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Regional Geology
The Gullfaks Oil Field located on the Gullfaks Fault Block that is part of the Western Flank of the Viking Graben which makes up the North Sea Rift System. The North Sea Rift System began to form during the Early Triassic when the Norwegian Greenland Sea Rift extended and started to rift in the North Sea area where the Viking Graben and Central Graben formed. The Viking Graben was formed during the Early Jurassic with the initiation of the North Sea Rift System. During the Late Jurassic, the extensions began to accelerate which subsided and submerged the North Sea area completely, forming a seaway from the North Atlantic to Southern England. Further Subsidence after the initial rifting was caused by Thermal Subsidence which is caused by loading of sediments at depths and the cooling of the crust post-rift phase. The Increased density leads to isostatic subsidence. The accelerated rifting peaked during the start of the Cretaceous. This caused rotational fault blocks around the Viking Graben to form. Syn-rift Cretaceous pelagic shales filled the new accommodation space that opened by the rifting. During the Late Cretaceous, the North Sea Rift System began to slow down in its rifting. While the Faeroe-Norweigan-Greenland Sea Rift System continued to rift and spread until Greenland finally broke away from Europe at the transition from the Paleocene to the Early Eocene. During which the North Sea Rift System became tectonically quiescent.

Stratigraphy
The base of the Gullfaks Oil Field is the Triassic Hegre Group which overlies a crystalline basement. The Hegre Group is about 1340 m thick. The Hegre Group is made up of the Lomvi/Teist Formation at the base and Lunde Formation at the top. The Hegre Group consists of medium-grained fluvial sandstones. The Lunde Formation is a moderate quality reservoir with low reservoir potential. The Statfjord Formation overlies the Hegre Group and is 180-200 m thick. This is a generally good to excellent reservoir formation. The members that make up the Statfjord Formation is the Raude, Eiriksson, and Nansen members. Raude Member is divided into three reservoir units. Raude 1 is the base unit of the Raude Member and of the Statfjord Formation. Raude 1 is a poor-quality reservoir that is made up of clay to silt grained material that coarsens upward. Raude 2 and 3 are very similar medium quality reservoirs, due to kaolinite crystallization in its pores. Raude 2 and 3 are made up of medium grained sands that were deposited in a dry setting .The Eriksson Member is also divided into three reservoir units though can be summarized as being a good to excellent reservoir that is consisted of coarse-grained meandering channel sandstones with a porosity of 28%. The Nansen Member has a reservoir quality of moderate to good that is consisted of well sorted sandstones from a beach or shoreline type environment.

Dunlin Group contains the Amundsen, Burton, Cook, and Drake formations. The Amundsen and Burton formations are made up of mudstones which are deposited during a marine environment. Amundsen and Burton formations are about 170-180 m thick. The Cook Formation is 70-120 m thick in the Gullfaks Oil Field. The Cook Formation is deposited in an regressive tidal/fluvial dominated deltaic environment.The sands becomes less interbedded further up in the formation and becomes cross stratified with planar laminated sandstones above it. The indicates that the environment is a wave dominated estuary on the upper portion of the Cook Formation. It has a poor to excellent quality reservoir that is divided into three reservoirs. Cook 1 is a mudstone with no reservoir potential. Cook 2 and Cook 3 are muddy sandstones with interbedded sand and shales with sandier sections upwards in the formation. Cook 2 though has some frequent layers of calcite-cemented sandstones which has a low porosity, ~2-8%. Thus, making the upper portions of the Cook Formation an excellent reservoir. The Drake Formation is made up of shales deposited in a marine environment and is about 75-120 m thick.

The Brent Group is made up of the Broom, Rannoch, Etive, Ness, and Tarbert formations. Each member having their own thickness ranges due to the Brent group being two separate delta systems. The Lower Brent Delta Lobe that formed the Broom, Rannoch, Etive, and Ness formations is a wave/fluvial dominated delta. While the Upper Brent Delta Lobe formed the Tarbert Formation and it is a tide/fluvial dominated delta. Broom Formation is 8-12 m thick consisting of shale or mudstone with thin layers of calcite cemented coarse grained sandstone and gravel. This formation is regarded to as the prodelta part of the delta lobe. Rannoch Formation is 50-90 m thick and is divided up into three units. The formation has similarities throughout the subunits, namely having hummocky cross bedding with small scale ripples throughout the formation thus interpreted as the shoreface part of the delta. Rannoch 1 is a coarsening upwards calcite cemented lenticular bedding. Rannoch 2 is a fine-grained sandstone and Rannoch 3 is medium-grained sandstone. There is calcite cemented beds scattered throughout Rannoch 2 and 3 though the reservoir quality is regarded to being good to excellent. Etive Formation is 14-40 m thick and is consistent of medium to coarse grained sandstones that was created by barrier bars which is evident by low-angled large-scale cross-stratification and grain size. The reservoir quality is excellent with porosities at 30-36% and permeability at 1000-4000 mD. Ness Formation is 85-110 m thick and is interpreted to be a delta plains type environment due to the start of coalbeds appearing in this formation. Ness Formation is divided into three units. Ness 1 is made up of interbedded coal beds with mud and sand stones which comes from the flooding when distributaries overflow evident by occasionally thick sandstone layers from channels. Ness 2 is coarsening upward sandy sequences that are deposited by crevasse splays, crevasse channel, and overbank flooding. The sandy sequences have good reservoir quality. Ness 3 is made up of mudstones and coal beds with occasional channel sands and lacustrine deposits. Tarbert Formation is 75-105 m thick, and is divided up into two units, lower and upper units. The lower unit is consistent with a tidal and shoreface environment where it is made up of shales, siltstone, coal beds, and medium-coarse grained sandstones that is often calcite cemented. The upper unit is consistent with an estuarine and sand bars which means that the reservoir quality is excellent for this unit. This unit is also the top of the stratigraphy, other than the upper cretaceous shale which seals the Gullfaks structure ,

