EPISODIC BASIN-WIDE HYDROFRACTURING OF OVERPRESSURED EARLY CENOZOIC MUDROCK SEQUENCES IN THE NORTH-SEA BASIN

A deformational fabric associated with the Early Cenozoic mudrock domi nated sequences of the North Sea is described. Seismic data show that the mudrock sequences are pervasively faulted over a large area of the basin axis in the central and northern North Sea. Mapping the faults using regional two- and three-dimensional data shows that the faults a re minor extensional structures, arranged in stratigraphically bound t iers in the deep water sequences and bounded by regional condensed seq uences (seals). The geometry of the faulting is complex. Faults are or ganized into cellular networks comprising polygonal prismatic and pyra midal forms. A gravitational mechanism of downslope collapse faulting is rejected on the grounds that the distribution of the deformed seque nces extends over large areas of formerly horizontal basin plain. A me chanism involving the repeated development of basin-wide overpressured compartments is proposed in which the cellular structure is related t o deformation patterns associated with a density inversion between the overpressured units and the overlying seal. Hydrofracturing is invoke d to account for the almost random strike distribution of the faulting . The region affected by the pervasive fluid expulsion faulting encomp asses almost half of the North Sea Basin. Basin-wide collapse of the o verpressured compartments could have resulted in a geologically rapid flux of enormous volumes of pore fluids. This mechanism has implicatio ns for concepts of steady-state compaction, heat flow in basins, hydro carbon transport and the genesis of lead-zinc ore deposits.