SEDIMENTOLOGY OF THE BRAE OILFIELD, NORTH SEA: FAN MODELS AND CONTROLS

Abstract

Submarine fans are important to the petroleum industry for their reservoir and source‐rock potential. There are a variety of fan types that can be characterized by different fan models. In assessing which model, if any, is appropriate for a particular area or field it is important to evaluate the controls that operate to determine the type of fan developed. Most present‐day fans that have been studied are deep‐water oceanic; more work is required on modern examples of shallow‐water, small‐basin fans which are particularly relevant to hydrocarbon exploration.Jurassic and Tertiary objectives are important in the North Sea and include both nearshore‐sandstone and submarine fan reservoirs. The Brae oilfield is an Upper Jurassic reservoir on the western faulted margin of the North Sea Central rift system. Detailed core, electric log, dipmeter and seismic data have been examined. The Brae field comprises at least three small overlapping submarine fans that form a sediment apron along the scarp margin and that were deposited by a variety of gravity‐flow processes in a shallow basin below wave base. Tectonic control on fan development has resulted in up to six fining upwards megasequences (50‐150m) within the overall fining upwards basin fill (300–600 m). There has been a large intermittent supply of coarse detritus across a narrow littoral zone and down a steep slope. Short, sedimentladen rivers supplied mud and plant débris, and a complex interdigitation of coarse‐and fine‐grained facies has resulted.The Brae fans are an important example of the fault‐controlled, shallow, small‐basin type and are most closely analogous to the Jurassic fans of East Greenland. Reservoir characteristics are dependent on diagenesis, facies and fault distribution, and vary markedly along the fault margin. A clear understanding of the controls and models is essential for continued exploration and development in this part of the North Sea.