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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Atlantic Ocean
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North Atlantic
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North Sea
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Viking Graben (5)
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Europe
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Western Europe
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Primary terms
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Atlantic Ocean
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North Atlantic
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North Sea
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Viking Graben (5)
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carbon
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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 fine-grained 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.
Reservoir Geology of the Upper Jurassic Brae Sandstone Member, Kingfisher Field, South Viking Graben, U.K. North Sea
ABSTRACT The Kingfisher field is located in Blocks 16/8a and 16/8d in the South Viking Graben 278 km (173 mi) northeast of Aberdeen. The field was discovered in 1972 by Shell/Esso well 16/8-1, which targeted a faulted anticlinal structure and encountered thin-bedded and poor quality reservoir sandstones within a dip closure mapped at the Base Cretaceous unconformity (BCU) level. The field was first appraised in 1984 by well 16/8a-4, which encountered a better quality Brae sandstone member reservoir interval to the northwest of the discovery well. Further appraisal wells 16/8a-8, 16/8a-9, and 16/8a-9z confirmed the presence of good quality reservoir sands across the central and western parts of the field. The exploration and appraisal drilling established the fluid contacts in the field and also critically helped to delineate the transition from good quality axial to poorer quality distal facies within the outer part of the Brae submarine fan system. Furthermore, data from development well drilling and reservoir behavior during production of the field have provided insights into the static and dynamic connectivity within the Brae sandstone member reservoirs as well as provided additional insights into the controls on reservoir quality and productivity. This chapter aims to build on the previous published information on the field by providing further details on the geological characteristics of the Brae sandstone member reservoirs within the Kingfisher field and how the reservoir architecture and properties, in addition to aquifer connectivity, have determined well and reservoir production performance and behavior.
ABSTRACT Anticlines along the western margin of the South Viking Graben in the Brae area of the U.K. North Sea form the structural components of large structural and stratigraphic traps within Upper Jurassic Brae Formation submarine fan deposits. Various interpretations of the origin of these anticlines and their attendant inboard synclines at the graben boundary have been previously published, with both gravity-driven processes and inversion being invoked. Based on regional interpretation of 3-D seismic data sets and analysis of thickness variations in uppermost Jurassic and Cretaceous sequences in numerous wells, it is concluded that gravity-driven processes were more important than inversion. Differential compaction of mudstone-rich slope deposits laterally adjacent to coarse clastic submarine fan reservoirs has resulted in the field reservoirs now being at slightly higher elevations than the finer grained deposits along the length of the anticlines. Compaction of the very thick sandstone- and mudstone-dominated successions in the basin center has also been greater than that of the more conglomeratic successions adjacent to the basin margin, where sequences are underpinned by the slope of the footwall, resulting in over-steepened slopes toward the basin on the outboard side of the anticlines. Movement of Permian salt that underlies the Jurassic (and Triassic) in the basin has also had significant broad effects on the Upper Jurassic structures, creating depressions and underpinning some anticlines. Continued slow subsidence of the basin-fill down the main graben boundary fault system in the under-filled rift during the latest Jurassic and Early Cretaceous, above changes in footwall slope (from eroded slope to graben-boundary fault, or, in the case of East Brae, across a plunging basement nose) is considered to be the primary cause of the anticlines and their inboard synclines. Reversal of movement along the main boundary fault, causing inversion of the graben-margin sequences, is considered unlikely as the primary mechanism for anticline formation. Additional movement down the graben-boundary fault system in the early Maastrichtian may have slightly tightened the anticlines. Final minor fault movement along the graben margin occurred in the mid Eocene, but this is unlikely to have significantly affected the Brae structures. Some of the anticlines provide evidence of the presence of the underlying thick reservoir sequences (due to differential compaction over conglomeratic sections), but not all positive structural features contain coarse clastic sediments.
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 downdip 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.