Abstract

A proposed general model for the episodic dewatering of thick shale successions is based on the recognition of a pervasive polygonal extensional fault network developed in the dominantly fine grained lower Tertiary of the North Sea basin. Seismic data show that the faults are arranged in stratigraphically bound structural units (tiers) that are delimited vertically by almost undeformed condensed sections, and are restricted in distribution to the lowest permeability slope and basin-floor facies. I propose an episodic three-stage mechanism to explain the fault genesis, involving (1) the development of basin-wide overpressured compartments, (2) a density inversion between the overpres-sured units and the overlying seal, and (3) natural hydraulic fracturing, pressure bleed-off, and resealing of the pressure compartment leading to a repeat of the cycle.

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