Transcript Diageneis - McGill University

Diagenesis
Francis, 2014
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Diagenesis
Diagenesis is the conversion of unconsolidated sediments
into rock. The transition from diagenesis to metamorphism
is somewhat arbitrary, but is typically taken to be in the
range 250o to 300oC at which point green phyllosillicates
with definite compositions, such as muscovite and chlorite,
become stable at the expense of mixed-layered clay
minerals.
This is also approximately the minimum
temperature required for mineral assemblages to represent
reaction equilibria, as opposed to the kinetic effects which
dominate the diagenesis regime.
The physical differences between unconsolidated sediments
and rocks are those of porosity, volume, and cohesion. The
changes during diagenesis are dominantly compaction,
dewatering, and cementation. Two idealized conceptual
end-member models for diagenesis can be postulated:
The rock represents an closed-system, except for the
expulsion of water, in which porosity reduction is achieved
by compaction and cementation by pressure solution.
The rock represents an open-system through which fluids
moved, porosity reduction is a result of infilling by cement
precipitated from the fluids. No compaction occurs.
Compaction clearly occurs in the real world and the amount of
the crystalline cement in most sedimentary rocks is small (< 10
%). The solubility of the main cementing agents, quartz and
calcite, in interstitial pore waters are so low, however, that they
are not capable of supplying even 1% cement. Clearly fluids
have moved through rocks and the end results of diagenesis
represent the accumulated interaction of pore fluids with the
sediments. The results of diagenesis are thus sensitive to fluid /
rock ratios and position with respect to fluid pathways. Many
dolomites, for example, represent rocks whose composition have
experienced a regional scale Mg enrichment due to the passage
of large volumes of fluids during diagenesis.
Diagenetic processes are also sensitive to local geothermal
gradients and the difference between hydrostatic and lithostatic
pressure gradients. For example, sandstone sequences with
impermeable mudstone horizons can develop over pressured
states in which hydrostatic pressure exceeds lithostatic pressure
and impedes compaction.
To make matters more complicated, many sedimentary rocks
show evidence for the development of secondary porosity due
to dissolution during diagenesis.
The development of
secondary porosity is an important factor in the preparation of
oil reservoirs.
Eodiagenesis (shallow burial , ~ 0 - km)
Biogenic Activity:
Reworking of sediment by burrowing organisms and conversion
of shell and other fossil fragments to micrite by boring organisms
such as algae.
Weathering:
The breakdown of feldspars to kaolinite and ferromagnesian
silicates to smectites (montmorillonte) that occurs during
weathering continues during early diagenesis. The fine clay
minerals can migrate to intergrain voids, while the fine silica
produced can lead to silica saturation in the pore fluids.
albite + water ------------------> kaolinite
+
silica + K
NaAlSi3O8 + H2O ---------> Al2Si2O5(OH)4 + 4SiO2 + 2K+
Physical Compaction and Dewatering:
Reduction of porosity due to compaction by grain realignment
and migration of fine clay minerals and micritic carbonate to
grain interstices.
Cementation
Early cementation occurs in some open systems where conditions favour chemical
precipitation. In warm water areas, CaCO3 may locally become sufficiently supersaturated
such that aragonite or high-Mg calcite will precipitate in the pore spaces to produce "hard
ground‘‘and "beach rock".
On the other hand, warm fluids rising from depth or volcanic zones become supersaturated in
silica as they cool, precipitating a silica cement in siliciclastic sediments. Sandstones rich in
volcanic glass clasts commonly develop early silica cement due to the release of silica during
weathering of these thermodynamically unstable glass fragments, which yields excess silica.
Cementation
Changes in oxidation fugacity can result in the precipitation of minor Fe cement, as pyrite in
reducing environments (high organic matter) or goethite and hematite under oxidizing
conditions (atmospheric exposure).
Overall, however, the amount of cementation in the eodiagenetic environment is probably
small, and tends to be locally developed when it occurs. Aragonite or high-Mg calcite
cements, as opposed to calcite, are indicative of the eodiagenetic environment, as is opaline
silica rather than quartz cement.
Mesodiagenesis (deep burial, 4 – 10 Km)
Compaction / Cementation
Compaction and decrease in porosity continue with increasing depth in sediments resulting in the explusion of large
volumes of water that must move upwards and outwards through the sedimentary pile. The compaction is achieved
by both physical and chemical means, and the final thickness of sedimentary rock beds is typically 0.5 to 0.75 that of
the thickness of the unconsolidated sediment layer. The expelled water moves upwards and outwards through the
sedimentary pile, reacting with their host sediments during their passage. Some horizons of the sedimentary pile will
be preferential fluid pathways (aquifers) and experience extensive alteration, while other more impermeable horizons
will remain relatively unaffected by the effects of fluid through put, but act as important aquatards.
