"THE GAS INDUSTRY has reserves, proved and potential, to carry it well into the 21st century." That cheery declaration came the other day from Paul E. Reichardt, the chairman of the Washington Gas Light Co. It's a startlings statement, is it not? Unfortunately, it belongs to the large category of startling statements that are are not necessarily correct. Mr. Reichardt is guessing, and the implications of his guesswork are profoundly misleading.

The basic question here, of course, is weather the present gas shortage is real. For those readers who might be in a hurry to catch the bus this morning, we offer here the short answer: Yes, it is real.But current handling - and mishandling - of reserve figures is worth a few minutes of your time, if you're among the large number of Americans who worry about energy policy.

Mr. Reichardt bases his view of the gas supply on the standard estimates of U.S. reserves. They average around 900 trillion cubic feet (tcf), and gas consumption is recent years has averaged a little over 20 tcf. Divide 900 by 20 and you come up with a 45-year supply - which takes you, along with the chairman of Washington Gas Light, well into the 21st century. Right?

Don't count on it. Those reserve estimates contain a couple of interesting assumptions. The most authoritative figures come from the U.S. Geological Survey. In a study two years ago it calculated that the known and measured reserves contain 237 tcf. Past experience suggests that these reserves may be proved bigger than the current estimates, as producers keep drilling and probing around the edges. If the historical pattern holds, they will turn out to contain an additional 200 tcf. That brings the total up to 437 tcf - but now we are counting tcfs that have not been precisely located.

In addition, the Geological Survey calculates that there is somewhere between 320 and 625 tcf in reserves still altogther underdiscovered. These figures, incidentally, represent a drastic reduction of the Survey's standard estimates as recently as five years ago. Even the current numbers are hotly controversial. Mr. Reichardt's good news is based on an inventory of reserves of which three-fourths have not yet been discovered, and some of which may not exist.

As gas is extracted from the porous rock and sand of an underground reservoir, the pressure falls - and the flow of gas falls with it. Last year the Federal Energy Administration published the prospects for the reserves that produced most of the country's natural gas, 17 tcg of it, in 1974. By this year, those same reserves will produce only about 13 tcf. By 1989, the flow from those reserves will be down to 34 tcf - less than the country uses today for home heating alone, not counting heating the bath water or driving the laundry.

In order to keep burning up 20 tcf of natural gas a year, the country will have to find 20 tcf in new reserves every year - proved reserves, not the "potential" discoveries and statistical extrapolations on which Washington Gas Light is happily counting. Since 1970, according to the gas industry, new additions to reserves have been running less than half that much. The country has increasingly been drawing down old reservoirs, and that is why production has been falling for the past three years. That trend won't be reserved quickly. Perhaps it won't be reserved at all. The policy issue is not whether to cut down the country's consumption of natural gas, but only where to cut it down - and at whose expense.