WEEKS ISLAND, La.-Long before President Carter had readied his energy conservation message for the nation, the Federal Energy Administration was quietly preparing to cache the equivalent of a year's supply of oil imports, most of it beneath the marshy wetlands of the Louisiana and Texas Gulf Coast.

The oil, some 1 billion barrels in several grades of crude, will compose the "strategic petroleum reserve? designed to safeguard the nation against the probable economic trauma of any future Arab oil embargo.

It will be stored in giant caverns hollowed out of salt domes - huge underground mushrooms of virtually pure rock salt that stipple the Gulf Coast basin.

Ironically, while the Strata beneath the Gulf Coast domes have yielded vast quantities of petroleum over the years, most of the oil to be stored inside the salt domes will probably come from the Middle East.

The salt-dome plan, though expanded and speeded up by the Carter administration, has actually been in preparation for more than a year, according to FEA officials.

"We looked at many other ways to store oil," said John Donnelly, public information officer with the FEA. "We looked at things like moored tankers and huge oil-filled bladders anchored on the ocean floor.

"But we came to the conclusion that salt storage would be better, cheaper, more accessible and have far fewer environmental risks than any other methods. The French and Germans have been storing oil in abandoned salt mines for years."

So far, according to FEA officials and attorney involved in procurement, the government has selected four salt-dome sites with a combined existing capacity of 301 million barrels. Even the smallest would hold more oil than 38 supertankers.

Another four "candidate sites" - including underground limestone mines in Central Rock, Ky., and Ironton, Ohio - have been designed as suitable but not yet chosen. Those four have a combined capacity of 92 million gallons.

Additional sites remain to be selected to meet the President's announced goal of 1 billion barrels of oil in the ground by 1980. The first caverns will be filled next April.

Three of the four chosen sites are brine caverns - former mines where salt was extracted by drilling deep into the dome, pumping fresh water down and evaporating the resulting brine. Several of these could be greatly enlarged by further use of this process, called brine leaching.

The other site is the Morton Salt mine here at Weeks Island, some 140 miles west northwest of New Orleans, where rock salt is mined by blasting huge rooms and tunnels through a salt dome more than two miles in diameter.

Negotiations are still under way for the lease or purchase of the Morton mine, which has been operated by one owner or another for nearly 75 years.

Some 89 million barrels could be stored here on two levels, the deeper of which is more than 700 feet underground.

The other chosen sites and their capacities are:

Bayou Choctaw, a 3,600-by-4,600-foot dome 12 miles southwest of Baton Rouge where Allied Chemical Co. has been mining brine sine 1934. Total capacity of the dome's 12 available salt caverns: 94 million barrels.

Bryan Mound, a 6,000-foot-diameter salt dome 2 1/2 miles southwest of Freeport, Tex., where Dow Chemical Co. operates brine mines. Existing capacity is 58 million barrels, with possible expansion available for another 100 million.

West Hackberry, a one-by-three-mile dome 20 miles southwest of Lake Charles, La., where Olin Chemical Co. produces brine. Five existing available caverns have a capacity of 60 million barrels, with possible expansion available for another 200 million.

While salt domes have been found over the years from the Midwest to the floor of the Gulf Coast domes have been chosen for their accessiblity to both refineries and transportation routes.

Most would be loaded and unloaded by either barge or pipeline, and are on or near existing waterway and pipeline routes to the mammoth refinery complexes around Houston and Baton Rouge.

Salt-dome storage is a growing phenomenon in the petrochemical field, particularly for natural gas, butane, propylene and other hydrocarbon fuels. In 1975 some 187 million barrels of liquefied natural gas were being stored in 24 salt domes in Texas,Louisiana and Mississippi, much of it in caverns leached specifically for storage purposes.

According to FEA officials, the $1-to-$2-per-barrel cost of creating a salt-dome storage cavity is far cheaper than utilizing any other form of storage with comparable safety features.

In addition, rock salt is generally impervious to liquid and gas, has a compression strength comparable to concrete, and moves like plastic to seal incipient fractures.

While the first salt dome in North America was discovered more than 100 years ago, the origin of the dome phenomenon is still not entirely understood.

Most are believed to be remnants of a million-year-old ocean that dried up eons ago, leaving the salt behind. As the volcanic activity and sedimentation piled thousands of feet of rock on top of the ancient seabed, the theory goes, the salt became plastic and gradually oozed up through weak spots in the crust above. More than 500 salt domes have been found in recent years in the Gulf Coast basin alone.