13.5 Late, Deep-Burial Diagenesis
netic systems. Here, only a few examples can be
briefly mentioned. They all come from Europe, because some general aspects of these locations are described in other sections of this book.
Middle Triassie shallow-marine limes tones, southem Germany. The classic Triassic sequences of central
Europe were deposited in a slowly subsiding intracontinental basin (cf. Sect. 12.3.2, Fig. 12.17). The Middle
Triassic (Muschelkalk Group) was characterized by a
major marine transgression leaving behind two
shallow-rnarine limestones sequences separated by
marine evaporites including halite. The relatively simple burial history and diagenetic evolution ofthe upper
limestone sequence was investigated in drill holes and
outcrops (Geng and Zeeh 1995, Fig. 13.25d). The cement sequence (A to D) reflects the initial influence of
an arid clirnate and the increasing burial depth and
temperature with time. Dolomitization occurred relatively early and is not pervasive. The deepest burial
and thus the peak of this development is reached at the
beginning of the Cretaceous. It is marked by the formation of saddle dolomite at temperatures of 80110°C. Then the Middle Triassic was slowly exhumed
due to uplift, except those parts buried deeply below
younger deposits in the Rhine graben (cf. Sect. 12.1,
Fig. 12.2).
Exhumation reactivated meteoric flow and led to a further calcite generation (E 1) and dedolomitization. Local subsidence in the graben and possibly the influence
ofhydrothermal fluids caused the generation offurther
saddly dolomite and siderite.
Upper Triassic fluvial sandstones, Paris basin (Fig.
13.26; cf. Fig. 14.10). Texturally and compositionally
immature sands (lithic arkoses or feldspathic litharenites) of distal alluvial fans, braided and slightly
meandering rivers were deeply buried (about 2 km).
They rest on strata free of lacustrine deposits and
evaporites and are covered by marine Mesozoic and
Tertiary sediments in the center of the Paris basin. Farther to the east, the Upper Triassic (Keuper Formation)
contains evaporites. The burial history of these immature sandstones was investigated in a representative
well (Fig. 13 .26b) which revealed the following sequence of diagenetic phases (Spötl et al. 1993):
- An early meteoric phase in semiarid climate characterized by extensive carbonate cementation, particularly dolocrete, and hematite (red-bed-type diagenesis ),
paleokarst and secondary porosity caused by leaching.
The basinal brine had a high
- A deeper burial phase (after compaction) in which
gravity-driven lateral fluid flow imported evaporitic
Keuper-brines from the uplifted eastem basin margin.
These brines had high ö 18 values and reached temperatures up to 140°C. They mixed with Keuper formation
693
water (lower salinity, low ö 18 values; Fig. 13.26c). The
resulting diagenetic minerals include ferroan carbonates, authigenic albite and K-feldspar, quartz, sulfates,
sulfides, and clay minerals.
- A second phase of long-distance lateral fluid flow,
but this time from the south (low salinity, low
ö 18 value). Mixing with the pre-existing basinal brine
reduced the salinity of the pore fluid to 50-100 g/l TDS
(concentration of the present-day brine).
- Fluids from the Keuper sandstone ascended along
faults within the basin center, crossed the Liassic
mudstones, and contributed to the cementation of
Dogger carbonates (Worden and Matray 1995).
In total, more than 20 different cements were identified
in the Keuper sandstones; some of them formed coevally or repeatedly, before hydrocarbons were
emplaced. The two different flow systems and fluid
mixing phases, operating during mesodiagenesis, are
inferred from the study of fluid inclusions, oxygen,
carbon, and strontium isotopes, C1IBr ratios, cathodoluminscence, and radiometric dating of illite.
Lower Permian (Rotliegend) sandstones. These
deeply buried sandstones (2-5 km) below the North
Sea, the northem Netherlands and northem Germany
have received much attention because they host large
amounts of hydrocarbons (mainly gas). The fluvial,
eolian, and partly lacustrine (playa) sandstones were
deposited in an arid continental environment on top of
the coal-bearing strata ofthe Carboniferous and below
the thick evaporites ofthe Zechstein Formation (Upper
Perrnian; cf. Sect. 6.4.3 and Fig. 12.17c). The diagenesis of these sandstones is not only controlled largely
by the depositional environment, but also displays
markedregional differences. Relativelyshallow-buried
fluvial and interdune sandstones ofthe southem Netherlands were rapidly cemented by dolomite, anhydrite
and quartz in an early meteoric regime and show no
influence ofPermian sea water or evaporites. Sands of
eolian dunes and dry sand flats have preserved good
porosities (mean value 17%) and permeabilities (mean
10 mD; cf. Fig. 13.21) under these conditions. More
porosity was lost by dolomite and anhydrite cementation than by pressure solution of quartz. In addition,
secondary clay minerals and hematite have reduced the
porosity. Isotope studies on drill cores from northem
Germany have revealed that the early Rotliegend cements resemble the cementation pattern 0 modem
playa lake basins.
At greater burial depths and in regions affected by
faulting, cross-formational and lateral fluid exchange
between the strata of uplifted horst blocks and subsided graben structures have brought about late cementation by quartz, Fe-carbonates, anhydrite and clay
minerals (e.g. authigenic illite). The isotopes of C, S
and Sr found in the cements indicate that small volumes of Zechstein-fluids have been injected. Some
cement types formed several times during the
netic systems. Here, only a few examples can be
briefly mentioned. They all come from Europe, because some general aspects of these locations are described in other sections of this book.
