Page 54 - Geology of Carbonate Reservoirs

DEPENDENT OR DERIVED ROCK PROPERTIES 35

rock categories on the basis of their porosity, permeability, and pore throat sizes —
is widely discussed in the petroleum engineering and log analysts ’ literature,
however.
A simple method for grouping pore characteristics is all that is required for a
basic reservoir rock description. The results can be combined with other data to

compare pore characteristics with rock classifications, capillary pressures, saturation

values, and borehole log signatures. Ideally, porosity classifications should compare

closely with rock classifications in order to reveal which kinds of rock data corre-
spond most closely with porosity data in order to identify rock properties that can
act as proxies for porosity. Ultimately, these rock – pore data sets could be compared
with petrophysical characteristics in order to achieve the goal of identifying and
mapping reservoir flow units (a version of petrophysical rock typing). Traditional

classifications of carbonate porosity were not designed for that purpose, but the

well - known schemes by Archie (1952) , Choquette and Pray (1970) , and Lucia (1983)
are useful to illustrate the evolution in thinking about carbonate reservoir pore
systems. Finally, a new classification based on end - member genetic categories —

depositional, diagenetic, and fracture porosity — is presented as a more useful
alternative.

2.4.1.2 The Archie Classifi cation

One of the first, if not the first, carbonate porosity classifications was developed by
G. E. (Gus) Archie (1952) , who pioneered the study of electrical resistivity in rocks,
developed the principles that led to the Archie saturation equation, and investigated
methods to integrate geological data with laboratory petrophysical data and bore-
hole log signatures. His objective was to illustrate relationships between rock and
petrophysical properties in reservoirs.
The Archie porosity classification is based on textural descriptions of reservoir

rock along with the “ character ” of any visible porosity. Three textural categories are
termed Type I, II, and III, and four classes for visible porosity are identified as classes

A through D. Class A has no visible porosity at 10 magnifications, class B has visible

pores between 1 and 10 μ m, and class C has visible pores larger than 10 μ m but

smaller than rotary cuttings (roughly, about 2.0 mm). Class D includes large visible
pores such as solution vugs larger than cuttings samples.
Archie described Type I carbonates as “ crystalline, hard, dense, with sharp edges
and smooth faces on breaking. ” Under the binocular microscope, these rocks have
a matrix made of tightly interlocking crystals that do not exhibit visible intercrystal-
line porosity. For practical purposes, these rocks correspond to today ’ s mudstones
and dolomudstones. The Solenhofen Limestone is a good example of this type of
rock. Type II rocks are described as “ earthy ” or “ chalky ” with grains not larger than
about 50 μ m in diameter (just below the finest silt size), and they are composed of

“ fine granules or sea organisms. ” These rocks correspond to today ’ s true chalk, or

mudstones and wackestones that have probably undergone diagenetic alteration to
attain the chalky appearance. Type III carbonates are “ granular or saccharoidal. ”
Saccharoidal is a somewhat arcane term derived from the Greek, σακκηαρoυ ,

meaning “ sugar. ” Many fine grained carbonates, especially dolostones, exhibit small

crystalline mosaics that reflect light like so many sugar crystals. Granular carbonates
include today ’ s grainstones and packstones.