Administration and oil industry sources said yesterday that the government's target of 240 million barrels of home heating oil and diesel fuel in primary storage has just about been reached, nearly three weeks ahead of the deadline set by Presidient Carter.

That news comes amid the first indications of softening of home heating oil prices on the spot markets and charges that some refiners -- particularly smaller ones -- are reprocessing and upgrading heating oil and diesel stocks into gasoline.

Several Department of Energy officials and American Petroleum Institute Director of Statistics Ed Murphy acknowledged yesterday that stockpiles have already, or soon will, exceed the 240-million-barrel goal. Actual figures on stockpiles levels today will not be available for about two weeks, they said, but given present production levels the 240-million-level would be hit right about now.

One DOE official said that most ofthe individual refineries have reported they are at or above the specific storage levels set for their companies by the government. Mobil, for example, informed DOE about a week ago that it had reached its goal.

John Buckley, president of Northeast Petroleum in Boston, one of the nation's largest home heating oil wholesalers, said that the 240-million-barrel goal "was never meant to be anything but an October inventory target," and added that he hoped the industry would keep refining at high level "until we hit about 255 to 260 million barrels."

Buckley, who also serves as the head of the fuel oil marketing advisory committee to the DOE, said "no one should be declaring a victory at 240 . . . we should be getting to 270 million barrels if we can. That's how we can put pressure on prices to drop."

In fact, Buckley said, spot prices for home heating oil and diesel fuel -- known as middle distillates -- have already dropped from a high of about 87 cents a few weeks ago to about 79 cents yesterday.

Some refiners, Buckley said, appear to be taking at least some of their middle distillate stocks, and running them through a refining process known as a "cat cracker" which up-grades them into gasoline. "They are trying to make gasoline now before OPEC prices go up again," Buckley said, "but we are watching and if their distillate production levels are slipping, we will start applying pressure."

The larger refiners do not seem to be upgrading their stocks, however.

"We have not been building up gasoline," said Gulf refining official Chuck Bowman.We have not been producing middle distillates at the expense of other products, we did it at the expense of customer demand. We held back distribution in the third quarter to meet inventory levels set by the government."

He said that at this point, however, Gulf is supplying its customers about 108 percent of what they purchased last year.

Nearly two-thirds of the heating oil needed for the winter months is traditionally not produced until after October, when the industry reaches peak storage. There is still some concern that a premature drop in distillate production could result in problems during a particularly harsh winter.