THE WORLD IS now burning up its oil at a rate of 900 billion gallons a year. The reserves still underground are vast, but at this rate they aren't going to last long. You have heard a lot about the current glut in the oil market and the possibility that prices will fall further. Neither the glut nor the falling prices are likely to continue forever.

The U.S. Geological Survey has been taking an inventory of worldwide oil resources, including estimates of the oil that remains to be discovered in the future. Using these estimates, scientists at the USGS arrive at two conclusions. First, the world has already burned about one-fourth of the recoverable oil that existed when the first well was drilled just over a century ago. Second, exploration has already found about two-thirds of the oil that it will ever discover. There will still be large and valuable discoveries in the years ahead, but it's not likely that any will be large enough to change this analysis significantly.

It means that, at the present rate of consumption, the world has roughly a 60-year supply of oil remaining. By the middle of the next century, for most uses, it must shift to other fuels.

But there's another implication here that will affect Americans long before the middle of the next century. It's spelled out in a recent paper by Charles D. Masters of USGS who, with his colleagues there, has been compiling these estimates. Most of the world's known oil reserves, he points out, are in the countries that touch the Persian Gulf, and the research at USGS indicates that future discoveries aren't likely to change that reality.

Most of the countries with oil -- such as the United States and the Soviet Union -- are producing it as fast as they can. The exceptions are OPEC countries, most of them in the Persian Gulf region. Iran and Iraq are constrained by their endless war. The others are holding down production deliberately, as a matter of national policy, to keep prices up and conserve supply. As the other countries rapidly consume their reserves, Mr. Masters of USGS points out, what's left of the world's oil will increasingly be concentrated in the Persian Gulf countries.

Americans would be imprudent to assume that the Saudis and OPEC have lost forever their influence over the oil market. The United States now accounts for one-fourth of the world's oil consumption, but has less than one-tenth of its oil reserves. It needs to move faster toward the alternatives to oil. Leaving this transition wholly to the market in the Reaganite manner is too perilous; the market, with its falling prices, is currently -- temporarily -- sending out misleading signals. For any period beyond the next several years, geology provides a more realistic guide to the ultimate limits of the resources.