Diagenesis and Metamorphism Flashcards
What is Diagenesis?
A series of processes that modify sediment during its lithification from loose sediment into rock.
What is primary and secondary porosity?
Primary porosity is the original porosity of the rock when it formed, and secondary porosity is the pore space created by subsequent processes such as fracturing.
What is the value of secondary porosity?
Economic value as it is critical for oil and gas formation.
What is metamorphism?
A change in the mineralogy and texture of rock which occurs in the solid state (due to thermal and mechanical energy).
At what point do diagenesis and metamorphism become distinguished?
Diagenesis involves all the changes to sediment up to the point the mineral change (generally 100-200°C and up to 10km in depth).
What is basic diagenesis?
When sediment becomes rock as cement grows in the pore space between the grains.
What are the main processes that occur during diagenesis?
Compaction, Cementation, Recrystallisation, Dissolution and Mineral replacement.
Compaction
What happens: Overburden pressure from accumulating sediments compresses the deeper layers.
Effects:
-Reduces pore space but not rock volume.
-Expels pore water.
-Aligns and reorients grains, especially clays.
What is overburden?
The reorganisation and rotation of grains to reduce volume (typically the long-axis moves to become perpendicular to the direction of compaction).
Why do silts and clays loose up to 90% of their volume compared to sandstones which loose 10% during compaction?
Sand is mainly quartz which is rounded in shape where as silts and clays are made up of clay and micas which are platy. This means that the silts and clays can be compacted way more.
What is pressure solution?
At mineral grain contacts the mineral dissolves, then re-precipitates as cement away from the high pressure contacts (the result is a net loss of porosity and is common in carbonates as they go into solution easily).
Cementation
What happens: Minerals precipitate from groundwater into the pore spaces between sediment grains.
Common cements:
-Calcite (CaCO₃)
-Quartz (SiO₂)
-Hematite (Fe₂O₃)
Effects: Binds grains together, turning loose sediment into hard rock resulting in a reduction in primary porosity.
What are concretions?
Spherical masses that are cemented more strongly than surrounding minerals.
-Typically spherical.
-Associated with organic material.
What the different types of cementation?
Local:
-Dissolution and reprecipitation
-Pressure solution
External:
-Brought by pumping
Recrystallisation
What happens: Original minerals change in size or shape without changing composition.
Example: Micrite (fine calcite mud) recrystallizing into sparite (coarse calcite crystals).
Effects: Increases crystal size, improves stability.
Dissolution
What happens: Unstable minerals dissolve in different temperatures, pressures and pore fluids, especially under acidic conditions.
Effects:
-Creates secondary porosity (new pores).
-Alters rock structure and flow paths.
Mineral Replacement
What happens: New minerals form in situ from dissolved ions in pore fluids.
Examples:
-Formation of glauconite or pyrite.
-Replacement of aragonite by calcite.
Effects: Alters mineral composition of the rock (may be same composition but is a polymorph).
What is dolomitisation?
A geological process where magnesium ions replace calcium ions in the mineral calcite (limestone), forming the mineral dolomite the result is an increase in pore space. This is economically important for hydrocarbons and water storage.
What factors impact the degree to which the processes of diagenesis occur?
-The original composition of the sediment.
-Pressure and temperature.
-Pore fluids.
-Porosity and permeability.
What processes occur during metamorphism?
Recrystallisation and Neo mineralisation.
Neo mineralisation
The formation of new minerals at the expense of existing ones (chemical reactions).
e.g. Shale turns to Schist (clays turn into garnet and micas into staurolites).
How can metamorphic rocks be used to understand deep Earth porcesses?
They are the most abundant rock in the Earths crust and they tend not to undergo retrograde metamorphism (revert back).
What is a protolith?
A precursor rock.
What happens to metamorphic grade with increasing depth?
Grade increases.
