666
pore water pressures. Thennobaric waters frequently
contain methane, carbon dioxide, and hydrogen sulfide
derived from the thermal alteration of organic matter
within the sediment.
Convective flow. This flow is density-driven and can
operate in different ways. It occurs in both relatively
shallow and deep burial depths:
- Near the surface when water ofhigher density (salinity) fonns a kind ofreflux system through sediments.
This is known from some modern salt lagoons separated from the open sea (cf. Sect. 6.4.2). Circulation of
sea water with slightly elevated salinity mayaiso occur
in carbonate platforms.
Beneath the North Island of the Great Bahama Bank, for
example, sea water with salinities between 38 and 42%0
flows eastward, mixes with colder groundwater of normal
salinity, and enters again the sea at water depths of 200 to
300 m (Whitaker and Smart ·1990). Such flow systems are
thought to be important for pervasive dolomitization of carbonate buildups (see below).
- Pore water flow in a circ1e or loop within a penneable sediment body may occur without the uptake of
water from outside or loss of water to an adjacent flow
system. Density differences may arise as a result of
differences in temperature andlor salinity (cf. Sect.
5.2). However, convection of fluid can only occur in
relatively large, penneable rock bodies if the difference in density is sufficiently high and, in addition, the
viscosity of the fluid is low enough to overcome friction within the porous medium. Convective mass transport can redistribute relatively large quantities of mass
in comparatively short times.
Theoretical calculations and some field observations conceming temperature gradients and pore water chemistry have
presented evidence that such flow systems are possible in
several sedimentary basins (Wood and Hewett 1984; Hanor
1987; Schwartz and Longstaffe 1988). For example, density
inversions in pore fluids on the Gulf coast ofTexas are sufficient to drive large-scale convective flow at rates of meters
peryear.
A quantitative evaluation of these large-scale flow systems is difficult, because the three-dimensional structure of
basin fill and its heterogenities are seldom sufficiently
known. The permeabilities ofthe different sedimentary facies
in particular vary markedly in space and time (Bethke 1989,
Bj0rlykke et al. 1989).
There are several other, so-called nonhydraulic mechanisrns, such as osmosis and ultra-filtration, which may
cause or influence fluid flow and pore water chemistry
in argillaceous sediments (NeuziI1986; Hanor 1987).
Although several experimental studies have been carried out in this field, the significance ofthese phenomena in large-scale natural systems is still poorly known.
Chapter 13 Diagenesis
13.2.7 Significance of the Various Flow Regimes
The importance of the various subsurface regimes depends on the type of sedimentary basin and its age and
evolution through time. Compactional and thennobaric
regimes are associated with the active depositional
phase of the basins, particularly in basins on continental margins, marine rift zones, or delta-fed basins
which subside significantly and are fi1led rapidly. If
this active phase comes to an end and is followed by
emergence ofthe basin fill due to sea-Ieve1 fall or the
onset of uplift, compactional and thennobaric flow
slow down and finally cease.
Depositionally inactive basins or hydrogeologically
maturing basins are then increasingly subjected to
gravity-driven, meteoric water circulation and therefore flushed by fresh water. Recharge commonly occurs along the uplifted margins ofthe basin, and meteoric flow is directed toward the less elevated basin
center. Pressure gradients versus depth commonly record hydrostatic conditions. Many basins all over the
world have evolved to the stage when gravity flow predominates, for example the Aquitaine basin and Paris
basin in France, the Pannonian basin in Hungary, and
several basins in North America.
13.2.8 Evolution ofPore Water Flow in
Various Depositional Systems
The young, slowly deposited sediments of deep-sea
basins far away from the continents, as weil as most
sediments of slowly subsiding epicontinental seas, exhibit only minor compaction flow and are barely thick
enough to develop overpressured thennobaric zones.
The emerged, more or less horizontally layered sediments of an epicontinental sea, for example, provide
little possibility for deeply circulating meteoric waters
(Fig. 13. 13a), as long as significanthydraulic gradients
are absent due to low relief. Hence, the evolving circulation systems are limited in extent both laterally and
vertically, excep~ for emerged carbonates which may
provide more effective conduits for ground water circulation. Sandstones in such sequences are usually thin
and contain small proportions of unstable minerals
which may be later leached and gain secondary porosity (see be1ow).
Thick, fluvial, and deltaic sediments along the margins of a continental rift basin are usually weil and
deeply flushed by meteoric waters during and after the
time of their deposition (Fig. 13 .13b). Their frequently
immature sediments derived from nearby uplifted rift
shoulders can thus be strongly affected by leaching
and alteration, as in the example of kaolinitization of
feldspar. Some compaction flow may originate in the
basin center and migrate $ome distance outward.
pore water pressures. Thennobaric waters frequently
contain methane, carbon dioxide, and hydrogen sulfide
derived from the thermal alteration of organic matter
within the sediment.
