5 - Diagenesis

8/8/2019 5 - Diagenesis

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Diagenesis

Chapters 5, 6, 7

Diagenesis

Definition

Controls on diagenesis

Zones, processes and products

Porosity

Organic matter

Summary

Diagenesis

Physical and chemical changes taking placein a sediment or sedimentary rock betweendeposition and either: a) metamorphism, orb) uplift and weathering

Sediment converted into consolidatedsedimentary rock

Low temperature near-surface processes tohigher temperature subsurface processes(


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Diagenesis vs. Metamorphism

Controls on Diagenesis

Movement of pore fluids Meteoric/surface waters into sedimentary

basins Potentiometric head defined by ground water

table above sea level, pore fluids will readilyflow into marine sedimentary basins

Thermal convection Inverse density gradient caused by thermal

expansion of water (batholiths, salt domes, etc.)

Compaction Porosity reduction drives interstitial waters

upward

Shallow Carbonate Diagenesis

Boggs 2001


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Boggs 2001

Bioturbation

Carbonates

Micritization Carbonate grains may be bored by fungi,

bacteria, algae

Fine-grained (micrite) carbonate(aragonite, high-mag calcite) may thenprecipitate in holes

In some cases, only exteriors of grainsaffected micrite rims/envelopes

In other cases, grains may be completelymicritized


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http://geology.uprm.edu/Morelock/GEOLOCN_/7_image/micrit.jpg

Micrite envelopes

Diagenetic Processes

Mesodiagenesis: four main processes:

Compaction

Dissolution

Precipitation

Recrystallization

Press and Siever 2001


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Diagenetic Processes

McIlreath and Morrow 1990

Compaction

Loosely packed sand porosity approaches25%; saturated mud 60-80% water. Porosityreduced during burial due to overburdenpressure

Fabrics may form identifiable in thin sectionincluding: deformation, distortion, flattening

Pseudomatrix formation when rock fragments

alter to clays under pressure looks like aprimary clay matrix

Pressure solution where grain boundariesundergo dissolution and crystallization

Compaction

Boggs 2001


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Concavo-Convex Contact

www.gly.uga.edu/railsback/PDFimage0208a.html0.27 mm

Sutured/Concavo-convex contacts

www.gly.uga.edu/railsback/PDFimage0212.html2.4 mm

Dissolution

Silicate and carbonate mineralsdissolved under conditions that are theopposite for cementation

Calcite and silicates show oppositebehaviour conditions for precipitation ofthe one are favourable for dissolution ofthe other


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Factors influencing the solubility ofFactors influencing the solubility of

CaCO3CaCO3 http://www.usask.ca/geology/classes/geol243/243notes/243week10a.http://www.usask.ca/geology/classes/geol243/243notes/243week10a.htmlhtml

Feldspar dissolution and calcite cement(high-pH conditions)

http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStratL1/slideshow_1_7.htm

Cementation

Development of new precipitates in pore spaces

Carbonates (calcite) and silicates (quartz) mostcommon, also clays in siliciclastic rocks

May be in response to groundwater flow,increasing ionic concentration in pore waters, andincreased burial temperatures

Overgrowths or microcrystalline cement whenhigh pore-water concentrations of hydrous silica

Iron oxide (hematite, limonite) determined byoxidation state


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Press and Siever 2001Calcite cement

Quartz overgrowths Dakota Fm.

Quartz overgrowths followed by calcite cement

http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStratL1/slideshow_1_16.htm


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Chlorite Cement

Boggs 2001

webmineral.com/specimens/picshow.php?id=1284

Illite cement

Cementation

Cementation of carbonates may takeplace in a variety of realms

Meteoric vadose/phreatic

Marine (phreatic) - seawater

Subsurface - basinal brines

Use fabric to help infer origin


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Carbonate CementsCarbonate Cements

http://sheba.geo.vu.nl/~imma/Project3.html

Pendant calcite cement

http://web.umr.edu/~greggjay/Carbonate_Page/LSGallery/pages/c-TF(F)b_10.htm

Bladed calcite cement followed by coarse spar


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Mineral Replacement

Dissolution of one mineral is replacedby another, simultaneously

No volume change

Carbonate replacement bymicrocrystalline quartz; chert bycarbonates; feldspars and quartz bycarbonates; feldspars by clay minerals

www-geoazur.unice.fr/PERSO/verati/Sericite

Mineral Recrystallization

Existing mineral retains originalchemistry but increases in size

Volume change

Amorphous silica to coarse crystallinequartz; fine lime mud into coarse sparrycalcite


