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Commercial use of salt domes in Louisiana and Gulf Shore Region

1. Geology of a salt dome
2. Commercial use of salt domes

• Natural Resources
  • Petroleum trap
  • Salt
  • Sulfur

• Space for storage and disposal
  • SPR storage sites
  • CO2 storage
  • Helium storage

Geology of a salt dome

Salt domes - columns of salt that intrude through overlying sediment units.

Salt domes are common geologic features within the Gulf Coast aquifer along the upper Texas Coast.

"The core of a salt dome forms a vertically elongate, cylindrical stock, consisting of 90 to 99 percent crystalline rock salt (halite). Cap rock composed of sulfate and carbonate minerals commonly overlies the crest of the salt stock and drapes down the uppermost flanks. Salt stock and cap rock are enclosed in sediments and sedimentary rocks of the Gulf Coast aquifer and deeper saline-water intervals. Salt-dome crests are generally one to three miles in diameter and buried at depths that range from land surface (essentially zero feet) to greater than 10,000 feet."

The salt originally formed bedded evaporite deposits in the ancestral Gulf of Mexico during the Jurassic period. A thick (greater than 20,000 feet) sequence of sedimentary rocks now overlies the salt source layer.

Salt, which is a low-density, ductile mineral, is gravitationally mobilized by sediment loading, forming a variety of upwelling structures, one of which is the cylindrical salt dome.

The zone of uplift near the dome is surrounded by areas of subsidence and downwarping. Faults and fractures are also common features of salt dome growth.

Salt domes form because salt has the ability to change shape and flow when placed under enough pressure. To develop a salt dome, the pressure on the salt must be high enough to enable it to intrude the overlying sediments. The pressure must be great enough to overcome the weight of overlying strata, the strength of overlying strata, frictional forces, and the force of gravity resisting uplift.

Natural Resources

Salt domes provide a variety of natural resources (Seni, 1986).

Structural deformation and cap rock formation have created prolific petroleum reservoirs. Oil and gas are trapped in uplifted strata surrounding or overlying salt domes and in the cap rock itself. In addition to petroleum, salt from the salt stock and sulfur from the cap rock are the main commodities derived from Gulf Coast salt domes in Texas . Salt domes also provide space for storage and disposal (Seni and others, 1985). Solution-mined caverns in the salt stock have been created both for brine production and for storage of various petroleum products, most commonly liquid petroleum gas. The volume of some storage caverns exceeds ten million barrels. Crude oil for the Strategic Petroleum Reserve is stored in caverns at several Texas Gulf Coast salt domes. Cavernous zones in cap rocks have been used for brine disposal (Seni and others, 1984c), and the potential for disposal of chemical wastes in salt caverns has been evaluated (Seni and others, 1984a).

Resource development and production can create geologic and hydrologic instabilities around salt domes (Seni and others, 1985). Land-surface subsidence, sometimes involving catastrophic collapse and sinkhole formation, is common where large amounts of sulfur, salt, and/or petroleum have been extracted from the salt dome (Mullican, 1988). High-volume brine disposal elevates cap rock fluid pressures in shallow intervals laterally adjacent to freshwater sands, reversing pre-development hydraulic gradients and creating the potential for aquifer contamination (Hamlin and others, 1988). Petroleum storage caverns in the salt stock have failed and leaked product into surrounding freshwater sands (Seni and others, 1984b, 1985).

Petroleum Reservoirs

Salt domes sometimes form pockets where hydrocarbons can get accumulated (image 1). Oil and gas are trapped in uplifted strata surrounding or overlying salt domes and in the cap rock itself.

A single salt dome can have many associated reservoirs at a variety of depths and locations around the dome. In addition to petroleum, salt from the salt stock and sulfur from the cap rock are the main commodities derived from Gulf Coast salt domes in Texas.

Salt from the salt stock

Some salt domes have been exploited by underground mining. These mines produce salt that is used as a raw material by the chemical industry and as salt for treating snow-covered highways.

Some salt domes are minded by solution. Refer to Salt Mining main article.

Sulfur from the cap rock

Salt domes are sometimes overlain by a cap rock that contains significant amounts of elemental sulfur. Some salt domes have enough sulfur in the cap rock that it can be economically recovered.

Space for storage and disposal

•Solution-mined caverns in the salt stock have been created both for brine production and for storage of various petroleum products, most commonly liquid petroleum gas.

•The volume of some storage caverns exceeds ten million barrels.

•Crude oil for the Strategic Petroleum Reserve is stored in caverns at several Texas Gulf Coast salt domes.

•Cavernous zones in cap rocks have been used for brine disposal.

SPR Storage Sites

•Emergency crude oil is stored at the Strategic Petroleum Reserve (SPR) in underground salt caverns at four major oil storage facilities in the Gulf Coast region of the United States, two sites in Texas (Bryan Mound and Big Hill), and two sites in Louisiana (West Hackberry and Bayou Choctaw).

•The caverns offer the best security and are the most affordable means of storage, costing up to 10 times less than above ground tanks and 20 times less than hard rock mines.

•These four sites have a combined authorized storage capacity of 714 million barrels.  

Potential of storing gas with high CO2 content in salt caverns

Salt caverns are one of the best options for the underground storage of gases because of salt rock's excellent sealing capabilities and mechanical properties, such as self‐healing when damaged or cracked.

Temperature naturally increases with depth in the Earth’s crust, as does the pressure of the fluids (brine, oil, or gas) in the formations. At depths below about 800 meters (about 2,600 feet), the natural temperature and fluid pressures are in excess of the critical point of CO2 for most places on Earth. This means that CO2 injected at this depth or deeper will remain in the supercritical condition given the temperatures and pressures present.

Carbon dioxide (CO2) can be stored underground as a supercritical fluid. Supercritical CO2 means that the CO2 is at a temperature in excess of 31.1°C (88ºF) and a pressure in excess of 72.9 atm (about 1,057 psi); this temperature and pressure defines the critical point for CO2.

The main advantage of storing CO2 in the supercritical condition is that the required storage volume is substantially less than if the CO2 were at “standard” (room)-pressure conditions.

Helium Storage

Salt domes also serve as national repositories for government reserves of helium gas.

Salt is the only type of rock that has a permeability so low that it can hold the tiny helium atoms.

Conclusion

•Salt domes are the source of numerous natural resources (oil, natural gas, salt, sulfur).

•Salt domes allow cheap and efficient storage of hazardous waste, oil and natural gas, helium and potentially CO2.

•Salt domes in Gulf Shore region have a large economical importance.

Image 1