Page 47 - Geology of Carbonate Reservoirs

28 CARBONATE RESERVOIR ROCK PROPERTIES

depositional porosity, they usually have comparatively simple intergranular pore
systems, and porosity is predictably related to facies geometry. However, because
they may have high intergranular porosity, they are susceptible to early cementation
and compaction that reduce pore and pore throat size. As rocks with high grain
content commonly occur near the tops of shallowing - upward cycles, they are rela-
tively easy to locate in repetitive sequences of these cycles. Some cycles terminate
in evaporite “ capping facies ” with pores in the underlying grainstones and pack-
stones plugged with gypsum, anhydrite, or halite. In those cases, diagenesis some-
times compensates for pore plugging at the cycle tops, because dolomitization
commonly accompanies evaporite formation and it may be linked with enhanced
porosity in midcycle wackestone and packstone facies.

2.3.2 Classification of Reef Rocks
The word “ reef ” still prompts animated discussion and disagreement among geolo-
gists. Much of the older terminology on reefs centers on the academic issue of
whether reefs are “ ecological ” or “ stratigraphic, ” as described by Dunham (1970) .
An ecological reef is built by constructor organisms that have the “ ecological poten-
tial ” to form wave - resistant frameworks. That is, they must be made of sturdy skel-
etal structures that grew presumably in the midst of breaking waves. Many reefs
throughout time grew in environments that were not exposed to breaking waves
and many biogenic buildups lack sturdy skeletal frameworks, especially buildups
constructed of micrite, or carbonate cement, or microbial thrombolites and stro-
matolites. Terminology is not a major issue for reservoir studies or for carbonate
sedimentologists who follow the more modern style of classifying all sturdy skeletal
buildups as frame - built , or skeletal reefs , and all of those buildups without sturdy
skeletal frameworks as reef mounds (Tucker and Wright, 1990 ). However, additional

detail is needed in reef classification schemes to describe the fundamental rock
properties of the reef as they relate to reservoir porosity, permeability, and
connectivity.
Reservoir characteristics in reefs vary with the type of constructor organisms,
with the relationship between constructor organisms and associated reef detritus,
and with growth patterns of reef complexes in response to prevailing hydrologic
conditions. Reefs built by calcified microbes, for example, have high proportions of

lime mud and cement but few skeletal framebuilders. Skeletal framework reefs, reefs
built up as repeated layers of pavement - like organic encrustations, and reefs formed

by the current - baffling and sediment - trapping action of benthic organisms such as
sea grasses and algae present unique rock fabrics and pore characteristics. Dense
encrustations by calcareous algae exhibit internal microstructures that differ from
those of porous sponge or coral skeletons. Patterns of reef growth vary in response
to the depth of the photic zone, to oxygenation and nutrient content, to turbidity,
and to water agitation by waves and currents. For example, modern corals grow in
sheet - like or dome - like fashion in deeper water because they need light for their
photosynthesizing symbionts, the zooxanthellae. Stromatoporoids, major construc-

tor organisms in Silurian and Devonian reefs, took on specific growth forms in
response to higher or lower levels of wave and current activity.
Facies patterns associated with reefs vary as a function of the hydrologic regime.
Shallow - water (in most modern oceans this is less than about 10 - m depth) reefs