Possible effects of glacial events on hydrocarbon accumulations

ErosionPressure loss, adjustment to hydrostatic orunderpressure Temperature loss

Transient effects: Pressure gradients, difference in contacts across subtle barriersHigh temperature gradient

Gascap expansion and gas exsolution

1000

2000

3000

1 20

Volumøkning av gassekspansjon, 1500 m erosjon

Dyp i meter

Barents Sea: Cooling commonly results in net leakage of gas from traps

Residual oil

Ice cover and cold periods

Low temperaturePossible permafrost and gas hydratesPossible pressure build-upPossible stress changeTransient low temperature

Hydrostatic pressure increasePossible gas hydratesPossible pressure build-upTransient slightly lower temperature ?

Vol gas = constant

Vol oil = constant

Tilting of oil – gas accumulation

Residual oil in water

When the field is tilted, the gas and oil will adjust to the new trap volume. If there is no migration, leakage or pressure change, the volume is preserved. When oil is swept by gas or water, some residual oil is left. Gas sweeps oil more efficiently than water, so oil shows in the gas zone are expected to be weaker than in the water.The paleo-contact reveals the position of the original contact. However, the paleocontact is not necessarily of the same age everywhere.

W E

Residual oil in gas

Sketch profile of Hammerfest Basin Fields

Snøhvit Askeladd Albatross Goliat

1500

2500

2000

1000

Fast migration and spill of hydrocarbons:

The impact of Plio-Pleistocene history on oil zones and residual oil in the Troll Field

Janka RomWenche Tjelta JohansenTrond Brekke (Brekke Chemo) Fridtjof Riis

Stavanger

Bergen

Troll field

35x55 km,

Located in the glacially eroded Norwegian channel (Norskerenna),

50 km from the coast

Belongs to the rim zone of Scandinavia which was affected by Cenozoic uplift

20 km

500 ms

Troll

migration

tilting

erosion

Viking Graben

Troll: Tectonic setting and migration routes

1km

2km

GNSR-119A

Northern connection Middle connection

Southern connection

TROLL EASTGas, 0 – 6 m oil

TROLL WESTGas province, 13-14 m oil

TROLL WESTOil province, 22-27 m oil

Pr/n-C17

9.0 (oil)

4.64.93.8

4.0 (Sognefj.)1.1 - 2.7

5.3

0.9 - 1.7

2.2 Sognefjd. and oil)

1.5 (Water zone)

19.0 (oil)2.5

1.5 (water zone)

1.9 (oil)

0.7 (Sognefjd.)

2.0 (oil)

1.10- 1.8

3.5 (Sognefjd.)2.3 (oil)

5.5 – 14.4 (Water zone

%CO2 in gas

2.3

1.7 0.6

1.2

2.0

0.3 – 0.6

1.2

1.7 – 3.0 0.50.3

0.3 –0.4

0.1 – 0.3

0.1

0.5 – 1.1

Biodegradation

Oil and residual oil: strong biodegradation in the W, weak in the east

Temperature from DSTBiodegradation is observed in shallow North Sea fields, limited to approx 1700 m below sea floor

Less biodegradation in Troll East than Troll west – supports a deeper initial burial of Troll East

Troll Paleo oil Well observations: thickness of residual oil below oil zone

,

Residual oil and paleo contacts in wells EOM data from Horstad et al.

0 50 mg/g EOM

1600

1550

?

?

4D seismic effect (1991-2005)

Low water saturations - orange colours.Residual oil zone - purple colour.

W E

Smoothed time thickness map of the difference between the black and the red reflector. Reflects thickness of paleo oil zone

Water saturation profile

Troll Paleo oil

STOIIP in paleo oil zone, assuming average Sw = 0,2

• TWOP: 125 mill Sm3

• Total: 423 mill Sm3

Map of Troll showing depth of conceptual paleo oil water contact

Vol gas = constant

Vol oil = constant

Tilting of oil – gas accumulation

Residual oil in water

When the field is tilted, the gas and oil will adjust to the new trap volume. If there is no migration, leakage or pressure change, the volume is preserved. When oil is swept by gas or water, some residual oil is left. Gas sweeps oil more efficiently than water, so oil shows in the gas zone are expected to be weaker than in the water.The paleo-contact reveals the position of the original contact. However, the paleocontact is not necessarily of the same age everywhere.

W E

Residual oil in gas

Top Sognefjordpresent60 m120 m

180 m250 m300 m500 m

Tilting of Troll

The animation starts with present day and geometrically tilts the structure back to 500 m tilt.(Mid Miocene?)

Reference level (grey horisontal surface) is based on the spill in the northern connection

Holocene tiltIsobases at the end of the Weichselian glaciation

20 m contours

Submerged beach

Younger Dryas isobases

Glaciation – tilting combined with pressure effects

100 m lowering of eustatic sea level:

10 bar pressure reductionTroll example: 7 % gas cap expansion4-5 m deeper gas oil contact

Ice stream in Norwegian channel 300 m higher than present sea level:

28 bar pressure increaseTroll example: 15 % gas cap contraction10 m shallower gas oil contact

Eustatic fallGlacial load

Glaciation effectson contact movements

40 m tilt

Eustatic fall

4-5 m downMax 25 m down

Glacial load

10 m upMax 10 m down

WEST

Eustatic fall

4-5 m downMax 15 m up

Glacial load

10 m upMax 30 m up

EAST

Timing of events

0

100

200

300

400

500

600

700

800

024681012

Ma

Tilt

re

lati

ve

to

pre

se

nt

da

y

800 m tilt Pliocene tilt 0,25 mm/year 500 m tilt 500m constant rate

Examples of possible uplift curves

Between 1,8 and 7,2 Ma

Between 1,2 and 4,4 Ma

First spill into Troll East

Between 180 and 300 m tilt

180 m300 m

• The paleo oil water contact in Troll was formed in the Neogene, probably prior to the Pleistocene and later than 6 Ma.

• At this time the field was tilted between 180 m and 300 m, and the gas volume was less than 60 % of the present volume. The corresponding rate of gas migration is in the order of 0,2 Mm3/year

• Glacial effects are not big enough to explain the paleo oil water contact

• In the western part of Troll, the oil zone may have been 20 – 25 m deeper in the Weichselian

• Oil shows in the gas zone mainly occur in the eastern part of Troll West, and were most likely formed during the Neogene tilt. A further study of these oil shows could give information about the thickness of the ice stream.

Conclusions