Geology Reference
In-Depth Information
selective pore systems. Solution pores are responsible
for the porosity of several of the worlds largest oil
fields.
cause a reduction in permeability. Pore-bridging ce-
ments seem to be more effective in reducing single-
phase permeability than pore-lining or pore-filling ce-
ments (Panda and Lake 1995).
Timing and location of porosity evolution
Porosity originates in different time intervals and
reflects the history of carbonate rocks (Box 7.3).
7.3.1.3 Porosity Measurements and Pore
Types in Thin Sections
7.3.1.2 Pore Geometry and Permeability
Porosity can be measured by physical methods or
in thin sections. The former method determines the vol-
ume of gas displaced from the pore spaces of a known
bulk volume of rock by capillary pressure. Informa-
tion on physical porosity measurements can be found
in Jennungs (1987).
Porosity and pore geometry are determined by mer-
cury capillary-pressure curves (Wardlaw and Taylor
1976, Kopaska-Merkel 1994), studies of three-dimen-
sional resin casts, and thin sections showing the size,
shape and distribution of the pores (Rieke et al. 1972;
Wardlaw 1976). Resin pore casts reveal a wide variety
of carbonate pore geometries (Walker 1978; Beckett
and Sellwood 1991). SEM techniques are ideal for ex-
amining both permeability and porosity. The combined
use of SEM and thin section information is useful in
illustrating porosity modifications due to solution and
dolomitization effects (Armstrong et al. 1980). Pore
throats are generally too small to be examined visually
in thin sections.
Pore geometry
influences the ability of the rock to
contain water and produce oil and gas. Reservoir prop-
erties are controlled by pore geometry, particularly the
mean size of pore throats (smaller spaces or constric-
tions of minimal cross-section area connecting larger
pores), and pore interconnections (Jodry 1972; Blief-
nick and Kaldi 1996). Reservoir characteristics depend
how pore spaces are arranged and interconnected, which
is a function of the size and shape of grains and crys-
tals. These criteria also control reservoir-rock produc-
tivity and recovery efficiency and must be included in
the differentiation of porosity types (Wardlaw and
Cassan 1978; Kopaska-Merkel 1994).
Note: Pores account for pore volumes, pore throats
for permeability!
Permeability
determines the ability of a fluid to pass
through a porous medium. It is measured as the rate at
which fluids pass through a sediment or rock. Perme-
ability is calculated according to Darcys Law and com-
monly expressed in millidarcy. It is largely related to
the size and shape of conducting pore spaces, the shape
and size of pore throats, and the specific surface area
within a pore space (Etris et al. 1988). Pore-throat size
is inversely proportional to capillary pressure (Wardlaw
and McKellar 1981; Kopaska-Merkel and Friedman
1988).
Flow rates decrease with the increase of fine-grained
matrix. Pure micrites exhibit the lowest permeability.
The micrite content of carbonate rocks, therefore, may
reflect original permeability conditions. In turn, early
cementation is strongly dependent on the primary car-
bonate facies types. These relationships are the basis
of the porosity classification proposed by Choquette
and Pray (1970). Flow rates depend strongly on pore
distribution: Interparticle and intercrystalline pores
should provide higher permeability than molds or
intraparticle pores. Pore geometry, especially the size
and the shape of the interconnections between adja-
cent pores, is a major controlling factor.
Permeability decreases during diagenesis because
of compaction and cementation. Intergranular cements
Quantitative determination of porosity from thin sec-
tions is commonly obtained by point counting or vi-
sual estimation. A useful method for a rapid and accu-
rate computer-assisted determination of porosity in a
two-dimensional, digitized image of thin sections is de-
scribed by White et al. (1998).
The relationships of thin-section porosity to mer-
cury-injection porosity and between mercury poros-
imetry and pore types were discussed by Wardlaw and
Li (1988), Etris et al. (1988), McCreesh et al. (1991),
and other authors combining Petrographic Image An-
alyis (Ferm et al. 1993; Ehrlich 1984) with statistical
analysis.
Differentiating porosity distribution and cement vol-
ume in thin sections and polished sections (Halley 1978)
is considerably facilitated by
staining techniques
(Ruz-
yla and Jezek 1987) and
digital imaging
(Ruzyla 1986;
Frykman 1992; Bowers et al. 1994; Carr et al. 1996;
White et al. 1998; Anselmetti et al. 1998). These meth-
ods allow open pore space (Pl. 30) as well as pores
closed by carbonate cements to be recognized and dif-
ferentiated.
