Geology Reference
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5.3.2.2 Descriptive Criteria of CalciteFilled
Microfractures
• Association with stylolites (Nelson 1981; Beach
1982; Pl. 25/4).
• Volumetric importance of calcite veins related to the
total rock.
• Differences in the occurrence of veins in various
beds.
• Relation of vein abundance and bed thickness (frac-
ture density can have an inverse logarithmic relation
to bed thickness, regardless of structural setting
(McQuillan 1973).
The following criteria are useful in describing and dis-
cussing of calcite veins:
• Spatial relations of fractures and grains (Simmons
and Richter 1976; Kranz 1983): (a) more crumbly frac-
tures (breaking around solid particles, intergranular and
intercrystalline cracks), or grain boundary cracks as-
sociated with grain boundaries, (b) sharp-edged frac-
tures (breaking through particles and intragranular and
intracrystalline cracks extending from a grain bound-
ary crossing into one or more other grains, Pl. 25/1).
Sedimentary texture and composition seem to be the
essential controls on the main fracture types of car-
bonate rocks.
• Occurrence of single microfractures or fracture sets
(composite crack-seal veins, Pl. 25/1) characterized by
the consistency of orientation.
• Morphology of fractures (straight, arched, branch-
ing, stepped, displaced, Pl. 25/4, 6).
• Existence of offsets on microfractures (indicating
shear, Pl. 25/3).
• Shape of upper and lower terminations (sharp, pinch-
ing out, irregular).
• Orientation of calcite veins with regard to bedding
planes (vertical, oblique, parallel).
• Occurrence of crosscutting fractures (Pl. 25/1, 6).
• Distribution, spacing and density of microfractures
and larger fractures (Gillespie et al. 1993). Microcrack
densities are expressed either as the number of cracks
per unit area or per grain. Crack densities increase with
increasing stress, the ratio of grain boundary cracks to
intragranular cracks decreases at higher applied loads.
These generalized density patterns, however, observed
predominantly in non-carbonate rocks, are not always
relevant for carbonate rocks because cracking can be
surpressed at higher pressures to the benefit of twin-
ning.
• Width and length of microfractures.
• Filling and sealing. Fracture filling should be de-
scribed with regard to (a) mineralogy, (b) inclusion
bands (Ramsay and Huber 1987), (c) cement types
(Muchez et al. 1994), (d) timing of filling generations,
(e) mode of vein-growth by various crack-seal mecha-
nisms (e.g. crack-seal fiber growth due to syntaxial
overgrowth of grains in microcrack walls and oriented
or random nucleation and initial growth of crystals: Cox
and Etheridge 1983; Ramsay 1980; solution cleavage,
Alvarez et al. 1978), and (f) dynamic effects (e.g. twin-
ning, curving of twin lamellae, undulose extinction).
• Spatial relations of microfractures with sedimentary
grains and structures (Pl. 25/2).
Relative age of joints and calcite-filled fractures.
Conventional analysis of geological fracture data
considers the orientation and distribution of fractures
measured on sample line/lines oriented normal to the
fracture strike and shown by stereonet projections.
Methods are described by McQuillan (1973), Huang
and Angelier (1989) and Gillespie et al. (1993).
Microfracture density can be expressed by (a) the
number of fractures intersecting a line in thin section,
hand specimen or core, or (b) the number of vertically
and obliquely oriented calcite veins related to a defined
thin-section area.
Geochemical and mineralogical criteria
Microfacies thin sections allow only a first evalua-
tion of calcite vein types, distribution and frequency.
Detailed analysis of fracture fillings demands that fluid
inclusions, isotopic composition and other geochemi-
cal criteria as well as SEM and TEM be investigated
(Marshall 1982; Dietrich and McKenzie 1983; Ramsay
and Huber 1987; Fairbairn and Ferguson 1992; Miga-
szewski et al. 1998; Suchy et al. 2000).
5.3.2.3 Significance of Microfractures in
Carbonate Rocks
Microfractures have great potential for evaluating the
postsedimentary history of carbonate rocks. The fol-
lowing points are of particular interest:
• Larger and smaller fractures in subsurface rocks are
known to impart the permeability necessary for the eco-
nomic recovery of hydrocarbons from reservoir rocks,
and some fractures provide a major part of reservoir
storage capacity (Drummond 1964; Narr and Currie
1982; Kulander et al. 1990). Fracturing of carbonate
rocks enhances permeability and porosity, both of which
are important in the reservoir potential. Many large hy-
drocarbon reservoirs are connected with fractures (e.g.
Longman 1985; Roehl and Weinbrandt 1985; Mc-
Quillan 1985; Irish and Kempthorne 1987).
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