Department of Petroleum Geology & Sedimentology,
Faculty of Earth Sciences,
King Abdulaziz University,
Jeddah, Saudi Arabia
EPS 421
CLASTIC SEDIMENTARY ROCKS
Lecture 11: Sandstone diagenesis
(compaction, cementation, authigenesis,
porosity and permeability).
Prof. Dr. Mahmoud A. M. Aref
SANDSTONE DIAGENESIS
Diagenesis, is divided into two broad stages:
1. Early diagenesis: for processes taking place from deposition
and into the shallow burial realm,
2. Late diagenesis: for those processes affecting the sediments at
deeper levels, and during and after uplift.
EPS 321 Lecture 11
DIAGENETIC TERMS
Syn- or eodiagenesis have been used for early diagenesis
Ana-or mesodiagenesis have been used for burial diagenesis
Epi- or telodiagenesis have been used for diagenesis after uplift
EPS 321 Lecture 11
PROCESSES OF SANDSTONE DIAGENESIS
„
Physical processes
„
(cementation; dissolution and
replacement of grains by other
minerals).
(compaction and pressure
solution)
both largely dependent on
depth of burial.
Chemical processes
They take place in the medium of water.
They depend on:
1.
Salinity,
2.
pH (a measure of the hydrogen ion
concentration)
3.
Eh (redox potential) of the water,
4.
The ability of water to move through
the sediment (dependent on the
porosity and permeability).
EPS 321 Lecture 11
Diagenetic water in the early stages of diagenesis
„
In the early stages of diagenesis, lasting for some 1000 to
100,000 years and affecting sediments to depths of around 1
to 100 m, pore waters are related to the depositional
environment; connate waters if marine, or fresh waters in the
majority of continental sedimentary environments. These
pore waters are soon modified by the breakdown of organic
matter and bacterial activity.
„
With marine sediments, for example, the initial stages of
diagenesis take place in oxidizing pore waters, which with
depth become reducing as oxygen is used up in bacterial
processes.
EPS 321 Lecture 11
Diagenetic water in deep burial
„
During deep burial, pore waters are modified further by
reactions with clay minerals, dissolution of unstable grains,
precipitation of authigenic minerals and mixing with waters from
other sources.
Burial diagenesis operates over millions of years and affects
sediments to depths of around 10,000 m, where temperatures
are in the region of 100°-200°C. Beyond this, processes of
burial metamorphism take over. In general, pore waters in
deeply-buried sediments are saline, neutral and alkaline.
EPS 321 Lecture 11
„
Diagenetic water in uplift
„
Processes taking place in sedimentary rocks on
uplift typically involve fresh ground waters with
low Eh and acid pH.
„
The extent of epidiagenetic processes depends
largely on the porosity and permeability, which
may well have been largely occluded during
burial diagenesis.
EPS 321 Lecture 11
Factors affecting sandstone diagenesis
™ The depositional environment,
™ composition
™ texture of the sediment
™ pore-fluid migrations,
™ the burial history
EPS 321 Lecture 11
The principal diagenetic processes of
sandstone
1)
compaction and pressure solution,
2)
silica and calcite cementation,
3)
clay mineral and feldspar authigenesis
4)
the formation of hematite coatings and
impregnations.
EPS 321 Lecture 11
The principal diagenetic processes of
sandstone
1)
compaction and pressure solution,
2)
silica and calcite cementation,
3)
clay mineral and feldspar authigenesis
4)
the formation of hematite coatings and
impregnations.
EPS 321 Lecture 11
1. COMPACTION AND PRESSURE SOLUTION
„
In the initial stages, compaction involves dewatering and a
closer packing of grains. Further compaction through
overburden pressure results in local fracturing and bending of
weak grains and the solution of grains at points of contact.
EPS 321 Lecture 11
COMPACTION AND PRESSURE SOLUTION
„
Pressure solution at grain contacts is minimal where the sediment is
cemented early, before deep burial, or where there is much matrix, since in
these cases the load is spread and the contact pressure reduced.
„
Pressure solution can take place once a rock is fully cemented to produce
irregular or sutured planes, known as stylolites. These cross-cut grains and
cements, and a thin layer of insoluble material is concentrated along the
stylolites.
