Evolution of Paleogene submarine fans of the North Sea in space and time

Abstract

Submarine fans of the Paleocene and Eocene have been a successful objective for hydrocarbon exploration in the Central and Northern North Sea since 1967. Over this period, knowledge of submarine fans has increased enormously, with fan models first appearing in 1972, seismic stratigraphy in 1977, and 3D seismic in 1981. In the present article, an attempt is made to combine sequence stratigraphy, seismic facies analysis and sedimentology to create an improved model of reservoir geometries and their variations in space and time. The Paleocene and Eocene of the UK North Sea have been subdivided into a number of sequences. Each of these starts with erosion on the shelf and fan deposition in the basin, and ends with a shale which can be correlated regionally. Four ‘second-order sequences’ ( Vail et al. 1977 ), the Andrew, Forties, Frigg/ Tay and Alba sequences, have been recognized on regional seismic data. The seismic facies characteristics of these sequences have been mapped to identify sand-prone areas of the basin. All fan sediment was derived from the northwest or the west, where an active uplift of 1–3 km of the UK mainland took place, together with a tilt to the southeast. The uplift has been related to the opening of the North Atlantic and the Thulean volcanic phase. Studies on cores, logs and seismic have demonstrated that all submarine fans in the Paleogene of the North Sea are channellized, with channels continuing near to the fan fringe. Soon after the initiation of fan deposition, the total sediment input reached a maximum. The initial channels lacked cohesive levees, due to the high sand content of the turbidites, which resulted in a rather erratic sand/shale distribution. During the later stages of fan deposition, the total sediment input decreased (because of a decreasing rate of shelf erosion) and the relative mud content increased, improving the stability of channel levees. Within these more stable channels, aggradation of massive sands could occur. These linear bodies of massive sand were turned into mounds after differential compaction, thus forming excellent stratigraphic traps. Consequently, the top of these submarine fans often displays a mounded appearance on seismic. This evolution, described for a single sequence, also occurs on a larger scale. A single Paleogene megasequence has been identified, of which the lowermost submarine fan (the Andrew Fan) is characterized by a very high sedimentation rate and very unstable channels, while the uppermost submarine fan (the Alba ‘Fan’) consists of isolated channel complexes only, now appearing as mounds of a maximum of 2 km wide, which are filled with up to 150 m of massive sand encased in shale. The most promising recent discoveries in this type of fan, have all been made since 1985: the Alba, Forth and Gryphon fields with Alba alone containing in place reserves of some 1.1 billion barrels. Isolated fan channel complexes occur mainly towards the top of the Paleogene megasequence, i.e. within the Eocene. They form the most promising, albeit difficult, future objective for stratigraphic traps within the North Sea Tertiary.