Seds - Diagenesis

Question 1

Diagenesis

Answer 1

The physical, chemical or biological alteration of sediments into sedimentary rock at relatively low temperatures and pressures that can result in changes to the rock’s original mineralogy and texture

Question 2

Diagenetic Process stages

Answer 2

• Compaction
• Burial
• Decomposition
• Alteration
• Dissolution
• Precipitation
• Lithification

Question 3

Diagenesis in sandstones:

Answer 3

• Chemical and physical compaction
• Decomposition – feldspars, micas
• Dissolution – internal structures lost (such as feldspars) and will turn to clay
• Partially dissolving a quartz crystal – causes rich silica around it – will precipitate later
• Initial compaction is mechanical compaction – fast but can only take you so far – about 22% porosity
• 80-90 degrees centigrade – quartz will begin to dissolve and fill the pore space – causing more compaction

Question 4

Mechanical compaction

Answer 4

• Goes from point contacts to long contacts to convex-concave compaction which is the most compaction – will lead to suturing – loss of material
• Early cementation will stop mechanical compaction – rocks unable to rearrange and push into eachother
• More compaction = minerals pushed together
o Quartz is hard so contact points will change (point, long, convex, concave, sutured)
o When too deep, the quartz will shatter and fracture (not common)
o Can bend micas

Question 5

Chemical compaction

Answer 5

Essentially movement of minerals to make space
o Minerals can be reprecipitated outside of crystals
• Pressure dissolution of long contacts – minerals dissolve into eachother – stylolites – dissolution surfaces
• Overgrowths
• Cements – reprecipitation of minerals from surrounding crystals
• For quartz = 90-120 degrees – 2-4km – chemical compaction
• Quartz cement overgrowth can reduce the porosity (90-130°C) – macroquartz
• Detrital and authigenic grain coatings can inhibit quartz cement overgrowth
• Microquartz coatings from biogenic silica (sponge spicules) (60-80°C) Authigenic clay mineral coatings grow at temperatures of
• 60-100°C for chlorite
• 70-90°C for illite

Question 6

Clastics vs Carbonates

Answer 6

 Clastics – smooth curve of porosity loss to depth – inflection point at around 2km when chemical compaction kicks in
 Carbonates – sharp change, loss a lot of porosity quickly
• Carbonates is soft, so they deform so stress is spread over a wide area so a reduction in compaction that is fairly severe occurs. Get a bit stronger as long contacts seen.
• 800m till chemical compaction
• For carbonates its temperature that’s important not depth

Question 7

Controls

Answer 7

• Depth  Pressure + Temperature
• Initial sediment  composition + texture
• Tectonic stresses  Can cause increases in pressure that are not due to depth
• Pore fluid

Question 8

Mechanical Compaction pressure:

Answer 8

Driven by vertical effective stress (VES)
Pore fluid pressure can reduce this stress - reduces compaction
Grain framework strengthening framework reduces VES - reduces compaction - calcite

Question 9

Controls on mechanical compaction ref

Answer 9

Grain size, shape and sorting, both in sandstones and mudstones, play an important role in determining the mechanical compaction prior to chemical compaction
Bjørlykke 2013