Page 295 - Reservoir Geomechanics

273 Stress fields

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and Schubert 2002). Zoback (1992) suggested that a roughly 40 counter-clockwise
rotation of horizontal stresses on the continental shelf offshore of eastern Canada was
due to superposition of a margin-normal extensional stress derived from sediment load
induced flexure.
Another illustration of the influence of lithospheric flexure on crustal stress is that
associated with post-glacial rebound in offshore areas at relatively high latitudes.
Grollimund and Zoback (2003) modeled the stress state in this portion of the North Sea
region to assess the affect of deglaciation on regional stresses. Figure 9.3 shows the
result of three-dimensional modeling of the stress field in this area. Figure 9.3a shows
that stresses induced by lithospheric flexure do a very good job of explaining both
the average E–W stress orientation observed in the northern North Sea as well as the
subtle swing of maximum horizontal stress orientations from WNW–ESE on the west
side of the Viking graben to ENE–WSW on the east side of the graben. We have also
considered data on the magnitude of the least horizontal principal stress obtained from
a study of approximately 400 wells offshore Norway. Note that the modeled ratio of
the magnitude of minimum horizontal compression to the vertical stress (Figure 9.3b)
compares favorably with measured values of the least horizontal stress magnitudes
observed in the northern North Sea (Figure 9.3c). Thus, regional variations of the mag-
nitude of the least principal stress also appear to support the hypothesis that the stress
field offshore Norway has been strongly affected by deglaciation.
In the sections below, I review stress states in some sedimentary basins around the
world to characterize stress magnitudes at depth in normal, strike-slip and reverse
faulting regions. As discussed in Chapter 1, Anderson’s faulting scheme defines the
relative magnitudes of principal stress. In Chapter 4,wesaw that one can predict stress
magnitudes at depth through utilization of simplified two-dimensional Mohr–Coulomb
failure theory and the concept of effective stress. As reviewed by McGarr and Gay
(1978), Brace and Kohlstedt (1980), Zoback and Healy (1984), Brudy, Zoback et al.
(1997) and Townend and Zoback (2000) numerous in situ stress measurements in areas
of active faulting have proven to be consistent with Coulomb faulting theory assuming
coefficients of friction in the range consistent with laboratory-determined values of
0.6–1.0 (Byerlee 1978). As discussed at the end of Chapter 4, this implies that the
state of stress in the areas in which the measurements were made is controlled by the
frictional strength of pre-existing faults. In other words, the state of stress in these areas
is in frictional failure equilibrium – there are pre-existing faults just at the point of
frictional sliding that control in situ stress states.

Normal faulting stress fields in sedimentary basins

Normal faulting stress states are observed in many parts of the world including the
Gulf of Mexico region of the United States (both onshore and offshore) and the central