Page 180 - Reservoir Geomechanics

163 Faults and fractures at depth

magnitude of the principal stress tensor (see Angelier 1979; Angelier 1984; Gephart
and Forsyth 1984; Gephart 1990; Michael 1987).
Figure 5.12 is a map showing S Hmax directions in the area shown in Figure 5.8.
The lines with inward pointed arrows are derived from wellbore breakouts (Chapters 6
and 9) and those with a circle in the middle show the P-axis of reverse-faulting focal
plane mechanisms. For slip on pure reverse faults, the horizontal projection of the P-
axis is quite similar to the S Hmax direction because the projection of the P-axis onto a
horizontal plane will be the same as the S Hmax direction regardless of either the choice
of nodal plane or the coefficient of friction of the fault. The S Hmax direction shown by
the heavy arrows was obtained from inversion of earthquake focal plane mechanisms in
the area enclosed by the rectangle (Finkbeiner 1998). Note that this direction compares
quite well with the stress orientations obtained from wells A–D, wellbore breakouts in
other wells and individual earthquake focal plane mechanisms. Because the majority
of earthquakes in this region are reverse faulting events, the direction of S Hmax is not
greatly affected by uncertainties in knowing either the coefficient of friction of the fault
or which nodal plane in the focal mechanism is the fault and which is the auxiliary
plane.