Silisciclastic Diagenisis Flashcards
(99 cards)
1
Q
At what depth, pressure and temperature does diagenesis takes place?
A
Ate around 6 km and at relatively low temperature (around ~300 c) and low pressure. Above 300 c and at high pressure you are altering the rock.
2
Q
What is an consequence of grain compaction/burial that has huge economical consequences in the energy sector?
A
Decrease in porosity
3
Q
What processes occur during diagenesis? (6 examples)
A
Compaction
Cementation
Grain repacking( distribution of grains change porosity)
Bioturbation
Pressure dissolution( sending minerals into solution at high pressure in pore water)
Authigenisis (growth of new minerals)
4
Q
Difference between compaction and repacking?
A
Compaction is different than repacking in the sence that compaction is the increase in weight from the overlaying beds and repacking is the change in the distribution of grains in order to occupy less space, witch results in decreased porosity.
Grain repacking is a consequence of compaction.
5
Q
What characteristics will dictate if diagenesis will occur?
A
Temperature
Pressure
Biomass (dictates who is stirring the sediment and also what is the chemistry of you sediment)
Pore water chemistry ( low ph- acidic- dissolves carbonate and high ph-alkaline-precipitate carbonate)
Sediment POROSITY and PERMEABILITY (this two dictates how your water behaves)
Lithology (mineral content)
6
Q
How depth affects salinity during diagenesis?
A
As pressure increases, the abundance of dissolve ions increase, thus increasing the salinity or the ppm of these ions dissolved in the pore spaces.
7
Q
What consequences the type of pore water can have during diagenesis ?
Fresh vs salt water?
!!!
A
Fresh water is very different from salt water so the chemistry of the pore water will be also different as well as the cement deposit in this different chemistry. As a consequence, the rocks formed in a terrestrial,marine and brackish settings are very different!
8
Q
What happens to carbonates in a acidic and alkaline environment?
A
Acidic- low pH- dissolve
Alkaline- High pH- precipitates
9
Q
What water driven rxns control early diagenesis of shallow-water siliciclastic sediments/ rocks in marine basins?
!!!
A
Marine vs freshwater vary different
Meteoric water-more acidic (pH~4.5-6)
Ocean water-alkaline (pH~8). Hence rxns are diff.
Different concentration of ions, hence some minerals will precipitate faster than others. Replacement rxns can also occur.
10
Q
What is diagenisis?
A
Physical and chemical changes that alter sediment after deposition.
Typically low temperature and low pressure
11
Q
What are the three types of diagenesis?
A
Eogenesis
Mesogenesis
Telogenisis
12
Q
What dictates Eogenesis?
A
Pore water interacts with the detrital assemblage at shallow burial under the influence of the depositional system.
13
Q
What is Eogenesis and at what level it takes place?
A
Shallow
- Reworking of grains (Bioturbation)
- Compaction (minimal!)
- Grain Repacking
- cementation(sometimes!)
14
Q
Grain repacking
A
Rearrangement of grains to maximize their energy.
15
Q
Explain how Caliche (hardpan) is formed?
!!!
A
It is a post depositional alteration due to Eogenesis right at the surfice
Calcite-cemented soil formed by evaporation of CACO3-rich ground water. The cement was left behind when the water evaporated.
16
Q
Caliche vs Silcrete
A
Caliche is made of water enriched in calcite. Cement is calcite.
Silcrete is made of water enriched in silica. Cement is silica
17
Q
Why Silcrete deposits are rare?
A
Because silica is not very soluble
18
Q
At what stage of diagenesis Silcrete is formed?
A
Eogenesis right at the surfice
Essentially same as caliche but cement is silica.
19
Q
Waht is a Beachrock and at what stage o diagenesis and what type of environment it is formed?
A
Eogenesis right at the surfice
Early cemented limestone
“Solid” rock prior to burial (CaCO3 cement)
Tidal environment.
20
Q
What are the environmental restrictions in a bechrock? (8)
A
pH Biomass Water depth Evolution and distribution of organisms Temperature Salinity Current strength Substrate
21
Q
Mesogenesis
A
Deeper
- Greater compaction
- More Cementation
- chemical alteration
Chemical compaction owing to pressure dissolution
The pore water chemistry can alter the sediments that are already there.
