Page 171 - Geology of Carbonate Reservoirs

152 DIAGENETIC CARBONATE RESERVOIRS

dolomite/calcite ratios, enhance porosity and permeability, and generally create
good reservoir rocks. This, along with the tetrahedral geometry of dolomite inter-
crystalline pore throats that we discussed in Chapter 3 , is probably the main reason
why dolostones have higher permeability than limestones with the same porosity.
Recall from Chapter 3 that the expression for capillary pressure can be written dif-
ferently for tetrahedral porosity in dolostones as compared to interparticle porosity
in limestones and sandstones. Petrographic examination shows that crystals in
porous dolostones are generally uniform in size and euhedral to subhedral in form.
Dolomite crystals do not usually exhibit compromise boundaries, but calcite crystals
almost always do. That is, dolomite crystals appear to have stopped growing when
initial contact was made with adjacent crystals — a phenomenon described as contact
inhibition by Wardlaw (1979) . It is probably the idiomorphic (well - formed) shape,
uniform crystal size, and absence of compromise boundaries between dolomite
crystals that give rise to the sucrosic texture described by Archie (1952) and that
make this rock type a more permeable reservoir than limestones with comparable
porosity (Wardlaw, 1979 ). Replacement of limestone by dolomite does not a priori
produce a net increase in reservoir porosity, however. This is particularly true if the

dolostone/limestone ratio is small. Moreover, it is difficult to prove that calcite was
removed from dolomitic rocks by dissolution because most dolomite crystals in
reservoir rocks show little evidence of corrosion or dissolution. Instead, they are
typically euhedral to subhedral and exhibit sharp crystal edges. Perhaps the volume
of dissolved calcite was small compared to the volume of dolomite remaining, and
because dolomite is less soluble than calcite it was comparatively unaffected. The
complex association between dolomite and reservoir porosity is reviewed by Sun
(1995) , who concludes that porosity evolution in dolomites involves both syndolo-

mitization dissolution and postdolomitization modification such as karstifi cation,
fracturing, and burial corrosion. This line of investigation was pursued by Saller and
Henderson (1998) , who described porosity increasing basinward in dolostone res-
ervoirs on the Central Basin Platform in West Texas. Those authors proposed that
dolomite was more abundant on the updip portion of the platform, where evapora-
tion had concentrated the seawater, and that such abundance allowed “ excess ”
dolomitization to occur in the form of replacement dolomite followed by dolomite
cementation. This scenario is similar to that described by Lucia and Major (1994)
for the Plio - Pleistocene carbonates of Bonaire, Netherland Antilles. Lucia and
Major emphasize that dolomitization of limestones does not lead a priori to increased
porosity and may, in fact, lead to decreased porosity after continued emplacement
of diagenetic dolomite cements. In their study of dolomitized rocks in the Permian
Basin, Saller and Henderson (1998) argued that as diagenetic fluids migrate basin-

ward, they could become less saturated with respect to dolomite after much of it
had been “ used - up ” by replacement and cementation. From this point basinward,
the migrating brines could trigger simultaneous dolomitization and dissolution of
CaCO 3 with the end result being more porous and permeable dolostones near the
shelf margin, where excess dolomitization as cement did not occur. Morrow (2001)
argued that the process described by Saller and Henderson (1998) could have
occurred in two stages: a massive episode of dolomitization on the updip platform

followed by downdip reflux of brines capable of forming dolomite but undersatu-
rated with respect to CaCO 3 in order to dissolve any nonreplaced limestone. The
take - home message for readers is this: as Wardlaw (1979) observed, dolostones are