Page 204 - Geology of Carbonate Reservoirs

FRACTURES AND FRACTURED RESERVOIRS 185

ing argument in support of the extension fracture mechanism involving high pore
pressure described by Lorenz et al. (1991) . On fractured carbonates in general,
Lorenz et al. (1997) conclude that extension fractures are common to virtually all
examples in their studies and that lithology exerts great control on fracture charac-

teristics. Dolomitic rocks fracture more easily than limestones, fine - grained rocks
fracture more easily than coarse - grained ones, thin beds are more prone to fractur-

ing than thick beds, and structural position has a great influence on fracture char-
acter. More fractures form along hinges of folds (areas of stress concentration) than
along limbs, for example. Importantly, they also note that multiple generations of
fracturing may occur in which older fractures are overprinted by younger ones,
especially in highly deformed rocks. The younger fracture sets may control reservoir
behavior because they are more likely to remain open and to be oriented parallel
to in situ stresses. Lorenz et al. (1997) frame their discussion of fractures in terms
of mechanical stratigraphic units to differentiate between lithostratigraphy and
mechanical stratigraphy. Sonnenfeld, in Lorenz et al. (1997) , described one type of
mechanical stratigraphic unit in the following terms: “ [a] hierarchy of stratigraphic
cycles that controls the distribution of small - scale evaporite collapse breccias, which
in turn create a fracture - prone mechanical stratigraphy . . . [with] the following
intrinsic factors: 1) thin bedding, 2) . . . dolomitic lithologies, 3) fine grain sizes, and

4) accentuated bedding - plane slip due to mechanical contrasts between . . . evaporite
collapse breccias and intervening competent . . . dolomite beds. ” Although regional
fractures are common in mildly deformed strata, tectonic fractures dominate in
moderately and severely deformed rocks.
Tectonic fractures commonly occur in predictable patterns determined by the
geometry of the associated faults or folds. Corbett et al. (1991) mapped fractures in
the Cretaceous Austin Chalk of Texas and established that four different structural

configurations had specific fracture patterns depending on the nature of the struc-
tural feature. The four structural types included anticlinal folds, monoclinal fl exures,
listric normal faults, and graben - in - graben normal faults (Figure 7.7 ). Stearns and
Friedman (1972) demonstrated that extension fractures occur along and parallel to
fold crests while conjugate sets of shear fractures typically occur along fold limbs
of anticlinal folds. The orientation of the respective fracture sets differs according
to the orientation of the maximum, intermediate, and minimum principal stresses.
Fractures associated with faults are parallel to fault slip planes and fracture density
is commonly higher on hanging walls of normal faults than on footwalls (Friedman
and Wiltschko, 1992 ). Fractures that result from mechanical compaction, or physical
diagenesis, also occur in predictable patterns along stratigraphic hinge lines such as

the inflection zones on monoclinal flexures. Fractures may form radial or concentric
patterns peripheral to and above or below buried reefs and mounds, where the beds
overlying the buildups have undergone differential compaction with respect to the
surrounding strata. Radial and tangential faults and fractures are commonly formed
as salt domes grew upward. Much of the fracturing at Ekofisk Field in the North

Sea was formed by salt tectonics (Farrell, in Lorenz et al., 1997 ). Differential com-
paction fractures around buried reefs are important contributors to total production
in the Mississippian mound reservoirs of the Williston Basin (Young et al., 1998 ).
The orientation of compaction fractures in the subsurface is not always obvious or
easy to deduce; consequently, image logs and borehole cores are essential for deter-
mination of fracture orientation, spacing, and density.