The Viking group is typically eroded away on structural highs such as the Gullfaks Structure. The Viking group is made up of the Heather and Draupne (Kimmberidge Clay) formations and was deposited during the Middle-Late Jurassic. The Heather Formation is 100 m thick and comprises of shales. The Draupne Formation is 200-400 m thick in the Viking Graben and is consisted of marine shales.

The Cretaceous Shales and marls caps the Gullfaks Structure. The Cretaceous Shales thickness ranges from 500-1000 m.

Structural Geology
The Gullfaks Field can be summarized into three main parts. The Western Domino Fault System, a heavily eroded Eastern Horst Complex, and the Transitional Accommodation Zone in between.

The Western Domino Fault System has major faults that are trending N to S with a slip direction of (090 ± 10) and a plunge of 25-30°. Between the major faults are minor faults which an come in four different orientation. The N-S striking minor faults that are sub-parallel to the main faults are related to either footwall or hangingwall collapse. If the dip is slightly steeper than it means the hangingwall collapsed, and if the dip is slightly less steep then it means the footwall collapsed. The E-W striking minor faults are signs of internal block deformation that are steeply dipping 45-90° with small throws that are mostly less than 50 m. The Diagonal minor faults are intermediate faults between the N-S and E-W striking faults. These typically stay within one fault block and have intermediate dips and throws.

The N-S minor antithetic faults are an uncommon fault type in the Gullfaks Structure. These have short throws that are <20 m but have an extremely steep to sub-vertical dips.

The Eastern Horst Complex also has main faults and minor faults. Though, minor faults are harder to map due to heavy erosion of the horst complex. The main faults are constantly N-S striking with a dip that goes either E or W at a 60-70° dip angle. The minor faults are sub-parallel to the main faults with steeply dipping angles at 45-70°.

The Transitional Accommodation Zone is a graben structure that is formed from a fold that is dipping west on its west limb and gentle to sub-horizontal east dipping east limb. The main faults in the Transitional Accommodation Zone are a continuation of the domino system with the same low angle eastern dipping faults (25-30°) with a west dipping fault that bounds the Eastern Horst complex that dips at a steeper 65°. Minor faults are typically N-S and E-W striking with a wide variety of dips.

Hydrocarbon:Sources, "Kitchens", Migration, Traps, and Reservoirs.
The main source of hydrocarbon the Draupne Formation. This formation has a HI value of 65-531 mg and a TOC of 1.7-9.6% all of which points to having very good hydrocarbon generating potential. The type of kerogen in this formation is 43-95% liptinite depending on where the sample was taken. Other sources of hydrocarbon sources are from the Heather Formation shales, marine shales of the Drake Formation, and the coals and shales of the Ness Formation.

There are three main “kitchens” for hydrocarbon maturation around the Gullfaks Field: The “Osenberg Kitchen” south, “Troll Kitchen” East, and “Møre Kitchen” North which cooks the hydrocarbon sources and releases the oil or natural gas depending on the temperature it was cooked.

The migration of hydrocarbon probably started producing and moving during the Palaeocene-Eocene. The migration paths from the three kitchens all leads to the Gullfaks Oil Field since the Gullfaks is the shallowest structure in the Tampen Spur area. The method of migration is “filling by spill only” which is hydrocarbons moving between each structural trap found around the Gullfaks Oil Field. Which eventually makes their way to the Gullfaks Structure and become trapped within Gullfak’s fault traps. This is evident by the decrease in gas and shallower hydrocarbon-water contact line up dip from the Brent Field to the Gullfaks Field.

The hydrocarbon that migrated from the "Kitchens" into the Gullfaks Structure are trapped by an erosional unconformity and fault trap combo. The Cretaceous shales caps the Gullfaks Structure and seals it off. The large amounts of faults in the Gullfaks Structure can also help trap hydrocarbon.

There are four potential reservoirs to contain trapped hydrocarbon: Statfjord Formation, Cook Formation, and Brent Group (Excluding the Broom Formation). The Statfjord Formation has generally moderate to excellent reservoir quality. The Raude Member is a noderate quality reservoir due to high amounts of kaolinite. The Eiriksson Member has good to excellent reservoir quality with porosities up 28% due to coarse grained channel sands and stacked fluvial channels. The Nansen Member is a moderate to good quality reservoir. The Cook Formation is an excellent reservoir because of little to no mud within its upper sandstones. The Brent Group is a good to excellent quality reservoir. The Rannoch Formation has good to excellent reservoir quality at the middle to upper units. Though there are scattered calcite cemented beds but not consistent between wells. South Gullfaks though there is a locally high mica concentration which reduces permeability in this formation. The Etive Formation has superior quality reservoirs due to its little to no mud sands. It has high porosities at 30-36% and permeabilities from 1000-4500 mD. The Ness Formation has good quality reservoirs at the coarsening upwards interdistributary bay fill deposits. The Tarbert Formation has excellent reservoir quality due to thick units of little to no mud sandstones that are found locally.