Middle Triassie shallow-marine limes tones, southem Germany. The classic Triassic sequences of central
Europe were deposited in a slowly subsiding intracontinental basin (cf. Sect. 12.3.2, Fig. 12.17). The Middle
Triassic (Muschelkalk Group) was characterized by a
major marine transgression leaving behind two
shallow-rnarine limestones sequences separated by
marine evaporites including halite. The relatively simple burial history and diagenetic evolution ofthe upper
limestone sequence was investigated in drill holes and
outcrops (Geng and Zeeh 1995, Fig. 13.25d). The cement sequence (A to D) reflects the initial influence of
an arid clirnate and the increasing burial depth and
temperature with time. Dolomitization occurred relatively early and is not pervasive. The deepest burial
and thus the peak of this development is reached at the
beginning of the Cretaceous. It is marked by the formation of saddle dolomite at temperatures of 80110°C. Then the Middle Triassic was slowly exhumed
due to uplift, except those parts buried deeply below
younger deposits in the Rhine graben (cf. Sect. 12.1,
Fig. 12.2).
Exhumation reactivated meteoric flow and led to a further calcite generation (E 1) and dedolomitization. Local subsidence in the graben and possibly the influence
ofhydrothermal fluids caused the generation offurther
saddly dolomite and siderite.
Upper Triassic fluvial sandstones, Paris basin (Fig.
13.26; cf. Fig. 14.10). Texturally and compositionally
immature sands (lithic arkoses or feldspathic litharenites) of distal alluvial fans, braided and slightly
meandering rivers were deeply buried (about 2 km).
They rest on strata free of lacustrine deposits and
evaporites and are covered by marine Mesozoic and
Tertiary sediments in the center of the Paris basin. Farther to the east, the Upper Triassic (Keuper Formation)
contains evaporites. The burial history of these immature sandstones was investigated in a representative
well (Fig. 13 .26b) which revealed the following sequence of diagenetic phases (Spötl et al. 1993):
- An early meteoric phase in semiarid climate characterized by extensive carbonate cementation, particularly dolocrete, and hematite (red-bed-type diagenesis ),
paleokarst and secondary porosity caused by leaching.
The basinal brine had a high
- A deeper burial phase (after compaction) in which
gravity-driven lateral fluid flow imported evaporitic
Keuper-brines from the uplifted eastem basin margin.
These brines had high ö 18 values and reached temperatures up to 140°C. They mixed with Keuper formation
693
water (lower salinity, low ö 18 values; Fig. 13.26c). The
resulting diagenetic minerals include ferroan carbonates, authigenic albite and K-feldspar, quartz, sulfates,
sulfides, and clay minerals.
- A second phase of long-distance lateral fluid flow,
but this time from the south (low salinity, low
ö 18 value). Mixing with the pre-existing basinal brine
reduced the salinity of the pore fluid to 50-100 g/l TDS
(concentration of the present-day brine).
- Fluids from the Keuper sandstone ascended along
faults within the basin center, crossed the Liassic
mudstones, and contributed to the cementation of
Dogger carbonates (Worden and Matray 1995).
In total, more than 20 different cements were identified
in the Keuper sandstones; some of them formed coevally or repeatedly, before hydrocarbons were
emplaced. The two different flow systems and fluid
mixing phases, operating during mesodiagenesis, are
inferred from the study of fluid inclusions, oxygen,
carbon, and strontium isotopes, C1IBr ratios, cathodoluminscence, and radiometric dating of illite.
Lower Permian (Rotliegend) sandstones. These
deeply buried sandstones (2-5 km) below the North
Sea, the northem Netherlands and northem Germany
have received much attention because they host large
amounts of hydrocarbons (mainly gas). The fluvial,
eolian, and partly lacustrine (playa) sandstones were
deposited in an arid continental environment on top of
the coal-bearing strata ofthe Carboniferous and below
the thick evaporites ofthe Zechstein Formation (Upper
Perrnian; cf. Sect. 6.4.3 and Fig. 12.17c). The diagenesis of these sandstones is not only controlled largely
by the depositional environment, but also displays
markedregional differences. Relativelyshallow-buried
fluvial and interdune sandstones ofthe southem Netherlands were rapidly cemented by dolomite, anhydrite
and quartz in an early meteoric regime and show no
influence ofPermian sea water or evaporites. Sands of
eolian dunes and dry sand flats have preserved good
porosities (mean value 17%) and permeabilities (mean
10 mD; cf. Fig. 13.21) under these conditions. More
porosity was lost by dolomite and anhydrite cementation than by pressure solution of quartz. In addition,
secondary clay minerals and hematite have reduced the
porosity. Isotope studies on drill cores from northem
Germany have revealed that the early Rotliegend cements resemble the cementation pattern 0 modem
playa lake basins.
At greater burial depths and in regions affected by
faulting, cross-formational and lateral fluid exchange
between the strata of uplifted horst blocks and subsided graben structures have brought about late cementation by quartz, Fe-carbonates, anhydrite and clay
minerals (e.g. authigenic illite). The isotopes of C, S
and Sr found in the cements indicate that small volumes of Zechstein-fluids have been injected. Some
cement types formed several times during the