13.3 Chemical Diagenesis
At this stage and even more so in the deep-burial
stage, differential compaction or tectonic motions may
create new conduits for fluid flow in indurated strata.
As a result, deeper pore water, e.g. from overpressured
zones, can escape upwards, or meteoric water circulation can start again and affect relatively deeply buried
sediment bodies. Particularly the second process, affecting limited zones within a large sediment body, can
generate both secondary porosity and filling of veins
with cement. Repeated dissolution events and cement
generations in the same fracture or in cross-cutting
veins have been described by several authors. Circulating meteoric waters saturated with respect to calcium
hydrogen carbonate can bring ab out dedolomitization.
Late diagenesis. In closed systems, pressure solution
and associated reprecipitation of the dissolved matter
as pore-filling cement slowly cease. However, water
circulation in sandy convection cells can lead to the
redistribution of quartz and carbonates, the latter being
precipitated at the lower end of the loop (Fig. 13 .20;
cf. Fig. 13.16f). Increased temperatures and pressures
favor the formation of illite, chlorite and albite in sandstones and claystones. The released Si0 2 and cations
allow some further cementation by quartz and
(Fe,Mn,Mg)-carbonates, e.g. in combination with CO 2
produced by thermocatalysis of organic matter (cf. Fig.
13.19, No. 11). Organic acids favor the precipitation of
illite. Significant amounts of water and cations expelled from mudrocks can contribute to the diagenesis
in overlying strata. The mudrocks in turn tend to become enriched in immobile elements (A1P3' Ti02, Zr,
and rare earth elements).
The influx of highly saline brines (high concentrations ofCl, S04, Na, Mg, Ca) from underlyingormore
remote evaporites can reactivate diagenesis. Fluids
flow along or across strata which have preserved some
permeability or have been fractured. Mixing with formation water of different composition creates either
new porosity or renewed cementation, including sulfate minerals, such as anhydrite, barite and celestite.
Fractures frequently show traces of bitumen and hydrocarbons which indicate that the diagenetic system
reached the oil window (cf. Fig. 13.15).
The processes operating during the different stages of
diagenesis are discussed for specific sediment types in
the Sects. 13.4 and 13.5.
Fig. 13.20. Principal processes during early, intermediate, and late diagenesis of sandstones and
calcarenites (overview). Early diagenesis is largely
controlled by the depositional environment and climate (continental vs. subaquatic, humid vs.
semiarid). Intermediate and late diagenetic phenom681
13.3.8 Porosity and Permeability of Sandstones
and Carbonate Rocks
The diagenesis of sandstones and various carbonate
rocks (grainstones, packstones, etc., cf. Sect. 3.4) is of
particular interest in hydrocarbon exploration and
hydrogeology, because these rocks provide reservoirs
and conduits for water, oil, and gas. One of the principal problems in petroleum exploration is the question
whether, for example, the sandstones maintained or
regained significant porosity and permeability, when
hydrocarbons started to migrate. It mayaiso be important to know whether meteoric water has circulated in
the deeply buried sandstones.
The initial porosity of sands and calcarenites (frequently between 25 and 40%) is first reduced by mechanical compaction, i.e., rearrangement and deformation of grains (cf. Fig. 13.22a; cf. Sects. 13.1.3 and
13.2). At increasing burial depths, the evolution ofporosity and permeability are mainly controlled by pressure solution and pore-filling cementation as well as
the creation of secondary porosity by dissolution
events and fracturing in the subsurface. The method
for discriminating between the various types of porosity loss and an example of a quartzose sandstone and
its loss in thickness are shown in Fig. 13.21d,e.
Even siliciclastic sands can lose a major part of their initial
porosity by grain rearrangement, i.e. twice as much as they
do by ductile grain deformation (McBride et al. 1991). This
type of porosity reduction preferentially takes place in the
uppermost 1000 m of their burial history. At greater depths,
pressure solution and cementation by quartz, kaolinitization
and illitization of feldspars and, to a minor degree, by carbonate become most important. At 3-4 km burial depth, compaction may completely cease. Permeability has dropped to
very low values at this stage, whereas some porosity is still
preserved. However, compaction is frequently retarded by
overpressure and the entry of hydrocarbons (e.g. Bjorlykke
1991; also see below).
Bioclastic carbonates containing metastable carbonate
phases may lose most oftheir initial porosity and permeability at shallow burial depths. Numerous case studies of carbonate petroleum reservoirs, containing information on porosities and porosity types, have been published in special
volumes (e.g. Roeh1 and Choquette 1985; Budd et al. 1995).
High porosities and permeabilities are frequently preserved
in skeletal shoals and reefs, whereas the sediments of tidal
flats and fine-grained slope and basinal carbonates commonly display moderate to low porosities and very low
permeabilities (Jordan and Wilson 1994). The initial porosity
of Jurassic ooid grainstones (-45%) of the Gulf of Mexico
was reduced mainly by pressure solution and to a smaller
degree by carbonate cementation (Heydari 1997).
ena in ± closed systems record increasing temperatures and pressures, but mayaiso become overprinted
by the opening of new conduits (fractures) for renewed meteroic flow or the influx of highly saline
brines. See text for further explanation
At this stage and even more so in the deep-burial
stage, differential compaction or tectonic motions may
create new conduits for fluid flow in indurated strata.
