Page 179 - Geology of Carbonate Reservoirs

160 DIAGENETIC CARBONATE RESERVOIRS

neomorphic change is not understood, but it appears to occur preferentially in
grainstones linked to paleo - highs. This association led to the interpretation that the
neomorphism was an early burial phenomenon produced by invasion of waters that
were out of equilibrium with the original, probably Mg - calcite, ooids (Ahr, 1989 ).

Other evidence suggests that some of the disequilibrium fluids were introduced late

in the burial history of the Cotton Valley reservoir (Dravis, 1989 ; Fretwell, 1994 ).
Regardless of the time of formation of the microporosity, it is a common and wide-
spread type of diagenetic porosity that serves as the principal pore type in many
gas reservoirs in carbonate rocks. It is not a good candidate pore type for oil reser-
voirs because the pore dimensions are in the micrometer range with pore throats
in the submicrometer size range. Capillary pressures associated with such miniscule
pore apertures are great enough to block oil movement.
Neomorphic microporosity is diagenetic but it lacks the characteristic vugs, molds,
caverns, channels, and other solution - related features that typify dissolution poros-
ity; therefore its origin is attributed to stabilization reactions rather than macroscale
dissolution. It is important to recognize that these micropores can store natural gas
and they can store formation water. A common example of bimodal porosity in
carbonates is large pores, such as molds and vugs, together with micrometer - sized
micropores. Bimodal porosity of this type has to be recognized by direct examina-
tion of rock samples. If it is not recognized, calculated S w will be too high. In such
cases, the saturation values are S wt , or total saturation, rather than S we , effective satu-
ration. Asquith and Jacka (1992) emphasized the difference between total and
effective saturation and the significance it has on estimating hydrocarbon volume.

6.4.2 Enhancement by Solution Enlargement

Solution - enlarged porosity includes (1) enlarged interparticle pores, (2) moldic
pores, (3) vugs, (4) channels (including solution - enlarged fractures), and various
combinations of (1) – (4) that may be completely or partially formed (e.g., partial
molds with some original grain material remaining). Solution - enlarged pores are
small - scale (millimeter range) features. Large - scale (centimeter to meter range)
phenomena such as caves, caverns, sinkholes, and collapse features are discussed
later. Reservoirs with solution - enlarged and karst porosity may be associated with
(1) surface unconformities, (2) present or ancient topographic highs that have
undergone exposure to vadose or phreatic dissolution in meteoric or mixing zones,
(3) fracture zones that may have been exposed to waters undersaturated with
respect to CaCO 3 , and (4) subsurface permeability pathways that allowed passage
of reactive fluids migrating from below (mesogenetic dissolution). Surface uncon-

formities are produced by subaerial weathering and erosion. During exposure, mete-
oric water percolates through porous rock beneath the unconformity. Pore types
and pore volumes created or influenced by this exposure depend to a great extent

on the duration of exposure, solubility of rock constituents, and the degree of dis-
equilibrium between water and rock. Saller et al. (1999) found that short (a few

thousand years) duration of exposure to dissolution had little effect on depositional
porosity, but that intermediate duration (50,000 – 130,000 years) modified the pore

system by forming micropores and channelized porosity. They found that long dura-
tion time correlated with extensive porosity reduction by cementation, although
some channelized pores were enlarged. In sum, those authors argue that intermedi-