STORING VERY LARGE volumes of oil, rapidly, turns out to be not quite so simple as the Carter administration had originally thought. It's easy enough in principle. You find a large underground cavern, pour in the oil and, in the event of an emergency like the Arab oil embargo, pump it back out to the refineries. The technology has been in use for decades. But it has never before been applied on the present gigantic scale, and the reserves have fallen far behind the administration's ambitious plans.

In late 1975, responding to the Arab embargo, Congress established the strategic petroleum reserve with a firm requirement that it was to contain 150 million barrels of oil by the end of this year. By the end of 1982 it was to contain about 500 million barrels - at that time, the equivalent of three months' imports. Unfortunately, the country's oil imports have nearly doubled since then. When Mr. Carter came into office, he immediately raised the targets for the reserve to 250 million barrels by the end of this year and 500 million by 1980. The idea was to demonstrate the vigor of the new administration. How is it coming along? Slowly, so far.

The Energy Department located a series of caverns along the Gulf Coast and began filling them last summer. The amount now in storage in only about 40 million barrels. The rate of filling the reservoirs is running around 225,000 barrels a day, although the Energy Department hopes to get it up to 800,000 barrels a day by the end of the year. The Energy Department currently hopes to get 125 million barrels in underground storage by the year's end, or in Jaunary at the latest. It will unfortunately fall short of the original statutory goal, let alone the Carter administration's much more ambitious one.

The delays have been, perhaps, inevitable. The reservoirs are caverns originally created by chemical companies in a process that uses water to mine salt and other soluble minerals. Fresh water is pumped into underground deposits, and the minerals are recovered from the outflow. When the government took over the caverns, most of them were full of brine. Pumping it out is easy enough, but getting rid of hundreds of thousands of barrels of it every day - without inundating the state of Louisiana - is proving more difficult. The Energy Department has been drilling deep wells to re-inject it into the earth. But the wells are having trouble handling the rising volumes.Eventually there are to be pipelines to carry it out to the Gulf of Mexico. But laying the lines takes time, and there is also the question of - ah, yes - the environmental impact. Some ecologists are worried about the effects on the Gulf of this vast dumping operation. The Environmental Protection Agency is brooding on the case.

Regardless of the justification, the delays give cause for concern. The Senate Energy Committee is planning correctly, to hold oversight hearings later this year. To fall behind in the oil-storage program only reinforces the worldwide impression that the United States does not take in its energy commitments seriously, and cannot be relied on to meet its targets.

But it's not only a matter of the political consequences. The domestic oil industry is now capable of producing only a little over half of the oil that Americans consume. If anything should happen - embargo, war or natural disaster - to interrupt the flow from foreign wells, this country's only shock absorber woudl be the strategic reserves. The present reserve represents less than six days' imports. No one can measure mathematically the probability that imports will be cut off in the future. But it has happened before, and only the most foolish will permit themselves to believe that it couldn't happen again.