EASC 2702: Lecture 19 Diagenesis

diagenesis

all chemical, physical and biological changes undergone by a sediment after its initial deposition, and during and after the sediment is changed into rock

• does NOT include weathering at surface or metamorphism at great depths

• Takes place at pressures up to several kilobars (mainly <10 km depth), temps. up to ~250-300 C

compaction

Volume loss and other changes caused by deep burial and the lithostatic load of overlying rocks.

• coincident with dewatering, breakage/bending of grains, pressure solution, mineralogical transformation of clays

dissolution

The dissolution of sedimentary minerals by pore fluids and/or high pressure during compaction.

precipitation/ cementation

The post-depostional precipitation of minerals; i.e. cements

replacement

Partial or complete replacement of one mineral by another

porosity

void space in rock

Volume of total pore space/ volume of rock sample * 100

typical- 5-25%

excellent- 25-35%

effective porosity

fluids can flow through

ineffective

pores not well connected

primary porosity

the porosity of sediment before deep burial compaction of the formation of cements (~20 40%)

secondary porosity

generated much later by the dissolution of earlier cements and/or grains.

permeability

ability of the rock to transmit fluids; proportional to flow rate through rock

strongly influenced by fractures and size of “pore throats”

eodiagenesis

Earliest stage of diagenesis occurring in shallow burial environment, following deposition: ≤ 100m, ≤ 50oC.

• Composition of pore water drives reactions and depends on depositional environment: if marine = saline, alkaline, reducing (carbonate, pyrite stable); if terrestrial = acidic, oxidizing (silica, FeOx stable).

• Precipitation: of carbonate and silica cements. Less common pyrite, chlorite, iron oxide, and “oddballs”.

• Dissolution: of carbonate and unstable silicate mineral grains (feldspar, amphibole, etc.)

Eodiagenesis: early cements

Quartz, Calcite most common. Also: Pyrite, Iron Oxide, illite/smectite, …

• can inhibit effects of compaction and/or later cements, preserving primary porosity

early calcite cements

Very common in near-surface marine setting; promoted by evaporation, increases HCO3- concentration

mesodiagenesis

Main stage of diagenesis occurring at significant depths and under high

temperature (0.1 – 10 km; ~50 – 250oC). High lithostatic pressure, and

significant compaction.

• Pore water is modified by dissolution of silicates: saline and alkaline with

high Ca, Sr, Mg and Si (less Na, SO4 and K).

• Precipitation: Dolomite, calcite and quartz cements.

• Dissolution: earlier cements and some grains to generate secondary

porosity.

sand → sandstone compaction ratio

1.1

mud → mudstone compaction ratio

~2 - 2.5

peat → coal compaction ratio

5-10?

compaction ratio

thickness of sediment that compacted down to 1 m of rock

• indicator of lost volume

permeability generally decreases…

with depth

sandstone compaction

• Breakage of brittle grains (quartz,

feldspar)

• Bending of ductile grains (micas,

lithics)

• Pressure Solution, suturing

stylolites

irregular sutured plane in rock, formed by “squeezing” of pore fluids until only insoluble residue (clays, organics) remain

mudstone compaction

• dewatering, first from pores, then form mineral lattice

• Smectite → mixed layer clays → illite

(dehydration, incorporation of K+)

@ 50 – 200oC; releases water

• Kaolinite → illite + chlorite

@ 120 – 150oC

common sandstone cements

• quartz- major cause of porosity loss

• calcite

• dolomite

• siderite

• hematite, limonite

less common sandstone cements

• Mn oxides

• pyrite

• gypsum

• barite

Fe-rich dolomite cements

form in deep burial setting due to higher Mg/Ca ratio in pore fluid, alkalinity and high temperatures – may replace calcite

How does dissolution create secondary porosity in sandstones?

• Silica cements can dissolve at depth, increasing pore space (though they may also re-precipitate).

• Carbonate cements dissolve when exposed to acidic fluids, often produced during organic matter breakdown and oil maturation.

• Framework grains (especially silicate grains) may be partially or completely dissolved, enhancing porosity.

replacement during deep burial diagenesis

• Replacement = dissolution of a mineral and simultaneous precipitation of a new mineral with no volume change.

• Often preserves original shapes or textures → pseudomorphs.

• Common replacements:

  • Feldspar → clay minerals

  • Feldspar → carbonate minerals

  • Feldspar → zeolites

Exhumation to surface (Telodiagenesis)

• ≤ 100m, ≤ 50oC; acidic meteoric pore waters.

• Dissolution, cementation; overlaps with weathering processes.