reservoir geomecanics

9 Stress fields – from tectonic plates
to reservoirs around the world
In this chapter, I discuss in situ stress fields at a wide variety of scales – from global
patterns of tectonic stress (with a brief discussion of the sources of large-scale tectonic
stress) to examples of normal, strike-slip and reverse faulting stress states in different
sedimentary basins around the world. The purpose of this review is to (i) illustrate
the robustness of the stress measurement techniques discussed in Chapters 6–8, (ii)
emphasize the fact that sedimentary basins are, in fact, found in normal, strike-slip
and reverse faulting environments (as discussed in Chapter 1) and (iii) demonstrate
that critically stressed faults are found in many sedimentary basins such that stress
magnitudes are often found to be consistent with those predicted on the basis of frictional
faulting theory (as discussed near the end of Chapter 4).
In this chapter I also review empirical methods used for stress magnitude estimation
at depth. Specifically, I provide an overview of some of the techniques being used for
estimating the magnitude of the minimum principal stress in normal faulting environments
(such as the Gulf of Mexico) for cases where direct measurements of the least
principal stress from extended leak-off tests and mini-fracs are not available. I discuss
in detail one particular model, the bilateral constraint, which has been widely used for
stress estimation at depth using values of Poisson’s ratio from geophysical logs. As discussed
in this section, this model is not based on sound physical principles and leads to
erroneous values of the horizontal principal stresses. Finally, because stress magnitude
information is needed as a continuous function of depth to address problems such as
wellbore stability during drilling (as discussed in Chapter 10), in the final section of
this chapter, I discuss a method for interpolation and extrapolation of measurements of
stress magnitude at selected depths based on the principal of constant effective stress
ratios.
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Global stress patterns
Knowledge of the magnitude and distribution of stress in the crust can be combined
with mechanical, thermal and rheological constraints to examine a broad range of
geologic processes. For example, such knowledge contributes to a better understanding