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Burial Dolomitization

Dolomite may form as a replacement of aprecursor limestone

Use textural relationships to determine origin Certain types of calcium carbonate may be

preferentially dolomitized

Dolomite may be a fracture/void spaceinfill

Problem: need mechanism for circulatinglarge volumes of Mg-rich water

Dolomite replacing matrix around micritized ooids

web.umr.edu/~greggjay/Carbonate_Page/DoloGallery/

Saddle (Baroque) Dolomite

http://www.uky.edu/KGS/emsweb/trenton/fieldwork.html


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Diagenetic Structures

Liesegangenbands - result from groundwaterprecipitates in porous sandstones

Concretions - nucleated, regular shapedrounded objects

Nodules - irregularly shaped rounded objects

Calcite, siderite, pyrite authigenesis aroundan organic nucleus

Geodes - concentric layers of chalcedony withinternal crystals of euhedral quartz or calcite

Indicators of Diagenetic Histories

Conodont color alteration (Harris, 1979) -Cambrian-Triassic phosphatic fossils from paleyellow (1; 300C)

Vitrinite Reflectance - resistant plant cells alteredunder T&P, and reflect more light the higher therank (100-240C)

Clay Mineral Transformation - stability of certain

clay minerals (>100C smectites form mixed-layer clays; >200C become illites; >300C onlymica remains)

Indicators of Diagenetic Histories

Zeolite facies - hydrous aluminosilicatesalteration (150C prehnite & pumpellyite)

Stable isotope ratios see next slide


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Isotopicsignature ofcarbonatecan indicatediagenetichistory

DiageneticZones -Shale

Diagenesis

Where multiple diagenetic episodes haveaffected a rock, it can be important toestablish the paragenetic sequence Detailed thin section observations

SEM images

Isotopic analyses of diagenetic phases

Etc.

Use to define burial history, fluid flowepisodes, etc.


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Almon&DaviesAlmon&Davies 19811981

SEM image of quartz

overgrowth (Q),chlorite (C), and

framboidal pyrite (P)

Diagenesis of Qtz Arenite


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Porosity/Permeability

Characterization of porosity andpermeability may be a an important

part of thin-section description How much?

What is origin?

Is porosity connected? (impliespermeability)

Burial and Porosity

Boggs 2001

Primary Porosity

Amount of void spaces within a rock

Primary porosity: a function of grain size, sorting,and packing


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Secondary Porosity

Development of pore spaces in rockthrough diagenesis

Deep diagenetic fluids dissolve lessstable framework grains or cement suchas carbonate, plagioclase, pyroxene,amphiboles, and rock fragments

Compression produces fractures

Secondary Sandstone Porosity

Porosity & Permeability

Porosity: % of void space in rock/sedimentthat may contain fluids

Total porosity all pore spaces

Effective porosity connected pores

Permeability: ability to transmit fluids

Units Darcies

Absolute/relative permeability a function ofporosity, texture, diagenesis, etc.


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Press and Siever 2001

Small pores, butinterconnected high permeability

Large pores, butnot connected low permeability

Moldic porosity


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Secondary porosity Dakota Fm.

Interparticle porosity Dakota Fm.

Sediment Porosity (%) Permeability

Gravel 25 to 40 excellent

Clean Sand 30 to 50 good to excellentSilt 35 to 50 moderate

Clay 35 to 80 poor

Glacial Till 10 to 20 poor to moderate

Rock Porosity (%) Permeability

Conglomerate 10 to 30 moderate to excellent

Sandstone, Wel l-sor ted, l it tle cement 20 to 30 good to very good

Average 10 to 20 moderate to good

Poorly sorted, Well cemented 0 to 10 poor to moderate

Shale 0 to 30 very poor to poor

Limestone, dolomite 0 to 20 poor to good

Cavernous limestone up to 50 excellent

Crystalline rock

Unfractured 0 to 5 very poor

Fractured 5 to 10 poor

Volcanic Rocks 0 to 50 poor to excellent

Porosity and Permeability


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Organic Diagenesis

Buried organic matter also undergoesdiagenetic transformations Peat -> coal (increase in wt % carbon,

decrease in volatiles)

Hydrocarbon generation Type (gas or oil) depends on temperature and

kerogen type

Kerogen set of complex organic compounds,composed of varying proportions of C, H, and O

http://www.uky.edu/KGS/coal/coal_information.htm


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van Krevelen plot

Summary

Diagenesis: Physical and chemical changestaking place in a sediment or sedimentaryrock between deposition and either: a)metamorphism, or b) uplift andweathering

Affects all sedimentary deposits

Siliciclastics, carbonates, organic, others

Summary

Three zones: Eodiagenesis early/shallow diagenesis

Mesodiagenesis deep burial

Teleodiagenesis uplifted

Key results: Compaction

Dissolution

Precipitation

Replacement


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Summary

Porosity

May be destroyed (compaction, cementation)

or created (dissolution, fracturing) during burial

Characterization of porosity type andconnectivity can be important

Diagenesis of organic deposits leads toformation of coals and hydrocarbons

5 - Diagenesis

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