Page 69 - Geology of Carbonate Reservoirs

50 CARBONATE RESERVOIR ROCK PROPERTIES

Smooth Serrate

Bell

Cylinder

Funnel

Figure 2.15 Log shape classification. When paired with resistivity log traces, these typical

gamma ray or SP log curves can be imagined to describe bell, cylinder, and funnel shapes.
Alhough these logs do not measure grain size directly, the bell, cylinder, and funnel shapes
are routinely used to identify fining - upward, uniform, and coarsening - upward textural trends

in siliciclastic sandstones. Maps showing trends in the distribution of log shapes at fi eld scale
are known as electrofacies maps. Log shapes are not reliable indicators of texture or facies
character in carbonate rocks; consequently, electrofacies mapping is generally limited to
sand – shale successions.

diagenetic facies from wireline logs alone; by extension, it is generally not possible
to make universally applicable electrofacies maps of carbonate reservoirs. Some
methods, such as the Schlumberger SPI ™ (secondary porosity index), have been
reasonably successful in estimating the different proportions of “ primary ” and “ sec-
ondary ” porosity with wireline logs. Anselmetti and Eberli (1999) used a similar
method that they named the “ velocity - deviation log. ” Their method incorporates
sonic and neutron - porosity or density logs to detect variations in log responses that
correspond to differences in pore characteristics. These methods have to be cali-
brated against real rocks and pore characteristics before the operator can be reason-
ably certain about the results. These methods notwithstanding, depositional and
diagenetic pore types in carbonates are not generally detectable by wireline log
traces because most carbonate porosity is not simply depositional and interparticle
in nature. Acoustic signatures are not unique to one pore type, radioactive minerals
common in sand − shale sequences are not usually present in carbonates, and there
are no reliable wireline log methods to measure depositional particle sizes in car-
bonates because carbonate particles are altered or obliterated by diagenesis. It is
more useful to focus on methods such as the NMR log that can measure pore
geometry in carbonates. Other challenges for the log interpreter include determin-
ing reliable petrophysical calculations in carbonates that have a variety of pore types
and sizes. Carbonate reservoirs can have bimodal (micropore − mesopore) systems
that require careful work to distinguish between effective water saturation, S we ,
and total water saturation, S wt (Asquith and Jacka, 1992 ). Calculating a reliable S w
depends on knowing which m (Archie cementation exponent) value to use. In res-
ervoirs with vuggy or moldic porosity, m may be 3 or 4, but in fractured reservoirs
it may be close to 1. The presence of certain minerals can infl uence petrophysical
calculations in carbonates, too, as emphasized by Major and Holtz (1997) . They