670
deviate from pure water at low temperatures. With these data
and the differences in the solute concentrations, the mass
transport from one site to another can be determined. For
further details see, e.g., Hanor (1994).
Another approach to this problem is the question how
many volumes of pore fluid flowing through a unit area of
sediment (e.g. 1 cm 2 ) are needed to dissolve minerals at one
site and to fill all sediment pores with cement at another site.
In the case of quartz, for example, the solubility increases by
2 ppm/°C (at 100°C). Assuming a geothermal gradient of
3°C1l00 mand a quartz density of2.7 g/cm 3 , a fluxof~3 x
10 8 cm 3 /cm 2 is required to import the silica from hotter strata
to overlying colder regions, which is needed to completely
cement a rock of 10% porosity (BjfIJrlykke and Gran 1994).
This means that a water colurnn of 1000 km (!) has to pass
through the sediment which appears to be absolutely unrealistic, unless there is long-term convection flow (see below).
It is often more likely that pressure solution and short-distance diffusion transport provide silica cement.
Meteoric Water Circulation
The prerequisites of meteoric water circulation and the
general laws controlling velocity and volume of this
flow (i.e. shallow and deep groundwater circulation)
have been mentioned in Sect. 13.2.6. Meteoric waters
commonly differ in their composition from connate
pore waters (cf. Sect. 13.3.6), before they approach a
thermodynamic equilibrium with the mineral phases
a10ng their flow parth. Intense and long-lasting meteoric flow can therefore be very effective in leaching
carbonate- and feldspar-bearing sediments, aided by
the uptake of carbon dioxide and organic acids from
the soil zone. Whereas leaching and creation of secondary porosity prevail elose to the recharge area of
these waters, reprecipitation of dissolved constituents
can take place after some flow. This mechanism is discussed further in Sect. 13.4.4.
Compaction Flow
Sedimentary sequences below the sea or large lakes as
well as deeper buried sediments, protected from meteoric inflow by overpressure or an impermeable to lowpermeable seal, may be affected only by compaction
flow (upward directed advection; cf. Sect. 13.2.3 and
Fig. 13. 12a,d) and diffusion (Sect. 13.3.4). These systems can be regarded as more or less elosed, and their
flow velocities and thus their advective solute transport
are very slow (cf. Fig. 13.16a).
As pointed out in Sect. 13.2.3, the velocity ofvertical compaction flow, v, and the filter velocity, v f = v· n, are commonly less than the sedimentation rate, SRw• Usingtheprevious example (velocity of compaction flow v 1 = 0.84 SRw>
SR,. = 40 m1Ma, porosity n1 = 0.3), the filter velocity of
upstreaming pore water is v f1 = 10 m1Ma. If, for example, the
pore fluid contains 20 mg/I or 20 g/m 3 dissolved silica, then
the transport rate of silica through a horizontal plane within
the sediment is 200 g/m 2 /Ma. This amount corresponds to a
Chapter 13 Diagenesis
0.074 mm thick layer of solid quartz or chert if all dissolved
silica could be precipitated within the plane. If a water colurnn with the same Si0 2 concentration as before, 1000 m in
height, passes through a unit area ofthe horizontal plane, it
may leave behind a quartz layer of7.4 mm at most. In reality,
only a small fraction of the dissolved silica can be precipitated as cement.
This solute transport is not enough to provide cement
for a sand layer of a few to tens of meters in thickness.
Similarly, limited compaction flow can barely dissolve
enough pre-existing cement in order to create secondary porosity. Even the relatively well soluble calcium
carbonate (100 to 1000 gim 3 ) cannot be transported by
compaction flow in sufficient quantities to account for
the 1arge vo1umes of carbonate cement observed in
rnany sequences. However, where thin sandy beds or
channel fills within muddy sediments act as lateral
drains ofupstreaming compaction flow (Fig. 16a), the
volume of pore fluid and the concentration of solutes
may match the requirements for significant cementation.
The efficiency of compaction flow in delivering
cement increases when it is highly saline as frequently
observed, e.g., on top of deeply buried evaporites.
Then evaporitic minerals such as anhydrite, barite, celestite and halite occur as cement minerals. Conversely, ascending low-centrated pore fluids can dissolve earlier cements and partieles of the matrix along
specific drains and fractures (see below).
In general, the volume of compaction flow further
decreases in deeply buried sediments which have already lost most oftheir initial porosity (cf. Fig. 13.10).
Under completely different circumstances, saline pore fluids
can precipitate minerals very rapidly. This is known as "scaling" from production wells in oil and gas fields which intermittently produce formation waters. Here flow lines and
valves can be blocked by mineral cements (e.g. Osbome et
al. 1994; Yuan et al. 1994). Rapid decrease in temperature
and pressure causes release of carbon dioxide and precipitation of calcium carbonate. Mixing of injected sea water (to
maintain reservoir pressure ) with incompatible brines my
lead to the precipitation of sulfates of Ba, Sr, and Ca.
Redistribution of Solutes by Convective Fluid Flow
Convection of pore fluids in a relatively large, elosed
circulation system (Sect. 13.2.6 and Fig. 13.16t) appears to be a rather common phenomenon in the
subsurface, even under conditions of normal geothermal gradients. Such long-term operating circulation
systems have been postulated by several authors in order to explain the volume of dissolved and redistributed cement in thick sandstone bodies, which otherwise are difficult to exp1ain. As a result of changing
solubilities with temperature, quartz will move from
hot source zones to cooler sinks, but carbonates from
cool sites to hot ones. Dissolution and reprecipitation
are particularly intense in anticlinal crests and
deviate from pure water at low temperatures. With these data
and the differences in the solute concentrations, the mass
transport from one site to another can be determined. For
further details see, e.g., Hanor (1994).
