Rock-Fabric
Classification
Classification
of Vuggy Pore Space
The addition
of vuggy pore space to interparticle pore space alters the petrophysical
characteristics by altering the manner in which the pore space is
connected, all pore space being connected in some fashion. Separate
vugs are defined as pore space that is interconnected only through
the interparticle porosity. Touching vugs are defined as pore space
that forms an interconnected pore system independent of the interparticle
porosity (Fig. 6).
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Figure
6. Geological/petrophysical classification of vuggy pore space
based on vug interconnection. The volume of separate-vug pore
space is important for characterizing the pore-size distribution. |
Separate-Vug
Pore Space
Separate-vug
pore space is defined as pore space that is 1) either within particles
or is significantly larger than the particle size (generally >2
x particle size), and 2) is interconnected only through the interparticle
porosity (Fig. 6). Separate vugs are typically
fabric-selective in their origin. Intrafossil pore space, such as
the living chambers of a gastropod shell; moldic pore space, such
as dissolved grains (oomolds) or dolomite crystals (dolomolds);
and intragrain microporosity are examples of intraparticle, fabric-selective
separate vugs. Molds of evaporite crystals and fossil molds found
in mud-dominate fabrics are examples of fabric-selective separate
vugs that are significantly larger than the particle size. In mud-dominated
fabrics, shelter pore space is typically much larger than the particle
size and is classified as separate-vug porosity whereas in grain-dominated
fabrics, shelter pore space is related to particle size and is considered
intergrain porosity.
In grain-dominated
fabrics, extensive selective dissolution of grains may cause grain
boundaries to dissolve, producing composite molds. These composite
molds may have the petrophysical characteristics of separate vugs.
However, if dissolution of the grain boundaries is extensive, the
pore space may be interconnected well enough to be classified as
solution-enlarged interparticle or touching-vug porosity.
Grain-dominated
fabrics may contain grains with intragrain microporosity (Pittman
1971). Even though the pore size is small, intragrain microporosity
is classified as a type of separate vug because it is located within
the particles of the rock. Mud-dominated fabrics may also contain
grains with microporosity, but they present no unique petrophysical
condition because of the similar pore sizes between the microporosity
in the mud matrix and in the grains.
Touching-Vug
Pore Space
Touching-vug
pore systems are defined as pore space that is (1) significantly
larger than the particle size and (2) forms an interconnected pore
system of significant extent (Fig. 6). Touching
vugs are typically nonfabric selective in origin. Cavernous, breccia,
fracture, and solution-enlarged fracture pore types commonly form
an interconnected pore system on a reservoir scale and are typical
touching-vug pore types. Fenestral pore space is commonly connected
on a reservoir scale and is grouped with touching vugs because the
pores are normally much larger than the grain size (Major et al.
1990).
Fracture porosity
is included as a touching-vug pore type because fracture porosity
is an important contributor to permeability in many carbonate reservoirs
and, therefore, must be included in any petrophysical classification
of pore space. Although fracturing is often considered to be of
tectonic origin and thus not a part of carbonate geology, diagenetic
processes common to carbonate reservoirs, such as karsting (Kerans
1989), can produce extensive fracture porosity. The focus of this
classification is on petrophysical properties rather than genesis,
and must include fracture porosity as a pore type irrespective of
its origin.
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