Clastic Diagenesis

Many definitions

The way in which a sediment adjusts to changing conditions of increasing pressure (mechanical or chemical compaction), increasing temperature, changing composition of interstitial pore fluids (cementation and compaction).

All occur during subsidence and burial.General trend:

Compaction and cementation: earlierReplacement: later

Definition

What processes play a role in sandstone diagenesis?

• Original depositional environment• Compaction• Mineral reactions• Diagenetic environment• Fluid flow• Organic matter evolution• Precipitation, replacement and destruction of

cements• Tectonic setting and basin evolution• Etc……..

Why bother?

• Nature and distribution of porosity & permeability determines extent of oil recovery

• How will secondary recovery fluids interact with the reservoir?

Section 1: Processes of Diagenesis

Processes• Physical

– Compaction– Fluid migration– Pressure solution

• Chemical– Mineral reaction– Mineral replacement– Cementation– Dissolution

• Physical and chemical processes are interdependent and cannot be easily separated.

Effect of compaction & cementation on primary porosity

• Reduction of bed thickness during compaction reduces porosity (surprise!)

• Two ways to do this:

– Mechanical compaction– Chemical compaction

Scherer, M. (Bulletin of the American Association of Petroleum Geologists, 1987)

Mechanical Compaction

Mechanical compaction

• Occurs at shallow burial depths (overburden).– Graton & Fraser (1935):

– Tightest packing of spheres is rhombohaedral (26% φ)» Loosest is cubic (48% φ)

– Most ssts= mix of shapes and sizes (not therefore a true representation)

Scherer 1987

• Assuming normal random packing in sands, under wet surface conditions:

• Primary porosity = 20.91 + (22.9/sorting)• N.B. roundness, sphericity, grain size only

of secondary importance.

Mechanical compaction generally more effective in mudrocks than sandstones:

• shales have a higher H20 content than sandstones: this is expelled rapidly by compaction.

Chemical compaction

Chemical compaction

• Mainly achieved by reprecipitation of minerals in remaining pore spaces

• Cementation more obvious in coarse clastics than mudrocks.

• Main process is pressure solution

Pressure Solution

• P3>P2>P1 > PW– where PW = pore fluid

pressure• Overgrowth forms (grey) by

diffusion of silica from the grain contact to the pore spaces.

Pressure Solution

• Development of sutured contact enhanced by presence of clays: enhance transport of ions away from site of pressure solution (diffusion network)

• Much more difficult to produce P.S. in clean ssts.• Quartz cementation tends to be pre-pressure

solution: Silica imported into the rock early in diagenesis; exported later.

• Ref: Houseknecht, 1988

Pressure solution

• Coarse clastics: high pressure contacts• Fine clastics (sandstones): higher surface

area in contact: fewer nucleation sites

• Thus: – Coarse clastics: rapid porosity loss early in

burial slowing down rapidly– Fine clastics rapid porosity loss at depth.

Authigenic cementation• Allogenic: during deposition• Authigenic grown in situ during diagenesis

• Rarely sufficient material in the rock to account for all cements present:

• Scavenges from fluids for cement formation:– Connate saline water (seawater)– Dissolution of soluble rocks (e.g. evaporites)– Percolating groundwater– Expulsion from shales– Mineral and organic reactions

The effect of cementation on porosity

• Fabric of the cement depends on how much of the mineral was present in soluble form at time of cementation: the degree of supersaturation of the pore fluid.

e.g. silica:

High levels of supersaturation, soluble forms precipitate

• opal, chalcedony form thin crusts and mosaics on grains

Low levels,

•quartz precipitates (low solubility) and slowly forms monocrystalline overgrowths

•Other common cements:

•Carbonates

•Sulphates

•Haematite

•Clay minerals

Determining relative timing of cementation

• Calculate the Minus Cement Porosity– High MCF = suggests early cementation– Low MCF = suggests later cementation

Why? Because compaction continues until stopped by cementation.

Sources of cement

• Silica– During early diagenesis:

• skeletal remains of diatoms and radiolaria(amorphous silica),

• glass shards, • quartz dust abraded during transportation

– Later• Mainly pressure solution

Sources of cement

• Carbonate– Dissolution of shelly material in sediments– Dissolution of nearby limestones

Sources of cement

• Sulphates– Primarily from evaporites– Usually find sulphate cements near these

• Gypsum• Anhydrite• Halite• Barytes• Celestite etc….

