Page 27 - Reservoir Geomechanics

12 Reservoir geomechanics

there is a close correspondence between the current stress field and large-scale active
faults in the region. Western California (discussed below) is such a region. However,
in other regions, the current stress state is not consistent with large-scale geologic
structures because those structures evolved during previous tectonic regimes, in some
cases, regimes that have not been active for tens, or even hundreds, of millions of years.
In fact, in some parts of the world there is a marked disagreement between currently
active tectonic stresses and the large-scale geologic structures defining oil and gas. One
example of this is the Tampen Spur area of the northern North Sea (mentioned below and
discussed in detail in Chapter 9) where earthquake focal mechanisms and direct stress
measurements indicate that there is currently a compressional (strike-slip and reverse
faulting) state of stress in much of the area, but the principal geologic structures are
those associated with extension and basin formation (normal faulting and subsidence)
at the time of opening of the North Atlantic in Cretaceous time, more than 70 million
years ago. As discussed in Chapter 9, the compressional stresses in this area appear to
arise from lithospheric flexure associated with deglaciation and uplift of Fennoscandia
in only the past 20,000 years. In some places in the northern North Sea, after tens of
millions of years of fault dormancy, some of the normal faults in the region are being
reactivated today as strike-slip and reverse faults in the highly compressional stress field
(Wiprut and Zoback 2000). The opposite is true of the eastern foothills of the Andes
in Colombia and the Monagas basin of eastern Venezuela. Although extremely high
horizontal compression and reverse faulting were responsible for formation of the large-
scale reverse faults of the region, the current stress regime is much less compressive
(strike-slip to normal faulting) (Colmenares and Zoback 2003).

Stress magnitudes at depth

To consider the ranges of stress magnitudes at depth in the different tectonic environ-
ments illustrated in Figure 1.2,itis necessary to evaluate them in the context of the
vertical stress and pore pressure, P p . Figure 1.4 schematically illustrates possible stress
magnitudes for normal, strike-slip and reverse faulting environments when pore pres-
sure is hydrostatic (a–c) and when pore pressure approaches lithostatic (overburden)
values at depth (d–f). At each depth, the range of possible values of S hmin and S Hmax are
established by (i) Anderson faulting theory (which defines the relative stress magni-
tude), (ii) the fact that the least principal stress must always exceed the pore pressure (to
avoid hydraulic fracturing) and (iii) the difference between the minimum and maximum
principal stress which cannot exceed the strength of the crust (which depends on depth
and pore pressure as discussed in Chapter 4). Note in Figure 1.4a, for an extensional (or
normal faulting) regime, that if pore pressure is close to hydrostatic, the least principal
stress can be significantly below the vertical stress (it will be shown in Chapter 4 that
the lower bound on S hmin is approximately 0.6S v ). In this case, the maximum horizontal
stress, S Hmax , must be between S hmin and S v . Alternatively, for the same pore pressure