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that the existence of these separate pressure regimes indicates that the faults
in this field are not open conduits, but there could be leakage through the
faults.
In the central block, the Hayes gas sand has a value X = 0.78 close to a fault
at a depth of about 11,300 ft (3445 m). Substituting zmax = 3445 m, pb =
2,300 kg m-3, and h = 0.8 into eq. 11.1, we find that uo must be less than 2
MPa (20 bars). [Similarly, Magara’s (1968) data lead to values of uo less than
2 MPa for Miocene mudstone in Japan.]
In areas of normal faulting, such as the U.S. Gulf Coast, the stress field is
such that the greatest principal stress is vertical, the least and intermediate,
horizontal. In the Midland field, the greatest principal effective stress (taking
pore-fluid pressure into account) at a depth of 3445 m is:
51 = PbgZ (1-1) = 15 MPa

and at about 1000 m, where the pore-fluid pressures on both sides of the
fault are normal hydrostatic, h = 0.5 and u1 = 11 MPa, approximately.
Figure 11-8 shows the Mohr diagram for the two possible conditions of
zero and 2 MPa tensile strength, plotted on either side of the axis. It is clear
that smaller differential stresses are required for shear failure without absolute

Fig. 11-9. Depths to which open fractures could occur in materials of tensile strengths to
2.5 MPa (25 bars) for various values of A.