Oseberg oil field

Oseberg is an offshore oil field with a gas cap in the North Sea located 140 km northwest of the city of Bergen on the southwestern coast of Norway.[1] The field, which is 25 km long by 7 km wide, was discovered in 1979 and its development is known to be one of the significant milestones in emergence of Norway's independent oil and gas industry.[2][3] The Oseberg field was named after Oseberg ship, one of Norway's most significant archeological discoveries. The ancient Viking ship from the early 9th century was discovered in a 1904 historical excavation of a burial mound at the Oseberg Farm, south of Oslo.[4]

An extensive subsea pipeline network called Oseberg Transport System (OTS) transports up to 765000 oilbbl/d from the Oseberg area to Sture terminal in Norway.[5] The sea depth in the Oseberg area is 100 meters.

Geology

Regional setting

The Oseberg field is one of a series of hydrocarbon accumulations within the Viking Graben. This part of the northern North Sea has been affected by two main rift episodes; in the Permo-Triassic and the mid-late Jurassic. This leads to complex variations of the earlier syn-rift sequence thickness across the later rift structures. In the Oseberg area, the major faults formed during these two phases dip in opposite directions and the size of the earlier basins are typically double the size of the later. The east-tilted Late Jurassic Oseberg structure is underlain by a larger-scale west-tilted Permo-Triassic half-graben.[13]

Reservoirs

The main reservoirs are sandstones of the Middle Jurassic Oseberg, Rannoch, Etive, Ness and Tarbert formations of the Brent Group. These are the deposits of prograding delta systems, with an overall shallowing upwards as the delta filled the available accommodation space. The Oseberg formation represents the deposits of a series of sand-rich deltas that advanced westwards from the Norwegian coastline. The rest of the group are the deposits of the massive northward prograding Brent delta system, which occupied the entire southern part of the Viking graben area.[14] There is a progressive transition in the depositional environment from subaqueous fans, through fan deltas and shoreface to delta plain.[15] The uppermost Tarbert formation may represent reworked delta plain deposits at the onset of the marine transgression shown by the overlying claystones of the Viking group.[16] The Brent Group reservoirs, which lie at depths between 2300 and 2700 m, generally have excellent properties with porosities around 20–30% and permeabilities ranging up to several darcys.[17]

The Statfjord formation, which is of Ryazanian to Sinemurian age is separated from the Brent Group by the Dunlin Group, which is a non-reservoir sequence. This means that there is normally no communication between the Statfjord formation and the Brent Group reservoirs. The Statfjord formation consists of massive sandstones. The lower boundary is taken at the transition from red continental shales of the Lunde formation to the overlying sandstones. The top of the unit, which consists of calcareous sandstones, shows a relatively sharp transition to the dark shales and siltstones of the overlying Dunlin group. The depositional environment changes from alluvial plain and braided stream deposits that make up the bulk of the formation to coarse sandstones with pebble lags and channeling, thought to indicate a coastal environment. The presence of glauconite and marine fossils in the uppermost sandstones indicate deposition in a shallow marine environment.[18]

Structure

The trap is a group of three easterly dipping tilted fault blocks associated with westerly dipping normal faults formed by rifting during the Late Jurassic on the east side of the Viking Graben. The three main fault blocks are known as Alpha, Gamma and Alpha North. The eastern boundary of the Oseberg fault blocks with the Horda Platform is formed by the major Brage fault.[17] The structural dip is an average 6–10 degrees.[7]

Seal

The top seal is formed by Upper Jurassic Viking group or Lower Cretaceous Cromer Knoll claystones.[19] There is also evidence in the Oseberg area for some element of fault seal.[20]

Sources

The oil in Oseberg is sourced from the Upper Jurassic Draupne formation, equivalent to the Kimmeridge Clay formation. Oils generated in at least two different 'kitchens' have been identified within the Oseberg area. The gas is likely to be sourced from over-mature Draupne formation in the deepest part of these kitchens.[19]

location map	North Sea
coordinates	60.49186°N, 2.82731°W
relief	yes
country	Norway
offonshore	Offshore
operators	Equinor
discovery	1979
startofproduction	1988
oil production m3/d	14121
producingformations	Upper Triassic to Lower Jurassic Statfjord formation; Middle Jurassic, Oseberg, Rannoch, Etive, Ness and Tarbert formations

Exploration history

Development history

Seismic data acquisition

Production

Geology

Regional setting

Reservoirs

Structure

Seal

Sources

Reserves

See also

External links

References