Replacement during diagenesis

• Mainly a later occurrence: deep burial• Pseudomorphs form (shape preserved)• Termed alteration when the replacement

mineral was part of the original mineral• E.g. K-feldspar Albite

Replacement of quartz

• High depths of burial• Calcite replaces quartz in alkaline

conditions of precipitation (quartz is more soluble in these conditions)

Shallow burial replacement

• Replacement can occur at shallow depths• E.g. surface weathering• Feldspar kaolinite (oxygen required)

The Diagenetic Sequence

• Succession of diagenetic events with increasing depth determined by the primary components available.

• In many cases, there is a regular sequence of cementation near surface:– Precipitation of silica cements, clay minerals and

feldspars– Quartz continues to precipitate for a long period– Carbonates and sulphates precipitate

Remember this!

• As rocks are buried, the evolve FLUIDS.• Hydrocarbons, gases, water

• Generally migrate through the most permeable units and fractures.

• Sandstones = most permeable units.

Porosity Loss

Enhancement of Porosity Loss

• Increasing temperature (increasing geothermal gradient accelerates diageneticalteration)

• Increasing pressure• Amount of fluid flow• Amount of matrix present

Porosity Gain

The development of Secondary Porosity

• Secondary φ critical in oilfields• Fracturing, shrinkage, dissolution• Dissolution of carbonates very important

• CaCO3 + H+ = HCO3- + Ca2+

• Assuming the dominant aqueous species is HCO3-

Dissolution of carbonate

• Two possibilities– Organic/ inorganic diagenesis produces CO2

– If pH fixed by mineral reactions (organics also) with fluid

• CO2 generates H2CO3 which dissociates, lowering the pH.• HCO3- increases and the reaction favours precipitation

– If pH not fixed• H2CO3 dissociates releasing H+ which decreases the pH,

favouring dissolution of carbonate.

Both are likely contributors

Section 2: Diagenetic Regimes

Near surface sandstone diagenesis

Near surface sandstone diagenesis

• Particularly occurs in semi-arid climates– Water table very low– Sediments are oxygenated for long period– Sparse vegetation : rapid erosion– Produces an immature sediment

• Large amount unstable minerals (feldspars, amphiboles etc)

Processes active near surface

• Clay infiltration• Intrastratal mineral dissolution• Replacement• Authigenic mineral growth

Clay Infiltration

• Sand near surface suffers a flood event:– Water is full of wash-load clays– These percolate through the sand and deposit

on grain surfaces– Clay plates arrange tangentially to the grain

surface– Produces a matrix-supported sediment: can

look like a debris flow, which it is not.

Intrastratal mineral dissolution

• Partial or complete dissolution of instable minerals

• Shows as solution pits on the grain surface• Occasionally, whole grain is removed.• If grain had a clay rim, the hollow rim often

remains visible in thin section.

Replacement

• Minerals with a dominant cleavage:– Clays replace along the cleavage planes, twin

planes and ultimately the whole grain– Tend to be Mixed Layer Clays (see later)_

Authigenic mineral growth• Dissolution of unstable minerals releases ions:• K+, Fe3+, Mg2+, Si4+, Al3+, Ca2+• May migrate from system OR precipitate as new

minerals• Commonest are:

– K-feldspar– Zeolites– Quartz– Calcite– M.L.C’s– Iron minerals

• Form growths in pore spaces and dissolution voids

Special Case: Red Beds

• A product of a more restricted form of near surface diagenesis

• In this case, the unstables dissolved are Fe-rich silicates such as:– biotite,– Amphibole– Pyroxene– olivine

• Near surface: these release Fe3+

•reprecipitated on grain surfaces as haematite: hence the red colour

•Fe3+ reduces near surface to Fe2+

•More mobile

•Fe2+ can be incorporated into carbonates•Ferroan calcite, ferroan dolomite, siderite (FeCO3)

•Also clays•Chlorite, smectite

Timing of reprecipitation is important:

• First mineral to form = limonite (yellow) which matures to become crystalline haematite Fe2O3

• Once formed, red colour may be leached by later fluids unless protected from further diagenesis: best is quartz overgrowths.

