Diagenesis of mudrocks and

clay minerals

S.Cirilli-SedPet 1

Diagenesis of clay minerals and mudrocksDiagenesis of clay minerals and mudrocks

�� Clay minerals can be modified and altered during early and late Clay minerals can be modified and altered during early and late diagenesis, and into metamorphism. diagenesis, and into metamorphism.

�� The main physical postThe main physical post--depositional process affecting the depositional process affecting the mudrocks as a whole is mudrocks as a whole is compactioncompaction. .

�� Compaction in mudrocks Compaction in mudrocks expels water and reduces the expels water and reduces the thickness of the deposited sediment by a factor of up to 10.thickness of the deposited sediment by a factor of up to 10.

�� When muds are deposited they contain in the region of 70When muds are deposited they contain in the region of 70--90% 90% water by volume. Compaction through overburden pressure water by volume. Compaction through overburden pressure soon removes much of the water so that at depths of 1000m or soon removes much of the water so that at depths of 1000m or so, the mudrocks contain around 30% water. Much of this is not so, the mudrocks contain around 30% water. Much of this is not free pore water but is contained in the lattice of the clay free pore water but is contained in the lattice of the clay minerals and adsorbed onto the clays. minerals and adsorbed onto the clays.

EPS 321 Lecture 13

about

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The main controlling process: COMPACTION and water loss

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Diagenesis of clay minerals and mudrocksDiagenesis of clay minerals and mudrocks

�� Further compaction through water loss requires temperatures Further compaction through water loss requires temperatures approaching 100approaching 100°°C and these are attained through burial at C and these are attained through burial at depths in the region of 2 depths in the region of 2 -- 4km. 4km. DehydrationDehydration of clays then takes of clays then takes place, accompanied by some changes in the clay mineralogy. place, accompanied by some changes in the clay mineralogy. Final compaction to give a mudrock with only a few per cent Final compaction to give a mudrock with only a few per cent water requires a much longer period of overwater requires a much longer period of over--burden pressure burden pressure with elevated temperatures. with elevated temperatures.

�� Evidence of compaction in mudrocksEvidence of compaction in mudrocks is provided by is provided by the fracture the fracture of shells, flattening of burrows, and the bending of laminae of shells, flattening of burrows, and the bending of laminae around shells and early diagenetic nodules.around shells and early diagenetic nodules.

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Diagenesis of clay minerals and mudrocksDiagenesis of clay minerals and mudrocks

�� Changes in clay mineralogy during diagenesisChanges in clay mineralogy during diagenesis take place take place principally through the rise in temperature accompanying principally through the rise in temperature accompanying increased depth of burial. increased depth of burial.

�� The main vertical change with depth is:The main vertical change with depth is:

1) 1) An alteration of An alteration of smectitessmectites to illite via mixedto illite via mixed--layer clays of layer clays of smectitesmectite--illite.illite. This alteration involves the incorporation of K+ This alteration involves the incorporation of K+ ions into the ions into the smectitesmectite structure and loss of interlayer water. structure and loss of interlayer water.

�� The process is largely temperature dependent and the The process is largely temperature dependent and the temperature at which temperature at which smectitesmectite begins to disappear is of the begins to disappear is of the order of order of 7070--9595°°CC; that is, at depths of ; that is, at depths of 22--3 km3 km. .

2) At slightly higher temperatures and greater depths, 2) At slightly higher temperatures and greater depths, kaolinite is kaolinite is replaced by illite and chlorite.replaced by illite and chlorite.

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S.Cirilli-SedPet

Diagenesis of clay Diagenesis of clay minerals and mudrocksminerals and 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:

� Illite & chlorite can survive

6

TEMPERATURE: controls clay minaral changings

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Argille ricche in sostanza organica e black shales

S. Cirilli: Petrografia del Sedimentario

Organic rich clays

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MUDSTONE'DIAGENSIS!Marine"Mudstone"Diagenesis""Zone'1:'0'='0.5m'V  Uncompacted,'high'porosity'V  oxic'pore'waters:'bioturba4on'V  Oxida4on'of'organics'by'aerobic'bacteria'forming'CO2''

'V  Zone'2:'0.5'–'10m'V  Zone'of'bacterial'sulfate'reduc4on''V  Anoxic'condi4ons'

'2CHO'+'SO4

2V'!'2CO2'+S2V'+2H2O'

Organic'material'

Sulfate'from'marine'water'

Bacterial'Ac4vity'

12C:'may'form'calcite'concre4ons'

S'V'may'form'H2S'

D.!vulgaris!

hQp://en.wikipedia.org/wiki/File:Dvulgaris_micrograph.JPG'

Bioturba4on'

hQp://pubs.usgs.gov/dds/ddsV033/USGS_3D/ssx_gif/bss8038.gif'

9"

S.Cirilli-SedPet 9

MUDSTONE'DIAGENSIS!Marine"Mudstone"Diagenesis""Zone'1:'0'='0.5m'V  Uncompacted,'high'porosity'V  oxic'pore'waters:'bioturba4on'V  Oxida4on'of'organics'by'aerobic'bacteria'forming'CO2''

'V  Zone'2:'0.5'–'10m'V  Zone'of'bacterial'sulfate'reduc4on''V  Anoxic'condi4ons'

'2CHO'+'SO4

2V'!'2CO2'+S2V'+2H2O'

Organic'material'

Sulfate'from'marine'water'

Bacterial'Ac4vity'

12C:'may'form'calcite'concre4ons'

