Page 175 - Reservoir Geomechanics

158 Reservoir geomechanics

a.
N

b.
0.4
m = 1.0 m = 0.6

t/S v

0.2

0
0 0.2 0.4 0.6 0.8 1
(s - P )/S v
n
p
Figure 5.10. (a) Stereographic representation of fault data detected through wellbore image
analysis in highly fractured granitic rock encountered in the Cajon Pass research well from 1750
to 3500 m depth (after Barton and Zoback 1992). (b) Representation of the same data utilizing a
three-dimensional Mohr diagram normalized by the vertical stress. While many fractures appear to
be critically stressed, most are not and thus reflect the rock’s geologic history (after Barton, Zoback
et al. 1995).

Earthquake focal mechanisms

In seismically active regions, important information can be obtained about the stress
field from earthquake focal mechanisms (also known as fault plane solutions). The
beach balls shown in Figures 1.2 and 5.1 that correspond to normal, strike-slip and
reverse faulting stress regimes are based on the pattern of seismic radiation resulting
from slip on a fault. A good description of earthquake focal plane mechanisms for
non-experts can be found in Fowler (1990).