Data di Pubblicazione:
2012
Citazione:
NON-MARINE CARBONATES: MICROBIALLY MEDIATED VS. ABIOTIC FABRICS AND POROSITY / G. Della Porta, F. Barilaro, E. Capezzuoli. ((Intervento presentato al convegno AAPG HEDBERG Conference Microbial carbonate Reservoir Characterization tenutosi a Houston nel 2012.
Abstract:
Non marine carbonate buildups have recently been the object of renewed interests from the academia and industry following the discovery of the South Atlantic hydrocarbon reservoirs. Non-marine carbonate buildups and microbial bioherms are characterized by a wide range of geobody types, fabrics and flow unit architectures and distributions with complex depositional and secondary pore systems. Carbonate buildups accumulate in large spectrum of terrestrial settings ranging from sublacustrine to subaerial spring environments.
Precipitation can be the result of 1) abiotic processes (evaporation, degassing, mixing of water masses supersaturated with respect to carbonate minerals) as well as the product of 2) biologically induced and influenced (organomineralization s.l.) precipitation mechanisms in association with photosynthetic cyanobacteria and green algae, heterotrophic bacteria and their biofilms (extracellular polymeric substances). 3) Biologically controlled precipitation by organisms secreting a carbonate skeleton can be present (e.g., bivalves, gastropods, ostracods) but might also be very scarce or absent according to the water extreme chemical and physical properties (e.g. hypersalinity, high alkalinity or high temperature) that might exclude most of the biota. Silica fixation by diatoms and carbonate precipitation associated with insect larvae occur in some continental aquatic depositional settings.
In lakes, carbonate buildups accumulate in a wide spectrum of water chemistries but predominantly in hydrologically closed basins, with alkaline to hypersaline conditions, in arid climate and tectonically active settings. Carbonate buildups can be distinguished in: 1) decimetre to meter scale bioherms, subparallel to the shorelines forming continuous belts traceable for hundreds meters where microbially mediated processes seem to predominate (e.g., hypersaline Holocene Great Salt Lake, Utah; schizohaline Eocene Green River Formation of Utah and Wyoming; freshwater to alkaline Miocene Ries Crater, Germany), 2) isolated meter to decameter scale mounds located at sites of sublacustrine groundwater spring discharge where the mixing of groundwater and lake water is the trigger for abiotic and biologically induced/influenced carbonate precipitation (e.g., late Pleistocene-Holocene freshwater to alkaline Pyramid Lake in Nevada, highly alkaline Mono Lake in California), 3) decimeter to decameter sheet-like to mound-shaped deposits associated with sublacustrine hydrothermal vents (e.g., Pyramid and Mono Lake, Miocene Southern Tuscany, Italy). In the latter case, the location of carbonate accumulation is often controlled by extensional and strike-slip faults and both abiotic and organomineralic precipitation processes take place. In terms of potential reservoir architecture, shoreline microbial bioherms can cluster forming continuous belts hundred to thousand meters in lateral extension, whereas carbonate mounds associated with mixing of lake and groundwater at sublacustrine springs are generally isolated, separated by siliciclastic conglomerate, sandstone and mudstone.
In subaerial continental settings, flowing water issuing from punctual or linear springs precipitates wedge-, mound-, and pinnacle-shaped carbonate deposits of decimeter to several tens of meters in size, occasionally up to hundreds of meters. Within the subaerial spring deposits, two groups can be distinguished: 1) Travertines are precipitated from water issuing from hydrothermal hot-springs (temperature > 20°C) and can typically drape the antecedent topography, accrete and prograde building mounds, fissure ridges, fans with terraced and smooth slopes, and cascades (e.g., Pleistocene-Holocene Southern Tuscany, Italy). In these fast-flowing settings rapid CO2 dega
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Keywords:
non marine carbonates ; microbialite ; fabrics
Elenco autori:
G. Della Porta, F. Barilaro, E. Capezzuoli
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