Page 121 - Petrophysics 2E

94 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

such as sandstones, conglomerates, and certain oolitic limestones [7].
However, it is important to emphasize that both types of porosity often
occur in the same reservoir rock.

VISUAL DESCRIPTION POROSITY CARBONATE ROCKS
IN
OF
The role played by the visual description of pore space in carbonate
rocks has changed considerably since the development of a method
for classrfying carbonate reservoir rocks in 1952 by Archie [SI. The
development of well logging technology has provided the petroleum
industry with effective and direct methods to measure the in-situ
porosity of a formation. The visual description of the pore geometry,
however, is still needed to estimate the effects of (1) the grain
size; (2) the amount of interparticle porosity; (3) the amount of
unconnected vugs; (4) the presence of fractures and cavities; and (5) the
presence or absence of connected vugs on the porosity-permeability
relationship and other petrophysical parameters of naturally fractured
reservoirs. Lucia presented field classification of carbonate rock pore
space based on the visual description of petrophysical parameters of
a large number of samples [9]. He also discussed basic geological
characteristics necessary for the visual estimation of particle size and
recognition of interparticle pore space, and connected and unconnected
vugs.
Figure 3.3 shows two common types of particle sizes based on
artificially prepared samples containing various kinds of carbonate
particles: large sand sized particles such as those found in packstone
or grainstone deposits, small silt-to-clay-sized particles such as mudstone
or wackestone [9]. The particle size of primary interest is that of the
supporting framework because interparticle porosity of the matrix rock is
controlled by the size of the particles. The concept of support in defining
particle size in dolomites is illustrated in Figure 3.4 [9]. If the dolomite
crystals form a continuous, supporting network, their size controls the
connected pore size. The dolomite crystal size is of primary interest when
it is the same or larger than the sediment particle size, such as observed
in dolomitized limestone or wackestone rocks. However, the sediment
particle size becomes of primary interest if the sediment particle size is
larger than the dolomite crystal size, as is usually the case in dolomitized
grainstones or packstones [9].
Recognition of intergranular porosity depends on the size and shape of
grains in the rock matrix. In coarsely grained rocks, the intergranular pore
space may be identified with the naked eye. In finely grained limestones
or dolomites, for example, the intergranular pores are more difficult