7.4 FLUID FLOW IN SEDIMENTARY BASINS
323
beneath unconformities, in overpressured clays, and where there has been extensive infiltration by meteoric water.
The composition of connate water varies considerably from seawater. It is normally
enriched in potassium, sodium, and chlorides, but contains fewer sulfates and magnesium. Connate waters also often contain less calcium than seawater, probably due to
precipitation of carbonate minerals. These differences demonstrate the evolution that
seawater undergoes when it is trapped in the pores of a subsiding sedimentary pile.
The last type of pore water to consider is juvenile or hydrothermal water. This is defined as water that is of deep magmatic origin, and has only just entered the hydrologic
cycle. Formerly, almost all epigenetic mineralization was attributed to precipitation
from magmatic fluids. Several lines of evidence, including fluid-inclusion analysis, show
that many epigenetic mineral deposits are precipitated from hot brines. Stable isotope
analysis shows that these brines are not always of magmatic origin, but have evolved
from connate waters. This is consistent with the fact that many epigenetic deposits, such
as Mississippi Valley ores, occur peripheral to evaporite basins. The source of the metals is often volcaniclastic or from normal sediments or even seawater. The metals have
then been concentrated by complex reactions in which organic matter and clay minerals often play catalytic roles. It has long been noted that oil field waters contain significant concentrations of metallic ions. The following reactions are often cited (e.g., Dunsmore, 1973; Eugster, 1985):
CaSO 4 + CH 4 ~CaCO 3 + H2S + H20.
The hydrogen sulfide may then react with metallic chlorides:
MeC12 + H2S ~ MeS + 2HC1,
where Me is a metallic cation.
This brief review of pore fluids shows their diversity and the complexity of their interrelationships. So far they have been considered to be at rest and in isolation. Now it
is time to discuss how they move within sedimentary basins.
7.4 FLUID FLOW IN SEDIMENTARY BASINS
There are three ways in which fluids move in sedimentary basins: by hydrostatic head, by
sediment compaction, and by convection (Fig. 7.14). These are now considered in turn.
7.4.1 Meteoric Flow
The concept of the piezometric surface, and the idea of meteoric water displacing
denser connate water, has already been described and illustrated (Fig. 7.11). This type
of fluid movement is naturally most important at shallow depths and in basins that have
been uplifted above sea level (Ingebritsen and Sanford, 1998). Hydrodynamic flow is
important as a mechanism for generating secondary porosity in weathering zones that
may ultimately be preserved beneath unconformities. It sometimes traps petroleum in
flexures that lack true vertical closure (termed "hydrodynamic traps") (Dahlberg, 1982),
and it is responsible for uranium roll-front mineralization (see Section 6.3.2.2.4).
323
beneath unconformities, in overpressured clays, and where there has been extensive infiltration by meteoric water.
The composition of connate water varies considerably from seawater. It is normally
enriched in potassium, sodium, and chlorides, but contains fewer sulfates and magnesium. Connate waters also often contain less calcium than seawater, probably due to
precipitation of carbonate minerals. These differences demonstrate the evolution that
seawater undergoes when it is trapped in the pores of a subsiding sedimentary pile.
The last type of pore water to consider is juvenile or hydrothermal water. This is defined as water that is of deep magmatic origin, and has only just entered the hydrologic
cycle. Formerly, almost all epigenetic mineralization was attributed to precipitation
from magmatic fluids. Several lines of evidence, including fluid-inclusion analysis, show
that many epigenetic mineral deposits are precipitated from hot brines. Stable isotope
analysis shows that these brines are not always of magmatic origin, but have evolved
from connate waters. This is consistent with the fact that many epigenetic deposits, such
as Mississippi Valley ores, occur peripheral to evaporite basins. The source of the metals is often volcaniclastic or from normal sediments or even seawater. The metals have
then been concentrated by complex reactions in which organic matter and clay minerals often play catalytic roles. It has long been noted that oil field waters contain significant concentrations of metallic ions. The following reactions are often cited (e.g., Dunsmore, 1973; Eugster, 1985):
CaSO 4 + CH 4 ~CaCO 3 + H2S + H20.
The hydrogen sulfide may then react with metallic chlorides:
MeC12 + H2S ~ MeS + 2HC1,
where Me is a metallic cation.
This brief review of pore fluids shows their diversity and the complexity of their interrelationships. So far they have been considered to be at rest and in isolation. Now it
is time to discuss how they move within sedimentary basins.
7.4 FLUID FLOW IN SEDIMENTARY BASINS
There are three ways in which fluids move in sedimentary basins: by hydrostatic head, by
sediment compaction, and by convection (Fig. 7.14). These are now considered in turn.
7.4.1 Meteoric Flow
The concept of the piezometric surface, and the idea of meteoric water displacing
denser connate water, has already been described and illustrated (Fig. 7.11). This type
of fluid movement is naturally most important at shallow depths and in basins that have
been uplifted above sea level (Ingebritsen and Sanford, 1998). Hydrodynamic flow is
important as a mechanism for generating secondary porosity in weathering zones that
may ultimately be preserved beneath unconformities. It sometimes traps petroleum in
flexures that lack true vertical closure (termed "hydrodynamic traps") (Dahlberg, 1982),
and it is responsible for uranium roll-front mineralization (see Section 6.3.2.2.4).