Geofisica

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 between deposition and either: a) metamorphism, or b) uplift and weathering Sediment converted into consolidated sedimentary rock Low temperature near-surfaceprocesses to higher temperature subsurface processes (100C smectites form mixedlayer clays; >200C become illites; >300C only mica remains)

Indicators of Diagenetic Histories
Zeolite facies - hydrousaluminosilicates alteration (150C prehnite & pumpellyite) Stable isotope ratios – see next slide

Isotopic signature of carbonate can indicate diagenetic history

Diagenetic Zones Shale

DiagenesisWhere multiple diagenetic episodes have affected a rock, it can be important to establish the paragenetic sequence
Detailed thin section observations SEM images Isotopic analyses of diageneticphases Etc.

Use to define burial history, fluid flow episodes, etc.

SEM image of quartz overgrowth (Q), chlorite (C), and framboidal pyrite (P)

Almon&Davies 1981

Diagenesis of Qtz ArenitePorosity/Permeability
Characterization of porosity and permeability may be a an important part of thin-section description
How much? What is origin? Is porosity connected? (implies permeability)Burial and Porosity

Boggs 2001

Primary Porosity
Amount of void spaces within a rock Primary porosity: a function of grain size, sorting, and packing

Secondary Porosity
Development of porespaces in rock through diagenesis Deep diagenetic fluids dissolve less stable framework grains or cement such as carbonate, plagioclase, pyroxene, amphiboles, and rock fragments Compression producesfractures

Secondary Sandstone Porosity

Porosity & Permeability
Porosity: % of void space in rock/sediment that may contain fluids
Total porosity – all pore spaces Effective porosity –...