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Abstract

Postdepositional remobilization and injection of sand are important processes in deep-water clastic systems. Features resulting from these processes are particularly well documented in the Paleogene of the central and northern North Sea, where large-scale sandstone intrusions significantly affect reservoir geometries and fluid-flow properties of sand and mudstone intervals throughout large areas. Large-scale sandstone intrusions seen in seismic data from the Paleogene of the North Sea can be grouped into three main categories based on their size, morphology, and relation to their parent sand body:

Type 1: Winglike sandstone intrusions are seen as discordant seismic anomalies that emanate from the sides and sometimes from the crests of steep-sided concordant sand bodies, which may be of depositional or intrusive origin. The intrusions may be as much as 50 m (164 ft) thick, and crosscut some 100–250 m (330–820 ft) of compacted mudstone section at angles between 10 and 35°. Winglike intrusions may form regardless of preexisting structures, but commonly exploit polygonal fault systems in the encasing mudstones.

Type 2: Conical sandstone intrusions are seen as conical amplitude anomalies that emanate some 50–300 m (164–1000 ft) upward from distinct apexes located a few meters to more than 1 km (0.6 mi) above the likely parent sand body. The intrusions may be as much as 60 m (196 ft) thick and are discordant to bedding along most of their extent, with dips ranging from 15 to 40°. The nature of the feeder system is conjectural, but may comprise subvertical zones of weakness such as blowout pipes or polygonal fault planes, whereas the intrusions themselves do not appear to be controlled by preexisting fault systems.

Type 3: Crestal intrusion complexes comprise networks of intrusions above more massive parent sand bodies. These intrusions are either too thin or too geometrically complex to be well imaged by seismic data. Despite the small scale of their component intrusions, crestal intrusion complexes may be volumetrically important.

Large-scale sandstone intrusions commonly terminate at unconformities such as base Balder (uppermost Paleocene), top Frigg (lower Eocene), or base Oligocene, where they may have extruded onto the paleo-sea-floor. Because sandstone intrusions are commonly highly porous and permeable, they are important as reservoirs and as efficient plumbing systems in thick mudstone sequences. Because the intrusions occur in unusual stratigraphic positions not predicted by standard sedimentary facies models, they may constitute drilling hazards by hosting shallow gas accumulations or by acting as sinks to dense and overpressured drilling fluids. Predrill prediction of the occurrence of large-scale sandstone intrusions based on seismic data and predictive models is thus vital to successful exploration of deep-water clastic plays.

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