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Ingrid Aarnes, Physics of Geological Processes, University of Oslo (Norway)
Henrik Svensen, Physics of Geological Processes, University of Oslo (Norway)
Stephane Polteau, Physics of Geological Processes, University of Oslo (Norway)
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There is a growing interest in understanding past climate changes, as this may lead to a better understanding of future challenges. One of the proposed mechanism for global warming and associated mass extinctions in the past (e.g. ∼250 Ma, ∼183 Ma and ∼55Ma) is venting of huge amounts of carbon gases (CO2 and methane) formed in contact aureoles of organic rich shales to the atmosphere. The metamorphism occurred during emplacement of Large Igneous Provinces and the gas was vented through pipe structures found in the Karoo Basin (South Africa), in basins offshore Norway, and in the Tunguska Basin in Siberia. During high temperature metamorphism, formation of carbon gases is preferred over liquid hydrocarbons. The aim of this study is to estimate under which conditions venting from contact aureoles occur. Understanding the conditions for fracturing of rocks on a local scale is the first step towards understanding the conditions for basin scale fracturing and vent formation. We have conducted detailed field work and borehole studies of sills and aureoles in the Ecca Group of the Karoo Basin. Geochemical parameters (pyrolysis, stable isotope analysis, and mineral chemistry) will be combined with numerical models to constrain aureole processes.
We show that fracturing of organic-rich aureoles occur during pressure buildup, resulting in horizontal and vertical mineralized veins. Carbon isotope data and the presence of bitumen in veins are fingerprints of the carbon degassing. Furthermore, the reduction of organic carbon in the black shale coincides with extensive veining. In addition to the gas production, expansion of pore fluids and thermal stresses contribute to the veining.
We have developed a numerical model to constrain the pressure increase from de-volatilization by using the kinetics of hydrocarbon formation and thermal stresses. In addition, formation of carbon gases from organic material is endothermic and may influence the maximum temperature obtained in the aureole. The rate of gas formation will be an important factor in predicting whether fracturing of the rock will occur, or if the pressure will simply diffuse away before any significant buildup and venting is possible.
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