Increase led compaction and increased temperature can change the salinity and pH, also altering the sediments around it
Pressure dissolution
22
Q
How deep the sediment need to be to start the diagenetic process?
A
It can happen at the surface. You don’t need a lot of mass to start packing things together
23
Q
What dictates mechanical compaction during early burial?
A
The type of sediment play an important role on how the gains are going to behave under pressure.
E.g., packing mica vs quartz
24
Q
What is the consequence of mechanical packing?
A
Reduction in porosity
25
What happens to the grains as a consequence of burial compaction?
Mechanical rotation
Grain rearrangement
Brakage of thin/soft grains reduces the overall volume of the rock
Reduce porosity!!!
26
Differential compaction
The difference between initial porosity of I consolidated sand (25-43%) and unconsolidated mud (60-80%) causes differential compaction
27
Rate of compaction is a function...
... of rate of sedimentation
The more sediment - the more pressure. Also, the velocity at which the sediment id deposit is very important.
28
Why mud behaves different than sand during compaction?
Because the incredible amount of porosity in mud means that is will behave differently ( different compaction)
29
How can you minimize the lost of porosity by sediment compaction?
!!!!
EARLY CEMENTATION to minimize compaction, than when the rock comes back during Telogenesis, the carbonate is dissolved by acidic ground water hence increasing the porosity of the rock. This way, hydrocarbons can migrate upwards.
When the rock was cemented can play an important role in what porosity will look like when the rock gets back to the surface.
30
In comparison with sand, mud tends to compact....
.... Twice as much.
When deposited mud usually ~ 60-80% water.
T final = ~ 1/2 T original
Sends tend to be ~25-30% porosity but 0-10% porosity post-burrial
31
Sends tend to be .......% porosity before burial and ......% porosity after burial.
....~25-30% ...... 0-10%....
32
Different compaction leads to ...... looking deposits. ( appearance )
Strange looking deposits
33
Concretion
Discrete, localized cemented body within a sedimentary unit, with sharp boundaries and generally ovoid in shape and flattened parallel to bedding.
Build up of minerals around a nucleus.
34
How can you infer relative time of formation of concretions?
It may be inferred from its relationship with the adjacent layers.
35
What controls the formation of concretions?
Pore water
- Temperature
- Pressure
- pH
- Available elements
36
Chemical compaction
It happens during Mesogenesis. Increased overburden- increasing sub vertical stress concentrated at framework grain boundaries. Localized stress enhances dissolution at point of contact.
Feldspar is a good example. Dissolve away at stress points, adding ions into the system to be precipitated elsewhere.
37
Precipitated is different than..... d........t
Deposit
38
What happens if you add pressure to a point-point contact?
Increase amount of pressure. You can deform de grains and suture them together. It is a compaction with a dissolution of the contact to the surrounding fluid.
39
What is the name given to a type of structure formed as a result of pressure dissolution?
Stylolite
40
Stylolites
| !!!
Compaction fabric caused by pressure dissolution.
41
Compaction Fabric
Is caused by pressure dissolution and for structures called Stylolites
42
Cement
Is a mineral precipitate in pore spaces
43
Which are the most common cements in sedimentary rocks?
Calcite and Silica
44
False or true
| Does all cements make a rock harder?
False, not all cements can make a rock harder.
45
Which cement can precipitate around any grain?
Calcite
46
Which type of cement preferably grows in quartz?
Silica prefers to grow around silica.
Quartz prefer to grow on quartz grains (particularly monocrystalline grains) to form quartz overgrowths
47
Quartz overgrowths
Is when silica grows around a quartz grain
48
Isopachous grain-rimming cement
| !!!
Uniformly distribution cement all around the grain.
Common in subaqueous environment
49
How does calcite grow in the pore space?
Calcite forms outside of the grain and grows into the middle of the grain
50
Do we need pressure to dissolve carbonate and silica?
No. We don't need pressure to dissolve things. pH of the water is very important .
