Page 184 - Geology of Carbonate Reservoirs

POROSITY REDUCTION BY DIAGENESIS 165

6.5.1 Pore Reduction by Compaction
Mechanical compaction results from overburden stress during burial and from tec-
tonism. Compaction effects are sometimes accentuated or exaggerated when disso-
lution and compaction act together to form stylolites by pressure acting concurrently
with dissolution. Graphs showing porosity decrease with increasing depth of burial
(Schmoker and Halley, 1982 ) suggest that compaction may be more infl uential than
cementation in reducing porosity. Of course, cementation takes place during burial,
so porosity reduction with depth is not a function of one variable. The relative
importance of compaction and cementation in reduction of porosity can be ascer-
tained by counting the number and kind of grain contacts in samples from different
burial depths to estimate the extent to which compaction has reduced original
intergranular porosity. Rocks with fl attened or penetrative grain contacts have sig-

nificantly more grain contacts per area and lower porosity than uncompacted rocks
where individual grain contacts are tangential and sparse, especially when counted
in 2D thin section views. Pore reduction by cementation can be crudely estimated
by measuring the 2D volume of cement in pore spaces along several transects across
a thin section. If compaction had a greater influence on porosity reduction, there

will be successively more contacts per grain with depth and the contacts will prog-
ress from being tangential contacts at shallow depths to penetrative and stylolitic
contacts at depth. As compaction continues with combined pressure and dissolution,
stylolites are formed. Generally, stylolites are more common in mud - supported
rocks (Dickson and Saller, 1995 ) than in grainstones and packstones, and in general,
they reduce porosity and permeability (Nelson, 1981 ). The literature is replete with
references to stylolites in Middle Eastern carbonate reservoirs and how they form
permeability barriers that can be used to zone or map reservoir fl ow units, baffl es,
and barriers. However, poststylolite diagenesis can create porosity and permeability
in previously tight rocks (Dawson, 1988 ). The take - home message is that one has to
look at the rocks to isolate the cause of reduced or enhanced porosity and permea-
bility. Wireline logs and seismic data cannot yet distinguish between cementation,
compaction, recrystallization, dissolution, and replacement.

6.5.2 Pore Reduction by Recrystallization

Neomorphic stabilization can enhance porosity as discussed earlier in reference to
the Overton gas field, Texas. However, neomorphism as a type of recrystallization

(inversion of aragonite or Mg - calcite to calcite) usually reduces porosity. Folk (1965)
described a form of coalescive neomorphism in which highly porous, “ felted ” net-
works of acicular microcrystals were recrystallized to produce larger crystals of
neomorphic microspar. Formation of microspar involves neomorphic ( “ new form ” )
crystal growth at the expense of smaller, precursor micrite particles. The process
forms a crystalline mosaic with abundant compromise boundaries and virtually no
intercrystalline porosity. Neomorphic microspar is the common form of calcite seen
in almost all stabilized, recrystallized lime muds, whether as muddy patches in grainy
rocks or in mudstones and wackestones. In carbonate reservoirs neomorphic
microspars are usually candidates for seals rather than reservoirs. In general, recrys-
tallization reduces porosity and permeability because the original sedimentary con-
stituents were composed of micrometer - sized crystals of metastable aragonite or