Page 177 - Geology of Carbonate Reservoirs

158 DIAGENETIC CARBONATE RESERVOIRS

Hydrocarbon inclusions Pores
and stains
Grains Stylolites
Oolites Metal sulfiedes, fluorite

Figure 6.6 A sketch of a hypothetical thin section showing associations of diagenetic attri-

butes characteristic of deeper burial (mesogenetic) diagenesis. The numbers refer to late
burial dissolution events that cross - cut the following numbered features: (1) saddle dolomite;
(2) coarse, burial calcite cements with hydrocarbon inclusions; (3) cemented fractures with
hydrocarbon inclusions in the cement; (4) along fractures that postdate stylolitization (note
the jagged stylolites that cut left to right across most of the drawing); (5) along stylolites
themselves; (6) along hydrocarbon stained fractures; (7) of cements associated with metallic

sulfides or fluorite; (8) of compacted grains; (9) of pressure - solution compacted grains; and

(10) of postcompaction cements. In short, late burial dissolution has cut across all previous
features including those formed during earlier moderate to deep burial. These examples
partly illustrate why carbonate reservoirs may have good porosity and permeability even at
great depths in the subsurface. (From Mazzullo and Harris (1992) . Reprinted with permission
of the American Association of Petroleum Geologists.)

and Harris, 1992 ). Undersaturation with respect to CaCO 3 in burial fluids is usually

because these fluids may be rich in CO 2 , H 2 S, or organic acids. A sketch showing ten

different occurrences of mesogenetic dissolution is shown in Figure 6.6 . This late
burial dissolution is one of the reasons that deeply buried carbonate reservoirs have
enough porosity and permeability to produce hydrocarbons, even though the stan-
dard “ dogma ” is that deeply buried carbonates typically have 5% or less porosity.
In short, pore enhancement by dissolution diagenesis can create a range of pore
sizes and shapes with widely ranging degrees of connectivity. A brief review of dia-
genetically enhanced porosity follows, tracing the development of pores ranging in
size from micrometer - sized micropores to karst caves and caverns.

6.4.1 Enhancement by Recrystallization

Decades ago, all diagenetic changes that affected texture, fabric, and even mineral-
ogy in carbonates were included in the umbrella term recrystallization . As discussed