Page 167 - Geology of Carbonate Reservoirs

148 DIAGENETIC CARBONATE RESERVOIRS

rhombohedra, depending on environmental conditions. Calcite varieties can also

form in crystal bundles such as radiaxial fibrous and fascicular optical calcite. Some
calcite cements are micritic or peloidal. Peloids are typically spheroidal in shape
and are a few micrometers in diameter. The spheroids consist of crystallites arranged
radially around grain centers, or nuclei. Most peloidal cements are interpreted to
result from microbial or biochemical processes because the peloids occur in cavities,

sometimes as geopetal fillings, within reefs and “ mud mounds. ” Microbial origin for

some carbonate deposits may be difficult to document; thus there is commonly some
controversy over their origin. The muddy interstices in these reef and mound envi-
ronments must have been fertile culture media for microbial life and there is usually
little or no evidence of animal life that could have produced peloids as fecal pellets.
There is no evidence of inorganic precipitation such as a continuous horizon of
peloidal grains indicating a sediment – water chemical interface. There is also no
evidence of mechanical abrasion to produce uniform, spheroidal, silt - sized grains
that typically fill sheltered cavities. Mechanically abraded grains, or intraclasts, are

generally not uniform in size and shape. Aragonite commonly crystallizes as needle -
like orthorhombic prisms only a few micrometers in size that can form grain - coating,
isopachous crusts, or rim cements (Figure 6.2 a). These pore - lining aragonite cements
indicate a marine phreatic origin. Botryoidal masses of aragonite crystals up to tens
of centimeters in length are commonly found in modern and ancient reefs (Figure
6.2 b), and some aragonitic cement is present in cave dripstones. Dolomite (Figure
6.2 c) typically crystallizes as simple rhombohedra in most surface and near - surface
environments but it may grow into a baroque form called saddle dolomite (Figure
6.2 d), particularly in the burial environment where water temperatures are 60 ° C

and higher (Radke and Mathis, 1980 ). A great deal of attention has been given to
“ hydrothermal dolomite ” in recent times. Some disagreement exists about the best

definition of “ hydrothermal ” and about the source of dolomitizing fluids that created

saddle dolomite. It is commonly associated with but not limited to fractured reser-
voirs in carbonates (Ahr, 1982 ). Saddle dolomite may also form as a by - product of
thermochemical sulfate reduction in nonfractured reservoirs. Some of the early
discussion on “ TSR ” - formed saddle dolomite was given by Machel (1987b) .

Recrystallization is the process by which crystal morphology is changed without
major changes in the mineral composition. Sometimes this process is called
“ neomorphism, ” a term coined by R. L. Folk (1965) , who defi ned neomorphism to
include both true recrystallization and mineralogical inversion. True recrystalliza-
tion is a change in crystal form without changes in mineralogical composition: for
example, micrometer - sized crystals of calcite micrite going through transformation
to millimeter - sized blocky calcite crystals (Figure 6.3 ) during what Folk (1965) called
aggradational neomorphism. Mineralogical inversion is not strictly recrystallization;
rather, as an example, it is the process by which a metastable mineral such as ara-
gonite or Mg - calcite undergoes both crystallographic and compositional change to
become ordinary calcite. Even though the basic composition of aragonite is CaCO 3 ,
it may have a different trace element and isotopic composition than its postneomor-
phism daughter product just as Mg - calcite will lose Mg to become ordinary calcite.
Aggradational neomorphism in microporous micrites can produce close - fi tting
crystal mosaics that have lower porosity and permeability than their precursor.
Finally, micritization, or grain diminution, is a form of degradational neomorphism.
One form of micritization, or at least a form of grain - size reduction, is sometimes