Page 33 - Reservoir Geomechanics

18 Reservoir geomechanics

S hmin and S v . Second, the orientation of the P (compressional), B (intermediate), and T
(extensional) axes (which are defined with respect to the orientation of the fault plane
and auxiliary plane) give an approximate sense of stress directions. Unfortunately, these
axes are sometimes incorrectly assumed to be the same as the orientation of S 1 , S 2 and S 3
but the P, B and T axes are only approximate indicators of stress orientation as discussed
in Chapter 5;nevertheless a collection of diverse focal mechanisms in a given area can
be inverted to determine a best-fitting stress field. Focal mechanisms from earthquakes
along plate-bounding faults, such as the San Andreas fault in California, cannot be used
to determine stress orientation because of their low frictional strength. In such cases,
the focal plane mechanisms are indicators of the kinematics of fault slip (and relative
plate motion) and not closely related to principal stress orientations (MacKenzie 1969).

Geologic stress indicators
There are two general types of relatively recent geologic data that can be used for in situ
stress determinations: (1) the orientations of igneous dikes or cinder cone alignments,
both of which form in a plane normal to the least principal stress (Nakamura, Jacob
et al. 1977)in the manner of a magma-filled hydraulic fracture (see Chapter 7); and (2)
fault slip data, particularly the inversion of sets of striae (i.e. slickensides) on faults as
for earthquake focal mechanisms as mentioned above. Of course, the term relatively
young is often quite subjective but essentially means that the features in question are
characteristic of the tectonic processes currently active in the region of question. In
most cases, data that are Quaternary in age are used to represent the youngest episode
of deformation in an area. Like focal mechanisms, a collection of observations of recent
fault slip can be inverted to find a best-fitting stress tensor.

Regional stress patterns

Zoback and Zoback (1980) showed that it was possible to define specific stress
provinces,regions of relatively uniform stress orientation and magnitude that corre-
late with physiographic provinces defined by the topography, tectonics and crustal
structure. Figure 1.5 shows maximum horizontal stress orientations for North America
taken from the WSM database. The legend identifies the different types of stress indi-
cators. Because of the density of data, only highest quality data are plotted (A and B
quality, as defined in Chapter 6, are distinguished by lines of different length). Where
known, the tectonic regime (i.e. normal faulting, strike-slip faulting or reverse faulting)
is given by the symbol color. The data principally come from wellbore breakouts, earth-
quake focal mechanisms, in situ stress measurements greater than 100 m depth, and
young (<2Ma old) geologic indicators. These data, originally presented and described
by Zoback and Zoback (1991)building upon work in the conterminous United States