Alkaline- dissolve silica
Acidic- dissolve calcite
51
........ also can change the pH. It alter the equilibrium and the solubility of CO2
Temperature
52
Secondary porosity
Porosity that has been created during diagenesis
53
Primary porosity vs secondary porosity
Primary porosity is the porosity you have in the rock to begin with, when the sediments were deposit.
Secondary porosity is the porosity that was created during diagenesis. Great reservoir rocks!! You want rocks with secondary porosity.
54
How secondary porosity is created?
Dissolution at depths can produce secondary porosity. Dissolution of entire grains.
During TELOGENESIS- Dissolution by slightly acidic, near surface fresh water can dissolve the cement causing secondary porosity
55
............ Great way to creat secondary porosity!!!
TELOGENESIS
Late stage of diagenesis
56
Define Diagenisis
* Physical and Chemical changes that alter sediment after deposition.
* Typically at relatively low Temperature and pressure
57
What processes occur during diagenesis?
- Compaction-
- Cementation-
- Grain repacking-
- Bioturbation
- Pressure dissolution
- Authigenesis (Growth of new minerals)
58
Define Authigenesis
Is the process by which minerals form in a sedimentary rock after its deposition
59
Difference between Compaction and Repacking
Compaction is the pressure exert over the sediment and repacking is the rearrangement of the grains in order to occupy less space (consequence of added weight/pressure))
60
What alterations may occur during Lithification?
* Physical
* Chemical
* Mineralogical
61
What are the major variables that affect rates/amounts of diagenetic alteration?
* Temperature
* Pressure
* Biomass Pore water Chemistry
* Sediment porosity
* Sediment Permeability
* Lithology (Mineral content)
62
What variations can be expected from pore water chemistry with increase in depth?
Pore water chemistry differ from basin to basin, but the included variations in abundance of such mineral forming ions such as: Si4+ , Al3+, Ca2+, K+, Mg2+, Na+ and HCO-3 (bicarbonate). Many of this ions increase in abundance with increase in depth, concomitant with increase in salinity.
63
What are the difference between marine and fresh water that control early diagenesis of shallow-water siliciclastic sediments?
Marine and freshewater compositions are very different.
pH - Rain water is slightly acidic (pH~4.5-6) and ocean water is alkaline (pH~8)
Ions present. Hence, reactions can be expected from ions present.
Concentration of Ions. Elemental concentrations become supersaturated w/respect to specific minerals in given waters. As a result some mineral will precipitate from aqueous solution; some faster than others; preplacement reactions can also occur. Altogether these 'water driven' reactions control early Diagenesis of shallow-water siliciclastic sediments/rocks in marine basins.
64
Define Eogenisis
Shallow
* Reworking of sediment
* Compaction
* Graing repacking
* Cementation (sometimes)
65
What happens with the precipitation of minerals during Eogenisis in marine reduced and oxidized environments?
Precipitation of new minerals:Marine: in reduced marine envronment (low O2) - formation of Pyrite (may form cement or replace other materials, such as woody fragments) in O2-rich pore waters, : Formation of Chlorite, Glauconite (greenish iron-silicate grains), Illite/Smectite clays, and Iron oxides can form (eg. red clays on the deep ocean floor) and preciptation of K-feldspar overgrowths, quartz overgrowth and carbonate cements.
66
What type of sedimentary structures can be formed as a result of Post-deposition alteration during Early Cementation?
Caliche (hard pan):
Calicite-cemented soil formed by evaporation of CaCO3-rich groundwater
* Nitrate Salts deposits in Chile Peru (mined for industrial uses)
Silcrete (Chert) Essentially the same as caliche, but cement is SiO2 (less commun). note: silica is not very soluble in water.
Beach rock
Early cementation of Limestone. Solid rock prior to burial (CaCO3 cement).
67
Explain the formation of Beackrock
Beach rock is formed in Tidal environment as tides bring water, water evaporates, increasing concentracions of calcium carbonate, which precipitate in porous to form a cement.
68
Define Mesogenisis and at what level they occur
Deeper
* Compaction
* Cementation
* Chemical Alteration (Chemical compaction due to Pressure Dissolution)
69
what happens to the sediment during burial compaction?
* Mechanical compaction occurs during burial Mechanical rotation, rearrangement of grains, and breakage of thin/soft grains reduces the overall volume of the rock and also Reduces Porosity.
