2307223 Sedimentology
Diagenesis, porosity & permeability
Sukonmeth Jitmahantakul
WEEK 7
Class schedule
8
CU expo
7
12-13
Class schedule - lab
Textbooks
Sediment pathway
From Nichols (2009)
Outline
1. Diagenesis
2. Porosity
3. Permeability
2307223 Sedimentology
www.geo.sc.chula.ac.th/2307223
Sediments are generally unconsolidated material at the time
of deposition and are in the form of loose sand or gravel, soft
mud or accumulations of the body parts of dead organisms.
Lithification
Lithification is the process in which sediments compact under pressure, expel
connate fluids, and gradually become solid rock.
Essentially, lithification is a process of porosity destruction through compaction and
cementation.
Lithification includes all the processes which convert unconsolidated sediments
into sedimentary rocks.
Diagenesis
The physical & chemical changes that
alter the characteristics of sediment after deposition
Diagenesis
Diagenesis is the change of sediments or existing sedimentary rocks into a
different sedimentary rock during and after rock formation (lithification), at
temperatures and pressures less than that required for the formation of
metamorphic rocks.
It does not include changes from weathering. It is any chemical, physical, or
biological change undergone by a sediment after its initial deposition, after its
lithification.
Diagenesis
All changes to sediment/sedimentary rock from the time of deposition to the
onset of metamorphism
Occur at relatively low temperatures, typically < 250º C, and at depths of up
to ~ 5,000 m
From Nichols (2009)
Diagenesis
Physical (burial) diagenesis - dissolution, pressure solution
Chemical diagenesis - cementation, dissolution, recrystallisation, replacement
Physical diagenesis - compaction
Changes in the packing of spheres can lead to a reduction in porosity and overall
reduction in volume.
From Nichols (2009)
Physical diagenesis - compaction
The accumulation of sediment results in the earlier deposits being overlain by
younger material, which exerts an overburden pressure that acts vertically on a
body of sediment and increases as more sediment, and hence more mass, is
added on top.
Loose aggregates initially respond to overburden pressure by changing the
packing of the particles; class move past each other into positions that take up
less volume for the sediment body as a whole.
This is one of the processes of compaction that increases the density of the
sediment and it occurs in all loose aggregates as the class rearrange themselves
under moderate pressure.
Physical diagenesis - compaction
Pore water in the voids between the grains is expelled in the process and
compaction by particle repacking may reduce the volume of a body of sand by
around 10%.
During compaction weaker grains, such as mica flakes or mud clasts in sandstone,
may be deformed plastically by the pressure from stronger grains such as quartz.
When muds are deposited they may contain up to 80% of water by volume: this is
reduced to around 30% under burial of a thousand metres.
Compaction has little effect on horizontal layers of sediment except to reduce the
thickness. Internal sedimentary structures may be slightly modified by compaction.
Physical diagenesis - compaction
Differential compaction where there is a lateral change in sediment type
differential compaction occurs.
Uncompacted
Compacted
From Nichols (2009)
Physical diagenesis - compaction
Compaction of layers within a mud rock around a concretion
From Nichols (2009)
Physical diagenesis - pressure solution
In sandstone and conglomerates the pressure is concentrated at the contacts
between grains or larger clasts creating concentrations of stress at these points.
In the present of pore water, diffusion takes place moving some of the mineral
material away from the contact and reprecipitating it on free surfaces of the
mineral grains.
This process is called pressure solution or pressure dissolution and it results in
grain becoming interlocked, providing a rigidity to the sediment, that is, it becomes
lithified.
Physical diagenesis - pressure solution
Pressure solution has occurred at the contact between two limestone pebbles.
From Nichols (2009)
Physical diagenesis - compaction effects
Compaction effects - the degree of compaction in an aggregate can be
determined by looking at the nature of the grain contacts.
From Nichols (2009)
Chemical diagenesis - dissolution
The processes of grain dissolution are
determined by the composition of the
grain minerals and the chemistry of the
pore waters.
The present of carbon dioxide in solution
will increase the acidity of pore waters
and leaching of compounds form organic
matter may also reduce the pH.
If dissolution happens before any
lithification occurs then all traces of the
calcareous fossil may be lost.
Dissolution of a fossil after centurion may
leave the mould of it - void or cast.
Most quartz dissolution occurs at grain
boundaries as a pressure dissolution
effect, but the silica released is usually
precipitated in adjacent pore spaces.
From FÜRSICH & PAN (2016)
Chemical diagenesis - cementation
Pore-filling minerals precipitated into voids within sediment/sedimentary rocks
Eogenetic cementation - very soon after deposition
Mesogenetic cementation - sediment buried and saturated with pore water
Telogenetic cementation - occurs during uplift
Chemical reactions take place between the grains, the water and ions dissolved in
the pore water. These reactions take place at low temperatures and are generally
very slow.
They involve dissolution of some mineral grains, the precipitation of new minerals,
the recrystallisation of minerals and the replacement of one mineral by another.
Chemical diagenesis - C-O isotope plot
From Warren et al (2014)
All data measured in central Thailand
increasingly negative oxygen values - increasing temperature (hotter fluid)
more negative carbon values - possibly indicative of the effects of catagenic carbon
Precipitation of cements
Cements are minerals precipitated within pore spaces during diagenesis.