Aeolian Red Beds

• Infiltration of wind-blown dust (mainly siliceous but contains iron oxides)

• A lot of fine silica abrasion dust is generated in deserts:– Good source of silica for precipitation of

protective quartz overgrowths.

Subsurface sandstone diagenesis

Subsurface sandstone diagenesis• Sand horizons: fluid pathways especially for

fluids from compacting mudstones• Fluids carry dissolved ions (cements)• The compacting sands can be a long distance

from shales (10’s of km)

As sand compacts…

• Some grains particularly susceptible to alteration by dissolution in acid waters: feldspars and volcanic lithic fragments (VLF’s)

• Feldspar illite kaolinite (releases K+, Si2+ for

cements, e.g. authigenic illite)• VLF’s smectite

Result

• Thus:– Any sand rich in feldspars and VLF’s generates

a lot of clay generated in diagenesis.– Clays flatten to form a matrix: reduces

permeability of the sedimentary rock.– Micas and mudstone clasts also flatten and

deform to reduce permeability (bend around grains)

Brief review of cements

• Silica• Clays• Carbonates

Silica• Dissolution of quartz • Dissolution of feldspars• Clay transformation (Smectite Illite)

• Reprecipitates as quartz overgrowths as long as the detrital quartz has NO CLAY COATINGS (nucleation problem)

• NB Clays good for silica dissolution; poor for reprecipitation.

• Formation of quartz overgrowths inhibits pressure solution and help to preserve remnant porosity

Clays

• SiO2 tetrahaedra in sheet form• Layers bound by OH-, Na, K, Ca, Mg• Base Exchange responsible for variations• Small g.s. / Large surface area• Surface charges cause coagulation/ dispersion• Charge proportional to grain size• Generally +ve at end points, -ve on flat surfaces

Clays in sandstones

• Common (90% ssts contain authigenic clays)• The commonest clays are:

– Shallow depth: • Smectite, montmorillonite, mixed layer clays (smectite-illite,

chlorite)

– Deeper levels• Illite and some chlorite

– Acid conditions: Kaolinite predominates, when pore water becomes more alkaline kaolinite transforms to illite (K+ from waters required)

What are mixed layer clays?

• Interlayered clays• E.g. smectite-illite (S/I) clays• There is a sequence of development:

• Progresses through increasing burial• Transformation begins at 70-100°C

The North Sea

• Oil migration into ssts predates illiteformation

To convert Kaolinite to Illite : need K+

Pore space plugged with hydrocarbons

Thus, No K+ migration and no illite.

Hydrocarbon emplacement has prevented illitization

This imparts a Thixotropic property to clays: liquid when disturbed, solid at all other times.

Clay Diagenesis

• Compaction: expulsion of fluids: mudrock is reduced to 10% original thickness.

• Stage 1: by 1km burial, only 30%vol H2O left:– Not free water: either crystal lattice water or adsorbed

onto crystal surface.

• Stage 2: To remove structural H2O – need 100°C+ (3km)– At this point H2O is removed and clay structure and

chemistry is changed.

Clay diagenesis

• Stage 3: Last H2O removed @ 150°C (even then a little remains: only sustained deep burial removes the last of the water

N.B. Temperature is the primary cause of clay mineral transformation.

Water loss from compacting mudrocks

Changes of clay minerals with increasing depth of burial and into metamorphism.

•Illite: no conversion but becomes more crystalline with burial

•Subvarieties of kaolinite convert to illite & chlorite with burial

•Phyllosilicates replace clay minerals during metamorphism:

•Laumontite, pyrophyllite replace clays

•Illite & chlorite can survive

Change in clay mineralogy of fine grained sediments through time

• Lower Palaeozoics/ Pre-Cambrian dominated by illite & chlorite (more stable).

• Upper Palaeozoic – present : wider mix of clays

• Temporal variation also enhanced by:– Land plants: changed the soil forming process,

surface weathering: reflected in a change of clay formation

So why study clays?

