Why Does Salt Rise Through Rock? It Is Lighter and It Flows

Thick salt deposits form where a sea evaporates, and they are then buried by sediment accumulating on top. Two properties then matter. Salt is less dense than compacted sediment and stays that way regardless of depth, since it does not compress further, while the rock above continues compacting and becomes heavier, so an unstable arrangement develops with lighter material beneath heavier. And salt is ductile under pressure, deforming by flow rather than by fracture over long periods, so it behaves as an extremely viscous liquid rather than as a rock. Given a weakness to exploit, it therefore rises, and the overlying sediment sinks around it to take its place.

The movement creates a recognisable set of structures:

• •A steep-sided column or plug rising through the overlying layers
• •Strata bent sharply upwards where the column pushed through them
• •A dome in the layers arched over the top of the rising mass
• •Faults radiating outwards over that dome as it was stretched
• •A cap of insoluble minerals where groundwater dissolved the salt top
• •Withdrawal basins where sediment sank into the space the salt left

These structures are the single most productive kind of oil trap and the reason is geometric. Oil and gas are lighter than the water in the surrounding rock and migrate upwards until something stops them, and a trap requires both an arched or tilted shape to collect them and an impermeable seal above. A rising salt mass supplies both, arching the overlying layers into domes and tilting the layers alongside it into wedges, while the salt itself is an essentially perfect seal since it is impermeable and self-healing. Enormous fields around the Gulf of Mexico, the North Sea, the Persian Gulf and offshore Brazil are associated with these structures, and locating them was the principal aim of early seismic surveying.

Locating a buried structure kilometres down required techniques that were developed largely for this purpose. Gravity surveying came first, since salt is less dense than the rock around it and therefore produces a measurably weaker pull directly above, which a sensitive instrument detects as a small deficit, and a portable instrument for measuring that pull was developed in the 1920s specifically for this search. Seismic surveying replaced it, sending sound into the ground and timing the reflections, which images the structure directly and which became the foundation of the whole exploration industry. Salt is awkward to image through, since sound travels through it far faster than through sediment and bends sharply at the boundary, and handling that distortion occupied computational geophysics for decades.

The salt itself has several uses beyond being a geological curiosity. It is mined directly where it comes close enough to the surface, and some of the largest salt mines in the world work these structures. Cavities are deliberately dissolved within them by pumping in water and removing brine, and the resulting voids are used to store enormous quantities of natural gas, crude oil and increasingly hydrogen, since a salt cavity is airtight, self-sealing and cheap compared with any constructed tank. Strategic petroleum reserves in the United States are held this way. The same cavities have been proposed and used for waste disposal, and one German site storing radioactive waste in a salt mine has required an extraordinarily expensive recovery operation after water entered it.

Buried salt stays less dense than the compacting rock above it and deforms by flow rather than fracture, so it rises through kilometres of sediment while the sediment sinks around it. The rising arches the overlying layers into domes and tilts adjacent layers, supplying both the shape and the impermeable seal that an oil trap requires. Dissolved cavities within them store gas, oil and hydrogen.