690
are connected with shallow-burial meteoric flow systems, even deeply buried sediments may experience a
secondary phase of meteoric circulation leading to similar results as mentioned for early diagenesis (leaching,
dedolomitization, etc.), particularly elose to fractures.
In addition, deeply operating flow systems may again
become more active due to increased hydraulic heads
and temperature gradients, or tectonic stress. Several
ofthe above mentioned processes can operate simultaneously or partially follow each other.
Although the general rules for these processes are
fairly well known, it still appears to be difficult to predict the deep-burial diagenetic evolution of an individual sequence with confidence. There is hardly a sedimentary section which behaves diagenetically in exactly the same way as another one. This problem is
demonstrated below by some case studies. In the following section the controversial interpretation of late
dolomitization only is addressed.
13.5.3 Large-Scale Fluid Flow Systems and
Late Dolomitization
Long-distance fluid flow. Fluid flow on a basin-wide
scale (basin geofluids) has been advocated recently by
a number of workers (e.g. Lawrence and Comford
1995; Monta ez et al. 1997) for the interpretation of
some diagenetic phenomena which otherwise are difficult to understand. One of these phenoma is the occurrence of highly saline pore fluids in sediment sections
which do not contain evaporites or brines derived from
dissolved evaporites originally present within the sequence. However, evporites are known in these cases
far away from the site of investigation. Therefore it is
assumed that large-scale, deep-reaching subsurface
flow systems have transported solutes (ineluding hydrocarbons) via permeable layers over long distances.
Another indicator of large-scale, deeply circulating
fluid flow is the high-temperature diagenesis observed
in some sequences which were never deeply buried
and could not have been affected by fluids from the
directly underlying rocks. In these cases, a kind of
basin-wide hydrothermal system must have developed
which transported not only heat, but also various solutes from deeper buried strata to marginal locations
where the basin fill is limited in thickness.
The high-temperature (up to 150°C) diagenesis ofintermediately buried (never more than 1.8 km) Pennsylvanian sandstones and limestones in the distal part of a foreland basin
(Cherokee basin, Kansas) can be explained only by ascending pore fluids from more central, deeper buried parts of the
basin (Wojcik et al. (1997).
Such flow systems are also discussed as a means oftransporting hydrocarbons over long distances (e.g. Bachu 1997;
Rostron et al. 1997; cf. Sect. 14.2).
Chapter 13 Diagenesis
These and additional findings have led to the development of large-scale conceptional models in which the
diagenetic evolution of the basin fill is mainly controlled by the tectonic setting and the hydrological
flow regimes operating in the basins (Heydari 1997):
- Passive margin settings. The burial diagenesis is
characterized by extensional tectonics, ± growth faults,
and relatively slow and steady subsidence. The hydrological flow regimes of these settings are (1) near-surface (up to 1 km depth), topographically-driven meteoric flow directed basinward (cf. Fig. 13.12d), (2)
compaction flow with a vertical and horizontal, stratabound flow component, and (3) thermohaline circulation (convection flow). Sediments deeper than 3 km
may become overpressured. Above this zone, the concentrations of cations and anions in the pore fluids
tend to increase with burial depth.
- Collisional margin settings (Fig. 13.25a) are characterized by compressional tectonics and thrust faulting,
extensive fracturing, and episodic fluid flow in response to tectonic loading. Hot saline and highly pressured fluids ("tectonic fluids") move laterally from the
collision zone toward the craton using permeable strata
and fracture zones as conduits. The may have taken up
some fluids from magmatic sourees, from metamorphie
processes, and dehydration water from altered clay
minerals.
- Post-orogenie settings (Fig. 13.25b) are dominated
by topographically-driven fluid flow and high flow
rates (1 to 30 mlyear). The groundwater composition
(1 to 100 g/l TDS, cf. Sect. 9.2) is controlled by rock
types.
The diagenetic evolution of rift basins resembles that
of passive margin settings, but may be complicated by
higher tectonic activity, stronger subsidence, and drastic changes in the depositional environment. The principles of the conceptual models can also be applied to
other basin settings. Considering long time periods,
some basin fills may experience two or even more fundamental changes in their diagenetic evolution.
The passive margin diagenetic realm is exemplified by the
northern Gulf of Mexico. The various phenomena of this
margin have been described by several authors (e.g. Ranor
1994; Reydari 1997; Land et al. 1997; Morse et al. 1997; see
also Sect. 6.4.8).
Rowever, this exarnple cannot be applied to all passive margins because it is in some ways specific due to the underlying
Jurassie evaporites, extensive salt structures, and high sedimentation rates.
The example from Kansas mentioned above (Wojcik et al.
1997) seems to represent mainly the collisional margin setting. The intermediate and late diagenetic history of the
Pennsylvanian (Late Carboniferous) limestones shows two
episodes ofbrine injection of differing composition and temperature.
