EOSC221 DIAGENESIS 1

Transcription

1 EOSC221 DIAGENESIS 1

2 LECTURE OUTLINE Introduc4on and Diagene4c Zones Sandstone Diagenesis Mudstone Diagenesis Marine Non Marine Carbonate Diagenesis Major Processes Diagen4c Environments Dolomi4za4on 2

3 INTRODUCTION The transforma4on of a sediment into a rock: Compac4on / recrystalliza4on / dissolu4on / authigenesis (growth of new minerals) and cementa4on Important economically in terms of porosity (Φ) and permeability (K) and in the forma4on of oil. Porosity and permeability of this sandstone has been greatly reduced by the precipita4on of calcite cement in between the pore spaces 3 hqp://

4 Diagene+c Zones: Different diagene4c processes will occur at different depths of burial and in different environments: marine vs terrestrial Different sediments will behave differently in these zones Phrea1c Zone 4 hqp://upload.wikimedia.org/wikipedia/commons/f/f2/vadose_zone.gif

5 Vadose Zone i. Clay infiltra4on from pore waters SANDSTONE DIAGENSIS ii. Mineral Solu4on: pi[ng and etching of unstable components 5

6 iii. Growth of Authigenic Minerals: Illite and Kaolinite the most common iv. Diagene4c reddening of sediments Caused by infiltra4on of hema4te a_er deposi4on or the dissolu4on of unstable minerals releasing Fe to form hema4te. 6 hqp://pubs.usgs.gov/dds/dds-033/usgs_3d/ssx_txt/figur27.htm

7 Phrea+c Zone i. Con4nued altera4on and dissolu4on of grains ii. Mechanical compac4on of framework grains pressure solu4on of quartz grains 7 hqp://en.wikipedia.org/wiki/file:pressure_solu4on_sandstone.svg

8 iii. Growth of cements: silica and calcite PPL - Silica o_en as quartz overgrowths in op4cal con4nuity - Calcite can be both an early or a late stage cement. XPL 8 hqp://

9 MUDSTONE DIAGENSIS Marine Mudstone Diagenesis Zone 1: 0-0.5m - Uncompacted, high porosity - oxic pore waters: bioturba4on - Oxida4on of organics by aerobic bacteria forming CO 2 - Zone 2: m - Zone of bacterial sulfate reduc4on - Anoxic condi4ons Bacterial Ac4vity 2CHO + SO 4 2-! 2CO 2 +S 2- +2H 2 O Bioturba4on D. vulgaris Organic material Sulfate from marine water 12 C: may form calcite concre4ons S - may form H 2 S 9 hqp://pubs.usgs.gov/dds/dds-033/usgs_3d/ssx_gif/bss8038.gif hqp://en.wikipedia.org/wiki/file:dvulgaris_micrograph.jpg

10 Zone 3: m - Zone of Organic Fermenta4on: no more sulfate reduc4on CH 2 O! CH 4 + CO 2 - Possible forma4on of Siderite - Expulsion of pore water by compac4on Zone 4: 1 2.5km - Breakdown of remaining organic maqer by decarboxyla4on - More pore water expelled 10 hqp://en.wikipedia.org/wiki/file:decarboxyla4on_reac4on.png

11 Zone 5: 2.5 7km - Zone of Hydrocarbon genera4on (about C) - Breakdown of unstable mineral components releases H 2 O - Release of H 2 O expels SiO 2, Mg 2+, Fe 2+ and any generated petroleum - At higher temps gas rather than liquid petroleum produced. Diagenesis - Metamorphism Zone 6: >7km - Zone of metamorphism - illite -> Sericite -> Muscovite (at 300 C) - Kaolinite -> Dickite/Nacrite -> Chlorite (at 200 C) Chlorite Mica Schist 11 hqp://en.wikipedia.org/wiki/file:chlorite_schist.jpg

12 DETRITAL INPUT ZONE 1 Is the rate of burial so slow that all organic carbon is oxidized near the sediment /water interface? NO YES RED/BROWN CLAYS NO Is deposi8onal water marine (SO 2-4 rich)? YES ZONE 2 Sulfate Reduc4on Is burial rate sufficiently slow for complete Fe 3+ conversion to pyrite? NO YES BLACK PYRITIC SHALES, CALCITE, DOLOMITE, PHOSPHATES Fermenta4on CH 4 CO 2 ZONE 3-4 Is burial rate sufficiently slow for extensive fermenta8on and decarboxyla8on? YES SIDERITIC MUDSTONES NO ZONE 5 Is burial rate sufficiently slow for extensive genera8on and migra8on of hydrocarbons? YES OIL and GAS PROSPECT ZONE 6 Metamorphism $ 12

13 Non-Marine Mudstone Diagenesis - Not so much SO Sulfate reduc4on not significant - Tend to produce siderite in early diagenesis - Siderite concre4ons may be a feature of freshwater environments The Mazon creek Tullimonster preserved in a Siderite nodule 13 hqp://

14 Major processes CARBONATE DIAGENESIS 1) Cementa4on: factors: CO 2, Mg/Ca ra4o, CO 3 supply 2) Neomorphism: Aggrading Calci4za4on Spar growing in micrite Calci8za8on Aggrading Shell recrystallized: internal detail lost 14

