ALBERT

Description: Thermogenic discharge into the atmosphere associated with the Karoo LIP event (183 Ma) is currently estimated to as much as 12,600 Gt of methane. In order to precise the degassing rates at the basin floor, we investigate the influence of the Karoo Basin stratigraphic physical properties, including a realistic initial TOC profile. Using state-of-the-art numerical models of hydrothermal fluid flow through porous rocks we investigate the formation and evolution of hydrothermal systems powered by sill intrusions. We take into account the energy consumption of the dehydration reaction and the thermogenic cracking in the sediments as well as the increasing pore space upon sediment dehydration. Permeability depends on porosity and, in addition, a simple Mohr-Coulomb rheology is used to simulate hydrofracturing. Our results indicate that fluid flow caused by a single sill intrusion emplaced in the deep organic-rich formation is insufficient to rapidly transport thermogenic methane to the basin floor. Instead the mobilized methane accumulates in shallower parts of the basin, where it may be later remobilized by shallower intrusions. Our new estimates suggest a reduction of one to two orders of magnitude in the amount of thermogenic gas emitted during the Karoo LIP event.