Page 162 - Reservoir Geomechanics

145 Faults and fractures at depth

core samples of the Antelope shale from the Buena Vista Hills field of the San Joaquin
basin (left side of Figure 5.2a), as well as outcrops along the coastline (Figure 5.2b),
are clearly associated with the presence of hydrocarbons. Thus, the enhancement of
permeability resulting from the presence of faults in the Monterey is critically important
for hydrocarbon production.
Figures 5.1b–d illustrates the idealized relationships between conjugate sets of nor-
mal, strike-slip and reverse faults and the horizontal principal stress (as well as the
corresponding Mohr circles). Recalling subjects first mentioned in Chapters 1 and 4
related to Andersonian faulting theory and Mohr–Coulomb failure, respectively, Fig-
ures 5.1b–d illustrate the orientation of shear faults with respect to the horizontal and
vertical principal stresses, associated Mohr circles and earthquake focal plane mech-
anisms associated with normal, strike-slip and reverse faulting. Lower hemisphere
stereonets (second column) and earthquake focal plane mechanisms (fifth column) are
described below. The first and fourth columns (map views and cross-sections) illustrate
the geometrical relations discussed in Chapter 4 in the context of equation (4.43). For
a coefficient of friction of 0.6, active normal faults (Figure 5.1b) are expected to strike
nearly parallel to the direction of S Hmax and conjugate fault sets are expected to be
◦
active that dip ∼60 from horizontal in the direction of S hmin . Strike-slip faults (Figure
5.1c) are expected to be nearly vertical and form in conjugate directions approximately
◦
30 from the direction of S Hmax .Reverse faults (Figure 5.1d) are expected to strike in
a direction nearly parallel to the direction of S hmin and dip approximately 30 in the
◦
S Hmax direction. The Mohr circles associated with each of these stress states (middle
column) simply illustrate the relative magnitudes of the three principal stresses (shown
as effective stresses) associated with each stress state.
There are three points that need to be remembered about the idealized relationships
illustrated in Figure 5.1. First, these figures illustrate the relationship between poten-
tially active faults and the stress state that caused them. In reality, many fractures and
faults (of quite variable orientation) may be present in situ that have been introduced by
various deformational episodes throughout the history of a given formation. It is likely
that many of these faults may be inactive (dead)in the current stress field. Because
currently active faults seem most capable of affecting permeability and reservoir per-
formance (see also Chapter 11), it will be the subset of all faults in situ that are currently
active today that will be of primary interest. The second point to note is that in many
parts of the world a transitional stress state is observed. That is, a stress state associated
with concurrent strike-slip and normal faulting (S Hmax ∼ S v > S hmin )in which both
strike-slip and normal faults are potentially active or strike-slip and reverse faulting
regime (S Hmax > S v ∼ S hmin )in which strike-slip and reverse faults are potentially
active. Examples of these stress states will be seen in Chapters 9–11. Finally, while the
concept of conjugate fault sets is theoretically valid, in nature one set of faults is usually
dominant such that the simple symmetry seen in Figure 5.1 is a reasonable idealization,
butis rarely seen.