Page 50 - Geology and Geochemistry of Oil and Gas

RESERVOIR ROCKS 27
Detailed procedure of determining friction factor ( f ) is discussed by Chilingarian
et al. (1996, pp. 4–8).
If permeability of porous reservoir rocks is proportional to the square of pore
diameter, in fractured ones it is proportional to cubed fracture opening.
A very good correlation between the porosity and permeability was established by
Chilingarian et al. (1992) by introducing two additional variables, irreducible fluid
saturation and specific surface area.
If fractures form in an anisotropic (laminated) medium, its permeability will differ
in different directions. Moreover, in some cases both porosity and permeability near
the fracture may significantly decrease. Studies conducted by Antonellini and Aydin
(1994) on samples from outcrops in the Utah National Park indicated that perme-
ability and porosity in the fault zone drops almost to zero (Fig. 2.4). Within the
compaction zone further from the fault porosity was less than 10% and permeability
was about 10 mD. Farther out, at a distance of just 5–6 mm, porosity was 26% and
permeability was 1,200 mD (average matrix values).
Fig. 2.5 displays permeability change in the direction perpendicular to the lam-
ination. Within the narrow zones adjacent to the destruction zones, the parallel
permeability k jj shows positive peaks, whereas the perpendicular permeability k ?
displays negative peaks, i.e., a decrease in the destruction zone.
Thus, not every fault causes an increase in porosity and permeability. In the
transition zone from the elastic to the plastic deformation, a decrease in both po-
rosity and permeability is observed. Drastic permeability anomalies emerge at the
boundary between plastic and rupturing deformations. Depending on the lithology,
direction, and type of the fault, these anomalies may have different signs. Perme-
ability and porosity in fault zones also depend on the conditions and the extent of

Fig. 2.4. Mesozoic Entrada sandstone, UT, USA. Permeability and porosity across a fault zone (after
Antonellini and Aydin, 1994). (Courtesy of the American Association of Petroleum Geologists.)