The Hegre Group – a complex stratigraphic interval: Example ......The Hegre Group – a complex...

The Hegre Group – a complex stratigraphic interval: Example from the Greater Sleipner AreaAnna Kulikova, Andreas Rittersbacher, Silvan Hoth, James Elgenes, Signe Ottesen and Annika Therese Ulfsdotter Heggheim

20 June 20Classification: Open

© Statoil ASA

Stratigraphy challenge

20 June 20172 Classification: Open © Statoil ASA

Salt tectonics

Permian fault blocks (Signe Ottesen, 2001) Top Zechstein -Top Sleipner Fm thickness map

15 km

15 km


Objectives of the study• Enhance the understanding of the deposition of a Triassic reservoir in

the Sleipner Area

• Enhance the decision making for potential Triassic exploration projects

Location and stratigraphy of the Sleipner Area


Facies from core: Proximal-medial part of a fluvial system

Cross bedding

Rip-up clasts

Erosional bases

Amalgamated channel fill deposits and paleosols

Paleosols with caliche

Cross bedding

Channel fill deposits


Facies from core: Distal alluvial plain

• A lot of mica –ponding, low energy

Terminal splay/ lake deltaAlluvial plain / ephemeral lake

• Gradationalboundaries

• Unconfined flow

Similar to examples of Smith Bank in UK sector (McKie, 2014)

• Parallel lamination

• Wave ripple & horizontallamination

• Roots, burrows

• Mudcracks and rip-up clasts

• Quiet setting


Facies modelDistal dry alluvial plain, terminal splays, lakes

• Unconfined flows from terminal splays

• Standing bodies of water - ephemeral lakes / ponds

• Aeolian?

Proximal to medial part of a fluvial system

• Channelized flow, maximum reworking

• Amalgamated sandstones, floodplain is

reworked and transported downstream

Cain & Mountney, 2009


Transport direction

Likely towards East / Southeast

Possible source from the Utsira High

Paleogeography is taken from McKie (2012) Figure 19a. Sediment dispersal is taken from McKie and Williams (2009). Active faults are taken from the Millennium Atlas (2003)


Intra Triassic lithological packages

• Heterogeneous

• Sandy

• Transitional

• Fine-grained (Smith Bank)

• Lack of biostratigraphic data

• Lithostratigraphic correlation


Combinations that occur at Sleipner Eastrare

Uncertainand rare

GRP1_3

P1_2

P1a/P1 P1

P1a

P1_3

P1_2

P1

P1a

P1_2

P1

P1a P1a

• If succession of identified packages is depositional and correct:

• Progradation of fluvial system has occurred

• Erosion could be an important factor in preservation of sandstones of the upper packages

• As packages are similar and thick –the fluvial system covered a large area and was flowing without restrictions.

rare

• Heterogeneous

• Sandy

• Transitional

• Fine-grained (Smith Bank)


Accommodation vs Sediment supply

Sandy amalagmated package

Transitional package

Fine grained package

• Progradation of the fluvial system• Decrease in subsidence rate or

increase in sediment supply or a combination of both

• NOTE that lateral change in facies could occur due to proximal to distal variations

Banham & Mountney, 2013


Evidence of erosion at the top of Triassic succession• Assuming that succession of lithological packages

is depositional and correct

• Different uppermost packages are missing in a number of the wells

• Unconformities have been observed at the top Triassic from dipmeter data

• Unconformities are also observed on seismic data

mis

sing

Dip

met

er d

ata

mis

sing

Dip

met

erda

ta

• Heterogeneous

• Sandy

• Transitional

• Fine-grained


1

Correlation and seismic section lines

5 km


trans

het/sandy

fine

trans

Well Correlation Line 1

On flanks ofan elevated pod

On top of the pod

1 2

3

• Sandy/heterogeneous

• Transitional

• Fine-grainedfines

4 5 6


Both wells drilled on flanks encountered

Skagerrak sandstones

Smith Bank

Older Skagerrakinterval is preserved

Jurassic Wedge, Active faultingLittle effect of

Jurassic erosion

Seismic section Line 1

BCUSkaggerakBase CoalSmith Bank (modelled)

• Sandy Skagerrak package is missing on top of the mini-basins and present on the flanks

• Faulting played a role as well

Skagerrak

2.5 km

1

2 34 6

No Skagerrak

Partly preservedSkagerraksuccession


Salt sediment interaction model

Sleipner Field example

2 3 4

SkagerrakSkagerrak

SALT

Smith Bank

1. Smith Bank – increased thickness in salt mini-basins2. Deposition of Skagerrak across the area with potential

depocentres or differential preservation above salt crestal collapse structures

3. Aalenian unconformity (Jurassic erosion) – partly or fully removes Skagerrak sandstone package

6 or 11

4 326 or 11

Model modified from McKie, 2014


Conclusions for the Sleipner Area

• Skagerrak fluvial system was active across the whole area and progradedtowards SE during formation of crestal collapse structures

• Erosion removed parts of Skagerrak deposits from tops of the inverted salt mini-basins

• Skagerrak sandstone is mainly preserved on the flanks of the inverted salt mini-basins and in the depocentres between the mini-basins, and partly on the top of the mini-basins

• Salt - sediment interaction models by McKie (2012, 2014) allow for the welland seismic observations made in the Sleipner Area

• Correlation of lithostratigraphic packages in absence of biostratigraphic data seems to be plausible, but uncertainty on the chronostratigraphy remains

• Hegre Gp is complex stratigraphic interval affected by a combination offluvial system dynamics, salt movements and Jurassic erosion


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Presentation title

The Hegre Group – a complex stratigraphic interval: Example from the Greater Sleipner Area

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The Hegre Group – a complex stratigraphic interval: Example from the Greater Sleipner AreaStratigraphy challengeSalt tectonicsObjectives of the studyFacies from core: Proximal-medial part of a fluvial systemFacies from core: Distal alluvial plainFacies modelTransport directionIntra Triassic lithological packagesCombinations that occur at Sleipner EastAccommodation vs Sediment supplyEvidence of erosion at the top of Triassic successionCorrelation and seismic section linesSlide Number 14Slide Number 15Salt sediment interaction modelConclusions for the Sleipner AreaSlide Number 21

The Hegre Group – a complex stratigraphic interval: Example ......The Hegre Group – a complex...

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