Convective flow. This flow is density-driven and can
operate in different ways. It occurs in both relatively
shallow and deep burial depths:
- Near the surface when water ofhigher density (salinity) fonns a kind ofreflux system through sediments.
This is known from some modern salt lagoons separated from the open sea (cf. Sect. 6.4.2). Circulation of
sea water with slightly elevated salinity mayaiso occur
in carbonate platforms.
Beneath the North Island of the Great Bahama Bank, for
example, sea water with salinities between 38 and 42%0
flows eastward, mixes with colder groundwater of normal
salinity, and enters again the sea at water depths of 200 to
300 m (Whitaker and Smart ·1990). Such flow systems are
thought to be important for pervasive dolomitization of carbonate buildups (see below).
- Pore water flow in a circ1e or loop within a penneable sediment body may occur without the uptake of
water from outside or loss of water to an adjacent flow
system. Density differences may arise as a result of
differences in temperature andlor salinity (cf. Sect.
5.2). However, convection of fluid can only occur in
relatively large, penneable rock bodies if the difference in density is sufficiently high and, in addition, the
viscosity of the fluid is low enough to overcome friction within the porous medium. Convective mass transport can redistribute relatively large quantities of mass
in comparatively short times.
Theoretical calculations and some field observations conceming temperature gradients and pore water chemistry have
presented evidence that such flow systems are possible in
several sedimentary basins (Wood and Hewett 1984; Hanor
1987; Schwartz and Longstaffe 1988). For example, density
inversions in pore fluids on the Gulf coast ofTexas are sufficient to drive large-scale convective flow at rates of meters
peryear.
A quantitative evaluation of these large-scale flow systems is difficult, because the three-dimensional structure of
basin fill and its heterogenities are seldom sufficiently
known. The permeabilities ofthe different sedimentary facies
in particular vary markedly in space and time (Bethke 1989,
Bj0rlykke et al. 1989).
There are several other, so-called nonhydraulic mechanisrns, such as osmosis and ultra-filtration, which may
cause or influence fluid flow and pore water chemistry
in argillaceous sediments (NeuziI1986; Hanor 1987).
Although several experimental studies have been carried out in this field, the significance ofthese phenomena in large-scale natural systems is still poorly known.
Chapter 13 Diagenesis
13.2.7 Significance of the Various Flow Regimes
The importance of the various subsurface regimes depends on the type of sedimentary basin and its age and
evolution through time. Compactional and thennobaric
regimes are associated with the active depositional
phase of the basins, particularly in basins on continental margins, marine rift zones, or delta-fed basins
which subside significantly and are fi1led rapidly. If
this active phase comes to an end and is followed by
emergence ofthe basin fill due to sea-Ieve1 fall or the
onset of uplift, compactional and thennobaric flow
slow down and finally cease.
Depositionally inactive basins or hydrogeologically
maturing basins are then increasingly subjected to
gravity-driven, meteoric water circulation and therefore flushed by fresh water. Recharge commonly occurs along the uplifted margins ofthe basin, and meteoric flow is directed toward the less elevated basin
center. Pressure gradients versus depth commonly record hydrostatic conditions. Many basins all over the
world have evolved to the stage when gravity flow predominates, for example the Aquitaine basin and Paris
basin in France, the Pannonian basin in Hungary, and
several basins in North America.
13.2.8 Evolution ofPore Water Flow in
Various Depositional Systems
The young, slowly deposited sediments of deep-sea
basins far away from the continents, as weil as most
sediments of slowly subsiding epicontinental seas, exhibit only minor compaction flow and are barely thick
enough to develop overpressured thennobaric zones.
The emerged, more or less horizontally layered sediments of an epicontinental sea, for example, provide
little possibility for deeply circulating meteoric waters
(Fig. 13. 13a), as long as significanthydraulic gradients
are absent due to low relief. Hence, the evolving circulation systems are limited in extent both laterally and
vertically, excep~ for emerged carbonates which may
provide more effective conduits for ground water circulation. Sandstones in such sequences are usually thin
and contain small proportions of unstable minerals
which may be later leached and gain secondary porosity (see be1ow).
Thick, fluvial, and deltaic sediments along the margins of a continental rift basin are usually weil and
deeply flushed by meteoric waters during and after the
time of their deposition (Fig. 13 .13b). Their frequently
immature sediments derived from nearby uplifted rift
shoulders can thus be strongly affected by leaching
and alteration, as in the example of kaolinitization of
feldspar. Some compaction flow may originate in the
basin center and migrate $ome distance outward.