Proximal Submarine Fan Reservoir Architecture and Development in the Upper Jurassic Brae Formation of the Brae Fields, South Viking Graben, U.K. North Sea

Abstract

Abstract Upper Jurassic proximal submarine fan deposits form the reservoirs of three large oil and gas condensate fields in Block 16/7a at the western margin of the U.K. South Viking Graben. High-relief (up to 1670 ft [∼510 m]) hydrocarbon columns are trapped by a combination of abutment against the graben margin fault system to the west, basinward slope away from the graben margin to the east, and lateral stratigraphic trapping against interfan fine-grained sediments. Gravel and sand were supplied to the fans by noncohesive debris flows and high-density turbidity currents down an eroded fault scarp from the platform area of the Fladen Ground Spur to the west. The South Brae and North Brae fields have pronounced conglomeratic channel systems in the upper part of the Brae Formation, which pass down-dip into thick, laterally extensive (up to 25 km [16 mi]) basin-floor sandstone fans that host three other large fields, Miller, Kingfisher, and East Brae. The Central Brae field is contained within a more cone-shaped fan, with thick conglomeratic deposits in the most proximal area that grade downdip into a thick sandstone package of relatively limited basinward extent (approximately 7 km [4 mi] downslope). The fan systems of the Brae Formation are contained within the Kimmeridge Clay Formation, which is a world-class source rock and also provides the top seal for the fields. Numerous exploration, appraisal, and development wells, supported by seismic imaging and reservoir pressure and production data, allow the architecture of the proximal fan systems to be established. The best developed channel systems at South Brae, which occur in the upper part of the reservoir, are approximately 1 km (0.6 mi) wide and are separated by thick units of interchannel mudstones and thin sandstones. These channels widen and become unconfined downdip where they merge into the basin-floor fan systems. Channel-fill sequences, which can total around 300 ft (91 m) in thickness, typically comprise a unit of very thick-bedded conglomerates with minor interbedded sandstones, which is overlain by a unit of thick-bedded sandstones up to 100 ft (30 m) thick. In individual South Brae reservoir layers, several channels that radiate from a single sediment source area can be found. In contrast, a single prominent channel system exists at the North Brae field, where the channel facies are similar to those at South Brae, but the total channel thickness reaches 800 ft (244 m). On the southern flank of the North Brae field, thick sandstone lobes occur that are connected to the central channel, and on the north side of the field, separate channel systems exist, which contain hydrocarbons that are not connected to the main field area. At Central Brae, laterally extensive, poorly confined channel systems were probably the depositional avenues in the upper part of the reservoir, but the bulk of the reservoir comprises a stacked sequence of downdip elongated tongues of conglomerates and thinner sandstones, which pass downslope into thick-bedded sandstones. Fan systems, which evolved through time, began to develop in the Brae area in the mid to late Oxfordian (from around 160 Ma), during the initial phase of intense rifting, but the increased organization in sediment dispersal patterns and the extension of the fans into the basin center occurred from the late Kimmeridgian (about 152 Ma) as rift extension ceased. Coarse clastic deposition during the later phases of the fans was periodically interrupted by deposition of mudstones, probably as a result of episodes of high relative sea level temporarily flooding the source areas. Central Brae was the first fan to be abandoned, followed by the South Brae system, and finally the North Brae system in the earliest late Volgian (around 144 Ma), when the Fladen Ground Spur was finally transgressed. Fan abandonment appears to have been relatively rapid as good quality turbidite sandstones occur very close to the top of the final depositional systems in both the basin-floor and proximal locations.