EPS 321 Lecture 11
The principal diagenetic processes of
sandstone
1)
compaction and pressure solution,
2)
silica and calcite cementation,
3)
clay mineral and feldspar authigenesis
4)
the formation of hematite coatings and
impregnations.
EPS 321 Lecture 11
2. A. SILICA CEMENTATION
1.
2.
Quartz overgrowth
Microquartz, megaquartz, chalcedonic
quartz and opaline silica.
1. Quartz overgrowth is the most common type of
silica cement. Silica cement is precipitated around
the quartz grain and in optical continuity, so that
the grain and cement extinguish together under
crossed polarizers. The syntaxial overgrowth
commonly gives the grain euhedral crystal faces.
„ In many cases the shape of the original grain is
delineated by a thin iron oxide-clay coating
between the overgrowth and the grain.
„ A thicker clay precipitate around the quartz grain
has inhibited precipitation of a syntaxial
overgrowth.
EPS 321 Lecture 11
SILICA CEMENTATION
„
In some cases the boundary between the grain and
overgrowth cement cannot be discriminated with the light
microscope, and the whole rock has the appearance of a
metamorphic quartzite.
„
One important feature arising from the early quartz
cementation of sandstones is that they are then able to
withstand better the effects of compaction and pressure
solution during later burial. In this way a moderate porosity can
be preserved which may be filled later with oil or gas.
EPS 321 Lecture 11
The origin of the silica for cementation
1. Pressure solution. Pore solutions become enriched in silica which is
then reprecipitated as overgrowths when supersaturation is achieved.
Quartz overgrowths in sandstones without pressure solution effects
may reflect significant upward migration of silica-rich solutions from
more distant sites of pressure solution, or indicate another source of
silica.
2. Dissolution of silica dust (grain abrasion), or other silicates (e.g.
feldspars, amphiboles and pyroxenes) and biogenic silica (e.g.
diatoms, radiolaria and sponge spicules ).
EPS 321 Lecture 11
The principal diagenetic processes of
sandstone
1)
compaction and pressure solution,
2)
silica and calcite cementation,
3)
clay mineral and feldspar authigenesis
4)
the formation of hematite coatings and
impregnations.
EPS 321 Lecture 11
2. B. CALCITE CEMENTATION
„
Calcite , dolomite and siderite.
„
Two main types of calcite cement are poikilotopic
crystals and drusy calcite spar.
„
Poikilotopic crystals are large single crystals, up to several centimetres
across, which envelop many sand grains. Drusy calcite mosaics consist of
equant crystals which fill the pores between grains, and typically show an
increase in crystal size towards the centre of the original cavity.
„
The cement may vary from a uniform to patchy distribution, to local
segregations and concretions.
„
As a result of calcite precipitation there is often a displacement of grains so
that the grains appear to 'float' in the cement.
„
Calcite may also be precipitated in cracks in grains and so force them to
split.
„
Apart from filling pores, calcite and the other carbonates may also replace
grains. Quartz grains cemented by calcite are often corroded and etched at
their margins, to produce irregularly-shaped grains.
EPS 321 Lecture 11
3. CALCITE CEMENTATION
„
Calcite cements are common in grain-supported sandstones, such
as quartz arenites, arkoses and litharenites. Calcite is frequently an
early diagenetic cement and the first cement. The early
precipitation of calcite inhibits later quartz overgrowth formation and
feldspar alteration and can result in total loss of porosity and
permeability. In other sandstones, calcite is a later precipitate,
postdating quartz overgrowths and authigenic kaolinite.
„
CaCO3 precipitation, taking place when the solubility product is
exceeded, often occurs through an increase in the activity of the
carbonate ion. In the very shallow subsurface, this may happen
through evaporation of vadose or near-surface phreatic ground
water. At depths, carbonate precipitation can be brought about by
an increase in the pH and/or temperature.
EPS 321 Lecture 11
The principal diagenetic processes of
sandstone
1)
compaction and pressure solution,
2)
silica and calcite cementation,
3)
clay mineral and feldspar authigenesis
4)
the formation of hematite coatings and
impregnations.
