Joints & Fractures
Joints & Fractures
Joints & Fractures
Like faults, joints result from a brittle response in rocks to stress. They are the most
conspicuous and omnipresent secondary structure of rocks exposed at the earth's
surface. In subsurface work, petroleum geologists use the term "fractures" to refer to
local ruptures of almost any kind that do not show enough offset to be called faults.
Geologists in the field distinguish joints from faults in a similar fashion, i.e., on the
sole criterion that no visible displacement has occurred along the planes of parting.
Ironically, the term "joint" was coined over two hundred years ago by workers in
British stone quarries. These men believed that the pleasingly regular, mutually
perpendicular planes were those across which individual blocks of rock were joined.
Most often, joints occur in sets of semi-regular spacing. They may be fully developed
and conspicuous or incipient ( Figure 1 , Illustration of joint face structures and
nomenclature).
Figure 1
Within a particular set, joints need not be parallel. Typically, jointing orientation is
related to position on a particular fold. Moreover, two or more sets frequently occur
together, comprising a joint system that essentially splits a specific rock body into an
assembled mosaic of blocks.
According to Stearns and Friedman (1972), fracture systems that show consistent
orientations and that pervade a large volume of rock can be divided into two broad
types: (1) those related to specific structures, and (2) regional orthogonal fractures.
The former type can most often be explained in terms of the stress system that
created the host structure. The latter type, however, is not well understood. Some
geologists have related it to epeirogenic movements, most notably plateau uplift, but
this does not appear to explain most cases of regional fracturing. Both types of
fractures, however, can enhance the reservoir quality of prospective formations.
Figure 1
In cases where fractures are mostly vertical or near-vertical (which is usually the
case), rose diagrams and histograms showing strike frequency are used.
Rose diagrams are often preferred, since they can be plotted directly on maps to
show the actual orientations and relative dominance of different fracture trends (
Figure 2 , Major fracture patterns in uplift areas of the Colorado Plateau.
Figure 2
Note the general NW-SE and NE-SW trends. These have been interpreted by some
researchers as indicating conjugate failure on a regional scale.). However, use of
stereo-grams is essential in cases where fractures dip at substantially less than 90º.
Stereograms also allow fracture and fault or fold data to be plotted and compared on
the same figure. This is extremely useful for establishing structural relationships in
fracture analysis. Notice that in part c of Figure 1 , fracture set Ill closely parallels the
host fold axis (point B). As a preliminary assumption, we might consider this set to
represent extensional fractures; sets I and II, therefore, most likely indicate conjugate
shear fractures.
Fault-Associated Fractures
Faults and fractures both represent stress-induced rupture of rock, and that when
they occur in association they can generally be related to the same stress field.
Fault movement itself generates shearing stresses that can induce fracturing. In
many instances, therefore, shear fractures can be considered miniature versions of a
particular fault. Thus, knowing the orientation of a fault means that one can
sometimes predict associated fracture trends. This also means that fracture
orientation will change with fault attitude.
As a general rule of thumb, faults that develop at shallow levels in especially
competent lithologies (e.g., dolomite) are more likely to have fractures associated
with them. However, it should be emphasized that no necessary relationship exists
between the displacement along a fault and the amount or intensity of fracturing.
Shear fractures are more likely to undergo displacement when associated with
faulting than with folding. Such movement can either increase permeability by
creating a poor fit between the two sides of the fracture, or decrease it by sealing the
fractures with gouge or even mylonite. As a rule of thumb, the intensity of fracturing
can be expected to be relatively equal in both upthrown and downthrown blocks of a
normal fault, but somewhat higher in the upthrown block of a reverse or thrust fault.
According to Stearns and Friedman (1972), several basic rules can be applied to
drilling a well such that the greatest number of natural fractures are encountered. As
shown in Figure 1 (Principal fracture patterns and their respective strain ellipses
associated with normal and reverse faults.
Figure 1
In each case, there are three possible fracture sets: two conjugate shear fractures
and one extensional fracture. One conjugate parallels the fault; the other is antithetic
to it. Note that the strike of fractures theoretically parallels that of the host faults or
faults.), the strikes of all three potential fractures will generally parallel that of the
host fault. For low-dipping faults, no deflection of the borehole is needed to intercept
the greatest number of fractures. As the fault attitude steepens, deflection of a well
toward the fault plane becomes more necessary ( Figure 2 , Diagram showing the
dependence of fracture orientation on fault attitude. N refers to normal fault, R to
reverse fault. Angles between shear conjugates are idealized.).
Figure 2
Fractures can develop during the early stages of deformation and may thus become
rotated. Later folding or faulting of a fractured section may obscure original structural
relationships. Most orogenic regions experience multiple episodes of deformation;
thus, later trends often overprint earlier structures. At times, the true connection
between fracturing and faulting will become clear only after these later deformational
effects have been "removed."
Fold-Associated Fractures
Other sets of fractures besides cleavage often characterize folded competent layers.
As an example, Figure 1 shows the variety of fractures seen in a small anticline in
southern Germany.
Figure 1
The fold shows an early stage of cleavage formation in finer-grained lithologies, and
various shear and extension fractures in the thicker, more competent sandstone
layers.
In all, five fold-related fracture patterns have been identified and analyzed (see
Stearns 1967). Figure 2 shows the derived axes of greatest and least principal
stresses for the two most common patterns (Patterns associated with folding (mostly
parallel); note that both patterns show a consistent geometric relation to bedding.).
Figure 2
Shown are b-axis lineations and elongated cobbles; a-axis striations due to interlayer
slip; minor folds with axes parallel to that of the host fold (B); a-c joints; and cleavage
planes.). Pattern B is essentially the same set of fractures rotated 90º, and indicates
that stretching takes place in the plane of dip. This pattern should recall the
distribution of strain shown in part b of Figure 4 (pure buckling).
Figure 4
According to Stearns and Friedman (1972), both patterns A and B can characterize a
single bed. Individual shear fractures of pattern A often occur as relatively isolated
features. They can cross an entire fold and extend hundreds of feet vertically, but are
also seen on many scales, even that of single grains. They show exceptional
consistency in their orientation on all scales, however, which means that statistical
plots show nearly identical patterns, whether data are taken from aerial photographs
or thin sections. Pattern B fractures are smaller (up to several meters long), but all
three fracture sets usually occur together.
Figure 5
What various fracture patterns tell us in a more general sense is that the same body
of rock is often subjected to several different states of stress during the folding
process. In every case, therefore, the specific relationship between a fracture pattern
and fold geometry must be carefully established, usually by statistical techniques.
Wellbore breakout is the term used to describe the spalling of rock that appears to
create elongation. To date, the data indicate that breakouts are relatively broad, flat
curvilinear surfaces that enlarge the wellbore on opposite sides to produce a final
elliptical shape ( Figure 1 , Proposed cause of wellbore breakout).
Figure 1
The two most current and accepted interpretations of this phenomenon attribute
breakout to (1) the intersection of the wellbore with natural fractures, and (2)
compressive shear failure due to stress relief, such that the direction of elongation is
parallel to the in situ minimum horizontal compressive stress ( Figure 1 ) (Zoback et
al. 1985). A growing consensus based on recent analyses strongly favors the second
interpretation.
The stress relief hypothesis is especially attractive, since it allows for relatively
straightforward derivation of the basic in situ stress field. This can have obvious
importance for explaining and predicting the orientation of hydraulically induced
fractures in low permeability reservoirs.