Page 199 - Geology of Carbonate Reservoirs

180 FRACTURED RESERVOIRS

σ
1
σ σ 3
A 3
σ
1

σ
σ 3 σ
B 1 σ 1
3

Figure 7.4 Typical fracture patterns in folds where maximum principal stress is oriented

vertically and horizontally: (a) type 1 pattern with maximum principal stress vertical; (b) type
2 pattern with maximum principal stress horizontal. Fracture patterns follow bedding rather
than fold geometry. (From Stearns and Friedman (1972) .)

Tectonic fractures associated with folding have been studied by many workers.
However, the study by Stearns and Friedman (1972) focused attention on the frac-
ture sets associated with folds that are important for exploration and development
models. They pointed out that two main sets of fractures are typical on anticlinal
folds. First, a set of conjugate shear fractures and an extension fracture indicating
are
that σ 1 is oriented in the dip direction of the bedding on the fold limb, σ 1 and σ 3
is normal to bedding (Figure 7.4 ). The other fractures
in the plane of bedding, and σ 2
consist of a conjugate set of shear fractures and an extension fracture, but the prin-
is parallel to the strike of
cipal stresses are oriented differently. In this case, σ 1
is oriented in the dip direction of bedding on the fold limb. Knowing
bedding and σ 3
these typical orientations of fracture sets on folds and faults can greatly aid
in forming models for exploration and development of tectonically fractured
reservoirs.
Regional fractures occur over large areas of the earth ’ s crust, they are always
normal to bedding planes, they show little offset along fracture planes, and they may
cross - cut local structures (Nelson, 2001 ). Their mode of origin is not well understood
but some workers suggest that they are formed by large - scale vertical movements
in the earth ’ s crust. Nelson describes regional fractures around the Colorado Plateau,
for example. He further notes that excellent reservoirs can result when regional
fractures are cut by later tectonic fractures.
Contractional fractures are extensional and tensional fractures formed as the
result of bulk volume reduction in the parent rock. Volume reduction can be pro-
duced by desiccation, syneresis, mineral phase changes, and temperature change.
Contractional fractures are not as important as tectonic and regional fractures in
hydrocarbon reservoirs.