Global Interparticle-porosity
Permeability Transform
Although fabrics
are divided into three petrophysical
classes, in nature there is no boundary between the classes.
Instead, there is a continuum from mudstone
to grainstone
and from 5
to over 500
mud-dominated dolostones
(Fig. 14b,c). Therefore, there is also a complete continuum of rock-fabric
specific porosity-permeability
transforms.
Figure
14. Continuum of rock
fabrics and associated porosity-permeability transforms.
(A) Rock-fabric
numbers ranging from 0.5 - 4 defined by class-average and
class-boundary porosity-permeability transforms. (B) Fabric
continuum in nonvuggy limestone.
(C) Fabric continuum in nonvuggy dolostone.
To model such
a continuum the boundaries of each petrophysical
class are assigned a value (0.5, 1.5, 2.5, and 4) (Fig.
14a) and porosity-permeability transforms generated. These transforms
were added to the class 1, 2, and 3, transforms and an equation
relating permeability
to a continuum of class values and interparticle porosity
is developed using multiple linear regressions. To avoid confusion,
the class values generated by this equation are termed rock-fabric
numbers. The resulting global transform is given below. This
equation is useful in calculating permeability from wireline logs
but is too detailed to be useful for routine classification of visual
descriptions.
Mud-dominated
limestones
and fine crystalline mud-dominated dolostones
occupy rock
fabric numbers from 4 to 2.5 (Fig. 14b,c).
The class number decreases with increasing dolomite crystal size
from 5
to 20
in mud-dominated dolostones, and with increasing grain volume in
mud-dominated limestones. Grain-dominated
packstones, fine-to-medium crystalline grain-dominated
dolopackstones, and medium crystalline mud-dominated dolostones
occupy the rock fabric numbers from 2.5 to 1.5 (Fig.
14b,c). The class value decreases with increasing dolomite crystal
size from 20 to 100
in mud-dominated dolostones and with decreasing amounts of intergrain
micrite as well as increasing grain size in grain-dominated packstones
and fine to medium crystalline grain-dominated dolopackstones. Grain-stones,
dolograinstones,
and large crystalline dolomites occupy rock fabric numbers 1.5 to
0.5 (Fig. 14b,c). The class value decreases
with increasing grain size and dolomite crystal size from 100 to
500.
Unusual Types
of Interparticle Porosity
Diagenesis can
produce unique types of interparticle porosity. Collapse of separate-vug
fabrics due to overburden
pressure can produce fragments that are properly considered
"diagenetic particles". Large dolomite crystals with their
centers dissolved can collapse to form pockets of dolomite rims.
Leached grainstones
can collapse to form intergrain fabrics composed of fragments of
dissolved grains. These unusual pore types typically do not cover
an extensive area. However, the collapse of extensive cavern systems
can produce bodies of collapse breccia that are extensive. Interbreccia-block
pores produced by cavern collapse are included in the touching-vug
category because they result from the karsting process (Kerans 1989).
Petrophysics
of Vuggy Pore Space
Petrophysics
of Separate-Vug Pore Space
The addition
of separate-vugporosity to
interparticle porosity alters the petrophysical characteristics
by altering the manner in which the pore space is connected, all
pore space being connected in some fashion. Examples of separate-vug
pore space are illustrated in figure 15.
Class
1 moldic separate-vug ooid grainstone
Class
1 intragrain microporous separate-vug ooid grainstone
Class
2 intrafossil separate-vug med xl grain-dominated dolopackstone
Class
3 grain mold separate-vug fossil wackestone
Microporosity
in an ooid (see arrow)
Class
1 intragrain microporous separate-vug ooid grainstone
Figure
15. Examples of separate-vug porosity (click
on each image to see a larger version)
Separate
vugs are not connected to each other. They are connected only
through the interparticle
pore space and, although the addition of separate vugs increases
total porosity,
it does not significantly increase permeability
(Lucia 1983). Figure 16a illustrates this principle. Permeability
of a moldic grainstone is less than would be expected if all the
total porosity were interparticle and, at constant porosity, permeability
increases with decreasing separate-vug porosity (Lucia and Conti
1987). The same is true for a large crystalline dolowackestone
in that the data are plotted to the left of the class 1 field in
proportion to the separate-vug porosity (Lucia 1983).
Figure
16. Cross plot illustrating the effect of separate-vug
porosity on air permeability. (A) Grainstones
with separate-vug porosity in the form of grain molds plot to
the right of the grainstone field in proportion to the volume
of separate-vug porosity. (B) Ooid grainstone with separate
vugs in the form of intragranular microporosity plot to the
right of the grainstone field.
This principle
is also true for intragrain
microporosity. Fig. 16b shows data from a Cretaceous ooid grainstone
from offshore Brazil with intragrain microporosity and intergrain
pore space (Cruz, 1997). The plot shows that the permeability
of the grainstone is less than would be expected if all the porosity
were interparticle.
Petrophysics
of Touching-Vug Pore Space
Examples of
touching-vug
pore types are illustrated in Fig. 17. Touching vugs can increase
permeability
well above what would be expected from the interparticle pore system
and are usually considered to be filled with oil in reservoirs.
Class
3 mud-dominated packstone with touching vugs of grain molds
connected by microfractures
Figure
17. Examples of touching-vug pore types (click
on each image to see a larger version)
Lucia (1983)
illustrated this fact by comparing a plot of fracture permeability
versus fracture porosity
to the three permeability fields for interparticle porosity (Fig.
18).
Figure
18. Theoretical fracture air permeability-porosity relationship
compared to the rock-fabric/petrophysical porosity, permeability
fields (Lucia, 1983). W = fracture width, Z = fracture spacing.
This graph shows
that permeability
in touching-vug
pore systems has little relationship to porosity. Typical porosity-permeability
cross plots from touching vugs have low (less than 6 percent) porosity
and display unorganized scatter. An example is illustrated from
a Pennsylvanian reservoir in West Texas (fig. 19).
Figure
19. Touching vug pore space from Sacroc field (Pennsylvanian),
West Texas. (A) Photo of core slab and (B) porosity-permeability
cross plot.