Page 221 - Geology of Carbonate Reservoirs

202 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

conditions, variations in seismic velocity through carbonates have been shown to
depend on pore characteristics (Wang, 1997 ), but electrofacies mapping methods
using gamma ray and resistivity log data that are widely used on siliciclastic reser-
voirs are not useful in carbonates because carbonates do not ordinarily contain
radioactive minerals and carbonate pores are not simply indicators of depositional

textures. With enough well control in an established field, log signatures of carbon-
ate rock and pore characteristics can be indicators of facies characteristics, especially
if the reservoir is facies selective and relatively free of complications or partitioning
by diagenesis. Perhaps density and neutron logs may be included with gamma ray
and resistivity logs to enable a kind of electrofacies mapping to be done with rea-
sonable success. Density – neutron crossplots are commonly done to identify rock
types in carbonates. The Schlumberger M - N plot involves sonic, neutron, and density
readings to help identify mineral components in complex carbonate lithology (Burke
et al., 1969 ) and modern computer programs such as the proprietary “ STATMIN ”
system can generate reasonably reliable lithological logs by processing digital data
from a “ basket of logs. ” The differences between carbonates and siliciclastics are
examined further in the following paragraphs where the three categories of rock

properties described in Chapter 1 — primary (fundamental), secondary (derived),
and tertiary — are reviewed to illustrate the unique attributes of carbonate
reservoirs.

8.1.1 Fundamental Rock Properties and Depositional Reservoirs
Fundamental rock properties include texture, composition, sedimentary structures,
taxonomic diversity, and depositional morphology. The last two properties are not
commonly listed as “ fundamental rock properties ” in most texts but they are impor-
tant attributes of sedimentary deposits that must be included in thorough reservoir
studies. Fundamental rock properties provide the basis for defining lithofacies, or

lithogenetic units that make up depositional reservoirs. Diagenetic and fractured
reservoirs are simply altered versions of the original depositional version. The most
reliable method for identifying these fundamental properties in carbonates is direct
observation of cores or cuttings. Cores provide enough sample volume to determine
sedimentary textures, grain types, sedimentary structures, and biota. Cuttings usually
provide enough volume to determine mineralogy, grain types, and estimates of
texture. Logs are not very helpful in identifying fundamental rock properties in
carbonates. Facies types can be identified in siliciclastic sandstones by using the

shape of the gamma ray and resistivity or, with older logs, the SP – resistivity log
traces. When the paired traces outline a bell, a funnel, or a cylinder, the correspond-

ing sandstone facies are assumed to be channel - fill, deltaic, or reworked sheet sands,
respectively. Other “ type curves ” are assumed to be indicators of other of sand –
shale depositional successions. The underlying assumption is that the gamma ray,
SP, and resistivity logs are sensitive to vertical changes in grain size. In fact, that
assumption is false. The logs are not sensitive to grain size. The gamma ray tool
measures natural radioactivity that issues from the K, Th, and U found in clay min-
erals that are commonly incorporated in shales and mudrocks. The tool does not
measure grain size. In fact, “ hot limes ” and “ hot dolomites ” are commonly found in
carbonate reservoirs where particle size has nothing to do with the presence of
natural radioactivity. The SP and resistivity tools likewise measure electrical proper-