomechanical limit of shales micro-fracturing?

Glaciation of NSB

New ideas & implications

Cavanagh & Haszeldine, 2014



New models

Hubbard et al., 2009 Clark et al., 2012


Glaciation of NSB

Recent revisions

Sejrup et al., 2014 (in press)

• Collective evidence points towards large independent Shetland-Orkney ice centre,

following BIS-FIS separation

• ~1 km thick with dynamic margins

• Strongly differs from early models

• Implications for glacial history and loading history of northern NSB

Hall, 2014

Bradwell & Stoker, in press


Clark et al., 2009

Glaciation of NSB

Summary

Boulton & Hagdorn, 2006

• Ice sheet evolution in NSB coalescence & separation

• Note position of Sleipner relative to ice margins (a,b,d)

• Northern NSB can be both ice free and ice centre within ~5 ka

• Very complex stress conditions experienced; unlike anywhere else in W Europe

NB: Still >>under-estimating ice thickness in Shetland sector


Some conclusions

• Glacial sediments In NSB are diverse & heterogeneous; porosity & permeability highly variable; discontinuities common across scale-range • Glaciotectonics common around (former) dynamic ice sheet margins in NSB; shallow strata disrupted up to 500 m below sea bed • Fluid pathways exist in glacial overburden; formation age unknown; fluid migration & seabed seepage occur at present day

• Tunnel valleys & ‘chimneys’ are most significant pathways; could affect seal integrity (& injectivity) • Former ice sheet margins have complex stress fields and rebound histories (esp. northern NSB)

• Neotectonics is often invoked (e.g. FIS margin) but evidence is still equivocal • Empirical data and models show/predict coalescence of ice sheets in northern NSB.

• Ice-free to thick ice-sheet cover in ~5 ka, central NSB; and rapid (marine) deglaciation <1 ka

• Still uncertainty about timing and pattern of deglaciation; and number of glaciations…


Acknowledgments

The whole GlaciStore Team!

Thanks for listening…

The role of glaciation and its implications for monitoring ... · “The North Sea Basin is the most important CO. 2 . storage region for the whole of Europe” Stuart Haszeldine,

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