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Fig. 6.55 Classic natural fracture (joint) systems in Shuaiba-analogue limestones near Cassis, Southern France
host rock - brittle rocks form joints more readily
- so joints are often bed-limited. Mechanical
stratigraphy is therefore important in understand-
ing joint patterns. Unlike fault networks the sta-
tistics of joint sets (in terms of frequency vs. size)
are typically lognormal. The classic 'naturally-
fractured reservoir' is typically a joint-network
reservoir, in which production is dominated
by flow from widespread, high-density fracture
networks such as those seen at outcrop in
Fig. 6.55 .
In addition to the dispersed joint systems
shown in Fig. 6.55 , more localised joint networks
occur in response to the development of other
structures. Two common relationships are
illustrated in Figs. 6.56 and 6.57 . The first is
fold-related, in which the hinge of a folded but
competent mechanical
tensile failure. Note how the underlying, more
ductile layer lacks the joints - it doesn't fail in
the same way.
The second is fault-related. This can produce
highly localised features - as in the open damage
zone case of the Douglas Field described in the
previous section. If the fault density is high,
however, or the reservoir rock brittle (as in car-
bonate reservoirs), fault-related fracturing can
generate widespread open fracture systems.
The expected flow behaviour of the three joint
systems shown in Figs. 6.55 , 6.56 , and 6.57 is
also very different - so once again the key is to
develop plausible fracture distribution concepts
before starting the modelling - simple sketches
are needed. These can be used as the basis for
choosing optimal model designs, of which there
are several to select.
layer is 'nudged' into
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