Carbonate Diagenesis. M.Geo.136b, Applications in hydrocarbon exploration Saskia Köhler, Patrick Ahlers

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1 Carbonate Diagenesis M.Geo.136b, Applications in hydrocarbon exploration Saskia Köhler, Patrick Ahlers

2 Carbonate in general 3 main components: 1) carbonate grains (aragonite, high- /low-mg calcite), 2) lime mud/micrite, 3) cement (calcite spar, fiberous calcite) Classification of limestones: (A) grain size, (B) after R.L. Folk, (C) after R.J. Dunham

3 Meteoric Realm Marine Realm Karsting, Springs Lacustrine Realm: Playalake, Open Lake System(Algea, biomineralization), Turbidites Fluviatile Realm Soil (e.g. calcretes, caliche due to Evaporation) Biomineralization: Reefs, molluscs (bivalves, gastropods), brachiopods, corals, echinoderms, bryozoans, calcerous algea, foraminifera Ooids, Peloids (fecal pellets) Microbinding of Sediment (e.g Stromatolithes) Pelagic and turbidite beds (deep water)

4 Moore, 1989

5 Early Diagenesis The Saturation of CaCO 3 changes with: Partial pressure of CO 2 Temperature --> decreases with depth Pressure --> increases with depth ph-value of seawater salinity Yu and Elderfield, 2007

6 Early marine burial Aragonite dissolves in the thermocline --> Ca 2+ and CO 3 2- become mobile Calcite precipitation --> cementation Below CCD water is still saturated with dolomite Frisch und Meschede, 2009

7 Dolomite replaces calcite --> Dolomitisation Above CCD --> Formation of Nodules Less permeability in cemented layers Alternation: carbonaterich and carbonatedepleted layers Moore, 1989

8 limestone, Mg-Calcite early cementation preserved structure B.A. Köhler, 2014

9 Cement chemistry controls crystal shape Moore, 1989

10 Dolomite Replacement of limestone: 2 CaCO 3 (limestone) + Mg 2+ CaMg(CO 3 ) 2 (Dolomite) + Ca 2+ 2 kinds of Dolomite: (1) Syndepositional (Penecontemporaneous), (2) Postdepositional Penecontemporaneous: host sediment is in its original depositional setting --> form under geochemical conditions of the depositional environment.

11 Primarily in shallow-marine to supratidal environments Mainly by direct precipitation from normal or evaporated seawater. Appearance: thin layers and lenses in evaporite lagoons/ lakes, supratidal crusts, fine-crystalline cements, replacements in peritidal sediments Boggs, 2009

12 Postdepotisional: form after the deposition --> the carbonate host rock hast removed from the zone of active sedimentation High porosity is needed --> migration of fluids Dolomitisation by fluids Early diagenetic dolomitisation: Very fine grained desiccation cracks, evaporites and their pseudomorphes Can take place soon after the deposition Preserves structures of the original sediment

13 Later diagenetic dolomitisation: Local replacement of certain grains or just the lime-mud matrix E.g. aragonite and high-mg calcite are dolomitised, low-mg-calcite fossils are not affected Rhombs of dolomite along stylolites Occurs in veins that cut through the limestone Dolomite of burial origin: Occurs in vugs and veins Curved crystal faces, prominent cleavage,light pink colour due to iron Porosity (left) in Dolomite and crystal structure (right) Scholle & Scholle, 2002 Adams, 1984

14 Importance of Dolomite for the Petroleum Industry Dolomitisation raises the pore volume In comparison to other limestones it has a very high porosity this makes dolomite a very good reservoir rock for hydrocarbons

15 DeDolomitization The replacement of dolomite forms limestone This process takes place near the surface (weathering) or in the underground (dissolution of evaporites) Adams, 1984

16 Isotopes δ 18 O, δ 13 C, δ 11 B, 87 Sr/ 86 Sr-ratio, Mg-isotopes, etc. Information about precipitation conditions : T, ph, fluid composition, rate of precipitation Distinguish zones: vadose, freshwater, mixed, marine Origin: aragonite or calcite bearing organism Grade of diagenesis Preservation of information

17 Swart, 2015

18 References Adams, Anthony E, William Scott MacKenzie, and C Guilford Atlas of Sedimentary Rocks under the Microscope. Longman; Wiley. Boggs, Sam Petrology of Sedimentary Rocks. Cambridge University Press. Frisch, W, and M Meschede Plattentektonik Kontinentverschiebung Und Gebirgsbildung: 196 P. Darmstadt (Primus). Moore, Clyde H Carbonate Diagenesis and Porosity. Vol. 46. Elsevier. Scholle, Peter A, and Dana S Ulmer-Scholle A Color Guide to the Petrography of Carbonate Rocks: Grains, Textures, Porosity, Diagenesis, AAPG Memoir 77. Vol. 77. AAPG. Swart, Peter K The Geochemistry of Carbonate Diagenesis: The Past, Present and Future. Sedimentology 62 (5): doi: /sed Tucker, Maurice E Sedimentary Rocks in the Field (3rd Edition). The Geological Field Guide Series. Böhm, F. et al Required but disguised: environmental signals in limestonemarl alternations, Paleogeography, Palaeoclimatology, Palaeoecology 189: Yu, J. and Elderfield, H Benthic foraminiferal B/Ca ratios reflect deep water carbonate saturation state, Earth and Planetary Science Letters 258: 73-86