A group of authors led by a Statoil ASA specialist in marine geology has proposed an unconventional theory for the origin of salt that could have far reaching implications for oil and gas exploration.
Masses of solid salt may form and accumulate under ground, independently of solar evaporation of sea water, Martin Hovland of Statoil and four other authors have suggested.
The article, “Salt formation associated with sub-surface boiling and supercritical water,” is published in the September 2006 issue of “Marine and Petroleum Geology.” The authors use as examples drilling results from the Deepsea Drilling Project Site 226 (Atlantis II Deep) in a marine setting in the central Red Sea and from Lake Asala near Danakil, Ethiopia, in a continental setting.
Underground precipitation
The Norwegian research team demonstrated how solid salt forms in high temperature/high pressure (HTHP) conditions when seawater circulates in hydrothermal systems in the crust or under piles of sediment.
It is the physical properties of supercritical water that stimulate the precipitation.
When water no longer boils because the pressure is too high, it enters the supercritical phase and attains a relatively low density (3 gm/cc).
For fresh water this occurs at temperatures above 374° C. combined with pressures above 221 bar. For seawater, the values are 405° C. and 300 bar (equivalent to a depth belowground of 2,000 m or below sea level of 2,800 m).
Both molecular theory and laboratory experiments prove that the solubility of salt in supercritical water is practically zero, Hovland noted. Therefore, when sea water enters into an HTHP (hydrothermal) convection cell, its salts precipitate out and accumulate within the surrounding rock fissures or sediment pores.
Geologists, whose current model for salt deposition and accumulation relies only on solar evaporation of seawater, have overlooked this novel hydrothermal outsalting mechanism.
The main beauty of the new model is its lack of demand for large ocean basins (such as the Mediterranean Sea) to evaporate up to 10 times for salt several kilometers thick to accumulate.
The hydrothermal process occurs totally independent of surface evaporation, as it relies only on high heat flow in the earth. Another strength of the model is its ability to predict how brines and sulfate (salt) masses are likely to have accumulated on other planets, such as Mars, where it is hard to account for there having existed a deep ocean that evaporated.
Hydrocarbon implications
The novel hypothesis on subsurface accumulation of salt has fundamental implications for hydrocarbon accumulation because it is a result of the processes that create sedimentary basins.
Basins are formed by tectonic movements, and the deep basinal faults are frequently associated with processes in the mantle, causing high heat flow in the fault systems. The faults allow the circulation of sea water and brines, driven by heat and gravity.
When brines come into the “supercritical zone” of water, water will gradually lose its ability to dissolve salt, and salt will accumulate in the fracture systems.
In traditional thinking, a prerequisite for the accumulation of salt is a dry climate, and this is contradictory to the frequently observed high rate of water-transported sediments into the basin.
The subsurface accumulation of salt opens up the possibility to have a wet climate that promotes a high rate of erosion and transportation of sediments into the basin, at the same time as salt accumulates in the subsurface. Rapid and extreme shifts in climate and sea level (e.g., the “Messinian Salinity Crisis”) are therefore not necessary prerequisites in the authors’ model.
This knowledge will have a fundamental impact on the interpretation of basin development that is a cornerstone in the exploration for hydrocarbons, Hovland said.
