Page 70 - Geology of Carbonate Reservoirs

TERTIARY ROCK PROPERTIES 51

found that the presence of gypsum in a Permian carbonate reservoir precluded the
use of the neutron and density logs for calculating porosity because the neutron log
measured bound water in gypsum as porosity. The low density of gypsum compared
to the surrounding dolomite and anhydrite dramatically increased the uncertainty
of what values to use in density calculations. Correcting S wt in bimodal porosity and
choosing m values for different pore types are discussed in Chapter 3 . The occur-

rence and significance of gypsum in carbonate reservoir rocks is discussed further
in Chapter 6 .

Many of the oil and gas fields around the globe were discovered decades ago.
As new technology and new knowledge become available, studies are made on old

fields to evaluate their economic potential as candidates for improved or additional
recovery. It is not always possible to find cores or cuttings to use in these studies;

therefore one of the most important but challenging tasks for the log interpreter is
to determine the lithology of a carbonate reservoir using only borehole logs. This
task is not easy nor does it always produce reliable results, especially when several
minerals are present in a single reservoir rock. A vivid example from this author ’ s
personal experience involved participating as the lead geologist on a team of engi-
neers and geoscientists who were competing with another team to resolve a dispute

over well spacing in an infill drilling (field development) program. The dispute cen-

tered on whether or not reservoir - quality rocks, mainly dolostones with high per-
centages of intercrystalline porosity, were present over large areas or were instead

widely scattered among nonreservoir - quality baffles and barriers. Lacking cores or
cuttings, the teams had to create lithological logs (synthetic rock descriptions) based
primarily on wireline log data. The teams reached significantly different conclusions

about lithological interpretations from identical logs from one reservoir zone in a
single field. Determining the proportions of carbonate and evaporite minerals when

they occur together is a well - known problem (Hashmy and Alberty, 1992 ). However,
if several minerals are present in the reservoir rock, as was the case in this author ’ s
experience, it is impossible to be completely certain about the presence and relative
proportions of limestone, dolostone, evaporite minerals, quartz, and clays from log
data alone. In such cases, cuttings or core samples are necessary to make direct
determinations of mineral composition to compare with log readings. If samples are
unavailable, data from different logs can be crossplotted to derive values that are
indicative of a particular rock type. One method in widespread use on carbonate

reservoirs is Schlumberger ’ s “ M - N ” plot, where the value of M derives from a
density - acoustic log crossplot and the value of N comes from a density - neutron
crossplot. The use of an M - N plot to determine lithology is described and illustrated
in Asquith and Krygowski (2004) . Rider (1996) points out that on the M - N plot,

shale and other minerals become separated into fields and porosity is eliminated.
The problem with the M - N plot, however, is that the geological value of the logs is
lost and there is, according to Rider, “ a tendency to rather obscure cross - plotting in
the vain hope of finding a unique ‘ shale point ’ or ‘ mineral point. ’ These points rarely

exist in nature. ”
Crossplotting can be done rapidly with modern personal computers (PCs) and it
is possible to process digital data from several kinds of logs simultaneously making
lithological interpretations easier and arguably more reliable. Included among many
examples of such interpretive programs in use today are those developed by such
companies as Petcom, Landmark, and GeoQuest. Most petrophysicists emphasize