13.3 Chemical Diagenesis
13.3.7 Mineral Reactions and
Major Diagenetic Regimes
Mineral Reactions
Ifthe thermodynamic equilibrium between the various
sediment partic1es, inc1uding earlier cement, and the
pore fluid is disturbed, then a number of minerals become instable. This is brought about by the influx of
pore fluid of different composition or changes in temperature and pressure. As a result, the instable minerals
slowly dissolve or are transformed into stable mineral
phases. During this process the minerals release specific cations, anions and water to the surrounding pore
fluid or they take up dissolved components from the
pore water.
The maj or silicate mineral reactions in diagenesis
are listed in a simplified way in Fig. 13.l9 (Nos. 1-7).
Most ofthese reactions occur at shallow burial depths,
where the relatively instable minerals (feldspar, mafic
minerals, smectite, and volcanic glass) are transformed
into more stable phases, mainly c1ay minerals. Some of
these, e.g. kaolinite but also smectite, are later, i.e. at
increased temperature and pressure, converted to illite
or chlorite.
The behavior of the major carbonate minerals has
been briefly mentioned in Sects. 3.4.7 and 6.4.6. Replacement of calcite by dolomite can take place in two
different ways:
(1) Uptake ofMg++ from the pore fluid and release of
Ca++ (Reaction No. 8, associated with a volume loss or
gain in porosity of 12%). This process is reversed during dedolomitization.
(2) Uptake ofboth Mg++ and C0 3 " from the pore fluid.
Then the volume of the newly formed dolomite increases by 75% (e.g. SalIer and Henderson 1998).
Carbonate cements containing Fe++ and Mn++ (ferroan
calcite and dolomite, ankerite and siderite; Fig. 13.l9,
No. 9) are precipitated at low redox conditions from
fluids with relatively high Fe++/Ca++ ratios (e.g. Morad
1998). Sulfate is normally not present under these conditions; it is either earlier reduced to sulfide by bacteria
under release of CO 2 (Fig. 13.19, No. 10), or it was
initially absent.
All of these processes imply water-rock interactions. Of particular interest is the release of Ca ++, Si0 2
(or a water-bearing compound such as H 4 Si0 4 ), and
H20 at greater burial depth (Reactions Nos. 3-5). This
allows the formation oflate quartz cement (apart from
pressure solution) and, in conjunction with the release
of CO 2 (Reactions Nos. 10 and 11) the precipitation of
late carbonate cement. The limited amount of re1eased
water can maintain some late compaction flow.
679
Major Diagenetic Regimes (Overview)
The most important diagenetic processes taking place
mainly in sandstones and calcarenites at early, intermediate and late diagenesis are surnmarized in Fig. 13.20.
Early diagenesis. As long as the sediment bodies are
completely submerged be10w sea water or fresh lake
water, they commonly maintain their initial thermodynamic equilibrium between sediment particles and pore
water for some time. During this phase, the sediments
are not subjected to marked diagenetic changes. An
exception to this rule is early dolomite replacing calcite (matrix-dolomite) which can form as long as pore
water is exchanged through well-permeable carbonates
with sea water. In addition, some quartz may precipitate if opaline silica was present in the primary sediment. The presence of organic marter commonly leads
to reducing conditions within the sediment and thus to
the formation of some pyrite.
As soon as parts of these sediment bodies emerge,
hydrologically open systems are established, allowing
meteoric water circulation. The same is true of all continental environments. The intensity of this water circulation and the resulting products of early diagenesis
strongly depend on the c1imate and the relief of the
system considered. Humid conditions lead to strong
flushing ofthe aquifers and thus to significant carbonate and feldspar dissolution along the main flow paths
(e.g. channel fills). This process is supported by soilderived carbon dioxide and organic acids causing a
first generation of secondary porosity and the export of
considerable amounts of dissolved marter to other areas. Limited amounts of more stable minerals, such as
calcite and kaolinite, are reprecipitated in areas of slow
fluid flow.
In contrast, continental or emerged basin fills in arid
to semiarid regions lose lirtle dissolved material by
meteoric flow. They tend to form near-surface
calcretes, dolocretes, and silcretes and, if affected by
sabkha brines, a first cement generation of calcite, dolomite, ankerite, chalcedony or quartz, gypsum and
anhydrite. Independent on the c1imate, iron released
from silicate minerals may form iron hydroxide which
can be transformed later to hematite (red bed
diagenesis ). If organic marter generates reducing conditions, ankerite and siderite can form during this and
later phases.
Intermediate diagenesis (mesodiagenesis). Due to mechanical compaction and interbedded low-permeable
muds, sediment bodies at intermediate burial depths
tend to approach c10sed diagenetic systems. Chemical
compaction resulting from pressure solution starts earlier in carbonates than in quartzose sandstones (cf.
Sect. 13.3.3) and can provide large amounts of carbonate and quartz cements (mineral overgrowths and voidfillings). Smectite is transformed to illite releasing
Si0 2 , cations and water.