S'V'may'form'H2S'

D.!vulgaris!

hQp://en.wikipedia.org/wiki/File:Dvulgaris_micrograph.JPG'

Bioturba4on'

hQp://pubs.usgs.gov/dds/ddsV033/USGS_3D/ssx_gif/bss8038.gif'

9"

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Zone'3:'10'–'1000m'V  Zone'of'Organic'Fermenta4on:'no'more'sulfate'reduc4on'

CH2O'!'CH4'+'CO2''

V  Possible'forma4on'of'Siderite'V  Expulsion'of'pore'water'by'compac4on'

'Zone'4:'1'–'2.5km'V  Breakdown'of'remaining'organic'maQer'by'decarboxyla4on'

''''V  More'pore'water'expelled'

hQp://en.wikipedia.org/wiki/File:Decarboxyla4on_reac4on.png'

10"S.Cirilli-SedPet 11

Zone'3:'10'–'1000m'V  Zone'of'Organic'Fermenta4on:'no'more'sulfate'reduc4on'

CH2O'!'CH4'+'CO2''

V  Possible'forma4on'of'Siderite'V  Expulsion'of'pore'water'by'compac4on'

'Zone'4:'1'–'2.5km'V  Breakdown'of'remaining'organic'maQer'by'decarboxyla4on'

''''V  More'pore'water'expelled'

hQp://en.wikipedia.org/wiki/File:Decarboxyla4on_reac4on.png'

10"

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Zone'5:'2.5'–'7km'V  Zone'of'Hydrocarbon'genera4on'(about'70'–'90°C)

'

V  Breakdown'of'unstable'mineral'components'releases'H2O'

V  Release'of'H2O'expels'SiO

2,'Mg

2+,'Fe

2+'and'any'generated'petroleum'

V  At'higher'temps'gas'rather'than'liquid'petroleum'produced.'

Zone'6:'>7km'V  Zone'of'metamorphism'

V  illite'V>'Sericite'V>'Muscovite'(at'300°C)'

V  Kaolinite'V>'Dickite/Nacrite'V>'Chlorite'

(at'200°C)'

hQp://en.wikipedia.org/wiki/File:Chlorite_schist.jpg'

Chlorite'Mica'Schist'

Diagenesis'V'Metamorphism'

11"

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Zone'5:'2.5'–'7km'V  Zone'of'Hydrocarbon'genera4on'(about'70'–'90°C)

'

V  Breakdown'of'unstable'mineral'components'releases'H2O'

V  Release'of'H2O'expels'SiO

2,'Mg

2+,'Fe

2+'and'any'generated'petroleum'

V  At'higher'temps'gas'rather'than'liquid'petroleum'produced.'

Zone'6:'>7km'V  Zone'of'metamorphism'

V  illite'V>'Sericite'V>'Muscovite'(at'300°C)'

V  Kaolinite'V>'Dickite/Nacrite'V>'Chlorite'

(at'200°C)'

hQp://en.wikipedia.org/wiki/File:Chlorite_schist.jpg'

Chlorite'Mica'Schist'

Diagenesis'V'Metamorphism'

11"

Zone'5:'2.5'–'7km'V  Zone'of'Hydrocarbon'genera4on'(about'70'–'90°C)

'

V  Breakdown'of'unstable'mineral'components'releases'H2O'

V  Release'of'H2O'expels'SiO

2,'Mg

2+,'Fe

2+'and'any'generated'petroleum'

V  At'higher'temps'gas'rather'than'liquid'petroleum'produced.'

Zone'6:'>7km'V  Zone'of'metamorphism'

V  illite'V>'Sericite'V>'Muscovite'(at'300°C)'

V  Kaolinite'V>'Dickite/Nacrite'V>'Chlorite'

(at'200°C)'

hQp://en.wikipedia.org/wiki/File:Chlorite_schist.jpg'

Chlorite'Mica'Schist'

Diagenesis'V'Metamorphism'

11"

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Sulfate'Reduc4on'

DETRITAL''INPUT'

RED/BROWN'CLAYS'

BLACK'PYRITIC'SHALES,'CALCITE,'

DOLOMITE,'PHOSPHATES'

SIDERITIC'MUDSTONES'

OIL''and'GAS'PROSPECT'

$

Is!burial!rate!sufficiently!slow!for!extensive!genera8on!and!migra8on!of!hydrocarbons?!

Metamorphism'

YES"

NO"

Is!burial!rate!sufficiently!slow!for!complete!Fe3+!conversion!to!pyrite?!

NO"

YES"

YES"

Fermenta4on'CH4'CO2'

NO"

Is!burial!rate!sufficiently!slow!for!extensive!fermenta8on!and!decarboxyla8on?!

NO"

Is!the!rate!of!burial!so!slow!that!all!organic!carbon!is!oxidized!!near!the!sediment!/water!interface?! YES"

Is!deposi8onal!water!marine!(SO42H!rich)?!

ZONE"1"

ZONE"2"

ZONE"3E4"

ZONE"5"

ZONE"6"

12"

YES"

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NonEMarine"Mudstone"Diagenesis"'

V  Not'so'much'SO42V'

V  Sulfate'reduc4on'not'significant'

V  Tend'to'produce'siderite'in'early'diagenesis''

V  Siderite'concre4ons'may'be'a'feature'of'freshwater'environments'

hQp://www.ucmp.berkeley.edu/carboniferous/mazon.html'

The'Mazon'creek'“Tullimonster”''preserved'in'a'Siderite'nodule' 13"

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