EPS 321 Lecture 11
4. FELDSPAR AUTHIGENESIS
„
Although in many sandstones, feldspars are altered
to kaolinite and illite, feldspar overgrowths do occur
on detrital feldspar grains. They are most common
on potash feldspars, but they also occur on detrital
albite grains.
„
For authigenic feldspar, alkaline pore waters rich in
Na or K, Al and Si are necessary. These elements
are largely derived from hydrolysis and dissolution of
less stable grains within the sediment.
EPS 321 Lecture 11
The principal diagenetic processes of
sandstone
1)
compaction and pressure solution,
2)
silica and calcite cementation,
3)
clay mineral and feldspar authigenesis
4)
the formation of hematite coatings and
impregnations.
EPS 321 Lecture 11
5. CLAY MINERAL AUTHIGENESIS
„
The importance of clay mineral precipitation within sandstones
has only been appreciated in the last few years.
„
Previously all clay in sandstones had been regarded as detrital.
„
The precipitation of even small amounts of clay in a sandstone
can have a great effect on the permeability and may seriously
reduce its reservoir potential.
„
Clay may also filter into a sandstone, carried down by pore
waters from muddy interbeds. Extensive infiltration drastically
alters the texture of the sediment and decreases the original
textural and compositional maturity.
EPS 321 Lecture 11
5. CLAY MINERAL AUTHIGENESIS
„
„
„
„
Illite and kaolinite are the most common authigenic clays in
sandstones, but montmorillonite, mixed-layer illitemontmorillonite and mixed-layer montmorillonite-chlorite also
occur.
Authigenic clay minerals occur as pore-filling cements and clay
rims up to 50 µm thick around grains.
The attenuation and absence of rims near and at grain contacts
demonstrates their diagenetic origin.
The precipitation of clay rims is usually an early or the first
diagenetic event, often predating quartz overgrowths or calcite
cementation.
EPS 321 Lecture 11
Environmental condition for clay mineral
authigenesis
„
For illite, alkaline pore fluids are required together with sufficient
K, Si and Al.
„
Kaolinite requires more acid pore waters and these can be
produced by flushing of the sandstone by fresh water, either
during an early burial stage if the sediments are continental, or
if marine, during uplift after a burial phase.
„
The ions for kaolinite and illite precipitation are largely derived
from the alteration of labile detrital minerals, in particular
feldspars.
EPS 321 Lecture 11
The principal diagenetic processes of
sandstone
1)
compaction and pressure solution,
2)
silica and calcite cementation,
3)
clay mineral and feldspar authigenesis
4)
the formation of hematite coatings and
impregnations.
EPS 321 Lecture 11
6. HEMATITE CEMENTATION AND PIGMENTATION
„
Many terrigenous clastic sediments are colored red through the
presence of hematite. In many cases these rocks were
deposited in continental environments (deserts, rivers,
floodplains, alluvial fans, etc.) and the term 'red beds' has been
applied to them.
„
The hematite typically occurs as a very thin coating around
grains, but also stains red infiltrated or authigenic clay minerals
and authigenic quartz and feldspar.
„
The hematite is chiefly amorphous or consists of micron-size
crystals. These features of the hematite, together with the
absence of hematite coatings at grain contacts, indicate a
diagenetic origin.
EPS 321 Lecture 11
Source And Origin Of Hematite Pigment In Red Beds
1) A detrital origin; that amorphous iron compounds formed
through moist tropical lateritic weathering in upland areas, are
transported and deposited along with the sediments and then
converted to hematite.
2) A purely diagenetic mechanism; whereby the iron is supplied by
intrastratal solution of detrital silicates such as hornblende,
augite, olivine, chlorite and biotite, and magnetite.
EPS 321 Lecture 11
Diagenetic environment of hematite
precipitation
„
In oxidizing diagenetic environment; the iron is
reprecipitated as hematite. The hematite has a red
colour and develops above the water table and below,
if the groundwater is alkaline and oxidizing.
„
In reducing diagenetic environment; the iron is present
in the more soluble ferrous state and if incorporated
into clays, rather than carried away in solution, it will
impart a green color to the sediments.