13.3.7 Mineral Reactions and
Major Diagenetic Regimes
Mineral Reactions
Ifthe thermodynamic equilibrium between the various
sediment partic1es, inc1uding earlier cement, and the
pore fluid is disturbed, then a number of minerals become instable. This is brought about by the influx of
pore fluid of different composition or changes in temperature and pressure. As a result, the instable minerals
slowly dissolve or are transformed into stable mineral
phases. During this process the minerals release specific cations, anions and water to the surrounding pore
fluid or they take up dissolved components from the
pore water.
The maj or silicate mineral reactions in diagenesis
are listed in a simplified way in Fig. 13.l9 (Nos. 1-7).
Most ofthese reactions occur at shallow burial depths,
where the relatively instable minerals (feldspar, mafic
minerals, smectite, and volcanic glass) are transformed
into more stable phases, mainly c1ay minerals. Some of
these, e.g. kaolinite but also smectite, are later, i.e. at
increased temperature and pressure, converted to illite
or chlorite.
The behavior of the major carbonate minerals has
been briefly mentioned in Sects. 3.4.7 and 6.4.6. Replacement of calcite by dolomite can take place in two
different ways:
(1) Uptake ofMg++ from the pore fluid and release of
Ca++ (Reaction No. 8, associated with a volume loss or
gain in porosity of 12%). This process is reversed during dedolomitization.
(2) Uptake ofboth Mg++ and C0 3 " from the pore fluid.
Then the volume of the newly formed dolomite increases by 75% (e.g. SalIer and Henderson 1998).
Carbonate cements containing Fe++ and Mn++ (ferroan
calcite and dolomite, ankerite and siderite; Fig. 13.l9,
No. 9) are precipitated at low redox conditions from
fluids with relatively high Fe++/Ca++ ratios (e.g. Morad
1998). Sulfate is normally not present under these conditions; it is either earlier reduced to sulfide by bacteria
under release of CO 2 (Fig. 13.19, No. 10), or it was
initially absent.
All of these processes imply water-rock interactions. Of particular interest is the release of Ca ++, Si0 2
(or a water-bearing compound such as H 4 Si0 4 ), and
H20 at greater burial depth (Reactions Nos. 3-5). This
allows the formation oflate quartz cement (apart from
pressure solution) and, in conjunction with the release
of CO 2 (Reactions Nos. 10 and 11) the precipitation of
late carbonate cement. The limited amount of re1eased
water can maintain some late compaction flow.
679
Major Diagenetic Regimes (Overview)
The most important diagenetic processes taking place
mainly in sandstones and calcarenites at early, intermediate and late diagenesis are surnmarized in Fig. 13.20.
Early diagenesis. As long as the sediment bodies are
completely submerged be10w sea water or fresh lake
water, they commonly maintain their initial thermodynamic equilibrium between sediment particles and pore
water for some time. During this phase, the sediments
are not subjected to marked diagenetic changes. An
exception to this rule is early dolomite replacing calcite (matrix-dolomite) which can form as long as pore
water is exchanged through well-permeable carbonates
with sea water. In addition, some quartz may precipitate if opaline silica was present in the primary sediment. The presence of organic marter commonly leads
to reducing conditions within the sediment and thus to
the formation of some pyrite.
As soon as parts of these sediment bodies emerge,
hydrologically open systems are established, allowing
meteoric water circulation. The same is true of all continental environments. The intensity of this water circulation and the resulting products of early diagenesis
strongly depend on the c1imate and the relief of the
system considered. Humid conditions lead to strong
flushing ofthe aquifers and thus to significant carbonate and feldspar dissolution along the main flow paths
(e.g. channel fills). This process is supported by soilderived carbon dioxide and organic acids causing a
first generation of secondary porosity and the export of
considerable amounts of dissolved marter to other areas. Limited amounts of more stable minerals, such as
calcite and kaolinite, are reprecipitated in areas of slow
fluid flow.
In contrast, continental or emerged basin fills in arid
to semiarid regions lose lirtle dissolved material by
meteoric flow. They tend to form near-surface
calcretes, dolocretes, and silcretes and, if affected by
sabkha brines, a first cement generation of calcite, dolomite, ankerite, chalcedony or quartz, gypsum and
anhydrite. Independent on the c1imate, iron released
from silicate minerals may form iron hydroxide which
can be transformed later to hematite (red bed
diagenesis ). If organic marter generates reducing conditions, ankerite and siderite can form during this and
later phases.
Intermediate diagenesis (mesodiagenesis). Due to mechanical compaction and interbedded low-permeable
muds, sediment bodies at intermediate burial depths
tend to approach c10sed diagenetic systems. Chemical
compaction resulting from pressure solution starts earlier in carbonates than in quartzose sandstones (cf.
Sect. 13.3.3) and can provide large amounts of carbonate and quartz cements (mineral overgrowths and voidfillings). Smectite is transformed to illite releasing
Si0 2 , cations and water.