Page 243 - Geology of Carbonate Reservoirs

224 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

isopach map of the overlying St. Louis Limestone) reveals that the environment on
the crest of the paleo - high was favorable for colonization by crinoids and bryozoans.

Interval isopach mapping is the key to finding depositional thicks that developed at
the right time and in the right structural setting. Development of Conley Field
depended also on recognizing that the bryozoan - rich rocks retained more porosity
than the crinoid - rich ones because syntaxial overgrowth cements fi lled pore spaces
in the crinoid - rich sediments.

8.4 DIAGENETIC RESERVOIRS

All carbonate reservoirs have been affected by diagenesis to some degree, including

the examples in the preceding section that were classified as hybrid - depositional
reservoirs. The distinction between depositional and diagenetic reservoirs is usually
a matter of degree, not kind. It is more realistic to classify slightly altered deposi-
tional reservoirs as hybrids with depositional attributes dominant. Once it is known
that diagenesis has changed half or more of the original pore size and shape, the

reservoir is classified as a hybrid with diagenetic attributes dominant. If the diage-
netic changes are so great that none of the original depositional characteristics can
be recognized, the reservoir porosity is purely diagenetic. Two good examples of
such cases are a dolostone reservoir with intercrystalline porosity that has no visible
relationship to depositional texture, fabric, facies, or other depositional characteris-
tics, and a karst or paleocave reservoir in which dissolution diagenesis cuts across
all preexisting rock textures, fabrics, and facies to create the massive caves, caverns,
and karst features.

8.4.1 Finding and Interpreting Diagenetic Reservoirs

Formulating geological concepts for exploration and development of hybrid reser-
voirs is not very different from developing concepts for exploration and develop-
ment in reservoirs with depositional porosity because depositional attributes are the
benchmarks that measure how severely diagenesis has modifi ed reservoir porosity.
Hybrids with depositional attributes dominant can be treated essentially the same
as if they were depositional reservoirs. Hybrids with diagenetic attributes dominant
require different strategies and concepts because flow units, baffles, and barriers will

not always “ line up ” with depositional facies boundaries. In these cases, it is neces-
sary to determine how severely diagenesis has altered depositional porosity, what
types of diagenesis were involved, in which diagenetic environment the changes
took place, and when during burial history the changes took place. Remember that
diagenesis includes all the changes that happen to rocks after deposition and before
metamorphism. Pore - altering changes that happened early during burial history will
have different characteristics than late burial diagenetic changes and the environ-
ments in which early diagenesis takes place will be distinctive. For example, early
diagenesis must take place in the vadose zone, in one of the phreatic zones, or the
shallow subsurface environment. Each environment leaves distinctive traces that
can be identified even if later alteration “ overprints ” the early products. Each envi-

ronment is usually associated with some geological event that brought the pristine
rocks in contact with disequilibrium waters that caused change. These events include