Another approach to this problem is the question how
many volumes of pore fluid flowing through a unit area of
sediment (e.g. 1 cm 2 ) are needed to dissolve minerals at one
site and to fill all sediment pores with cement at another site.
In the case of quartz, for example, the solubility increases by
2 ppm/°C (at 100°C). Assuming a geothermal gradient of
3°C1l00 mand a quartz density of2.7 g/cm 3 , a fluxof~3 x
10 8 cm 3 /cm 2 is required to import the silica from hotter strata
to overlying colder regions, which is needed to completely
cement a rock of 10% porosity (BjfIJrlykke and Gran 1994).
This means that a water colurnn of 1000 km (!) has to pass
through the sediment which appears to be absolutely unrealistic, unless there is long-term convection flow (see below).
It is often more likely that pressure solution and short-distance diffusion transport provide silica cement.
Meteoric Water Circulation
The prerequisites of meteoric water circulation and the
general laws controlling velocity and volume of this
flow (i.e. shallow and deep groundwater circulation)
have been mentioned in Sect. 13.2.6. Meteoric waters
commonly differ in their composition from connate
pore waters (cf. Sect. 13.3.6), before they approach a
thermodynamic equilibrium with the mineral phases
a10ng their flow parth. Intense and long-lasting meteoric flow can therefore be very effective in leaching
carbonate- and feldspar-bearing sediments, aided by
the uptake of carbon dioxide and organic acids from
the soil zone. Whereas leaching and creation of secondary porosity prevail elose to the recharge area of
these waters, reprecipitation of dissolved constituents
can take place after some flow. This mechanism is discussed further in Sect. 13.4.4.
Compaction Flow
Sedimentary sequences below the sea or large lakes as
well as deeper buried sediments, protected from meteoric inflow by overpressure or an impermeable to lowpermeable seal, may be affected only by compaction
flow (upward directed advection; cf. Sect. 13.2.3 and
Fig. 13. 12a,d) and diffusion (Sect. 13.3.4). These systems can be regarded as more or less elosed, and their
flow velocities and thus their advective solute transport
are very slow (cf. Fig. 13.16a).
As pointed out in Sect. 13.2.3, the velocity ofvertical compaction flow, v, and the filter velocity, v f = v· n, are commonly less than the sedimentation rate, SRw• Usingtheprevious example (velocity of compaction flow v 1 = 0.84 SRw>
SR,. = 40 m1Ma, porosity n1 = 0.3), the filter velocity of
upstreaming pore water is v f1 = 10 m1Ma. If, for example, the
pore fluid contains 20 mg/I or 20 g/m 3 dissolved silica, then
the transport rate of silica through a horizontal plane within
the sediment is 200 g/m 2 /Ma. This amount corresponds to a
Chapter 13 Diagenesis
0.074 mm thick layer of solid quartz or chert if all dissolved
silica could be precipitated within the plane. If a water colurnn with the same Si0 2 concentration as before, 1000 m in
height, passes through a unit area ofthe horizontal plane, it
may leave behind a quartz layer of7.4 mm at most. In reality,
only a small fraction of the dissolved silica can be precipitated as cement.
This solute transport is not enough to provide cement
for a sand layer of a few to tens of meters in thickness.
Similarly, limited compaction flow can barely dissolve
enough pre-existing cement in order to create secondary porosity. Even the relatively well soluble calcium
carbonate (100 to 1000 gim 3 ) cannot be transported by
compaction flow in sufficient quantities to account for
the 1arge vo1umes of carbonate cement observed in
rnany sequences. However, where thin sandy beds or
channel fills within muddy sediments act as lateral
drains ofupstreaming compaction flow (Fig. 16a), the
volume of pore fluid and the concentration of solutes
may match the requirements for significant cementation.
The efficiency of compaction flow in delivering
cement increases when it is highly saline as frequently
observed, e.g., on top of deeply buried evaporites.
Then evaporitic minerals such as anhydrite, barite, celestite and halite occur as cement minerals. Conversely, ascending low-centrated pore fluids can dissolve earlier cements and partieles of the matrix along
specific drains and fractures (see below).
In general, the volume of compaction flow further
decreases in deeply buried sediments which have already lost most oftheir initial porosity (cf. Fig. 13.10).
Under completely different circumstances, saline pore fluids
can precipitate minerals very rapidly. This is known as "scaling" from production wells in oil and gas fields which intermittently produce formation waters. Here flow lines and
valves can be blocked by mineral cements (e.g. Osbome et
al. 1994; Yuan et al. 1994). Rapid decrease in temperature
and pressure causes release of carbon dioxide and precipitation of calcium carbonate. Mixing of injected sea water (to
maintain reservoir pressure ) with incompatible brines my
lead to the precipitation of sulfates of Ba, Sr, and Ca.
Redistribution of Solutes by Convective Fluid Flow
Convection of pore fluids in a relatively large, elosed
circulation system (Sect. 13.2.6 and Fig. 13.16t) appears to be a rather common phenomenon in the
subsurface, even under conditions of normal geothermal gradients. Such long-term operating circulation
systems have been postulated by several authors in order to explain the volume of dissolved and redistributed cement in thick sandstone bodies, which otherwise are difficult to exp1ain. As a result of changing
solubilities with temperature, quartz will move from
hot source zones to cooler sinks, but carbonates from
cool sites to hot ones. Dissolution and reprecipitation
are particularly intense in anticlinal crests and