The most common being silica (quartz), carbonates (calcite) and clay minerals
Carbonate minerals may precipitate as cements if the temperature rises or the
acidity decreases, and silica cementation occurs under increased acidity or cooler
conditions.
Growth of cement preferentially takes place on a grain of the same composition.
Precipitation of cements - cement fabrics
Matrix
Fine-grained material deposited
simultaneously with larger
particles.
Generally appears as darkercoloured detritus between grains.
vs
Cement
A chemical precipitate between
grains formed from pore-water
after deposition.
Chemical diagenesis - cementation
Pore-filling minerals precipitated into voids within sediment/sedimentary rocks.
Quartz
Hematite
Calcite
Chert
Limonite
Aragonite
Chalcedony
Phosphate
Mg-calcite
Opal
Clay
Dolomite
Glauconite
Siderite
Chemical diagenesis - cementation
Cement can be homogeneous, chemically pure, specific fabrics, multiphased or
zoned
Cement
Quartz grain
Chemical diagenesis - cementation
Cement can be heterogeneous.
Hematite cement
Quartz cement
Chemical diagenesis - recrystallisation
The in situ formation of new crystal structures while retaining the basic chemical
composition is the process of recrystallisation.
This is common in carbonates of biogenic origin because the mineral forms
created by an organism are not stable under diagenetic condition.
The recrystallised grains will commonly have the same external morphology as the
original shell or skeletal material, but the internal microstructure may be lost in the
process.
Recrystallisation of the siliceous hard parts of the organisms occurs because the
original structures are in the form of amorphous opaline silica, which recrystallises
to microcrystalline quartz.
Chemical diagenesis - replacement
The replacement of a grain by a different mineral occurs with grains of biogenic
origin and also detrital mineral grains.
For example, clay minerals may completely replace the volume of the original
feldspar grain or silicification in carbonate rocks
Diagenesis summary
A sediment body deposited on land or in the sea normally undergoes
significant modification before it becomes a sedimentary rock
Physical, chemical and biological processes act on the sediment at scales
that range from the molecular to basin-wide
Generally these processes change sediment into sedimentary rocks by
compacting loose detritus and adding material to create cements that bind
the sediment together
Chemical changes occur to form new minerals and organic substances, and
physical processes affect the layers on large and small scales
An important product of these post-depositional processes is the formation
and concentration of fossil fuels: coal, oil and natural gas
Porosity and permeability
Porosity and permeability are related
properties of any rock or loose
sediment.
Most oil and gas has been produced
from sandstones. These rocks often
have high porosity, and are usually
“high perm” too.
Porosity and permeability are
absolutely necessary to make a
productive oil or gas well.
The petroleum geologist must stay
focused on the porosity and
permeability of the prospective
reservoir.
A greatly magnified image of a sandstone as seen in a thin
section of the rock under the microscope. The rock sample was
injected with blue-colored epoxy that is seen here filling pores
which are interconnected (permeable). After plastic is injected and
solidified, the rock sample is cut and polished on a glass slide.
This particular sandstone contains grains of quartz (white),
calcite, and feldspar (shades as brown). The sample is
exceedingly porous and permeable. The grains are loosely
packed and there is very little cement filling the space between
the grains. The arrow indicates possible pathways for fluid
movement.
Porosity and permeability
Porosity consists of the tiny spaces in the rock that hold the oil or gas.
Permeability is a characteristic that allows the oil and gas to flow through the rock.
You need both porosity and permeability to make a producing oil or gas formation.
Microscopic structure of shale, sandstone, and limestone with water in pore spaces.
Note differences in scale among views of each rock type.
From Tom Grace, TERC
Porosity
Volume % of open space
Total porosity (Ø) = (bulk volume - grain volume) / bulk volume
Effective porosity = interconnected pores
Ineffective porosity
Effective porosity
Total porosity > Effective porosity
Conventional view of porosity
0-5% Negligible
5-10% Poor
10-15% Fair
15-20% Good
20-25% Very Good
From Saxena & Mavko (2016)
Primary porosity
Controlled by:
Degree of Uniformity of Grain Size - Sorting
Shape of the Grains
Method of Deposition (Manner of Packing)
Compaction
Cementation
Porosity varies with sorting
Porosity varies with packing
Cubic Packing 47%
porosity in the ideal
situation
Rhombohedral Packing
26% porosity in the ideal
situation
Secondary porosity
Additional open space developed after sedimentation:
Dissolution
Dolomitization
Fracturing
Modification to porosity
Sandstone
Carbonate
Pressure Solution
Compaction – 2 to 20%
Cementation
Solution
Fracturing
Recrystalization – Dolomitization
Fracturing
Shale
Compaction – 50%
Bound Water Expulsion
Cementation
Secondary porosity
Additional open space developed after sedimentation:
Dissolution
Dolomitization
Fracturing
Permeability
Ability of fluids to flow through Porous Media
Permeable – Large Well-Connected Pores
Impermeable – Smaller, Fewer or Less Interconnected Pores
Permeability values
Poro-perm measurements
Porosity can be measured by logs (density, neutron)
Permeability needs cores to do lab injection tests
From Tom Grace, TERC
Unconventional or conventional
The distinction between conventional and unconventional resources is a matter
of permeability.
2307501 Basin analysis
Unconventional vs conventional reserv.
2307501 Basin analysis