Rock-Fabric
Classification
The classification
used here has been presented by Lucia (1983, 1995, 1999). The foundation
of the Lucia classification is the concept that pore-size distribution
controls permeability and saturation and that pore-size distribution
is related to rock fabric. Lucia (1983) showed that the most useful
method of characterizing rock fabrics petrophysically is to divide
pore types into pore space located between grains and crystals,
called interparticle porosity, and all other pore space, called
vuggy porosity (Fig. 2). Interparticle porosity is defined as pore
space located between grains or crystals and is not significantly
larger that than the particles. Vuggy porosity is pore space that
is within grains or crystals or that is significantly larger than
grains or crystals; that is pore space that is not interparticle.
Vuggy pore space is further subdivided into two groups based on
how the vugs are interconnected; (1) vugs that are interconnected
only through the interparticle pore network are termed separate
vugs and (2) vugs that form an interconnected pore system are termed
touching vugs. Vugs are commonly present as dissolved grains, fossil
chambers, fractures, and large irregular cavities. Although fractures
may not be formed by depositional or diagenetic processes, fracture
porosity is included because it defines a unique type of porosity
in carbonate reservoir rocks.
 |
Figure
2. Petrophysical classification of carbonate pore space. |
In order to
relate carbonate rock fabrics to pore-size distribution, it is important
to 1) determine which of the three major pore-type classes are present,
interparticle, separate-vug, or touching-vug, 2) measure the volume
(porosity) of interparticle and separate-vug pore space, and 3)
characterize the rock fabric of each pore type petrophysically.
Classification
of Interparticle Pore Space
Interparticle
porosity is defined as pore space located between grains or crystals
that is not significantly larger than the particles (generally <2
x particle size). The pore-size distribution can be described in
terms of particle size, sorting and interparticle porosity (Fig.
3). The volume of interparticle pore space is important because
it relates to pore-size distribution.
 |
Figure
3. Geological/petrophysical classification of carbonate interparticle
pore space based on size and sorting of grains and crystals.
The volume of interparticle pore space is important because
it relates to pore-size distribution. |
Lucia (1983)
showed that particle size can be related to mercury capillary displacement
pressure in nonvuggy carbonates with more than 0.1 md permeability,
suggesting that particle size describes the size of the largest
pores (Fig. 4). Whereas the displacement pressure characterizes
the largest pore sizes and is largely independent of porosity, the
shape of the capillary pressure curve characterizes the smaller
pore sizes and is dependent on interparticle porosity (Lucia 1983
 |
Figure
4. Relationship between mercury displacement pressure and average
particle size for nonvuggy carbonate rocks with permeability
greater than 0.1 md (Lucia, 1983). The displacement pressure
is determined by extrapolating the capillary pressure curve
to a mercury saturation of zero. |
The relationship
between displacement pressure and particle size (Fig. 4) is hyperbolic
and suggests important particle-size boundaries at 100 and 20 m.
Lucia (1983) demonstrated that three permeability fields can be
defined using particle-size boundaries of 100 and 20 ,
a relationship that appears to be limited to particle sizes of less
than 500 m (Fig. 5). Lucia (1995) named these three fields as petrophysical
class 1 (>100 )
class 2 (100-20 ),
and class 3 (< 20 ).
These class designations will be used throughout this presentation.
 |
Figure
5. Porosity-air permeability relationship for various particle-size
groups in nonvuggy carbonate rocks (Lucia, 1983).
|
Recent work
has shown that permeability classes can be better described in geologic
terms if sorting as well as particle size is considered. The approach
to size and sorting used in this petrophysical classification is
similar to the grain-/mud-support principle upon which the Dunham's
(1962) classification is built. Dunham's classification, however,
is focused on depositional texture whereas petrophysical classifications
are focused on contemporary rock fabrics which include depositional
and diagenetic textures. Therefore, minor modifications must be
made in Dunham's classification before it can be applied to a petrophysical
classification.
Instead of dividing
fabrics into grain support and mud support as in Dunham's classification,
fabrics are divided into grain-dominated and mud-dominated (Fig.
3). The important attributes of grain-dominated fabrics are
the presence of open or occluded intergrain porosity and a grain-supported
texture. The important attribute of mud-dominated fabrics is that
the areas between the grains are filled with mud even if the grains
appear to form a supporting framework.
Grainstone is clearly a grain-dominated fabric, but Dunham's packstone
class bridges a boundary between large intergrain pores in grainstone
and small interparticle pores in wackestones and mudstones. Some
packstones have intergrain pore space and some have the intergrain
space filled with mud. The packstone textural class must be divided
into two rock-fabric classes: grain-dominated packstones that have
intergrain pore space or cement and mud-dominated packstones that
have intergrain spaces filled with mud (Fig. 3).
Grainstone
= grain supported with no intergrain lime mud present.
Grain-dominated packstone = grain supported
with both intergrain pore space and lime mud.
Mud-dominated packstone = grain supported
with intergrain volume filled with lime mud.
Wackestone = mud supported with more
the 10 percent grains.
Mudstone = mud supported with less than 10 percent grains.
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