Page 190 - Geology of Carbonate Reservoirs

DIAGNOSING AND MAPPING DIAGENETIC RESERVOIRS 171

graphic feature (unconformity surface or upper part of a paleoaquifer). Exposed
highs subjected to dissolution can provide CaCO 3 that migrates down the hydraulic

gradient to be precipitated as pore - filling cement. In some cases it may be possible
to identify where along the paleowater table certain cements were precipitated by
examining their geochemistry, petrography, and cathode luminescence (Grover and
Read, 1983 ). Similarly, the timing, associated paleohydrological environments, and
controls on replacement by dolomite, and dolomite stabilization (neomorphism)
were investigated by Barnaby and Read (1992) and Monta ñ ez and Read (1992) ,
respectively. Meyers ’ (1974) pioneering work on “ cement stratigraphy ” in carbonate
burial environments showed that it is possible to identify and correlate individual
calcite cement types over several square miles by their petrographic and geochemi-
cal characteristics.
Patterns of compaction and pressure solution can be somewhat more diffi cult to
identify because they are the result of mechanical and chemical processes acting
simultaneously. Meyers (1980) was able to determine the timing of compaction in
Mississippian limestones from New Mexico on the basis of their physical appearance
in chert replacements for which relatively precise age dates were available. That is,
he found compacted and broken constituent grains within chert replacements of a

specific age but not in rocks of other ages. He found that depositional porosity was
reduced by at least 50% and up to as much as 75% in the compacted rocks. Fur-
thermore, he was able to determine that burial compaction took place between
Mississippian deposition and burial until Permian times. The methods used by
Meyers (1980) can be employed to determine the amount of porosity reduction by
mechanical and chemical compaction, the timing of compaction events, and perhaps
the spatial extent of compaction - reduced porosity at reservoir scale. Because many
diagenetic processes are linked to subaerial exposure surfaces or paleo water tables,
clues to diagenetic porosity reduction can be found when those features are
recognized.

6.6 DIAGNOSING AND MAPPING DIAGENETIC RESERVOIRS

Diagenesis usually follows more than one pathway in creating, enhancing, or reduc-
ing porosity. Hybrid pore types are probably the rule rather than the exception.
Original (depositional) porosity may be reduced, destroyed, or enlarged (enhanced).
Purely diagenetic reservoirs such as blanket dolomitization of limestone parent
rocks cannot be traced directly back to depositional rock properties. Hybrid pore
systems where depositional attributes are dominant will exhibit either enlarged or
reduced intergranular, intragranular, fenestral, shelter vug, keystone vug, and “ reef ”
pores. Diagnostic procedures to evaluate this type of hybrid reservoirs are essen-
tially the same as those used to identify and map depositional reservoirs. In short,
reservoir porosity is facies selective but diagenetically altered. Maps of depositional
facies are still proxies for porosity.
If diagenesis does not follow facies, lithostratigraphic, or chronostratigraphic
boundaries, then some other cause – effect relationships must be examined. The less

obvious boundaries of diagenetic reservoirs can be identified by examining paleo-
structure, paleohydrology, and relative timing of diagenetic events. Further diagnos-
tic methods include recognition, classification, and thin section measurement of pore