Physical Compaction
Deformation and/or crushing of weak grains such volcanic lithic fragments, micas, and altered feldspar, combined
with the movement of fine-grained material into interstices
Pressure Solution
Under conditions in which the pore fluid pressure is less than the lithostatic pressure there will be a
preferential dissolution of quartz or calcite grains at high stress points where they touch and a reprecipitation of quartz or calcite cement in the adjacent interstices. Opal and aragonite cements are
absent at this stage, replaced by quartz and calcite. This re-crystallization results in intergrowths
between grains and is probably the dominant cementation process in sedimentary rocks.
Stylolites in Recrystallized
Limestones and Dolomites
In carbonates, the extent of recrystallization,
or ‘neomorphism‘ can be extreme,
especially in the case of dolomites, resulting
in a rock in which little evidence of the
original fragments remain. Stylolites are
contorted sutures or seams of insoluble
material; such as clay minerals, Fe oxides,
and organic matter; thought to be produced
by extensive pressure solution and
recrystallization.
Stylolites Produced by Pressure
Solution in Dolomite
Classification of Organic Matter (kerogen) in Sedimentary Rocks
Sapropelic
Catagenesis
kerogen type
Humic
algal
amorphous
herbaceous
Type I
Type II
Type II
woody
coaly
Type III
H/C
1.7 - 0.3
1.4 - 0.3
1.0 - 0.3
0.5 - 0.3
O/C
0.1 - 0.02
0.2 - 0.02
0.4 - 0.02
0.1 - 0.02
Environment
marine, delta, lacustrine
delta, lacustrine
Fossil fuel
oil, sapropels
oil, gas
At depths of 1500 to 4000m (T = 60 - 130oC), solid
kerogen (organic matter, < 10% of black shales)
becomes converted into mobile hydrocarbon fluids that
migrate into the pore spaces.
Catagenesis is a
distillation process in which high H/C fluids are
released leaving behind a refractory low H/C solid
residue (vitrinite). The progress of these reactions in a
rock can be measured by measuring the spectral
reflectance of the residual vitrinite. There is a range of
kerogen types, reflecting the type of original organic
matter and the environment of deposition.
terrestrial
gas, tar
humic
coal
Clay Mineral Maturation
As temperature rises in the accumulating sedimentary pile, a point is
reached at which the weathering reactions reverse and the
progressive recrystallization of the clay minerals stable at the
surface begins with increasing depth.
kaolinite
+ silica + K
Al2Si2O5(OH)4 + 4SiO2 + 2K+
100oC
200oC
kaolinite
mixed layers with pyrophyllite / illite
smectite
mixed layers with chlorite, illite
albite + water
NaAlSi3O8 + H2O
300oC
muscovite /
paragonite
chlorite/
muscovite
This is a gradual process with intermediate members characterized
by randomly mixed layers of the end-member phyllosilicates.
During this progression, the recrystallizing phyllosilicates react and
become intergrown with the original clastic grains, tending to blur
their margins and acting as a cementing agent (typical in Paleozoic
greywackes).
Clay Minerals
300oC
kaolinite/smectite
Illite
I
S
Chl
Q
K
=
=
=
=
=
illite
smectite
chlorite
quartz
kaolinite
Replacement / Dissolution
Open systems with high fluid through put, and
thus high fluid / rock ratios, can result in the
replacement of original minerals, such as
carbonate by dolomite, resulting in a 5% increase
in porosity. Furthermore, porosity may actually
increase with depth in carbonates at temperatures
above 100oC, the point at which the maturation of
organic matter (catagenesis) gives off CO2,
resulting in increased solubility of CaCO3
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Telodiagenesis (uplift)
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dolomite porosity
Late stage diagenetic processes occur in rock sequences that have been uplifted above sea level. These
sequences are infiltrated with meteoric water and can be divided into two important zones:
Vadose
- above the water table, water undersaturated.
- oxidation & weathering
Phreatic
- below the water table, pore spaces completely filled with fluid.
- dissolution resulting in secondary porosity
The contact between these two zones, the water table, is typically a zone of intensive carbonate dissolution
because of the mixing between meteoric and ground waters, this is the horizon along which caves form in
karst terranes.