Stratigraphic Relationships Between Upper Jurassic Submarine Fan Sequences in the Brae Area, U.K. North Sea: The Implications for Reservoir Distribution

Abstract

ABSTRACT Six large oil and gas condensate fields are present within the Upper Jurassic Brae Formation in the Brae area of the North Sea. All of the reservoir rocks are either sandstones or conglomerates deposited on partially coalesced submarine fans located at the western margin of the Viking Graben. Reservoirs occur in proximal, mid and distal fan facies, and are trapped by a combination of structure and shale-out at fan margins. The stratigraphic relationships between the fields are explained. The correct identification of the fan margins is critical to the accurate mapping of field limits and 40 the identification of new plays. INTRODUCTION In the Brae area of the southern Viking Graben (U.K. Blocks 16/3,16/7 and 16/8 - Figs. 1 and 2A) there are six large oil and gas condensate reservoirs either under production (South, North and Central Brae), in the initial phase of development drilling (Miller) or awaiting development (East Brae and "East Miller"). The total recoverable reserves of these fields is around1.2 billion BBLS of oil and 3.5 TCF of gas. Five of these fields have strong stratigraphic trapping elements, and only one (East Brae) exhibits complete structural closure. All of the reservoirs are contained in the Upper Jurassic Brae Formation1, which comprises a complex interlayered sequence of conglomerate, sandstone and mudstone units that form an eastwards thinning wedge at the western boundary of this Mesozoic rift basin. Since the first published account of the South Brae Reservoir2 the debate on the interpretation of the Brae Formation has been intense. However, in recent years authors have agreed that the reservoir rocks were deposited within submarine fan complexes3,4,1,5. A recent series of papers6,7,8 has also provided many additional details on the geology of the Brae Formation from the early development phase of the three producing fields in Block 16/7a. As yet no publication has sought to place the fields together within the complex Upper Jurassic stratigraphic framework of this prolific hydrocarbon province. In this paper we aim to remedy this and to describe the relationships between the fields, indicating the stratigraphic components of their trapping mechanisms. GEOLOGICAL SETTING OF THE RESERVOIR SEQUENCES The sedimentary basin into which the coarse clastics of the Brae Formation were transported and deposited was formed during the earliest late Jurassic phase of rifting in the southern Viking Graben. Earlier extension may have taken place during the Triassic9, however the extensive paralic, coal-bearing sequences of Bajocian to Bathonian age (Sleipner Formation) that are widespread throughout the south Viking crabenlo attest to a relatively flat, stable depositional surface close to sea level during the middle Jurassic. The succeeding late Jurassic deposits represent an overall transgressive marine sequence. The lowermost, Callovian to early Oxfordian, deposits comprise shallow marine sandstones and mudstones (the Hugin Formation) and deeper marine mudstones and turbidite sandstones (the Heather Formation); these form a marked eastwards thinning wedge deposited during the initial late Jurassic phase of extension and fault block rotation. The overlying Kimmeridge Clay and Brae Formations (Fig. 31, which also form an eastwards thinning sedimentary wedge, albeit less pronounced than the underlying sequence, were deposited as water depths increased during the later stages of continued graben development.