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Abstract: Polygonal Fault and Fracture Systems in North Sea Tertiary Reservoirs

Joseph Cartwright

High resolution 3-D seismic datasets from the UK sector of the North Sea have been used to investigate anomalous high density fault and fracture sets affecting Lower Tertiary elastic sequences throughout the basin axis. Most of the known hydrocarbon accumulations are located in the region with this deformation pattern. Detailed mapping with 12.5 to 25 m line spacing and resolution of fault throw down to 5 m has revealed fault patterns analogous to shrinkage cracks in desiccated mud flats. Fault patterns vary from survey to survey, and are clearly influenced by factors such as gross permeability of the faulted interval, local setting (slope versus basin plain), and relationship to underlying rift/inversion structures. The basic polygonal fault pattern is, however, a fundam ntal characteristic of all the surveys interpreted so far.

Faults imaged in section are exclusively extensional and range from 5 m (lower resolution limit) to 100m throw. Core studies indicate that a smaller hierarchy of equivalent structures is likely to be present wherever seismic shows the macro fault pattern in place. Faults are organized in vertical sets or tiers that are layer-bound, and relate to the sequence-stratigraphic organization. Reservoir units are often offset by the faults but, in many cases, faults sole out into thick reservoir packages. Top seal claystone units are intensely faulted without exception. Fluidised reservoir sands have been injected up fault planes in some areas, and these are sometimes found to be oil bearing. Oil/water contacts are known to be displaced at fault contacts, indicating effective lateral seal wit in reservoir compartments.

A model to explain the occurrence and geometrical organization of these faults is presented here. The polygonal fault geometry is explained as a consequence of radial tensile stress conditions developed at shallow burial depths. These stress conditions are established during episodic collapse of sequence-bound overpressure cells. The fault system is believed to be an intrinsic product of episodic compaction and dewatering of the dominantly fine-grained depositional systems of the basin axis. The presence of such a fault system has serious implications for development strategies. It also has implications for general models of shale compaction and for allied processes such as primary and secondary migration, basin-wide fluid flux, and for seal capacity.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France