Page 57 - Geology of Carbonate Reservoirs

38 CARBONATE RESERVOIR ROCK PROPERTIES

percentage values are assigned for abundance. In the end, one can represent indi-
vidual pore categories by a code string that names the pore and indicates the degree
of fabric selectivity, pore size, direction of diagenetic change, and estimated
abundance.
Since the work by Choquette and Pray (1970) , the literature on carbonate dia-
genesis has grown and terms such as eogenetic, telogenetic, and mesogenetic are

not widely used. Diagenetic environments are classified on the basis of their rela-
tionship to the fresh and marine water tables and on their water chemistry. Above
the water table is the vadose zone , below the water table is the phreatic zone , and
below the phreatic zone is the subsurface burial zone . Deep and shallow burial are

subjective terms, as there is no unique mineralogical or fabric definition to distin-
guish between them. Qualitatively and subjectively, the burial domain can be divided
into shallow - and deep - burial environments with the shallow - burial environment
differentiated from the overlying meteoric phreatic zone by its different water
chemistry and somewhat greater overburden pressure and temperature. The deep -
burial environment is distinguished by its significantly different water chemistry and

its elevated pressure and temperature: 60 – 200 ° C, for example. Effects of pressure
can be interpreted from the style of grain contacts, grain breakage, and stylolitiza-
tion. Temperatures of burial diagenetic products can be determined by stable isotope
geochemistry or fluid inclusion geothermometry. Porosity affected by burial diagen-

esis may be recognized by exotic mineral cements or pore - fillings such as fl uorite
and sphalerite. Exotic crystal habits can also indicate high temperatures. Saddle
crystal dolomite usually indicates deeper burial conditions. Deeper burial water is
influenced by upward migrating fluids expelled from buried sediments. This burial

water is unaffected by phreatic flow and it usually imparts distinctive trace element
and isotopic signatures to minerals that crystallize at depth (Dickson et al., 2001 ).

Burial compaction can produce fitted or penetrative rather than tangential grain
contacts, along with other pressure - solution fabrics such as stylolites. Some of these
diagenetic characteristics are treated in the Choquette – Pray classifi cation. Finally,
fracture porosity is classified as non - fabric - selective. Normally, fractures cut across

depositional and diagenetic fabrics, but in the case of some dolomite – limestone rock
combinations, dolomite may fracture selectively because it behaves as a more brittle
material than limestone.

In sum, the Choquette – Pray classification is a useful method to describe carbon-
ate porosity but it was not designed to aid in determining the spatial distribution of
different pore types. For example, fabric - selective pores in crystalline dolostones are
treated identically as intergranular pores in an oolite grainstone, yet the origin of
the two pore types — diagenetic and depositional, respectively — is signifi cantly dif-
ferent and requires different strategies for correlating the pore types at stratigraphic
scale. The non - fabric - selectivity criterion is also insufficient to differentiate between

mechanical fractures and large - scale diagenetic features such as caverns and con-
nected vugs, all of which exhibit extreme petrophysical behavior and which may
require different methods for correlation between wells within a fi eld.
The Choquette – Pray classification has limited usefulness in determining relation-

ships between rock and petrophysical properties because it focuses on fabric selec-
tivity, drawing one to relate reservoir porosity to rock fabric. As discussed earlier,
rock fabric may represent mechanical sedimentation, biological growth processes,
or diagenetically produced crystallinity. Fabric selectivity that is also equivalent to