Many countries including the US have underground strategic gas & oil reserves. At present the US has 4 salt dome petroleum storage sites in Texas and Louisiana along the Gulf coast. Salt domes are convenient and inexpensive for underground storage of crude oil and gas. Salt mining has been done from underground workings of drifts and shafts employing underground miners. It has also been accomplished by hot water injection into the formation and retrieval of NaCl brine. Sometimes the salt dome is capped with a layer of elemental sulfur which is recoverable.

The dome formations, called ‘diapirs‘, are usually 1/2 to 5 miles across and rise to 500 ft to 6000 ft below the surface. The discovery of these salt formations coincides with the discovery of oil at Spindletop Hill near Beaumont, TX, in 1900. The current inventory in the SPR as of March 29, 2024, is 363.6 million barrels.

The US acquired the sites in 1977 after the 1973 oil crisis hoping to buffer large disturbances in the crude oil supply. In October of 1973, the Organization of Arab Petroleum Exporting Countries (AOPEC or OPEC) embargoed any country that supported Israel in the 4th Arab-Israeli War. The US was embargoed after Richard Nixon requested 2.2 $ billion for support of Israel’s war effort. At about the same time, after the US had disconnected the dollar from gold, OPEC Ministers found that with the free-floating dollar, their income was lagging because they did not have institutional mechanisms to raise prices with fluctuations in the market price.

Oil storage is all very interesting but what’s the deal with salt domes themselves? What do they look like and where are they found? Why do they form domes?

The graphic above is from a 1984 Atlas of Salt Domes produced at UT Austin. It shows the intrusive nature of an idealized salt dome. Salt strata are able to deform upwards due to the weight of the formation on the lower density and lower strength salt (halite) layer. The term “salt tectonics” has been used.

The collision of supercontinents Laurentia and Gondwana lead to compressional folding of strata and the formation of anticlines and synclines in the areas now lying in eastern Utah and western Colorado.

There are several types of diapirs: Magma, Salt and Mud. Salt diapirs start out as layers of salt sediment deposited. Here ‘salt’ refers to a number of ionic solids previously precipitated from an inland ocean. Changing sea levels and evaporation of shallow marine waters lead to the deposition of dissolved and suspended solids forming shales, dolomite, anhydrite, sodium and potassium salts, and black shale.

These salt strata do not consist purely of halite, NaCl, but rather contain other evaporites like hydrated gypsum, CaSO₄ ^.2 H₂O, and anhydrous gypsum or anhydrite, CaSO₄. naturally, there may be layers of silt or sandstone included in the salt formation. If the salt strata form an anticline, oil, gas and water (or brine) may accumulate in the diapir.

Salt is less dense, has greater ductility and is more buoyant than the surrounding country rock. Under great pressure from the surrounding rock, salt can begin ductile flow in the direction of lower pressure which may be upwards. The salt formation has greater ductility than the surrounding rock, so the salt is extruded into faults and sediment layers above. This has been called ‘salt tectonics’.

Salt diapirs are more common than most of us realize. On the Gulf Coast they are found on both sides of the coastline and further inland. Sometimes they are associated with oil and gas deposits. Salt strata are found in Utah below Arches National Park and further south in the Moab valley and southeast into the Paradox Valley in western Colorado. The salt strata can be thousands of feet thick originating from marine transgressions of the Jurassic Sundance Sea. This is not to be confused with the later Cretaceous Western Interior Seaway.

The Sundance Sea moved in and out 5 times with a hiatus of erosion between transgressions.

At Arches National Park, over time the ground beneath the area had been rising, fracturing the sandstone above and producing the fin formations at the surface. The exposed rock, called the Entrada sandstone, is part of the Pennsylvanian Paradox Formation. The fins are comprised of many layers of sandstone. If the broad face of the sandstone fins erode enough, a hole can be formed giving the namesake arches. An aerial photograph shows parallel fin formations over a wide area.

If the reader has never explored Utah, by all means do so. It’s just gorgeous.