Submarine salt dissolution in the Santos Basin, offshore Brazil

Clara Rodríguez, Christopher Aiden-Lee Jackson , Rebecca E. Bell, Atle Rotevatn, Malcolm Francis

Salt dissolution occurs when evaporite-dominated rocks come into contact with NaCl-undersaturated fluids. Salt dissolution can positivity and negatively impact hydrocarbon and mineral exploration, seismic imaging, drilling, and structural restorations in salt-bearing sedimentary basins. However, due to typically poor seismic imaging and a lack of borehole data, few studies have analysed the detailed morphology of salt dissolution-related features (i.e., salt karst) and how this relates to intrasalt stratigraphic heterogeneity, and associated deformation within post-salt overburden. Here we integrate high-quality 3D seismic reflection data, a regional 2D seismic reflection line, and borehole data from the Santos Basin, offshore Brazil to characterise salt dissolution-related features at the crest of buried salt diapirs.
We recognise: (i) flat (<10◦), halite-dominated crests mainly characterised by up to 100 m tall, sub-circular mounds, likely comprising insoluble evaporite; (ii) rugose, evaporite-interbedded crests mainly characterised by up to 100 m deep, oval-to-circular sinkholes above more soluble evaporite units; and (iii) up to 60 m tall breccia pipes, capped by collapse-related sinkholes within the overburden above the flat and rugose crests. Reverse-basin modelling suggests salt dissolution occurred in a fully submarine environment in water depths of 1900 m (± 100 m), which, in combination with seismic-stratigraphic-relationships in post-salt strata, suggests dissolution occurred due to (i) superjacent NaCl-undersaturated seawater which penetrated exposed, thin (up to 60 m) roofs; and (ii) lateral updip migration of formation fluids from flanking submarine channels and lobes. We are therefore able to demonstrate a direct link between the intrasalt stratigraphic heterogeneity, and the style of salt karst, and related deformation in post-salt sedimentary overburden, providing evidence for widespread dissolution of salt in a fully submarine environment.