15 3) Dissolu4on 4) Compac4on 5) Dolomi4za4on: conversion from CaCO3 -> CaMg(CO 3 ) 2 Guilin, China hqp://en.wikipedia.org/wiki/file:1_li_jiang_guilin_yangshuo_2011.jpg Stylolites hqp://en.wikipedia.org/wiki/file:stylolites_mcr1.jpg Dolomite 15 hqp://en.wikipedia.org/wiki/file:7092_pieskovna_dolinka_pri_hradis4_pod_vratnom_dolomit.jpg hqp://en.wikipedia.org/wiki/file:stylolite_oehrlikalk_1b.jpg

16 Diagene+c Environments Three main environments: Meteoric / Marine / Burial 1) Meteoric: probably more important than the marine zone i. Upper Vadoze zone: Zone of infiltra4on: acidic rain water causes dissolu4on of high Mg calcite and Aragonite ii. Lower Vadoze zone: Zone of Percola4on: recrystalliza4on of unstable components and precipita4on of meniscus cements Pendent forms hqp://sepmstrata.org/carbonatediagenesisgallery/carbdiagenesisgallery.html Original aragonite shell recrystallized to low Mg Calcite hqp://en.wikipedia.org/wiki/file:geopetalcarboniferousnv.jpg 16

17 iii. Phrea4c zone: Permanent water satura4on - More Low Mg Cements - Ini4al phrea4c cements o_en Isopachous - Later phrea4c cements tend to be coarser blades - Syntaxial overgrowths on some grains coarser blades Isopachous Syntaxial overgrowths 17

18 Meniscus cement Rain Water Table METEORIC VADOSE Air in pores Ocean Isopachous cement METEORIC PHREATIC Pores saturated with water Pendent cement Ocean MIXING ZONE MARINE PHREATIC MARINE: Mostly Aragonite / high Mg calcite cements METEORIC: Low Mg calcite cements 18

19 2) Marine - Micri4za4on of grains on sea floor - Cementa4on forma4on of hardgrounds / beachrock - Can pendent cements in beachrock hqp://en.wikipedia.org/wiki/file:cretaceous_hardground.jpg - Cements o_en aragonite / High Mg Calcite - Can get similar isopachous cements (blades and fibres) as in meteoric hqp://sepmstrata.org/carbonatepar4cles/pages/164-micri4za4on.html Cretaceous hardground from Texas with encrus4ng oysters and bivalve borings. 19

20 Beachrock 20 hqp://en.wikipedia.org/wiki/file:reunion_saint-leu_beachrock(detail).jpg hqp://en.wikipedia.org/wiki/file:reunion_saint-leu_beachrock.jpg

21 3) Deep Burial - Cements fill remaining porosity - Calcite made available by forma4on of stylolites - Porosity can drop from 20 5% - Tend to ferroan (iron rich) calcite cements (anoxic pore water) - O_en mosaic in form - Possible aggrading neomorphism - patches of spar in micrite. 21 hqp:// hqp://

22 Original Carbonate Grain Deeper burial, oven ferroan rich equant mosaic crystals Larger bladed later vadose or early deep burial cement First genera1on vadose cement 22

23 Dolomi+za+on - Penecontemporaneous - altera4on to dolomite soon a_er deposi4on of the limestone - 2CaCO 3 +Mg 2+ -> CaMg(CO 3 ) 2 + Ca 2+ - Direct precipita4on - Not repeated under lab condi4ons - Ca 2+ + Mg 2+ +2CO >CaMg(CO 3 ) 2 - Deep burial diagenesis and altera4on of limestone The Dolomites Range, Italy 23

24 A. Penecontemporaneous 1) Evaporite-Brine Residue Model I. Reflux Model - Gypsum (CaSO 4 ) forma4on increases Mg:Ca ra4o - Resultant brine sinks into sediment - Problem: not observed in nature Supra4dal High Tide See figure in text book Mg RICH brine Mg DEPLETED brine OCEAN Dolomi4zed Zone 24

25 II. Evapora4ve Pumping - Sabkha environment - Evapora4on draws brines Sabkha, Tunisia Halite Crust EVAPORATION Gypsum and Anhydrite Stromatolites common OCEAN Saline Groundwaters 25 hqp://en.wikipedia.org/wiki/file:sebkha_el_melah_-_2001.jpg

26 2) Groundwater Mixing Model - Brackish water in zone where fresh mixes with salt water - May produce condi4ons suitable for dolomi4za4on - In all cases dolomi4za4on o_en removes or degrades original deposi4onal structures / features Rainfall Meteoric porewaters Sea Level Marine porewaters 26

27 3) Sea water Model - Tidal effects or rising sea level pump sea water through sediment - Pores con4nually flushed with fresh marine water - Mg in sea water may allow for dolomi4za4on Meteoric porewaters Rising Sea Level Marine porewaters 27

28 B. Deep Burial Dolomi1za1on - Majority of dolomites show ghost textures - Must be altera4on of original limestones - Probably in in deep burial se[ngs - Occurs over millions of years DOLOMITE PEAK, ALBERTA 28

29 GLOSSARY All lecture material is potentially examinable. It is up to you to know unfamiliar terms / names / people. Use this space to create your own lecture glossary TERMS / NAMES DEFINITION 29

30 TERMS / NAMES DEFINITION 30