As a result, deeper pore water, e.g. from overpressured
zones, can escape upwards, or meteoric water circulation can start again and affect relatively deeply buried
sediment bodies. Particularly the second process, affecting limited zones within a large sediment body, can
generate both secondary porosity and filling of veins
with cement. Repeated dissolution events and cement
generations in the same fracture or in cross-cutting
veins have been described by several authors. Circulating meteoric waters saturated with respect to calcium
hydrogen carbonate can bring ab out dedolomitization.
Late diagenesis. In closed systems, pressure solution
and associated reprecipitation of the dissolved matter
as pore-filling cement slowly cease. However, water
circulation in sandy convection cells can lead to the
redistribution of quartz and carbonates, the latter being
precipitated at the lower end of the loop (Fig. 13 .20;
cf. Fig. 13.16f). Increased temperatures and pressures
favor the formation of illite, chlorite and albite in sandstones and claystones. The released Si0 2 and cations
allow some further cementation by quartz and
(Fe,Mn,Mg)-carbonates, e.g. in combination with CO 2
produced by thermocatalysis of organic matter (cf. Fig.
13.19, No. 11). Organic acids favor the precipitation of
illite. Significant amounts of water and cations expelled from mudrocks can contribute to the diagenesis
in overlying strata. The mudrocks in turn tend to become enriched in immobile elements (A1P3' Ti02, Zr,
and rare earth elements).
The influx of highly saline brines (high concentrations ofCl, S04, Na, Mg, Ca) from underlyingormore
remote evaporites can reactivate diagenesis. Fluids
flow along or across strata which have preserved some
permeability or have been fractured. Mixing with formation water of different composition creates either
new porosity or renewed cementation, including sulfate minerals, such as anhydrite, barite and celestite.
Fractures frequently show traces of bitumen and hydrocarbons which indicate that the diagenetic system
reached the oil window (cf. Fig. 13.15).
The processes operating during the different stages of
diagenesis are discussed for specific sediment types in
the Sects. 13.4 and 13.5.
Fig. 13.20. Principal processes during early, intermediate, and late diagenesis of sandstones and
calcarenites (overview). Early diagenesis is largely
controlled by the depositional environment and climate (continental vs. subaquatic, humid vs.
semiarid). Intermediate and late diagenetic phenom681
13.3.8 Porosity and Permeability of Sandstones
and Carbonate Rocks
The diagenesis of sandstones and various carbonate
rocks (grainstones, packstones, etc., cf. Sect. 3.4) is of
particular interest in hydrocarbon exploration and
hydrogeology, because these rocks provide reservoirs
and conduits for water, oil, and gas. One of the principal problems in petroleum exploration is the question
whether, for example, the sandstones maintained or
regained significant porosity and permeability, when
hydrocarbons started to migrate. It mayaiso be important to know whether meteoric water has circulated in
the deeply buried sandstones.
The initial porosity of sands and calcarenites (frequently between 25 and 40%) is first reduced by mechanical compaction, i.e., rearrangement and deformation of grains (cf. Fig. 13.22a; cf. Sects. 13.1.3 and
13.2). At increasing burial depths, the evolution ofporosity and permeability are mainly controlled by pressure solution and pore-filling cementation as well as
the creation of secondary porosity by dissolution
events and fracturing in the subsurface. The method
for discriminating between the various types of porosity loss and an example of a quartzose sandstone and
its loss in thickness are shown in Fig. 13.21d,e.
Even siliciclastic sands can lose a major part of their initial
porosity by grain rearrangement, i.e. twice as much as they
do by ductile grain deformation (McBride et al. 1991). This
type of porosity reduction preferentially takes place in the
uppermost 1000 m of their burial history. At greater depths,
pressure solution and cementation by quartz, kaolinitization
and illitization of feldspars and, to a minor degree, by carbonate become most important. At 3-4 km burial depth, compaction may completely cease. Permeability has dropped to
very low values at this stage, whereas some porosity is still
preserved. However, compaction is frequently retarded by
overpressure and the entry of hydrocarbons (e.g. Bjorlykke
1991; also see below).
Bioclastic carbonates containing metastable carbonate
phases may lose most oftheir initial porosity and permeability at shallow burial depths. Numerous case studies of carbonate petroleum reservoirs, containing information on porosities and porosity types, have been published in special
volumes (e.g. Roeh1 and Choquette 1985; Budd et al. 1995).
High porosities and permeabilities are frequently preserved
in skeletal shoals and reefs, whereas the sediments of tidal
flats and fine-grained slope and basinal carbonates commonly display moderate to low porosities and very low
permeabilities (Jordan and Wilson 1994). The initial porosity
of Jurassic ooid grainstones (-45%) of the Gulf of Mexico
was reduced mainly by pressure solution and to a smaller
degree by carbonate cementation (Heydari 1997).
ena in ± closed systems record increasing temperatures and pressures, but mayaiso become overprinted
by the opening of new conduits (fractures) for renewed meteroic flow or the influx of highly saline
brines. See text for further explanation