Every day, as thousands of barrels of crude oil splash into salt dome caverns along the Texas-Louisiana coast, Americans buy a bit more security against an interruption of foreign oil supplies.
As of now, Richard D. Furiga says, 215 million barrels are stored in five salt domes - four in Louisiana, one in Texas - less than one-third of the government's goal of 750 million barrels by 1989.
Half a million barrels a day pour into the caverns, said Mr. Furiga, deputy director of the Strategic Petroleum Reserve (SPR) program, in a telephone interview.
Ninety percent of that oil is foreign, with Britain's North Sea wells leading the way, followed by Mexico, Libya, Nigeria, Saudi Arabia and other suppliers. Domestic oil from Alaska accounts for the other 10 percent.
Born after the 1973-74 Arab oil embargo, the SPR sputtered along for years, with no crude at all flowing into the domes at times when world oil supplies were especially tight.
Now, with a worldwide glut and major producing nations anxious to sell more oil, the Reagan administration is plunging ahead on the project.
Shortly, says Mr. Furiga, a new batch of contracts should be announced, amounting to 100,000 barrels a day of new supplies destined for the SPR.
Steady buildup of the government-owned SPR contrasts with the action of some major oil companies, whose stocks have soared with low demand.
Oil companies generally are reducing stocks, partly to cut high interest charges on inventories, partly to protest high prices demanded by some members of the 13-nation Organization of Petroleum Exporting Countries (OPEC).
Some cash-hungry OPEC members - notably Nigeria - have discounted the price of their oil, leading to pricing chaos within the cartel.
Now OPEC has agreed on a unified price, accompanied by a Saudi Arabian pledge to reduce production enough gradually to bring supply and demand into balance. Saudi Oil Minister Ahmad Zaki Yamani says OPEC's action should wipe out the oil glut by next spring.
The White House and Congress, meanwhile, are taking advantage of a weak oil market to buy up SPR supplies at relatively bargain prices.
By late January, Mr. Furiga says, Phase I of the SPR program - calling for 248 million barrels of stored oil - should be completed.
Phase II calls for construction work to expand the storage capacity of the five salt domes by an additional 290 million barrels. During the construction period, said the SPR official, the daily fill rate is expected to be 200,000 barrels, down from the current half a million.
Phase III, providing for another 212 million barrels of storage and bringing the total up to 750 million barrels, will be finished by 1989, assuming work proceeds as expected.
Ultimate SPR goal, Mr. Furiga said, is a billion barrels, but no funding has been approved beyond the 750 million barrel mark.
Construction of facilities to store the oil is a budget expense, with House and Senate still wrestling over how much to appropriate for fiscal 1982, which began Oct. 1.
Purchase of the oil itself, however, is what the government calls an ''off-budget'' item, to be financed through the sale of US Treasury bonds.
Some critics charge that this is simply a device to make the 1982 budget deficit look smaller than it really is. The US government - which means taxpayers - foots the bill in either case.
Currently the US imports about 6 million barrels of oil daily, not all of which is subject to supply interruption for political reasons.
Such suppliers as Britain (North Sea), Mexico, Venezuela, and Canada, for example, would not cut off oil shipments to the US, in the event that Arab producers did so.
To arrive at ''days of protection,'' the amount of oil stored in the SPR should not be divided by total US consumption of oil - roughly 16 million barrels daily - but by the amount of oil that might be lost through another Arab embargo.
At worst, if all Arabs, plus Nigeria and Indonesia, joined in an embargo (which is unlikely), the US would lose just under 50 percent of its total oil imports, or about 18 percent of total consumption.