Possible effects of glacial events on hydrocarbon accumulations Erosion Pressure loss, adjustment to hydrostatic or underpressure Temperature loss Transient effects: Pressure gradients, difference in contacts across subtle barriers High temperature gradient Gascap expansion and gas exsolution

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 temperature Possible permafrost and gas hydrates Possible pressure build-up Possible stress change Transient low temperature Hydrostatic pressure increase Possible gas hydrates Possible pressure build-up Transient 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

Fast migration and spill of hydrocarbons: The impact of Plio-Pleistocene history on oil zones and residual oil in the Troll Field Janka Rom Wenche Tjelta Johansen Trond 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 EAST Gas, 0 – 6 m oil TROLL WEST Gas province, m oil TROLL WEST Oil province, m oil

Pr/n-C (oil) (Sognefj.) Sognefjd. and oil) 1.5 (Water zone) 19.0 (oil) (water zone) 1.9 (oil) 0.7 (Sognefjd.) 2.0 (oil) (Sognefjd.) 2.3 (oil) 5.5 – 14.4 (Water zone %CO2 in gas – – – – – 1.1 Biodegradation Oil and residual oil: strong biodegradation in the W, weak in the east

Temperature from DST Biodegradation 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 mg/g EOM ? ?

4D seismic effect ( ) 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 Sm 3 Total: 423 mill Sm 3 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 Sognefjord present 60 m 120 m 180 m 250 m 300 m 500 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 tilt Isobases 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 reduction Troll example: 7 % gas cap expansion 4-5 m deeper gas oil contact Ice stream in Norwegian channel 300 m higher than present sea level: 28 bar pressure increase Troll example: 15 % gas cap contraction 10 m shallower gas oil contact Eustatic fall Glacial load

Glaciation effects on contact movements 40 m tilt Eustatic fall 4-5 m down Max 25 m down Glacial load 10 m up Max 10 m down WEST Eustatic fall 4-5 m down Max 15 m up Glacial load 10 m up Max 30 m up EAST

Timing of events 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 m 300 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