Page 165 - Geology of Carbonate Reservoirs

146 DIAGENETIC CARBONATE RESERVOIRS

hydrocarbons liberated from kerogen are oil rather than gas. Traditional metamor-
phic minerals and facies do not form at temperatures and pressures that accompany
organic metamorphism. A good example of hydrothermal mineralization in the
metamorphic domain rather than the sedimentary diagenetic domain is the green-

schist metamorphic mineralization that occurred at temperatures of about 390 ° C
in subsurface sandstones around the Salton Basin geothermal area (Helgeson, 1968 ;
Hulen et al., 2003 ). Metamorphic petrologists suggest that the onset of metamor-
phism is marked by the appearance of muscovite, epidote, albite, lawsonite, laumon-
tite, or pyrophyllite (Ehlers and Blatt, 1982 ). Although all of these minerals do not
form at the same temperatures and pressures in the subsurface, they are reliable
indicators of metamorphism and would not be expected to occur together with oil

or gas. That said, one would probably never find these minerals in sedimentary car-
bonates, particularly in reservoir rocks. In practice, deep - burial diagenesis in carbon-
ate reservoir rocks never reaches the level of conventionally defi ned metamorphism

even though minerals such as metallic sulfi des, fluorite, and saddle dolomite may be

formed during the invasion of deep - burial fluids into carbonate reservoirs. Miner-
alization of this type is called MVT (Mississippi Valley type) diagenesis. It is a form

of mineralization that commonly occurs in association with the influx of hydrocar-

bons and their associated mineralizing fluids during migration from source rock to
reservoir.
6.1.2 Diagenetic Processes
The mechanisms of diagenesis may be mechanical, biological, or chemical, or several
of them in some combination. An example of mechanical diagenesis is volume
reduction by compaction during burial. In fine - grained rocks such as lime mud-

stones, compaction is usually accompanied by expulsion of interstitial water, reduc-
tion in thickness of beds or laminae, and an increase in bulk density. In granular
rocks or some reef rocks, compaction can modify grain packing; it can cause penetra-

tive grain - to - grain contacts (fitted contacts) to form or may cause grain breakage.
Stylolites (Figure 6.1 ) are produced by the combination of compaction and dissolu-
tion and are common in all carbonate rock textures. After chemical diagenesis,

mechanical compaction has the greatest influence on carbonate reservoir porosity.
Biological diagenesis consists mainly of bioerosion, or the grinding, rasping,
boring, and otherwise eroding of rock surfaces by plants and animals. Some organ-
isms may produce secretions that dissolve rock. Bioerosion is particularly visible in
some modern tropical carbonate coasts, where notches and overhangs have been
formed at water level along steep, rocky shorelines by the combination of bioerosion
and breaking waves. Bioerosion is relatively unimportant as an agent of change in
carbonate reservoir porosity but it may produce a large percentage of the mud frac-
tion in sediments from some depositional settings.
Chemical diagenesis is the most important agent of change in carbonate reser-
voirs. It is a process of rock - and - water interaction that proceeds in rates and direc-
tions dictated by the equilibrium between the rocks and the water. The main
processes in chemical diagenesis are dissolution, cementation (precipitation), recrys-
tallization, and replacement. Dissolution occurs when the rock – water system is out
of equilibrium. In such a case, the water is undersaturated with respect to CaCO 3 .
For example, meteoric water dissolves CaCO 3 until saturation equilibrium is reached