Abstract

Polygonal faults attributed to three-dimensional (3-D) volumetric contraction of muddy sediments during early burial are widespread within the Eocene-lower Miocene succession of the United Kingdom central North Sea. The analysis of a 3-D seismic survey encompassing the Eocene Alba field in the central North Sea has allowed us to investigate (1) the influence of polygonal faults in surrounding mudrocks on the geometry of the Alba deep-water sandstone reservoir and (2) how the presence of the reservoir sandstone influences the polygonal fault pattern above and below the reservoir. The main reservoir in the Alba field is an elongate (12 km long, 1-2 km wide, up to 90 m thick), fine-grained, massive sandstone body that may have been deposited in a deep-water channel or slope gully. Although depositional processes likely have been responsible for the dominantly linear sandstone distribution, the current reservoir geometry is largely controlled by the location of polygonal faults in the surrounding hemipelagic mudstones. Sandstones interpreted as injected along faulted margins of the Alba field indicate that faulting facilitated remobilization and sand injection during early burial, further modifying the reservoir shape. Unusual, isolated 1-km-wide subcircular mounds to the west of the main field also are attributed to sand withdrawal and remobilization during early burial. On a mapped marker horizon in the mudrocks 80-120 m above the reservoir there is a marked decrease in polygonal fault density compared to areas away from the reservoir. On a horizon in mudrocks within 5-50 m of the base of the reservoir there is an increase in horizon disruption due to small faults directly below the sand body. Changes in polygonal fault density and pattern thus may indicate the presence of sandstones and may be a useful exploration tool for explorationists searching for subtle Eocene deep-water sand bodies that typically are poorly imaged on seismic data in the North Sea.

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