• Give a good overview of thermal alteration in a basin

• Illite crystallinity measurements: main tool for thermal examination

• Clays in pores very important in assessment of poroperm characterisation of sandstones

Poroperm• Kaolinite

– Forms booklets– Greatly reduces

porosity but spaces between layers allow flow: do not reduce permeability

Kaolinite booklets in pore spaces of sandstone. Note significant φ between layers

Poroperm

• Illite– Forms hair-like

threads– Low overall

volume: porosity not reduced much

– Form matted webs: permeability reduced

Authigenic illite cement in Lower Permian Rotliegendes sandstone, North Sea: c. 1.9km

Poroperm

• Smectite(montmorillonite)– Forms honeycombe

structures– Some reduction of

both porosity and permeability

Typical authigenic smectite showing honeycombe structure. Smooth fusing of edges distinguishes from chlorite

Poroperm• Chlorite

– Many forms: platy, honeycombe, roseatte…

– Some reduction of both porosity and permeability

Commonest morphology for authigenic chlorite. Individual idiomorphic plate-like crystals.

Carbonate Diagenesis

Carbonates• Very common cement in sandstones• Uncommon in quartzose sandstones (i.e clean

quartzites)• Main cement responsible for φ and K reduction

(fluid flow barriers)

• Remember: Fine grained sediments release ion-rich fluids (Ca2+, Mg2+,, HCO3

- ): reprecipitate as cement.

• THUS: carbonate cements particularly common near sst/mst contacts.

Carbonates

• Carbonates are more soluble than silicates: diagenetic fabrics generally more complex– Aragonite: very unstable– Dolomite: CaMg(CO3)2: termed Ankerite when Fe-rich– Calcite: ferroan and non-ferroan, High Mg and Low

Mg. – Siderite: rare in a carbonate rock

• Marine calcite: “high” Mg content (Ca90Mg10)CO3

Non-skeletal grains

• Ooids (high wave energy environment)• Peloids• Oncoliths (aggregates of debris)

Lime muds (micrite)

• Origin:– Chemical precipitate– Disaggregation of shells– Microbial/ bacterial micritization

Environments of Carbonate diagenesis

• Subsea• Subaerial• Subsurface

(a) Subsea diagenesis: carbonates

• By far the most important process in subseadiagenesis:

• Micritization of shells. • A number of mechanisms are suggested for

this

Physical breakdown of material

• Unlikely:• Tendency would be to recrystallize rather

than break down

Algal action (Bathurst)

• Small-scale borings in shell surface filled by micrite

• Therefore the action of alga builds up a “micrite envelope”

Bacterial action (Purdy)

• Algae aren’t present in areas where micritization is identified most (10-15 cm under surface)

• Hence bacterial breakdown of shelly material is more likely

• Micritization is known to happen early in burial (2-300yr after settling)

• Almost all skeletal material undergoes this to some degree• No connection between composition and degree of

micritization

High Mg calciteEchinoidea

AragoniteCorals

Aragonite + Low Mg calciteGastropods

High Mg calciteForaminifera

AragoniteHalimeda

High Mg calciteCorraline algae

Composition←Increasing m

icritization

Material

Ease of micritization probably related to size (echinoids: large

plates)

Subsea cementation

• Thought impossible until Ginsbruck & Shim found cemented material in coral reef a few hundred years old. Bermuda, 1970

• Persian Gulf: large areas of sea floor cemented 1-60m deep.– Layering occurs– Beer bottles etc in cemented crust (probably

mapping students)

Main cement: aragonite – acicular crystals in porosity

Also: High Mg calcite – blocky crystals

Problem: The sea here is saturated with aragonite, why does Mg calcite

precipitate?

• Most likely: aragonite → calcite conversion– Aragonite: tighter lattice – conversion would

release 8% CaCO3 to maintain volume• Hard limestone crusts are buckled:

supporting evidence for volume increase

Aragonite → High Mg Calcite + 8% CaCO3

Different classes of cement occur: can be used to map

environments

• Shallow Marine• Supertidal

– Phreatic environment– Vadose environment

Shallow Marine cementation• Forms ISOPACHOUS/ SYNTAXIAL CEMENT

– Crystals have same orientation as fossil on which they grow

• Common in crinoids, resulting in:

single calcite crystal

Supertidal Phreatic

• Below the water table• MENISCUS CEMENT

Supertidal Vadose

• Above the water table• MICROSTALACTITIC CEMENT

droplets of cement

Concretions

• Carbonate concretions common in shales• Form before compaction (bedding visible within

them)• Oriented parallel to bedding• Uncertain origin:

– Organic material common in concretions– Disintegration produces alkaline conditions locally

(NH3) decreasing CaCO3 solubility– No depletion of CaCO3 ever demonstrated around

concretions: dubious theory

(b) Subaerial carbonate diagenesis

• Second largest area of carbonate diagenesis• Most important process is cementation• Occurs when calcareous muds elevate

above water and meteoric waters attack it

METEORIC WATER

SOIL

CALCAREOUSMUD

HUMIC ACID

Meteoric H2O collects humic acid in soil zone

Humic acid dissolves CaCO3 in mud zone

High Mg CaCO3 and aragonite preferentially removed

i.e. Dissolution of fossils and cavities

• Low Mg becomes saturated in the meteoric fluids:

• Replaces High Mg calcite and aragonite

• Thus: any ascending water in dry periods will be Low Mg calcite-rich

– causes deposition of low Mg calcite at higher levels at later stages

Distinguishing subaerial and subsurface cementation

• Isotopic analysis• 13C/12C and 18O/16O ratios (δ13C , δ18O)• Plots of such data are now more refined

– Allow distinction between marine/ non-marine environments

– Depth of water

δδ

1318

C/

O

13C

deep sea carbonates

any atmospheric calcite

(c) Subsurface cementation and diagenesis

• Absolute mechanisms not fully understood• Cementation occurs continuously: new term

applied – Lithification: production of a solid limestone with no porosity

• However:– Modern lst muds have 40 – 70% φ whilst lsts

never have >5%– Cannot be blamed on compaction (delicate

fossils preserved)– Where does the CaCO3 come from to close the

space?

Pressure Solution and Stylolitization

• P.S. is a popular choice for pore closure– Any strained part of a crystal becomes highly soluble:– CaCO3 migrates to areas of lower energy

• Stylolites: impurities in the lsts along a layer become stress razors (pressure points)– Interpenetration occurs (see graphic next)– Definitely a solution phenomenon since stylolites can

cut fossils– Material dissolved is a calcite source

Measurements in the local U.W.L. show 30% thickness reduction due to stylolitization

• This releases a lot of calcite for cementation

Recrystallization/ Neomorphism

• Change in crystal shape/ orientation without compositional change

• The orientation of the first (seed) crystal govern the direction in which further crystals grow.

• The calcite scalenohaedral phase is the fastest growing

• Upright axes survive in most cases so most pore filling calcite crystals have c-axis normal to surface

(a) (b)

(c)

a and c terminate

Triple Junctions• Boundary of 3 crystal faces.

– Most common angle of contact is 180° in cements

– Commonly, one crystal terminates by growing into another

– Most common angle in recrystallized crystals is 120° - more stable due to reduced surface angles

stops

Patterns in Clastic Diagenesis

Quartz cements

• Timing is temperature controlled• Can be as low as 80°C – thus the presence

of qtz cement infers temps in excess of this

Common environment cement characteristics

Quartz dominatedShallow marine

Quartz cement + zeolite cementDeep marine

More carbonate + quartzFluvio-deltaic

More claysFluvial sst

Generally high % quartz cementAeolian sst

Main cements in sandstones

• Pore coating– Quartz

• low birefringence• No cleavage• Clear/ colourless• Optically continuous with substrate• euhedral

Main cements in sandstones

• Pore-filling– Carbonate

• Calcite– Usually early– Usually fills porosity completely– Calcrete: terrestrial cementation – fills porosity and

corrodes silicates• Dolomite

– Fe dolomite: typically forms rhombs– Fe calcite– Siderite: typically smaller brown rhombs

Main cements in sandstones• Pore-filling

– Clays• Kaolinite Al2Si2O5(OH)4

• Colourless, low birefringence, low reief• Illite KAl3Si3O10(OH)2

• Low relief, mod. Birefringence, colourless, fringe-structure

• Chlorite-smectite: solid solution series• (MgFeAl)(AlSi)O10(OH)4 –(NaKCa)(MgFeAl)(SiAl)O10(OH)8

• Chlorite: greenish fringe• Smectite: colourless to greenish, pore filling,

open matrix