Cracked and full of sand: insights into the development of fractured basement reservoirs west of Shetland

Bob Holdsworth1 , Ken McCaffrey1, Eddie Dempsey2, Kit Hardman1, Jon

Hunt3, Martin Feely3, Giuseppe Palladino4, Giacomo Prosser5,

Christine Siddoway6

With thanks to Andy Conway, Andrew Robertson, Catherine

Witt, Simon Richardson

The findings presented here are the interpretations of Durham University & not necessarily those of the Joint Venture or the individual companies themselves

1 Durham University, UK 2 Hull University, UK 3 UC Galway, Eire 4 University of Aberdeen, UK 5 Università Basilicata, Potenza, Italy 6 Colorado College, USA

Basement reservoirs: issues

Globally, fractured basement reservoirs widely known but underexploited

Oil stored within fracture systems as crystalline basement rocks have low porosity + permeability

Oil has migrated in from adjacent sedimentary source rock

Seismic imaging issues

Limited core & industry knowledge

How does the oil get in?

Passive fracture conduits or fault driven migration (or both)?

Clair Ridge seismic

Clair core 206/7-A2

Lancaster Hurricane Energy

Liaohe ‘buried hills’China

Johan Sverdrup Offshore Energy Today

Witt et al 2010

Rona Ridge

Known fields: Clair, Lancaster, Halifax, Victory

Clair: Multi-billion barrel, potential field life > 25 yrs

Studied Rona Ridge basement & cover cores + thin sections + new geochronology + preliminary FI analyses + fracture attributes + poroperm analyses (not discussed here)

Clair

Lancaster Trice 2014

Series of uplifted footwall blocks of fractured basement bounded Mesozoic normal faults

Moy & Imber 2009

Halifax

Victory

Example basement core (206/7a-2): lithologies All cores 100mm wide

Grey granodioritic gneiss (55%)

Grey dioritic gneiss (5%)* Green mafic gneiss (10%)* Pink granitic gneiss (15%) Foliated porphyritic

granite (15%) ~20% interlayered on cm-

mm scales

* often occur as enclaves

Radiometric age constraints & correlatives

U-Pb zircon ages carried out by Ritchie et al. (2011), Morton et al. (unpubl) & this study

Tight Neoarchaean age range

Same age as Lewisian protolith in mainland Scotland, but no isotopic trace of Proterozoic (Laxfordian) events

Faroe-Shetland block of Ritchie et al. (2011) – correlates with Central Greenland Craton (CGC)

Basement crops out onshore in N Roe-Uyea, Shetland [NR on map]: same age (Strachan et al. unpubl)

Implies presence of ‘northern Laxford Front’ just N of Scotland

CGC

N Roe, Shetland (NR)

ca 2.73-2.83 Ga

Ages ca 2.74 - 2.75 Ga

Early geological history 1) Coarse grained gneisses with textures consistent with early granulite-upper amphibolite facies metamorphism: ductile folds, shear zones ca. 2.8Ga

2) Local evidence of younger, lower T ductile events: age unknown

3) ‘Early’ brittle deformation: a) grey cataclasite; pseudotachylyte; b) epidote-qtz veins

4) Found as clasts in cover (Dev-Carb Clair Gp, Jurassic)

1 2

3b

3a

4

All cores 100mm wide

Main set of oil-bearing faults/fractures Red-brown qtz-adularia-carbonate-pyrite microbreccias & veins

Associated with: - Sediment-filled open fractures & injections - Larger shear fractures (often poor preservation in core) - Vuggy qtz-carbonate-pyrite fractures/veins full of oil

Dominant structures in basement – also recognized in Dev-Late Jurassic cover sequences

Rona Sst Clair Gp Basement

Basement All cores 100mm wide

Basement fracture mineralization sequence Red scale bar = 500mm

Standout feature #1a: Primary sedimentary fracture fills

Cover-hosted

Jur Rona Sst host

Dev Clair Gp host

10mm

Basement-hosted

All cores 100mm wide

Y Way-up criteria: grading, geopetal fills

Youngs up core in vertical wells, across core in horizontal wells

Y

Y

Basement-hosted

Cover-hosted

Jur Rona Sst host

Dev Clair Gp host

Y Red scale bar = 500mm

Red scale bars = 1000mm

All high porosity, filled with oil

Post-Jurassic based on age of youngest host rock cut by such fracture fills Likely Late Cretaceous?

