Abstract

Synrift to early postrift Upper Jurassic submarine fan sequences form the reservoirs of numerous large oil and gas condensate fields in the South Viking Graben. The largest of these fields are in the Brae area, on the western side of the graben. Here, proximal conglomerate and sandstone facies of the Brae Formation host the South Brae, Central Brae, and North Brae fields, each within its own discrete submarine fan unit. More distal, basin-floor sandstone facies derived from the later episodes of South Brae and North Brae fan activity host the Miller, Kingfisher, and East Brae fields. Interfan areas comprise thick sequences of finegrained sediments, which provide very significant lateral stratigraphic trapping elements for all the fields. An extensive well and seismic data set now allows a more detailed tectonostratigraphic evaluation of the Jurassic reservoir sequences in the context of the development of the graben and footwall than was previously possible.

The submarine fans resulted from the uplift of the Fladen Ground Spur footwall to the west, with the consequent erosion and redeposition into the graben of very large volumes of gravel, sand, and mud. A prerift sequence of the Bathonian alluvial to paralic Sleipner Formation, which culminated with deposition of an extensive coal unit, extends across the graben and was probably also deposited on the footwall. Late Jurassic rifting began in the early Callovian, with deposition of the Hugin Formation in a shallow marine setting, with sand and mud supplied from the low-relief platform area to the west. Episodes of abrupt but slight deepening of the basin, caused by initial fault movements at the graben boundary, are suggested by numerous sharp-based coarsening-upward sequences within this formation. Following a period of apparent quiescence, when the Fladen Ground Spur may have been flooded, the main rift phase began in the late Oxfordian when subsidence of the graben margin and uplift of the footwall resulted in a deep marine trough and subaerial relief on the footwall probably totaling several thousand feet (hundreds of meters). Early submarine fan systems are likely to have been relatively unorganized cones of conglomerate and sandstone deposited from noncohesive debris flows and high-density turbidity currents. Fan systems became more organized upward as accommodation space close to the graben margin was filled following the climax of rifting in the late Kimmeridgian, and two large proximal to basin-floor fan systems developed at South Brae and North Brae, with conglomeratic channels in the proximal areas and sheetlike sandstone units on the basin floor. In the later stages of Brae Formation deposition, the top of the footwall is likely to have been close to sea level, which allowed periodic flooding of the source area and deposition of regionally extensive, relatively thin mudstone units across the fans, which act as internal reservoir baffles within fields. At the peak of fan deposition, during the early Volgian, the three main fan systems in the area (the South, Central, and North Brae fans) plus several smaller fans were all active. However, fans became inactive sequentially, with deposition first on the Central Brae, then on the South Brae, and finally on the North Brae fans ceasing relatively abruptly as the Fladen Ground Spur was progressively transgressed. Deposition of mudstones of the Kimmeridge Clay Formation, which are the hydrocarbon source rocks and the top seals for the fields and with which the Brae Formation interdigitates, continued after fan deposition ceased, into the earliest Cretaceous.

The current sub-Upper Jurassic basement rock types of the footwall in the immediate area of the Brae fields comprise well-lithified Devonian sandstones and a significant but minor area of Silurian granite. However, the origin of the coarse clastic detritus, particularly the sands, within the Upper Jurassic fan systems was not simply a result of erosion of these rock types. Regional mapping and provenance studies suggest that a considerable thickness of Middle Jurassic, Triassic, and Permian sedimentary rocks previously overlay the present-day basement rocks of the footwall. These strata were probably almost completely eroded from the area immediately west of the fields where footwall uplift is likely to have been the greatest and redeposited into the graben during the Late Jurassic.