700
sediments) are less than in the underlying Paleozoic strata. In
the area of regional discharge (B), temperature increases rapidly with depth in the basin iill. Under specific conditions,
temperatures in the fIuid-discharge area of a foreland basin
may be higher than in deeply buried strata closer to the recharge area. This was inferred from the study of coal ranks in
the Carboniferous foreland basin of Wales (Gayer et al.
1998).
Local areas not affected by this basin-wide redistribution
of heat by groundwater fIow may be of interest for the exploration of oil and gas. Such areas displaythe original temperature field and thus indicate the presence of impermeable
rocks favorable for the retention ofhydrocarbons.
A subsurface temperature distribution sirnilar to that in
Fig. 13.30a was calculated for the Paleozoic Illinois basin
(Fig. 13.30b, Bethke 1989). It is also caused by regional,
long-distance groundwater circulation.
The marked influence ofthe permeability or hydraulic
conductivity of the rock or sediment mass on the distortion of conductive heat flow is demonstrated for a
situation with high relief (Fig. 13 .30d,e). If hydraulic
conductivity is relatively high, the thermal gradients in
the basin fill are significantly affected by groundwater
circulation. All these thermal effects have to be taken
into account in the interpretation of inorganic and
organic processes in diagenesis.
In the model, the difference in elevation between groundwater recharge and discharge zones was assumed to be 500 m
over a distance of a few km (Smith et al. 1989). If the sediment mass is homogeneous and has a very low permeability
(k= 1 0. 18 m 2 , corresponding with a hydraulic conductivity of
about 10. 6 mlday or 0.4 mmlyear), conductive heat fIow lines
and the geothermal gradient are only slightly disturbed (Fig.
13.30d). However, if the permeability increases by three
orders of magnitude (hydraulic conductivity around 0.4
mlyear), conductive heat lines and thermal gradients are
strongly affected (Fig. 13.30e). Other examples are reported,
for example, by Chapman and Rybach (1985).
13.8 Special Methods and
Processes in Diagenesis
13.8.1 Temperature, Degree, and
Age of Diagenetic Reactions
Classical methods for identifying dissolution phenomena and cement minerals are thin sections (cf. Fig.
13.27), their staining, X-ray diffraction, and various
geochemical methods.
In addition, scanning electron microscopy and
microprobe analysis can be used. The different stages
in the growth of carbonate cements and their varying
contents of iron and manganese are frequently studied
by using cathodoluminescence microscopy (cf. Fig.
13.27c). Digital image analysis has become acommon
method for investigating porosities and pore geometries.
Chapter 13 Diagenesis
The maximum temperature a sediment body has
experienced can be determined by several methods. If
particulate organic matter is present, its state of
diagenesis, defined by the vitrinite reflectance, is a
function of temperature and time (cf. Chap. 14).
Traces of organic matter in conodonts (biogenic phosphate) and their degree of maturation can indicate the
maximum burial depths of Plaeozoic to Triassic sediments.
Fluid inclusions, frequently present in secondary
crystals precipitated in rock fractures or in large pores,
are a very common objective in the study of late
diagenesis. At normal temperatures the inclusions consist of a fluid phase and a gas phase. By heating them
under the microscope, one can observe that the fluid
and gas are transformed into a homogeneous fluid.
From the homogenization temperature, corrected for
the effects of pressure and considering other sources
of error, one can reconstruct the temperature and pressure at the time the inclusion was formed (e.g. Roedder
1984).
Stable isotopes, for example the Ö 18 0 values of carbonate cements, can be used to deterrnine the temperature and burial depth at the time the cement mineral
was formed (e.g. Heydari 1997). However, one has to
be cautious, because these values may be affected by
the mixing of pore fluids derived from sea water and
meteoric water. In addition, one must take into account
that the isotopic composition of ocean water changed
from the Paleozoic to the present time (Garrels 1986;
Veizer et al. 1986; Carpenter and Lohmann 1989). For
example, the Ö 18 0 values increased, but carbon and
sulfur isotopes fluctuated during Phanerozoic time
(Schidlowski et al. 1984). To investigate such a question, one must search in ancient sediments for primary
minerals or early cements which formed in equilibrium
with the former ocean water and were preserved unaltered. On the other hand, these isotopes, including
those of strontium, allow identifying the mixing of
pore fluids originating from different formations.
The timing 0/ thermal events can be deterrnined by
dating cement or recrystallized minerals containing
unstable isotopes. The age of late illite formation has
been assessed in some examples by K-Ar dating (e.g.
Budeyet al. 1992; Zingmann et al. 1998).
In specific cases, fission tracks in apatite, other minerals, and volcanic glass (e.g., Naeser 1979; Odin
1982; Faure 1986) can indicate the time ofheating and
thus an unusual event in diagenesis. Fission tracks are
caused by nuclear particles emitted from uranium-238
impurities. Theyare obliterated if the temperature exceeds a certain limit (for example 100 to 120°C for
apatite) and start to form again and grow with time if
the temperature drops below this boundary. Hence,
fission tracks indicate that the rock sampie has been
exposed to elevated temperatures a certain time ago.
sediments) are less than in the underlying Paleozoic strata. In
the area of regional discharge (B), temperature increases rapidly with depth in the basin iill. Under specific conditions,
temperatures in the fIuid-discharge area of a foreland basin
may be higher than in deeply buried strata closer to the recharge area. This was inferred from the study of coal ranks in
the Carboniferous foreland basin of Wales (Gayer et al.
