Page 186 - Geology of Carbonate Reservoirs

POROSITY REDUCTION BY DIAGENESIS 167

Figure 6.8 Photomicrograph of saddle dolomite replacing micritized ooids in the Jurassic
Cotton Valley Formation at Teague Townsite Field, Texas. The dark stains at the margins of
the replacive saddle dolomite are residual hydrocarbons, suggesting that the dolomite - forming
liquid was associated with the hydrocarbon migration “ front. ” The width of the photo is
1.5 mm.
6.5.4 Pore Reduction by Cementation

Cementation can occur several times in the diagenetic history of a carbonate rock
beginning with cementation in the marine environment just after deposition and
continuing through vadose, shallow, intermediate, and deep - burial environments.
The literature on early marine cementation is too extensive to summarize here, but
one of the early and dramatic examples of marine cementation is in a paper by
Shinn (1969) . He described a marine cement crust on a soft drink bottle found on
the shallow seabed of the Persian Gulf. At the time, this discovery was particularly
noteworthy because a controversy in carbonate sedimentological circles during the
1950s centered on whether submarine cementation could actually happen. Marine
phreatic cements usually coat the entire perimeter of constituent particles, giving
rise to the name “ isopachous ” marine cement. Original mineralogy of marine
cements in today ’ s oceans consists primarily of Mg - calcite and aragonite in various
crystal habits ranging from micrite to needle crystals to botryoidal masses.
The mineralogy and crystal forms of carbonate cements change as water chem-
istry and diagenetic environments change from marine phreatic to meteoric phreatic
to shallow and deep subsurface waters. Folk (1974) was one of the first to emphasize

the importance of Mg content in interstitial water, salinity of the water, and environ-
ment of diagenesis (vadose, phreatic, or subsurface) as controls on the mineralogy
and crystal form of the resulting cements. Marine phreatic cements in today ’ s oceans
are Mg - calcite or aragonite because abundant Mg favors precipitation of aragonite