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In situ stress regimes with lithology-dependent and depletion effects 175

new equations is that Poisson’s ratio of the rock is much easier to be
obtained than the coefficient of friction of the fault.
5.3.4 Lithology-dependent minimum and maximum
horizontal stresses
To illustrate the relationship between lithology and horizontal stresses, the
minimum and maximum horizontal stresses can be schematically plotted in
Fig. 5.9 based on the explanation in the previous sections. It can be
observed in Fig. 5.9A that in normal faulting regime, a sandstone normally
has a smaller minimum horizontal stress because of its smaller Poisson’s
ratio. By contrast, a shale has a higher minimum horizontal stress. There-
fore, the shale can be used as a barrier of hydraulic fracture propagation
when hydraulic fracturing is performed in the adjacent sandstone (Zhang
et al., 2018). It should be noted that in the strike-slip and reverse faulting
stress regimes, this lithology-dependent effect in the horizontal stresses may
be smaller (Fig. 5.9B and C) or even reverse because of the larger tectonic
stresses. In highly tectonically stressed environment both the minimum and
maximum horizontal stresses can approach or exceed the overburden stress,
and tectonic strain effect on the sandstone/shale sequence becomes
significant (Blanton and Olson, 1999). Therefore, in strong tectonic
compressional settings, a mechanically stiffer formation (with higher
Young’s modulus), commonly a sandstone, may be more stressed by the
tectonic stresses than a softer formation, such as a shale. The following
equation can be used to illustrate the impact of the tectonic strains on the
horizontal stresses:
E
s x tect ¼ ðε x þ nε y Þ (5.19)
1 n 2
(A) (B) (C)
Stress Stress Stress
shale shale shale
sandstone sandstone sandstone
shale σ h σ H σ σ V shale σ h σ V σ H shale σ V σ h σ H

sandstone sandstone sandstone
shale shale shale
shale shale shale
Depth Depth Depth
Figure 5.9 Schematic representation of the lithology-dependent horizontal stresses in
three different faulting stress regimes: (A) Normal faulting; (B) Strike-slip faulting; and
(C) Reverse faulting.