Page 244 - Geology of Carbonate Reservoirs

DIAGENETIC RESERVOIRS 225

such things as uplift and exposure, exposure after sea - level lowering, submergence
after sea - level rise, or exposure to mineralized waters that resulted in precipitation
of distinctive cements, among other types of change. The characteristics of early
and late diagenesis and the environments in which they occur are discussed in
Chapter 6 .
Interpreting purely diagenetic reservoirs can be tedious if not truly diffi cult
because unraveling the history of diagenesis usually requires determining ancient
hydrological history rather than structural or depositional histories. For example,
both dolomitization and dissolution are accomplished in nature by rock – water
interaction. Dolomitization results when precursor carbonates are exposed to Mg -

rich fluids and Mg is exchanged for some of the Ca in the precursor. The paleohy-

drological question is: When and where did the Mg - rich fluids originate and what
ancient hydrological mechanism brought them in contact with the precursor
rocks?
Dissolution occurs when undersaturated fluids are brought in contact with

carbonates. When and where did those fluids originate and how did they come in

contact with the carbonates? An appreciation of these unknowns puts sharp focus
on the value of interpreting the time of diagenesis by studying cross - cutting relation-
ships in thin sections. Seemingly insoluble problems can be simplified by establishing

the timing of diagenetic changes that had the greatest impact on porosity. Knowing
the timing of the diagenetic events relative to the time of deposition and early lith-

ification can enable workers to concentrate on the paleohydrological regime that
caused the changes to occur. Dolomitization may have occurred soon after deposi-

tion, as in the seepage - reflux model, which postulates that Mg - enriched brines move

by density flow outward and downward from coastal lagoons through porous and
permeable shoreline carbonates. In this case, it is relatively simple to determine
when and where the fluids originated and how they came in contact with the precur-

sor rocks. Cave formation and massive dissolution are common at the top of the
meteoric phreatic zone and, in some climates, in the vadose zone (McIlreath and
Morrow, 1990 ). Most of the literature describes porosity associated with collapsed
paleocaves and karst that formed in continental environments; however, recent
studies (Smart and Whitaker, 2003 ) indicate that caves formed in mixing - zone envi-
ronments along marine coastal margins may be better analogs for collapsed paleo-
cave and karst porosity.
Mapping ancient water tables is more difficult than identifying hypersaline

lagoons that were adjacent to shorelines at or near the time of deposition because
lagoons and shorelines have distinctive lithofacies. Ancient water tables do not.
Tracing water tables depends on cement mineralogy, cement crystal habit, trace
element content of calcite or dolomite cements, isotopic analyses of cements, and
the physical appearance of rocks. Identifying these characteristics requires samples
from borehole cores or outcrops. Microscopic study of cements and their links with
paleohydrology gave rise to a specialized branch of petrography known as “ cement
stratigraphy. ” Meyers ( 1974 ) did pioneering work on cement stratigraphy of some
Mississippian skeletal grainstones in the Sacramento Mountains of New Mexico to
unravel the history of cementation and its relationship to paleohydrology. Later,
Grover and Read (1983) , among others, used cathodoluminescence to distinguish
between burial cements formed in different zones within an ancient aquifer. Cath-
odoluminescence is sensitive to the trace element composition of carbonate cements,