Red scale bars = 100mm

Green scale bar = 1000mm

10 mm

Standout feature #1b: Secondary injected sediment ‘slurries’

Qtz-calcite cemented with ‘dusty’ Fe oxide coatings on clasts

Some contain bituminous ‘clasts’

Some can be traced back to primary ‘collapsed’ sediment-filled cavities

Red scale bars = 500mm

Green scale bar = 10mm

Standout Feature #2: Cockade textures Microbreccia clasts surrounded by zoned cements in qtz-carb-adularia-pyrite veins & sediment injections

These are ‘cockade textures’: widely recognised in low T (50-350°C) , near-surface hydrothermal systems

Weissenbach (1836)

Review in: Frenzel, M. & Woodcock, N.H. (2014)

Cockade breccia: product of mineralisation along

dilational faults. Journal of Structural Geology, 68 (Part A). pp. 194-206. ISSN ISSN:

0191-8141

Red scale bars = 100mm

Purple scale bar = 1000mm

Cockade textures: significance

Repeated fluxing of fluids through incompletely cemented fracture systems/cavities

Long term presence of open fracture systems through which mineralizing fluids can easily & repeatedly migrate

Cut-effect

Suspension in fluid

Accretion & rolling in fluid

PLUS

Red scale bar = 100mm

Standout feature #3: Vuggy cavities/fills

Very common, filled with oil Fracture intersection rhomboids Partial to complete cockade style fills (qtz, calcite, pyrite, sediment)

Vugs previously attributed to ‘late’ reactivation & dilatancy of fractures NO EVIDENCE Left-over cavities formed as fracture system floods with oil, shutting down mineralization

All cores 100mm wide

Red scale bar = 500mm

U-Pb dating of calcite fills: Clair, Victory

Variable Uranium concentrations reflects zoned calcite crystals

Suggests spread of ages of hydrothermal mineralization in different sections of Rona Ridge over at least 20 Ma in Late Cretaceous

U-Pb geochronology conducted via the LA-ICP-MS method at NERC Isotope Geosciences Laboratory (NIGL) method in Roberts & Walker (2016).

90.2 ± 3.9 Ma

71 ± 2.4 Ma

West of Shetland oil system: generation & migration events

Lamers & Carmichael 1999

Basin Modelling: oils formed 60-80Ma

Victory

VICTORY FIELD: 3 pulses: ~80 Ma (oil-bearing fluid - 125˚C) ~72 Ma (108-86˚C oil-bearing) ~60 Ma (<50˚C aq fluid)

Mapping fluid charge - Ar-Ar Ksp dates

Age of oil – Re-Os dates

Finlay et al. 2011

Finlay et al., 2011

94.1 - 73.4 U-Pb calcite (this study)

Findings consistent with published results suggesting Late Cretaceous oil generation (from Jurassic source) using basin modelling & geochronology

Mark et al., 2005

Clair 206/8-16 – Calcite vein

Fluid Inclusion Studies - Aqueous Fluid Inclusions

Clair 206/7A-2 – Quartz vein Microthermometric studies of fluid inclusions provide salinity & minimum trapping temperatures (TH) of the trapped fluids in crystals (qtz, calcite)

Calcite-hosted fluids (low to moderate T <150°C) Quartz-hosted fluid in basement (high T ~215°C)

Consistent with evidence of high T fluid pulses in Clair Group reported by Baron et al. (2008)

Plane polarised light

Plane polarised light

Two phase aqueous FI liquid and vapour (L>V)

L V

Cross-cutting trails

Hydrocarbon-Bearing Fluid Inclusions (HCFI) Clair 206/8-16 – Calcite vein

HCFI observed in calcite veins show yellow-green fluorescence colour indicating an API Gravity of ~25-35

Vapour bubble volume fraction: ~15% and TH :~ 90-120 °C ~ similar to North America black oil to North America volatile oil high CO2

Assuming hydrostatic conditions, aqueous + oil trapping pressures in calcite (using isochores) range from 2.6km to 3.4km, but could be shallower if overpressuring events occurred

After Bourdet (2008)

Plane polarised light

Combined plane polarised and ultraviolet light

Two phase hydrocarbon-bearing liquid and vapour (L>V)

L

V

Yellow-green fluorescence

What drove fluid migration?

