Page 185 - Geology of Carbonate Reservoirs

166 DIAGENETIC CARBONATE RESERVOIRS

Mg - calcite. In short, highly porous parent sediment composed of metastable arago-
nite, Mg - calcite, or small calcite particles with high surface area to volume ratio will
undergo coalescive or porphyroid neomorphism with attendant loss of interparticle
porosity.

6.5.3 Pore Reduction by Replacement
Replacement of precursor carbonate minerals by silica, anhydrite, sulfi de minerals,
and dolomite can reduce original porosity by replacing both the original mineral and
the original rock fabric at the expense of depositional or pre - replacement diagenetic
porosity. Anhydrite, saddle dolomite, and silica are common replacement minerals
in carbonate reservoirs. Anhydrite is the stable form of calcium sulfate at depths
below about 3000 feet in the subsurface (Hardie, 1967 ), where it may be a burial
transformation of gypsum. Anhydrite may occur as massive beds, nodules, and pore

fillings, and as replacements that transgress grain boundaries and pore walls. Bedded

or massive anhydrite forms during deposition and may mark specific parts of strati-
graphic successions such as the tops of shallowing - upward successions or parase-

quences. Massive anhydrite layers may be identifiable by distinct signatures on
wireline logs or may otherwise be predicted to occur as cycle capping beds in para-

sequences identified by their sequence - stratigraphic stacking patterns. This type of
anhydrite is typically dewatered gypsum that has been altered during burial. Pore -
filling and replacement anhydrite usually form during early burial diagenesis as

sulfate - rich brines percolate downward through porous and permeable carbonates.
Pore fillings result from precipitation of gypsum or anhydrite from the migrating

brines. As brine migration continues, more and more pore spaces are filled and reac-
tions between the sulfate - rich water and the carbonate rock result in replacement
of carbonate by anhydrite. This type of anhydrite may occur in beds up to several
meters below the tops of shallowing - upward cycles, where the anhydrite derives from
interstitial brines that drained downward from exposure surfaces, restricted lagoons,
or tidal flats. Pore - fi lling and replacement anhydrite can dramatically reduce depo-

sitional porosity. On the positive side, anhydrite plugging may form seals to prevent
hydrocarbon leakage from reservoir - quality rocks below. It is relatively common in

shallowing - upward successions to find the best porosity and permeability in beds
beneath cycle - capping, anhydrite - cemented grainstones. Anhydrite may also come
from the deep - burial environment and form as a late burial cement or replacement
derived from upward - migrating fl uids that invaded the reservoir from below.
Saddle dolomite, silica, and sulfide replacements are not common in the shallow

burial domain; instead, they usually indicate deeper - burial diagenetic replacements.
Saddle dolomite occurs in two different settings: (1) fractured reservoirs (see Figure

8.18 ) in which deep subsurface fluids migrate up the fracture systems and into res-
ervoir pores, and (2) nonfractured reservoirs (Figure 6.8 ) in which thermochemical
sulfate reduction (TSR) is interpreted to be a major factor influencing the formation

of saddle dolomite (Machel, 1987b ). Silica replacement as chalcedony is common
in deep - burial replacements and may develop in association with saddle dolomite,

fl uorite, sulfide minerals, and hydrocarbons. Hydrothermal silicates also are known
as replacements for evaporite minerals (Ulmer - Scholle et al., 1993 ); consequently,
early burial sulfates such as anhydrite may be found to have been replaced by chal-
cedony or other varieties of quartz during later burial alteration.