Page 220 - Geology of Carbonate Reservoirs

ROCK PROPERTIES AND DIAGNOSTIC METHODS 201

and accurate determinations of mineralogical composition in multicomponent res-
ervoir rocks. One might wonder why mineralogical composition is important in
carbonate reservoirs because they consist mainly of calcite or dolomite. The answer
is that porosity and permeability in some reservoirs is highly dependent on miner-
alogical composition, such as in those that produce exclusively from intercrystalline
porosity in dolostones. In those reservoirs small percentages of “ accessory minerals ”

may significantly alter reservoir quality. Calcite, anhydrite, clay minerals, quartz, or
other minerals generally have a negative influence on reservoir quality in dolostone

reservoirs because those minerals usually plug pore spaces that would otherwise be
open. Without direct observation to confi rm mineralogical composition, the risk of
error increases in direct proportion to the number of different minerals that may
be in the rocks and to the reliability of the logging or seismic methods to estimate
mineralogical composition.

8.1 ROCK PROPERTIES AND DIAGNOSTIC METHODS

Carbonates and siliciclastics have very different origins. That fact is important
because the ways in which rock properties are interpreted to identify carbonate
depositional and diagenetic environments and facies and how those characteristics
determine reservoir behavior depend on understanding those differences. Three of
the most obvious differences were recognized by Ham and Pray ( 1962 ): (1) carbon-
ates form locally, within the basin of deposition; (2) carbonates are almost exclu-
sively composed of biological constituents; and (3) carbonates are highly susceptible
to diagenesis. The consequences of those differences are not often discussed in the

literature, yet they have profound impact on the final carbonate reservoir rock
composition, texture, and reservoir properties. Carbonate rock composition, at least
in reservoir rocks, is almost exclusively calcite (limestone) or dolomite (dolostones)
or a mixture of the two minerals. Carbonates are not classified on the basis of

mineral content as are siliciclastics, consequently, it is not possible to assign a tec-
tonosedimentary history to a carbonate rock based on its mineral composition.
Because carbonate depositional textures reflect biological (skeletal) characteristics,

chemically precipitated crystalline fabrics, or alteration of porosity by diagenetic
effects, they do not indicate a history of transportation, abrasion, and size sorting
as do siliciclastics and it is risky to interpret depositional environments in carbonates
on the basis of textural characteristics alone. That said, it is common practice to

assume that mud - rich rocks reflect low - energy environments and grain - rich rocks

reflect high - energy environments as reflected in the Dunham classification for detri-

tal carbonates. Rock fabrics in carbonates can be created or influenced by chemical,

mechanical, biological, or depositional processes. Terrigenous sandstone fabrics and
porosity are almost exclusively the result of detrital sedimentation with only limited
alteration by mechanical or chemical diagenesis during burial. Carbonate rocks may
behave as brittle or ductile material under stress, but they behave quite differently
than most siliciclastic rocks because they generally have crystalline fabrics while
siliciclastics have granular fabrics. Microscale deformation of crystalline material
involves crystal lattice structures such as twin planes or dislocation zones, along with
crystal boundary patterns. Deformation of many granular rocks involves grain
rotation and translation without deformation of crystal lattices. Under research