5.7.10.3 Constraints on Diagenetic Dolomite
Formation
While early diagenetic dolomite is normally relatively
fine-grained, late-diagenetic dolomite usually forms
larger crystals, often well-defined dolomite rhombs.
The smaller crystals are formed by rapid crystal
growth under conditions of high supersaturation. The
larger crystals are formed by slow crystal growth from
a few nucleation centres at a very low degree of
supersaturation combined with deep burial.
As mentioned previously, hydration of Mg
2+
decreases with increasing temperature, making Mg
2+
more readily available for the dolomite structure.
Dolomite might then be formed at very low
Mg
2þ
=Ca
2þ ratios; at about 80
C the ratio can be as
low as 0.1. Although dolomite can precipitate in
solutions with low Mg
2þ
=Ca
2þ ratios, the question
remains of how sufficient magnesium is added for
the dolomitisation to take place. The amount of magnesium in solution in the deeper part (>2–3 km burial
depth) of sedimentary basins is in most cases very low
and is far from adequate as a source for large-scale
dolomitisation. Water from compaction of mudstones
and shales is probably also insufficient to supply much
magnesium for dolomitisation. Primary high-Mg calcite will be a source of Mg to form dolomite at depth.
There is only a little magnesium in the porewater of
sedimentary basins apart from in the vicinity of
evaporites. Probably most of the dolomitisation occurs
near the surface where the magnesium comes from
seawater. Fine-grained dolomite, formed at an early
stage, may be dissolved at depth and recrystallise as
coarser-grained dolomite, in which case there is no need
to postulate a magnesium supply deep in the basin.
We often find dolomite enriched along stylolites,
probably because dolomite is less soluble than calcite
and the solution and precipitation round a stylolite will
concentrate clay minerals which, in turn, may release
some magnesium.
If the composition of the porewater later shifts
towards a low Mg
2þ
=Ca
2þ ratio, dolomite may dissolve and calcite reprecipitate. There are a number of
cases where distinctly dolomite-type rhombs are found
to consist of calcite. One common cause of reversed
dolomitisation – often called dedolomitisation – is
porewater coming from gypsum which is dissolving.
This gives the porewater a high Ca
2+ concentration.
However, many people have recommended that the
term “dedolomitisation” should be dropped, and the
positive term “calcitisation” be used instead.
5.7.10.4 The Significance of Dolomitisation
For many years there has been intensive research into
the processes which lead to dolomitisation. A great
deal remains to be learnt, however, before we really
understand the precise conditions for dolomitisation so
that we can predict the extent of dolomite in sedimentary basins. The reason for this great interest is that
dolomitic carbonate rocks are very important
reservoirs for oil and gas. The dolomitisation process
may create secondary porosity because dolomite has a
greater density than calcite so if an identical number of
mol dolomite is precipitated as in the original calcite,
we would get approximately 12% smaller volume and
an equivalent increase in porosity. The dolomitisation
process assumes, however, that calcium is removed
and magnesium introduced, and there is then no reason
why there should be an increase in porosity proportional to the difference in density, since there is no
reason why the same number of mol dolomite should
be precipitated as were removed by the calcite
dissolving. Since dolomitisation involves large-scale
percolation of porewater, we may also have net
increase in the porosity associated with this process.
Micritic limestones have in most cases too low
porosity and permeability to be regarded as reservoir
rocks except when fractured. Tectonically fractured
limestones may become cemented in relatively short
geologic time before the migration of petroleum. Fractured dolomites are, however, more stable and likely to
remain open longer because of the lower solubility of
dolomite.
5.7.11 Formation of Carbonate Sediments
Rich in Siderite and Chamosite
(Ironstones)
Iron carbonates like siderite (FeCO 3 ) are stable carbonate phases with rather low solubility (Berner
1981). In the presence of sulphides, however, most
of the iron will be precipitated as iron sulphides (i.e.
pyrite or marcasite). Precipitation of siderite and other
iron carbonates is therefore restricted to settings where
the porewater has a low content of reduced sulphur
(sulphides), as in freshwater and below the sulphatereducing zone.
