Page 291 - Reservoir Geomechanics

270 Reservoir geomechanics

subjected to uniform stress orientation or a uniform pattern of stress orientations
(such as the radial pattern of stress orientations in China) are referred to as first-
order stress provinces (Zoback 1992).

Sources of crustal stress

As alluded to above, stresses in the earth’s crust are of both tectonic and non-tectonic,
or local, origin. The regional uniformity of the stress fields observed in Figures 1.5, 9.1
and 9.2 clearly demonstrate the tectonic origins of stress at depth for most intraplate
regions around the world. For many years, numerous workers suggested that residual
stresses from past tectonic events may play an important role in defining the tectonic
stress field (e.g., Engelder 1993). We have found no evidence for significant residual
stresses at depth. If such stresses exist, they seem to be only important in the upper few
meters or tens of meters of the crust where tectonic stresses are very small.
In the sections below the primary sources of tectonic stress are briefly discussed.
Although it is possible to theoretically derive the significance of individual sources of
stress in a given region, because the observed tectonic stress state at any point is the
result of superposition of a variety of forces acting within the lithosphere, it is usually
difficult to define the relative importance of any one stress source.

Plate driving stresses
The most fundamental sources of the broad-scale regions of uniform crustal stress are
the forces that drive (and resist) plate motions (Forsyth and Uyeda 1975). Ultimately,
these forces arise from lateral density contrasts in the lithosphere. Lithospheric plates
are generally about 100 km thick, are composed of both the crust (typically about 40 km
thick in continental areas) and the upper mantle, and are characterized by conductive
heat flow. They are underlain by the much less viscous asthensophere.
The most important plate-driving processes resulting in intraplate stress is the ridge
push compressional force associated with the excess elevation (and hot, buoyant litho-
sphere) of mid-ocean ridges. Slab pull (a force resulting from the negative buoyancy
of down-going slabs) does not seem to be transmitted into plates as these forces appear
to be balanced at relatively shallow depths in subduction zones. Both of these sources
contribute to plate motion and tend to act in the direction of plate motion. If there is
flow in the upper asthenosphere, a positive drag force could be exerted on the litho-
sphere that would tend to drive plate motion, whereas if a cold thick lithospheric roots
(such as beneath cratons) this may be subject to resistive drag forces that would act to
inhibit plate motion. In either case the drag force would result in stresses being trans-
ferred upward into the lithosphere from its base. There are also collisional resistive
forces resulting either from the frictional resistance of a plate to subduction or from the