Some sediment sections from deep drill holes in the
Variscan foreland basins of Europe have revealed a multi-
are connected with shallow-burial meteoric flow systems, even deeply buried sediments may experience a
secondary phase of meteoric circulation leading to similar results as mentioned for early diagenesis (leaching,
dedolomitization, etc.), particularly elose to fractures.
In addition, deeply operating flow systems may again
become more active due to increased hydraulic heads
and temperature gradients, or tectonic stress. Several
ofthe above mentioned processes can operate simultaneously or partially follow each other.
Although the general rules for these processes are
fairly well known, it still appears to be difficult to predict the deep-burial diagenetic evolution of an individual sequence with confidence. There is hardly a sedimentary section which behaves diagenetically in exactly the same way as another one. This problem is
demonstrated below by some case studies. In the following section the controversial interpretation of late
dolomitization only is addressed.
13.5.3 Large-Scale Fluid Flow Systems and
Late Dolomitization
Long-distance fluid flow. Fluid flow on a basin-wide
scale (basin geofluids) has been advocated recently by
a number of workers (e.g. Lawrence and Comford
1995; Monta ez et al. 1997) for the interpretation of
some diagenetic phenomena which otherwise are difficult to understand. One of these phenoma is the occurrence of highly saline pore fluids in sediment sections
which do not contain evaporites or brines derived from
dissolved evaporites originally present within the sequence. However, evporites are known in these cases
far away from the site of investigation. Therefore it is
assumed that large-scale, deep-reaching subsurface
flow systems have transported solutes (ineluding hydrocarbons) via permeable layers over long distances.
Another indicator of large-scale, deeply circulating
fluid flow is the high-temperature diagenesis observed
in some sequences which were never deeply buried
and could not have been affected by fluids from the
directly underlying rocks. In these cases, a kind of
basin-wide hydrothermal system must have developed
which transported not only heat, but also various solutes from deeper buried strata to marginal locations
where the basin fill is limited in thickness.
The high-temperature (up to 150°C) diagenesis ofintermediately buried (never more than 1.8 km) Pennsylvanian sandstones and limestones in the distal part of a foreland basin
(Cherokee basin, Kansas) can be explained only by ascending pore fluids from more central, deeper buried parts of the
basin (Wojcik et al. (1997).
Such flow systems are also discussed as a means oftransporting hydrocarbons over long distances (e.g. Bachu 1997;
Rostron et al. 1997; cf. Sect. 14.2).
Chapter 13 Diagenesis
These and additional findings have led to the development of large-scale conceptional models in which the
diagenetic evolution of the basin fill is mainly controlled by the tectonic setting and the hydrological
flow regimes operating in the basins (Heydari 1997):
- Passive margin settings. The burial diagenesis is
characterized by extensional tectonics, ± growth faults,
and relatively slow and steady subsidence. The hydrological flow regimes of these settings are (1) near-surface (up to 1 km depth), topographically-driven meteoric flow directed basinward (cf. Fig. 13.12d), (2)
compaction flow with a vertical and horizontal, stratabound flow component, and (3) thermohaline circulation (convection flow). Sediments deeper than 3 km
may become overpressured. Above this zone, the concentrations of cations and anions in the pore fluids
tend to increase with burial depth.
- Collisional margin settings (Fig. 13.25a) are characterized by compressional tectonics and thrust faulting,
extensive fracturing, and episodic fluid flow in response to tectonic loading. Hot saline and highly pressured fluids ("tectonic fluids") move laterally from the
collision zone toward the craton using permeable strata
and fracture zones as conduits. The may have taken up
some fluids from magmatic sourees, from metamorphie
processes, and dehydration water from altered clay
minerals.
- Post-orogenie settings (Fig. 13.25b) are dominated
by topographically-driven fluid flow and high flow
rates (1 to 30 mlyear). The groundwater composition
(1 to 100 g/l TDS, cf. Sect. 9.2) is controlled by rock
types.
The diagenetic evolution of rift basins resembles that
of passive margin settings, but may be complicated by
higher tectonic activity, stronger subsidence, and drastic changes in the depositional environment. The principles of the conceptual models can also be applied to
other basin settings. Considering long time periods,
some basin fills may experience two or even more fundamental changes in their diagenetic evolution.
The passive margin diagenetic realm is exemplified by the
northern Gulf of Mexico. The various phenomena of this
margin have been described by several authors (e.g. Ranor
1994; Reydari 1997; Land et al. 1997; Morse et al. 1997; see
also Sect. 6.4.8).
Rowever, this exarnple cannot be applied to all passive margins because it is in some ways specific due to the underlying
Jurassie evaporites, extensive salt structures, and high sedimentation rates.
The example from Kansas mentioned above (Wojcik et al.
1997) seems to represent mainly the collisional margin setting. The intermediate and late diagenetic history of the
Pennsylvanian (Late Carboniferous) limestones shows two
episodes ofbrine injection of differing composition and temperature.
Some sediment sections from deep drill holes in the
Variscan foreland basins of Europe have revealed a multi-