Page 271 - Geology of Carbonate Reservoirs

252 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

Now we have a common language about carbonates and reservoirs. We learned
how and where carbonate rocks form. We learned about fundamental, derived, and
tertiary rock properties that must be studied in order to get reproducible results in
our research on carbonates and to communicate those results in understandable

terms. Some of those terms include classification terminology for both rocks and

porosity. In particular, we learned that a genetic classification of carbonate porosity
is a necessary and powerful tool to help construct geological concepts to explain
the depositional, diagenetic, and tectonic histories that created reservoirs and
aquifers.
We found that many fundamental rock properties are related to reservoir proper-
ties. Porosity and permeability are derived properties that correlate directly with
depositional sedimentary texture, with sedimentary structures, and to a degree with
carbonate grain types. We focused attention on the rock properties that correspond

most closely with reservoir characteristics that, in turn, influence reservoir quality.
There we learned about saturation, wettability, capillarity, capillary pressure, and
how those reservoir characteristics are related to fundamental, derived, or tertiary
rock properties. After learning how rock and reservoir properties are related, we
learned how carbonate sediments are formed, how sediment accumulations become
stacked in stratigraphic arrays, and how the stratigraphic units can be correlated,
mapped, and exploited as reservoir units. We found that seven basic depositional
successions may be deposited in lateral arrays that characterize two end - member
depositional platform types — ramps and shelves. The variety of environments that
exist on these platforms produces that array of depositional successions, each of
which represents specifi c locations on the platforms. The lateral array of these suc-
cessions — from beaches and dunes to basinal rhythmites — is predictable for ramps
and shelves. Given the location of an ideal depositional succession, we can predict
the locations of other successions in updip or downdip directions. Each of the seven
ideal depositional successions on ramps and shelves has characteristic textures, grain
types, sedimentary structures, and ranges of taxonomic diversity that provide key
information to use in constructing depositional reservoir models for ramps and
shelves.
Knowing the array of ideal depositional successions across ramps or shelves
enables us to construct two - dimensional facies models for the platforms. Time is the
third dimension. Time and the accompanying sedimentation, erosion, or nondeposi-
tion comprise the 3D depositional model — the stratigraphic succession. We made a
conceptual jump from rock and reservoir properties and from 2D sedimentary
blankets that encompass only small increments of time to 3D stratigraphic units that
may include thousands or millions of years ’ worth of deposition. The mental jump
led to principles of stratigraphy, correlation, geological time, and sequence stratig-
raphy, or chronostratigraphy. Part of dealing with stratigraphy is that sedimentary

successions are created and influenced by sediment source and supply (the carbon-
ate factory), by platform subsidence or uplift, and by relative sea - level fl uctuations.
These large - scale phenomena usually occur in cyclical fashion so that the strati-
graphic record is replete with cyclical repetitions of depositional successions. Tracing
only the successions that have the greatest reservoir potential requires understand-
ing of the total process – response system.
The basic information about sedimentology and stratigraphy led to the heart of
this book — the three end - member types of carbonate porosity and how profoundly