Rock-Fabric/Petrophysical
Relationships
Petrophysics
of Interparticle Pore Space
Limestone
Rock Fabrics
Examples of
nonvuggy limestone petrophysical rock fabrics are illustrated in
Fig. 7. In grainstone fabrics, the pore-size distribution is controlled
by grain size; in mud-dominated fabrics, the size of the micrite
particles controls the pore-size distribution. In grain-dominated
packstones, however, the pore size distribution is controlled by
grain size and by the size of micrite particles between grains.
Figure 8 illustrates all the data for limestones compared with the
permeability fields. Grainstone and mud-dominated fabrics are reasonably
well-constrained permeability fields. Although grain-dominated packstone
fabrics plot at an intermediate location between grainstones and
mud-dominated limestones, they show more variability because of
the large grain size difference. Grain size of grain-dominated packstone
ranges from 400 microns for oncoid fabrics to 80-150
for fine peloid fabrids.
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Class
1 ooid grainstone
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Class
1 ooid grainstone
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Class
2 peloid grain-dominated packstone
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Class
2 oncoid grain-dominated packstone
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Class
3 fossil mud-dominated packstone
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Class
3 rudist floatstone
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Figure7. Examples of nonvuggy
limestone rock fabrics.
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Class
3 Orbitolind wackestone
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Despite the
considerable scatter in the data, grainstone, grain-dominated packstone,
and mud-dominated fabrics are reasonably well constrained to the
three permeability fields. Whereas grain size and sorting define
the permeability fields, the interparticle porosity defines pore-size
distribution and thus the permeability within the field. Systematic
changes in intergrain porosity by cementation, compaction, and dissolution
processes will produce systematic changes in pore-size distribution
and result in systematic changes in permeability. Therefore, permeability
in nonvuggy limestones is a function of interparticle porosity,
grain size, and sorting.
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Figure
8. Porosity-air permeability cross plot for nonvuggy limestone
fabrics compared with the three permeability fields.
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Dolomite
Rock Fabrics
Examples of
nonvuggy dolostone petrophysical rock fabrics are illustrated in
figure 9. Dolomitization can change the rock fabric significantly.
In limestones, fabrics can usually be distinguished with little
difficulty. If the rock has been dolomitized, however, the overprint
of dolomite crystals often obscures the limestone fabric precursor.
Precursor fabrics in fine-crystalline dolostones are easily recognizable.
However, as the crystal size increases, the precursor fabrics become
progressively more difficult to determine.
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Class
1 peloid med xl dolograinstone
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Class
1 ooid med xl dolograinstone
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Class
2 peloid med xl g.d. dolopackstone
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Class
2 peloid med xl g.d. dolopackstone
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Class
3 f. xl dolowackestone
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Class
2 med xl dolowackestone
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Class
1 lg xl dolowackestone
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Class
1 lg xl dolograinstone
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Figure
9. Examples of nonvuggy dolostone rock fabrics. |
Dolomite crystals
(defined as particles in this classification) commonly range in
size from several m to >200 .
Lime mud particles are usually <20
in size. Thus, dolomitization of a mud-dominated carbonate fabric
can result in an increase in particle size from <20
to >200
(Fig. 9). The plot of interparticle porosity
against permeability (Fig. 10a) illustrates
the principle that, in mud-dominated fabrics, permeability increases
as dolomite crystal size increases. Finely crystalline (average
15 m) mud-dominated dolostones plot within the class 3 permeability
field. Medium crystalline (average 50 )
mud-dominated dolostones plot within the class 2 permeability field.
Large crystalline (average 150 )
plot in the class 1 permeability field.
Grainstones
are usually composed of grains much larger than the dolomite crystal
size (Fig. 9) so that dolomitization does not
have a significant effect on the pore size distribution. This principle
is illustrated in Fig. 10b where interparticle
porosity is plotted against permeability measurements from dolograinstones.
The grain size of the dolograinstones is 200 .
Fine crystalline dolograinstone, medium crystalline dolograinstone
and large crystalline dolograinstone all plot within the class 1
permeability field. The large crystalline mud-dominated dolostones
also plot in this permeability field, indicating that they are petrophysically
similar to grainstones (Fig. 10a).
A cross plot
of interparticle porosity and permeability measurements from fine
to medium crystalline grain-dominated dolopackstones is presented
in Fig. 10c. The average grain size is 200 .
The data plot in the class 2 permeability field. The medium crystalline
mud-dominated dolostones also plot in this field (Fig.
10a).
Figure 11 illustrates
all dolomite data compared with permeability fields. Dolograinstones
and large crystal dolostones constitute the class 1 permeability
field. Grains are very difficult to recognize in dolostones with
a > 100
crystal size. However, because all large crystalline dolostones
and all dolograinstones are petrophysically similar, whether the
crystal size or the grain size is described makes little difference
petrophysically. Fine and medium crystalline grain-dominated dolopackstones
and medium crystalline mud-dominated dolostones constitute the class
2 permeability field. Fine crystalline mud-dominated dolostones
constitute the class 3 field.
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Figure
11. Composite porosity-air permeability cross plot for nonvuggy
dolostone fabrics compared with the three permeability fields. |
The dolomite
permeability fields are defined by dolomite crystal size as well
as grain size and sorting of the precursor limestone. Within the
field, permeability is defined by interparticle porosity. Systematic
changes in intergrain and intercrystal porosity by predolomite calcite
cementation, dolomite cementation, and compaction will systematically
change the pore-size distribution, resulting in a systematic change
in permeability. Therefore, dolomite crystal size, grain size, and
sorting define the permeability field, and interparticle porosity
defines the permeability.
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Figure
12. Composite porosity-air permeability cross plot for nonvuggy
limestones and dolostones showing transforms for each class.
See text for equations. |
Limestone
and Dolomite Combined
Data from limestone
and dolomite rock fabrics are combined into one interparticle-porosity
permeability cross plot in figure 12. The fabrics that make up the
class 1 field are (1) limestone and dolomitized grainstones and
(2) large crystalline grain-dominated dolopackstones and mud-dominated
dolostones. An upper particle size limit of 500
is imposed but not well defined. The upper limit is imposed because
as the grain size increases the slope of the porosity-permeability
transform approaches infinity and permeability becomes independent
of porosity.
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Figure
13. Petrophysical/rock-fabric classes based on similar capillary
properties and interparticle-porosity/permeability transforms. |
Fabrics that make up the class 2 field are (1) grain-dominated packstones,
(2) fine to medium crystalline grain-dominated dolopackstones, and
(3) medium crystalline mud-dominated dolostones. The class 3 field
is characterized by mud-dominated fabrics (mud-dominated packstone,
wackestone, and mudstone) and fine crystalline mud-dominated dolostones.
This arrangement of petrophysically similar rock fabrics is illustrated
in figure 13.
Transforms are
presented below for each combined permeability field (Fig.
12).

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