EPISODIC BASIN-WIDE FLUID EXPULSION FROM GEOPRESSURED SHALE SEQUENCESIN THE NORTH-SEA BASIN

A proposed general model for the episodic dewatering of thick shale su ccessions is based on the recognition of a pervasive polygonal extensi onal fault network developed in the dominantly fine grained lower Tert iary of the North Sea basin. Seismic data show that the faults are arr anged in stratigraphically bound structural units (tiers) that are del imited vertically by almost undeformed condensed sections, and are res tricted in distribution to the lowest permeability slope and basin-flo or facies. I propose an episodic three-stage mechanism to explain the fault genesis, involving (1) the development of basin-wide overpressur ed compartments, (2) a density inversion between the overpressured uni ts and the overlying seal, and (3) natural hydraulic fracturing, press ure bleed-off, and resealing of the pressure compartment leading to a repeat of the cycle.