* Porosity: proportion of rock occupied by fluid (e.g. air, water, oil) rather than solid material; or void ratio. Difference between initial porosity of unconsolidade sands (25-43%) and unconsolidade mud (60-80%) causes differencial compaction
* Rate of compaction is a function of rate of sedimentation
70
What causes differencial compation?
Difference between initial porosity of unconsolidade sands (25-43%) and unconsolidade mud (60-80%) causes differencial compaction When deposited, mud usually ~ 60-80% water: tfinal= ~ 1/2 toriginal
* Sand tends to be ` 25-30% posrosity: 0-10% porosity post-burial
* Muds tend to compact twice (2x) as much as sands
71
What is a concretion?
Discrete, localized cemented body within a sedimentary unit, with sharp boundaries and generally ovoid in shape and flattened parallel to bedding.
72
Define porosity
Porosity: proportion of rock occupied by fluid (e.g. air, water, oil) rather than solid material; or void ratio.
73
How to determine relative timing of formation of concretions?
Relative timming of formation can be inferred from its relationship with adjancent uncemented layers.
74
Explain scaping structure and what type of environment it is related to?
Are structures forming mostly due to a mass wasting event where the organism has to dig his way out to prevent from been burried, thus leaving a trace behind. Deltic environment
75
Grazing vs Feeding
* Grazing is when an organisms is feeding of sediment, shoving in his mouth as he goes along.
* A feeding trace could be a borrow, a hole where the organism grab something and eats something in his way by.
76
How pore water dictates the rock forming process?
Pore water dictates the type of cement and the type of rock that will be formed.
* Salinity
* pH
* Ions present in pore water
* Concentration of certain minerals
* As a consequence some minerals will precipitate from aqueous solution; replacement reactions can also occur.
77
What water driven reactions control early diagenis of shallow-water siliciclastic sediments/rock in marine basins?
Poor water dictates the type of cement and the type of rock that will be formed.
* Salinity
* pH
* Ions present in pore water
* Concentration of certain minerals
* As a consequence some minerals will precipitate from aqueous solution; replacement reactions can also occur.
78
What processes and consequences can occur during mesogenisis?
* Mechanical and Chemical compaction
* Interstitial waters influenced by increasing T and P and from products of burial rxn in adjacent and enclosing sediments
* Growth of cements
* Destruction of primary porosity
79
What effect mesogenisis have on pore water ,clays, organic matter and hydrocarbons?
Compction and decrease in porosity
80
What happens with the precipitation of minerals during Eogenisis in non-marine oxidized environments?
Non-Marine: O2-rich pore watersIron Oxides (Geothite,Hematite) creates red beds. Also formation of Kaolinite clay minerals and precipitation of Quartz and Calcite cements may take place in this environment.
81
Define Diagenisis
* Physical and Chemical changes that alter sediment after deposition.
* Typically at relatively low Temperature and pressure
82
What processes occur during diagenesis?
- Compaction- Cementation- Grain repacking- Bioturbation- Pressure dissolution- Authigenesis (Growth of new minerals)
83
Difference between Compaction and Repacking
Compaction is the pressure exert over the sediment and repacking is the rearrangement of the grains in order to occupy less space (consequence of added weight/pressure))
84
What 3 fundamental alterations may occur during Lithification?
* Physical
* Chemical
* Mineralogical
85
What are the major variables that affect rates/amounts of diagenetic alteration?
* Temperature
* Pressure
* Biomass
Pore water Chemistry
* Sediment porosity
* Sediment Permeability
* Lithology (Mineral content)
86
What variations can be expected from pore water chemistry with increase in depth and what happens to the concentration of ions with depth?
Pore water chemistry differ from basin to basin, but the included variations in abundance of such mineral forming ions such as: Si4+ , Al3+, Ca2+, K+, Mg2+, Na+ and HCO-3 (bicarbonate). Many of this ions increase in abundance with increase in depth, concomitant with increase in salinity.
87
What are the difference between marine and fresh water that control early diagenesis of shallow-water siliciclastic sediments?
Marine and freshewater compositions are very different.
pH - Rain water is slightly acidic (pH~4.5-6) and ocean water is alkaline (pH~8)
Ions present. Hence, reactions can be expected from ions present.
