Carbonate fault cement can lead to effective sealing of faults that act as conduits for upward fluid flow. In conductive faults, fluid flow, and thus cementation, is typically focused along fault intersections, extensional steps, and fault terminations. Based on the analysis of four carbonate-cemented faults, I propose that carbonate fault cements form by 1. the microbial oxidation of hydrocarbons and 2. exsolution and degassing of CO2 during rapid upward fluid flow. Depending on the quantity and distribution of oil migration, microbial oxidation of oil can lead to discontinuous cements and thus ineffective seals. In contrast, oxidation of migrating methane is observed to lead to effective fault seal. Degassing of CO2 during upward flow can also lead to effective fault seal provided pressure gradients are steep in the flow direction. Under subsurface conditions, steep pressure gradients are expected for coseismic seal failure and rapid fluid flow.


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