208
N.-M. Hanken et al.
Formation
While early diagenetic dolomite is normally relatively
fine-grained, late-diagenetic dolomite usually forms
larger crystals, often well-defined dolomite rhombs.
The smaller crystals are formed by rapid crystal
growth under conditions of high supersaturation. The
larger crystals are formed by slow crystal growth from
a few nucleation centres at a very low degree of
supersaturation combined with deep burial.
As mentioned previously, hydration of Mg
2+
decreases with increasing temperature, making Mg
2+
more readily available for the dolomite structure.
Dolomite might then be formed at very low
Mg
2þ
=Ca
2þ ratios; at about 80
C the ratio can be as
low as 0.1. Although dolomite can precipitate in
solutions with low Mg
2þ
=Ca
2þ ratios, the question
remains of how sufficient magnesium is added for
the dolomitisation to take place. The amount of magnesium in solution in the deeper part (>2–3 km burial
depth) of sedimentary basins is in most cases very low
and is far from adequate as a source for large-scale
dolomitisation. Water from compaction of mudstones
and shales is probably also insufficient to supply much
magnesium for dolomitisation. Primary high-Mg calcite will be a source of Mg to form dolomite at depth.
There is only a little magnesium in the porewater of
sedimentary basins apart from in the vicinity of
evaporites. Probably most of the dolomitisation occurs
near the surface where the magnesium comes from
seawater. Fine-grained dolomite, formed at an early
stage, may be dissolved at depth and recrystallise as
coarser-grained dolomite, in which case there is no need
to postulate a magnesium supply deep in the basin.
We often find dolomite enriched along stylolites,
probably because dolomite is less soluble than calcite
and the solution and precipitation round a stylolite will
concentrate clay minerals which, in turn, may release
some magnesium.
If the composition of the porewater later shifts
towards a low Mg
2þ
=Ca
2þ ratio, dolomite may dissolve and calcite reprecipitate. There are a number of
cases where distinctly dolomite-type rhombs are found
to consist of calcite. One common cause of reversed
dolomitisation – often called dedolomitisation – is
porewater coming from gypsum which is dissolving.
This gives the porewater a high Ca
2+ concentration.
However, many people have recommended that the
term “dedolomitisation” should be dropped, and the
positive term “calcitisation” be used instead.
5.7.10.4 The Significance of Dolomitisation
For many years there has been intensive research into
the processes which lead to dolomitisation. A great
deal remains to be learnt, however, before we really
understand the precise conditions for dolomitisation so
that we can predict the extent of dolomite in sedimentary basins. The reason for this great interest is that
dolomitic carbonate rocks are very important
reservoirs for oil and gas. The dolomitisation process
may create secondary porosity because dolomite has a
greater density than calcite so if an identical number of
mol dolomite is precipitated as in the original calcite,
we would get approximately 12% smaller volume and
an equivalent increase in porosity. The dolomitisation
process assumes, however, that calcium is removed
and magnesium introduced, and there is then no reason
why there should be an increase in porosity proportional to the difference in density, since there is no
reason why the same number of mol dolomite should
be precipitated as were removed by the calcite
dissolving. Since dolomitisation involves large-scale
percolation of porewater, we may also have net
increase in the porosity associated with this process.
Micritic limestones have in most cases too low
porosity and permeability to be regarded as reservoir
rocks except when fractured. Tectonically fractured
limestones may become cemented in relatively short
geologic time before the migration of petroleum. Fractured dolomites are, however, more stable and likely to
remain open longer because of the lower solubility of
dolomite.
5.7.11 Formation of Carbonate Sediments
Rich in Siderite and Chamosite
(Ironstones)
Iron carbonates like siderite (FeCO 3 ) are stable carbonate phases with rather low solubility (Berner
1981). In the presence of sulphides, however, most
of the iron will be precipitated as iron sulphides (i.e.
pyrite or marcasite). Precipitation of siderite and other
iron carbonates is therefore restricted to settings where
the porewater has a low content of reduced sulphur
(sulphides), as in freshwater and below the sulphatereducing zone.
208
N.-M. Hanken et al.