The Department of Energy yesterday claimed the Exxon Co., in a 33-month period, had overcharged refiners $70.8 million for crude oil produced in two fields in Alabama and Florida.

In charging Exxon with what its lawyers called "one of the largest single overcharges" on crude oil sales in the United States, the Energy Department said Exxon violated "applicable ceiling prices" established by the federal government for the period from March 1, 1974, to Dec. 31, 1976.

The charge against Exon covers oil the company produced from onshore fields in Santa Rosa and Escambia Counties in the Florida Panhandle and in neighboring Escambia County in Alabama.

The notice of "probable violation" of overcharge was issued by the Energy Departments Economic Regulatiory Administration, which said that Exxon "miscalculated" the price it could charge for oil produced in its two fields during the period mentioned.

Under Energy Department regulations, a producer of domestic crude oil could qualify in the questioned period for the higher world oil price for oil it produced above and beyond what it had been producing in these domestic oilfields before 1972.

The notice issued yesterday alleged that Exxon said it was producing less oil from the two fields before 1972 than the Energy Department says it was producing. The Energy Department said it had run an "intensified" audit of Exxon's production figures to arrive at his allegation.

Energy Department lawyers said the overcharge "was by no means intentional," declaring that it was "not an irrational" oversight by Exxon. Lawyers said the overcharge involved about $6 a barrel for more than 11 million barrels of oil.

Lawyers said that 75 per cent of the overcharge involved the sale of crude oil by Exxon to its own refineries. The rest came from sale of oil to other major oil refineries and independent refiners, none of which was named in the notice of overcharge.

The notice was the first one issued by the Energy Department as part of an intensified audit of 34 oil companies begun by the Office of Special Counsel, which was activated by orderof Energy Secretary James R. Schlesinger Jr. Dec. 4.