Concentration of Ions. Elemental concentrations become supersaturated w/respect to specific minerals in given waters. As a result some mineral will precipitate from aqueous solution; some faster than others; preplacement reactions can also occur. Altogether these 'water driven' reactions control early Diagenesis of shallow-water siliciclastic sediments/rocks in marine basins.
88
What happens with the precipitation of minerals during Eogenisis in marine reduced and oxidized environments?
Precipitation of new minerals:Marine: in reduced marine envronment (low O2) - formation of Pyrite (may form cement or replace other materials, such as woody fragments) in O2-rich pore waters, : Formation of Chlorite, Glauconite (greenish iron-silicate grains), Illite/Smectite clays, and Iron oxides can form (eg. red clays on the deep ocean floor) and preciptation of K-feldspar overgrowths, quartz overgrowth and carbonate cements.
89
What type of sedimentary structure can be formed as a result of Post-deposition alteration during Early Cementation?
Caliche (hard pan):
Calicite-cemented soil formed by evaporation of CaCO3-rich groundwater
* Nitrate Salts deposits in Chile Peru (mined for industrial uses)
Silcrete (Chert) Essentially the same as caliche, but cement is SiO2 (less commun). note: silica is not very soluble in water.
Beach rock
Early cementation of Limestone. Solid rock prior to burial (CaCO3 cement).
90
Explain the formation of Beackrock
Beach rock is formed in Tidal environment as tides bring water, water evaporates, increasing concentracions of calcium carbonate, which precipitate in porous to form a cement.
91
what happens to the sediment during burial compaction?
* Mechanical compaction occurs during burial Mechanical rotation, rearrangement of grains, and breakage of thin/soft grains reduces the overall volume of the rock and also Reduces Porosity.
* Porosity: proportion of rock occupied by fluid (e.g. air, water, oil) rather than solid material; or void ratio. Difference between initial porosity of unconsolidade sands (25-43%) and unconsolidade mud (60-80%) causes differencial compaction
* Rate of compaction is a function of rate of sedimentation
92
How to determine relative timing of formation of concretions?
Relative timming of formation can be inferred from its relationship with adjancent uncemented layers.
93
Explain scaping structure and what type of environment it is related to?
Are structures forming mostly due to a mass wasting event where the organism has to dig his way out to prevent from been burried, thus leaving a trace behind. Deltic environment
94
How poor water dictates the rock forming process?
Poor water dictates the type of cement and the type of rock that will be formed.
* Salinity
* pH
* Ions present in pore water
* Concentration of certain minerals
* As a consequence some minerals will precipitate from aqueous solution; replacement reactions can also occur.
95
What water-driven reactions control early diagenesis of shallow-water siliciclastic sediments/rock in marine basins?
Poor water dictates the type of cement and the type of rock that will be formed.
* Salinity
* pH
* Ions present in pore water
* Concentration of certain minerals
* As a consequence some minerals will precipitate from aqueous solution; replacement reactions can also occur.
96
What processes and consequences can occur during mesogenisis?
* Mechanical and Chemical compaction
* Interstitial waters influenced by increasing T and P and from products of burial rxn in adjacent and enclosing sediments
* Growth of cements
* Destruction of primary porosity
97
What effect mesogenisis have on pore water ,clays, organic matter and hydrocarbons?
Compction and decrease in porosity
98
What happens with the precipitation of minerals during Eogenisis in non-marine oxidized environments?
Non-Marine: O2-rich pore watersIron Oxides (Geothite,Hematite) creates red beds. Also formation of Kaolinite clay minerals and precipitation of Quartz and Calcite cements may take place in this environment.
99
What type of sedimentary structure can be formed as a result of Post-deposition alteration during Early Cementation?
Caliche (hard pan):
Calicite-cemented soil formed by evaporation of CaCO3-rich groundwater
* Nitrate Salts deposits in Chile Peru (mined for industrial uses)
Silcrete (Chert) Essentially the same as caliche, but cement is SiO2 (less commun). note: silica is not very soluble in water.
Beach rock
Early cementation of Limestone. Solid rock prior to burial (CaCO3 cement).