EPS 321 Lecture 11
Other diagenetic minerals in sandstone
diagenesis
„
Other diagenetic minerals of less or only local importance are
sulphates and sulphides. Gypsum and anhydrite occur as
cements in sandstones where there are evaporite beds in the
sequence; otherwise they are rare. Celestite (SrSO4) and barite
(BaSO4) rarely occur as cements. Pyrite occurs in many
sandstones but only as an accessory diagenetic mineral.
EPS 321 Lecture 11
DIAGENETIC SEQUENCES
„
Within any sandstone formation, the sequence of
diagenetic events can be simple, involving only one
mineral precipitate, or highly complex, involving many
stages of precipitation and replacement.
EPS 321 Lecture 11
Factors controlling the path of diagenesis in sandstones
1.
the depositional environment,
2.
sediment composition and texture,
3.
the porewater chemistry,
4.
depth of burial
5.
timing of uplift.
The relative timing of diagenetic events in sandstones is important
in terms of the introduction of hydrocarbons. If a sandstone's
porosity is occluded by early cementation, then it cannot act
as an oil reservoir. Diagenetic processes take place in an
aqueous medium so that the influx of oil terminates
diagenesis and prevents further reactions.
EPS 321 Lecture 11
POROSITY AND PERMEABILITY
„
Two important aspects of sedimentary rocks are their porosity and
permeability.
„
Porosity is a measure of the pore space and two types are defined:
Absolute porosity Pt =
Effective porosity Pe =
(bulk volume - solid volume)
bulk volume
interconnected pore volume
x 100
x 100
bulk volume
„
Absolute porosity refers to the total void space, but since some of this will be
within grains, Effective porosity is more important. Effective porosity
determines the reservoir properties of a rock, together with permeability, the
ability of a sediment to transmit fluids.
„
Porosity is a basic feature of a sediment or rock whereas permeability depends
on the effective porosity, the shape and size of the pores and pore
interconnections (throats), and on the properties of the fluid itself, i.e. capillary
force, viscosity and pressure gradient.
EPS 321 Lecture 11
POROSITY AND PERMEABILITY
„
Two major types of porosity; primary and secondary porosity.
„
Primary porosity is developed as the sediment was deposited and
includes inter-and intra-particle porosity.
„
Secondary porosity develops during diagenesis by solution and
dolomitization, and through tectonic movements producing fractures
in the rock.
„
Primary porosity in sandstones is principally interparticle porosity,
dependent on the textural maturity of the sediment, controlled largely
by depositional processes and environments, and to a lesser extent
on compositional maturity.
EPS 321 Lecture 11
POROSITY AND PERMEABILITY
„
In general, the primary porosity increases as the grain size increases, the
sediment is better sorted and more loosely packed, and the clay content
decreases. The clean, well-sorted, loosely packed sands of beaches and
aeolian dunes can have porosities in excess of 50%, and they have high
permeabilities too.
„
Fine-grained sediments of high effective porosity, sorted siltstones for
example, and chalks, often have low permeabilities since capillary forces
prevent the flow of fluids through the small pore throats.
„
The effect of compositional maturity relates to the breakdown of unstable
grains; this can increase the porosity if the grains are simply dissolved out,
but in most cases the porosity is reduced through the formation of clay
minerals and other alteration products.
EPS 321 Lecture 11
Factors that decrease the porosity of sandstones
„
Once the clastic sediments are deposited, there are two processes
(compaction and cementation) which lead to a decrease in porosity: On a
broad scale, these processes bring about a gradual decrease in porosity
with increasing depth of burial.
„
Compaction takes place from a few metres below the sediment surface, and
results in a closer packing of grains, and eventually at depths of 100's to
1000's of metres to pressure solution and interpenetration of grains.
„
Cementation is the principal process of porosity loss in sandstones. Silica,
calcite and clay can all be precipitated as cements, filling pores and
decreasing both porosity and permeability.
„
Most non-carbonate petroleum reservoirs occur in sandstones which have
been only partially cemented and so retain much of their depositional
porosity.
„
Porosities necessary for good petroleum reservoirs are 20 to 35%.
EPS 321 Lecture 11