1998).
Local areas not affected by this basin-wide redistribution
of heat by groundwater fIow may be of interest for the exploration of oil and gas. Such areas displaythe original temperature field and thus indicate the presence of impermeable
rocks favorable for the retention ofhydrocarbons.
A subsurface temperature distribution sirnilar to that in
Fig. 13.30a was calculated for the Paleozoic Illinois basin
(Fig. 13.30b, Bethke 1989). It is also caused by regional,
long-distance groundwater circulation.
The marked influence ofthe permeability or hydraulic
conductivity of the rock or sediment mass on the distortion of conductive heat flow is demonstrated for a
situation with high relief (Fig. 13 .30d,e). If hydraulic
conductivity is relatively high, the thermal gradients in
the basin fill are significantly affected by groundwater
circulation. All these thermal effects have to be taken
into account in the interpretation of inorganic and
organic processes in diagenesis.
In the model, the difference in elevation between groundwater recharge and discharge zones was assumed to be 500 m
over a distance of a few km (Smith et al. 1989). If the sediment mass is homogeneous and has a very low permeability
(k= 1 0. 18 m 2 , corresponding with a hydraulic conductivity of
about 10. 6 mlday or 0.4 mmlyear), conductive heat fIow lines
and the geothermal gradient are only slightly disturbed (Fig.
13.30d). However, if the permeability increases by three
orders of magnitude (hydraulic conductivity around 0.4
mlyear), conductive heat lines and thermal gradients are
strongly affected (Fig. 13.30e). Other examples are reported,
for example, by Chapman and Rybach (1985).
13.8 Special Methods and
Processes in Diagenesis
13.8.1 Temperature, Degree, and
Age of Diagenetic Reactions
Classical methods for identifying dissolution phenomena and cement minerals are thin sections (cf. Fig.
13.27), their staining, X-ray diffraction, and various
geochemical methods.
In addition, scanning electron microscopy and
microprobe analysis can be used. The different stages
in the growth of carbonate cements and their varying
contents of iron and manganese are frequently studied
by using cathodoluminescence microscopy (cf. Fig.
13.27c). Digital image analysis has become acommon
method for investigating porosities and pore geometries.
Chapter 13 Diagenesis
The maximum temperature a sediment body has
experienced can be determined by several methods. If
particulate organic matter is present, its state of
diagenesis, defined by the vitrinite reflectance, is a
function of temperature and time (cf. Chap. 14).
Traces of organic matter in conodonts (biogenic phosphate) and their degree of maturation can indicate the
maximum burial depths of Plaeozoic to Triassic sediments.
Fluid inclusions, frequently present in secondary
crystals precipitated in rock fractures or in large pores,
are a very common objective in the study of late
diagenesis. At normal temperatures the inclusions consist of a fluid phase and a gas phase. By heating them
under the microscope, one can observe that the fluid
and gas are transformed into a homogeneous fluid.
From the homogenization temperature, corrected for
the effects of pressure and considering other sources
of error, one can reconstruct the temperature and pressure at the time the inclusion was formed (e.g. Roedder
1984).
Stable isotopes, for example the Ö 18 0 values of carbonate cements, can be used to deterrnine the temperature and burial depth at the time the cement mineral
was formed (e.g. Heydari 1997). However, one has to
be cautious, because these values may be affected by
the mixing of pore fluids derived from sea water and
meteoric water. In addition, one must take into account
that the isotopic composition of ocean water changed
from the Paleozoic to the present time (Garrels 1986;
Veizer et al. 1986; Carpenter and Lohmann 1989). For
example, the Ö 18 0 values increased, but carbon and
sulfur isotopes fluctuated during Phanerozoic time
(Schidlowski et al. 1984). To investigate such a question, one must search in ancient sediments for primary
minerals or early cements which formed in equilibrium
with the former ocean water and were preserved unaltered. On the other hand, these isotopes, including
those of strontium, allow identifying the mixing of
pore fluids originating from different formations.
The timing 0/ thermal events can be deterrnined by
dating cement or recrystallized minerals containing
unstable isotopes. The age of late illite formation has
been assessed in some examples by K-Ar dating (e.g.
Budeyet al. 1992; Zingmann et al. 1998).
In specific cases, fission tracks in apatite, other minerals, and volcanic glass (e.g., Naeser 1979; Odin
1982; Faure 1986) can indicate the time ofheating and
thus an unusual event in diagenesis. Fission tracks are
caused by nuclear particles emitted from uranium-238
impurities. Theyare obliterated if the temperature exceeds a certain limit (for example 100 to 120°C for
apatite) and start to form again and grow with time if
the temperature drops below this boundary. Hence,
fission tracks indicate that the rock sampie has been
exposed to elevated temperatures a certain time ago.