..whilst widespread development of dilational pull aparts suggests that fluid movement is fault driven

Local development of ‘explosion microbreccias’ & occurrence of injected sedimentary fills suggests transient overpressuring episodes…

Red scale bars = 500mm

Microcrystalline silica fault rocks associated with larger shear fractures = source of injected cockade microbreccias

Evidence for seismogenically-driven fluid flow?

Implies a tectonically-driven cyclic

overpressuring mechanism

All cores 100mm wide

Blue scale bar = 100mm

Red scale bars = 500mm

Earthquake Hydrology

It is well known that major hydrological changes follow earthquakes

Associated with a whole range of large-scale geological phenomena such as: • Liquefaction • Formation of new springs • Increased stream discharge • Change in groundwater levels

Not well understood…attributed to STATIC & DYNAMIC strain effects…but…. Canterbury Earthquakes 2010-11 & Meizhou County,

Guandong, China, day after Boxing Day 2009

Relationship to regional (seismogenic) rifting ?

Geochronological evidence for hydrothermal & hydrocarbon fluid migration events in Rona Ridge/eastern FSB ca 60-94Ma (Late Cretaceous)

Strong regional evidence for active rifting at this time.

Did lateral oil migration into Rona Ridge & Clair occur via near-surface active seismogenic fault systems?

Trice 2014

Fluids, sediment, oil sucked in Injection, migration

Muir-Wood & King 1993 ‘Seismic pumping’

Open dilatant fractures associated with near-surface faults Analogue modelling

Holland et al. 2011

Van Gent et al. 2010

Theory: Mohr envelopes

Tensile

Hybrid

Shear

4D imaging & analysis

Note deep penetration & connectivity of open

fracture cavities

Onshore analogue, Calabria

Tyrrhenian Basin

Ionian Basin

Back Arc Basin

N

Montenat et. al., 1991

Conceptual model

Submarine fault scarp + stepping basement benches + prolific sediment ingress

Tonalite basement formed in old subduction-related arc-basement complex

Cut by arrays of sediment-filled fractures, related to Messinian (L Miocene)-age basin development

110m

Messinian Unconformity

3D Exposure at coastal and cliff top level

Tonalite Basement: 5-50% Sediment filled fractures volume overall

Copanello

Straits of Messina

1

2

3

4

b

a

e

d

1. Early shearing forms cataclasite & pseudotachylyte (a), prior to exhumation

2. Passive sediment infilling of fracture cavities (b,c) during FW/HW collapse, fractures cut early basin fills

3. Reactivation of basin-bounding normal faults with injection of sediment slurries (d).

4. Continued FW collapse and passive infilling (e), prior to deposition of Messinian basin fill

5. Note ‘missing’ basin fill

c

Fill, fault, inject; repeat Modified after Montenat et. al., 1991

Laminated sediment

Injected slurry

Laminated sediment

Laminated sediment

Breccia fill

Friction melts

Cracked & full of sand Upper crustal fracturing in

low permeability host rocks

Surface Tensile open +

sediment fills

Hybrid ‘leaky’

dilational jogs

Shear –injection

veins

Not to scale

Precambrian age & affinities of Rona Ridge basement established

Upper Cretaceous basement oil charge related directly to near-surface system of open fractures that also host

contemporaneous qtz-adularia-calcite-pyrite hydrothermal mineralisation

These (and ONLY these) fractures consistently show: – Primary sedimentary fills & injected slurries; – Zoned cockade textures in veins and fills;

– Vuggy cavities

A long-lived, highly permeable system of open fractures that persist to the present day – NO late reactivation required!

Good onshore analogues found below unconformities capping crystalline rocks

Evidence that seismogenic faulting drove fluid flow

Do seismogenic basement fractures act like a beating heart driving oil migration during rifting? (based experimental sample in van Gent et al. 2009)