Page 523 - Petrophysics

GEOLOGICAL CLASSIFICATIONS OF NATURAL FRACTURES 49 1

to the maximum principal stress and at an obtuse angle to the
direction of minimum compressive stress.
(b) Extension fractures exhibit a sense of displacement perpendicular
to and away from the fracture plane. They are formed perpendicular
to the minimum stress direction. They too result when the stresses
in the three principal directions are compressive, and can occur in
conjunction with shear fracture.
(c) Tension fractures also exhibit a sense of displacement perpendicular
to and away from the fracture plane. However, in order to form
a tension fracture, at least one of the principal stresses has to be
tensile. Since rocks exhibit significantly reduced strength in tension
tests, this results in increased fracture frequency.

Classification based on paleostress conditions: The geological classification
of fracture systems is based on the assumption that natural fractures
depict the paleostress conditions at the time of the fracturing. Based
on geological conditions, fractures can be classified as in the following
paragraphs.

Tectonic fractures: The orientation, distribution, and morphology of
these fracture systems are associated with local tectonic events. Tectonic
fractures form in networks with specific spatial relationships to faults
and folds. Fault-related fracture systems could be shear fractures formed
either parallel to the fault or at an acute angle to it. In the case of the
fault-wedge, they can be extension fractures bisecting the acute angle
between the two fault shear directions [2, 51. The intensity of fractures
associated with faulting is a function of lithology, distance from the fault
plane, magnitude of the fault displacement, total strain in the rock mass,
and depth of burial.

Fold-related fracture systems exhibit complex patterns consistent with
the complex strain and stress history associated with the initiation and
growth of a fold [6]. Fracture types in fold-related systems are defined in
terms of the dip and strike of the beds.

Regional fractures: These fracture systems are characterized by long
fractures exhibiting little change in orientation over their length. These
fractures also show no evidence of offset across the fracture plane and
are always perpendicular to the bedding surfaces. Regional fracture
systems can be distinguished from tectonic fractures in that they generally
exhibit simpler and more consistent geometry and have relatively larger
spacing.
Regional fractures are commonly developed as orthogonal sets with
the two orthogonal orientations parallel to the long and short axes
of the basin in which the fractures are formed. Many theories have