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Overpressure - Extension

Under Fault seal it was shown that less steeply dipping normal faults can seal and 4DGeoStress can detect increasing seal by analysing the average dip of the fault plane for each isochore. Numerous papers on the Gulf of Mexico show examples of this and also demonstrate that over pressure increases with depth as down to basin faults flatten into shaley sequences.

 

The pressure-depth graph (top left) is from the North Sea and shows pore pressures at 1.7 times the normal gradient and in many fields, the gradient can reach 2.2 (as in the Gulf of Mexico), but the former is experiencing compression today and the latter, extension.

 

A quadrant of the deformation ellipse (bottom left) shows there is no fault seal and normal pore pressure on shallow, active, vertical normal faults because Sh/SV is low but exceeds the tensile strength of the fault. Fault leak continues to occur until approximately the conjugate shear angle of 60° (equal to a fault plane dip of 60° from the vertical) is reached as shown by the curved fault surface due to Sh/SV increasing and the effects of clay smear. Comparison with published Gulf Coast data indicate pore pressure reaches 1.8 times normal at fault dips less than 30° and a fault seal probability of 80% has been assigned to this. That probability increases as fault dip approaches sub-horizontal.