Page 272 - Geology of Carbonate Reservoirs

CONCLUSIONS 253

they influence reservoir characteristics. Depositional reservoirs refl ect depositional
processes. Sedimentology and stratigraphy are the primary tools needed to distin-
guish facies and diagnose reservoir characteristics.
After the depositional successions and stratigraphic relationships are worked out
and after identifying facies that are predicted to have good reservoir potential, it is
sometimes unfortunately discovered that the predicted reservoir performance was
not achieved. Depositional characteristics may not always govern reservoir charac-
teristics; rather, the postdepositional changes that enhanced, created, or reduced
porosity and permeability instead govern reservoir quality. Diagenetic porosity and
accompanying permeability are commonly the key factors that determine carbonate
reservoir quality. Fractured reservoirs are not only distinctive in their mode of
origin, but fracture porosity and permeability are computed in different ways than
for depositional and diagenetic reservoirs.
Many variations are possible for these three end - member types of reservoirs and

there is no single model that satisfies all conditions for all genetic reservoir types.
In fact, some reservoirs may have geological characteristics that are so specialized
that their case histories are applicable only to those unique situations. This leads to
a philosophical point: the most important thing in working with carbonate reservoirs
is to keep an open mind, a fertile imagination, and a basic understanding of the main
processes that create end - member pore types in carbonates. Avoid the temptation
to choose an “ analog ” or a look - alike. Analyze and diagnose the reservoir on the
basis of its fundamental rock properties, how those rock properties relate to reser-
voir (petrophysical) characteristics, how the reservoir units are distributed in strati-
graphic space on different platform geometries, and whether diagenesis or fracturing
has infl uenced reservoir behavior. This requires not only a keen mind, good obser-
vational powers, and analytical thinking but also direct observation of rock samples
in cuttings or full - diameter cores.
Studying carbonates can be greatly rewarding because they contain about half
of the hydrocarbons on the planet, they are aquifers in many parts of the world,
they are hosts to sulfide ores on several continents, they are used for construction

material and road metal, they are an essential ingredient in cement, some forms of
calcite are used in optics (the original nicol polarizing prism was cut from Iceland
Spar), and dolomite is sold as a health - food supplement. Reservoirs and aquifers in
carbonate rocks are intriguing. They can pose challenges to the best geoscientist.
They can be complex but not so complicated that they are difficult to understand.

A closing reminder is that probably 60% of the already - discovered oil around the
world still remains in place and at least half that is in carbonate reservoirs. The
knowledge of how to extract some of the remaining oil at prices competitive with
exploration is within our grasp. The key is in understanding the spatial distribution
of porosity and permeability, the combined amounts of which defi ne fl ow units,

baffles, and barriers. More accurate maps of poroperm systems can be developed to

improve efficiency of sweeps made by injected fluids as they move from injection
to extraction wells. It seems certain that the economics of the petroleum industry
will increasingly focus on optimizing development of new discoveries and especially

on improved recovery in established fields. Improved knowledge of carbonate pore
systems will also aid in exploring for and management of groundwater resources in
carbonate aquifers. It will aid in developing better ways to predict contaminant
dispersal in groundwater aquifers. It is just waiting to be done.