Page 223 - Geology of Carbonate Reservoirs

204 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

successions is present at a location. Using the methods outlined in Chapters 4 – 6 ,
the 3D anatomy of reservoirs with depositional and some types of hybrid porosity
can be reconstructed. In practice, because cores are rarely available for reservoir
studies, wireline logs are usually the primary source of information for stratigraphic
correlation, petrophysical calculations, and lithological logs, including computer -
generated, multimineral lithological logs. Seismic data is the primary method to
identify structures and large - scale stratigraphic features in the subsurface. When
used together, information from logs, seismic records, and direct examination of
rocks can enable geoscientists and engineers to construct the best possible reser-
voir model.

8.1.3 Derived Properties: Porosity and Permeability
Determining porosity – permeability relationships in carbonate reservoirs requires
different methods than those used for siliciclastics because porosity in carbonates
can be any of three types or some combination of them. Although some rare sand-
stone reservoirs and aquifers have intragranular porosity in leached feldspars,
porosity in siliciclastic sandstones is nearly always intergranular such that semilog
plots of sandstone permeability versus porosity are linear with comparatively little

point scatter away from the best - fit line. This close correspondence between mea-
sured φ and k from core analyses is routinely used to estimate siliciclastic reservoir
permeability in fields where one or two cores were taken but most wells have log

data only. Estimates of permeability are obtained from semilog plots of measured
k versus φ , which were obtained from routine core analyses. An equation in the form
of y = mx + b is obtained from linear regression of the points on the φ – k plot. To
estimate k in wells without core analyses, log - derived φ values are “ plugged into ”
the just - established linear regression equation and estimates of k are “ backed - out. ”
This method is also used on carbonate reservoirs, because in many cases it is the
only method available to estimate k in the absence of core data. However, caution
must be exercised with the estimated values because the method assumes that a
semilog linear relationship exists between porosity and permeability. This is not true
for carbonate reservoirs that lack interparticle porosity such as those with intrapar-
ticle, fenestral, vuggy, moldic, channel, fracture, and dual porosity. It may be valid
for some intercrystalline pore systems in pure dolostones with comparatively large
crystal sizes.

8.1.4 Tertiary Properties and Petrophysical Characteristics

Tertiary properties are indirectly measured by wireline logs and to an extent by the
seismograph. As mentioned before, logging tools do not make direct measurements
of primary or secondary rock properties; they measure other properties that serve
as proxies for them. Proxy parameters include acoustic transmissivity, electrical
properties, magnetic resonance characteristics, and natural radioactivity. Modern
logging tools provide relatively good data for interpreting lithology and porosity in
many carbonate reservoirs except those with multicomponent mineralogical com-
position where dolomite, calcite, quartz, clay minerals, and anhydrite may be present
in varying percentages. Recent advances in software applications reportedly can
compute accurate lithology in rocks with as many as five minerals. Although lithol-