A high-ranking Energy Department lawyer says a key provision in a proposed settlement of federal overcharge claims against the Mobil Oil Corp. is "unacceptably generous" because it would make it "virtually impossible" to press civil actions against Mobil as a result of "certain other investigations" of oil resellers still in progress, according to government documents.

The provision is contained in a side letter to a proposed consent order that would allow Mobil to settle for $27 million charges that it claimed excessive costs totaling $920 million to justify prices it charged customers between Jan. 1, 1973, and Jan. 27, 1981, while oil was still under federal price controls.

Excessive cost claims, another Energy Department source noted, are not the same as overcharges. The source said at a maximum Mobil might have overcharged customers $100 million as a result of the cost claims. The proposed settlement thus would appear to be in line with the Reagan administration's stated intent to try to recover 10 to 30 cents on the dollar on overcharges.

The side letter, however, provides that Mobil also would be exempted from any civil action resulting from contacts it may have had with 12 crude oil resellers currently being investigated by the Energy Department unless a criminal conviction is obtained stemming from those contacts and all appeals are exhausted.

Nancy C. Crisman, the Energy Department's assistant general counsel for regulatory litigation, said in a Jan. 12 memorandum to Thomas C. Newkirk, deputy general counsel for enforcement and litigation, that she thought "the added provision was unacceptably generous because it made it virtually impossible to prosecute Mobil for reseller violations." She said, however, she had been informed by another government attorney in the case that "the provision is a 'deal-breaker'--that is, Mobil will not sign the consent order without the provision."

Neither Crisman nor Newkirk could be reached yesterday for comment. An Energy Department source, however, said that in settling two dozen other cases involving overcharge claims against oil companies the government had not previously agreed to a provision like that in the side letter.

Milton C. Lorenz, the Energy Department special counsel for the overcharge cases, said the Mobil case was "still an unsettled matter."

"There still are some additional negotiations that have to go on regarding certain provisions of the consent order," he said.

Lawmakers who have been highly critical of the Reagan administration's handling of these overcharge cases, however, expressed particular concern over the proposed Mobil settlement yesterday.

Chairman John D. Dingell (D-Mich.) of the House Energy Committee and Rep. Albert Gore Jr. (D-Tenn.), ranking majority member of the House Energy subcommittee on investigations, sent Energy Secretary Donald P. Hodel a letter urging him not to sign the proposed settlement.

"These settlements are getting more outrageous as they go along," Gore said. "This is for less than three cents on the dollar plus the added sweetener of suspending all enforcement actions against Mobil for its controversial crude reseller transactions. I think this constitutes a sham against the public."

"The claim that we might settle $920 million in claims for $27 million is totally inaccurate," said Herbert Schmertz, vice president of public affairs for Mobil. "In addition to the $920 million, we had an excess of $1 billion of unrecovered costs on our books and if any court had disallowed any of the $920 million, it would have been offset by the additional $1 billion in costs."

Congressional sources also expressed concern that the proposed Mobil settlement could set a precedent for settling $7 billion in alleged cost-claim violations involving 11 other major oil companies, including Exxon Corp. and Texaco Inc.