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.
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. There is a progressive transition in the
depositional environment from
subaqueous fans, through fan deltas and shoreface to delta plain. 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. 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. 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.
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. There is also evidence in the Oseberg area for some element of fault seal.
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. ==Reserves==