Commodity enrichment - the role of salt
In terms of total mass in sedimentary basins, the proportion of evaporite across the world’s Phanerozoic basins is rarely more than 2%. Today we have comprehensive documentation that salt horizons, their brines, associated dissolution and alteration conduits control significant economic associations of oil, gas and metals in volumes well above that expected from its inherently low levels in the Earth's lithosphere (Warren, 2016).
Salt punches above its weight (Warren, 2016)
- 50% of world’s carbonate reservoirs are tied to evap[orite (seals, traps and source rocks)
- All the world’s supergiant oil and gas fields in thrusts (seals and structural traps).
- All supergiant sedimentary copper deposits (halokinetic brine focus)
- 50% of the world’s giant SedEx deposits (halokinetic brine focus).
- 80% of giant MVT deposits (sulphate-fixer & brine interface)
- World’s largest Phanerozoic Ni deposit (meta-igneous – Noril’sk).
- Many more substantial IOCG deposits (meta-evaporite, brine and hydrothermal)
Why?
This high degree of commodity enrichment runs counter to evaporites making up no more than 2% of the world's Phanerozoic sediments. The exact why or how for many of these commodity associations is still not well understood. Most geologists working with oil, gas or metal buildups in a salt-rich basin will come to have a suspicion, and for some, the conviction, that salt or its subsurface alteration plays a role in defining the position or enrichment level of the commodity of interest. Evaporite masses in the subsurface, especially if halite-dominant, enable both physical and chemical alterations, which tend to improve economic prospectivity. The unique properties of salt in the diagenetic realm tend to facilitate, focus and stabilise processes that lead to elevated levels of accumulations of various commodities. That is, evaporites in a basin tend to enhance the volume of oil, gas or metals in an accumulation, but are not necessarily the direct cause of the accumulation/precipitation (Salty Matters March 13, 2017).