A COUPLE OF WEEKS ago the Federal Energy Administration poured the first barrels of oil into the nation's strategic petroleum reserve. They were the first of a billion. The reserve is to be the country's insurance against future embargoes and disruptions of the imported oil on which the national economy crucially depends. It's not inexpensive insurance: The cost will run in the range of $20 billion. You will pay part of it in taxes, and part of it in a slight rise in the price of oil products. But even at $20 billion it is a very reasonable premium, in relation to the risk.

The petroleum reserve is a good idea that Congress wrote into the 1975 Energy, Act, but this gigantic project is getting off to a somewhat slower start than Congress had originally hoped. President Carter, though, has now set an ambitious schedule under which the Federal Energy Administration will be putting away oil underground at a rate, by next year, of more than 600,000 barrels a day. That urgency is altogether deserved.

The first delivery was Saudi Arabian crude, and it went into a salt dome at a place called West Hackberry, in southeastern Louisiana near the Gulf Coast. This cavern alone will eventually hold some 60 million barrels. (How much is that? By way of comparison, the U.S. currently is using just under 18 million barrels of oil a day, of which a bit over eight million are imported.) In an emergency, the stored oil in the West Hackberry cavern could be suckedout and fed into refineries at a rate of 400,000 barrels a day.

It's the kind of vast engineering project that Americans admire, and there's already a tendency here in Washington to suppose that the United States will immediately become embargo-proof. But it isn't quite so easy. First of all, it will be the work of years to fill the reserve. Even under the accelerated timetable it will reach the halfway mark, 500 million barrels, only at the end of 1980. As things are going now, 500 million barrels represents just about six months' deliveries to the United States from the Arab countries. The 1973-74 embargo, if you were wondering, lasted five months.

As the reserves are built up, they will greatly increase the risk and cost of the oil weapon to any country that tries to invoke it. They will provide a shield against political threats and skirmishing tactics like holding up a few tankers for a few weeks. Any government that contemplates the oil weapon will have to be prepared to cut off the flow - and forego revenues - for a painfully long time.

The reserves are insulation against sudden shocks, but they have their limitations. They are not going to be much help against the slow sqeeze on world supplies that is predicted for the 1980s. As the world is forced gradually to rely on more expensive and less accessible sources of oil, it's conservation that will count. Valuable though the reserves will eventually be, they still exist mainly on paper. It will take years of stock-building before the insurance policy becomes fully effective.