Lecture 9 - Sedimentary notes PDF

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Lecture 9: Sandstone DiagenesisRecap:- Diagenesis is the processes that take place after thesediment has been deposited- There are physical and chemical changes alter thesediment- Occur at relatively low temperatures and pressures- Lithification is a diagenetic process- Around 0-200 degrees and 0-8K...

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Lecture 9: Sandstone Diagenesis Recap: - Diagenesis is the processes that take place after the sediment has been deposited - There are physical and chemical changes alter the sediment - Occur at relatively low temperatures and pressures - Lithification is a diagenetic process - Around 0-200 degrees and 0-8Kba pressure - Factors affecting diagenesis: tectonic and depositional environments

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All of the factors above, affect diagenesis and what sediments form Need to consider the chemistry of fluids Rainwater tends to be acidic, oxidising Seawater tends to be alkaline, oxidising Oil field tends to be alkaline, reducing

1) The eogenetic environment:

- Is the depositional environment. - Can be wet, dry, oxidising, reducing etc. - Particularly significant where biological agents are active a) Marine eogenesis: - Salinity is around 35% - pH > 7 - So4-2 and HCO3- is relatively high - Pyrite (FeS2) common where H2S and ironbearing minerals are present - Many different types of environments within a marine eogenesis: Anoxic mud, anoxic pore waters, oxygenated pore waters b) Continental eogenesis: - pH varied - Lower HCO3 - and SO42- Pore waters acidic where chem. weathering intense - High conc. of dissolved species but

Processes that occur: Compaction (shaking) - Changes of the packing lead to a reduction in porosity and a reduction in volume - Can just shake the material can reduce porosity Compaction (chemical) - Where you apply stress on the grain

- Materials that have clay cement loose porosity much more rapidly than materials with quartz cement .

- Increased pressure causes compaction Temperature: - Higher temperatures of +10 degrees can increase reaction rates of X2,X3 (carbonates are the exception however — faster in colder waters) Pore water chemistry: - Pore water is subject to rapid change due to the fact that: accelerated dissolution, accelerated precipitation, formation water circulation, fresh percolating down, connate being squeezed upwards and outwards, thermal convection, downward dilution — (large variety) - Salt sieving: where fine sediments especially clays act as semi-permeable membranes — Uncharged water molecules can pass through — Unbalanced charges are filtered out into clay mineral sites. - Where you get to 1km down most bacteria die — the alteration is purely chemical: polymerisation, polycondensation (humic and fulvic acids) and then insolubalisation to humin - Humin produces kerogen, the precursor to petroleum - Organic material does not stop as long as burial stays the same Cementation: silica - A lot of the geological record is cemented with silica: where does it all come from? - Silica sources include: dissolution of silicates during weathering, dissolution of planktonic secreters, dissolution of sandstones, pressure solution, chemical reaction — need a high pH for this dissolution to occur - To get a high percentage quartz cement need a low matrix cement - Pressure solution (quartz overgrowth) occurs as point contacts, pressure occurs, go into solution and then precipitate out where pressure is lower in pore spaces - Silica cementation occurs much in the eogenetic and early mesogenetic ranges, silica predicates as fluids cool and pressure is lower, also predicates out at lower pH Cementation: carbonate - Carbonate cements are also common, sources include: Dissolution of carbonate skeletons in sediment, Dissolution of limestones in a succession, Cations from dissolution of silicate minerals - Textures include syntaxial overgrowth, mosaic 3) Telogenetic environment - Deeply buried sediments are uplifted and eroded — go back up to the surface - T and P lowered - Pore waters generally oxidising and meteoric - Organic material oxidised and lost - Mesogenetic minerals and pore waters are out of eqm with the surface so change rapidly - Chemical weathering takes hold - If know the tectonic area, can say something about the burial history and phases of porosity in that sequence - As burial occurs you loose porosity - Some phase of secondary porosity increases, then looses this secondary porosity - Fine grained material looses its porosity quicker than coarser grains

- Clays can infiltrate sediments in many different ways...