The Nigerian government is demanding that the three largest foreign oil companies operating in that West African country pay a $6.5 billion fine for allegedly leaving 183 million barrels of oil in the ground instead of pumping it out for marketing by the state-owned petroleum firm.

A government report released Thursday at the end of a two-month investigation accussed Shell, Mobil, and Gulf of contributing to a loss of oil revenues between 1975 and 1978.

Under informal arrangements, the companies were to produce oil that would go to the government's Nigerian National Petroleum Corp. for marketing.

The legal status of the report was not immediately clear. Nigerian sources suggested last month that they expected the matter to go to court if the companies dispute the government's interpretation.

There were no indications yesterday that Nigeria's efforts to fine the companies would have any impact on oil exports to the United States. Nigeria is the second supplier of oil to the United States after Saudi Arabia. America consumes half of Nigeria's daily 2 million barrel production.

Of that production, 60 percent is by Shell, 15 percent by Gulf and 16 percent by Mobil in combination with Phillips. The remaining production is spread among seven other companies.

The government ordered the investigation to remove a cloud of suspicion that has hung over the Nigerian National Petroleum Corp., and the former military government, since allegations of a missing $5 billion in government oil revenues surfaced in March.

A Gulf spokesman confirmed a report yesterday from Lagos by Agence France-Presse about the report's findings. The Gulf official said the company's Nigeria-based managers were served yesterday with papers outlining the findings of the investigation.

The Gulf spokesman said the managers do not know how the Nigerian government arrived at the $6.5 billion figure, why the companies are being fined or how much each of the companies is supposed to pay.

"I don't have any information on what the basis of the calculations might be," he said.

Officials of Shell and Mobil could not be reached for comment.

The five-member investigative committee appointed by Nigerian President Shehu Shagari concluded in their report released Thursday that the $5 billion had never been missing and that the allegations arose from Nigerian journalists' misinterpretation of an auditor's report. Shagari, elected last August, succeeded the military government in October.

During the public hearings held by the investigative committee, an outside expert alleged that the government's petroleum corporation did not have sufficient technical staff to monitor the production of foreign oil companies and that some of them were juggling figures for their own gain.

The committee's report said investigation revealed that Shell, Gulf and Mobil had never signed any formal contractual agreement on oil production with the Nigerian government.

The committee charged that from 1975 to 1978, about 183 million barrels of production were withheld from the Nigerian corporation without its knowledge.

The government further charged Shell, Gulf and Mobil with having collected operation fees from the state for the 183 million barrels of oil when, in fact, that oil never was produced. The committee added that if the oil had been pumped from the ground, Nigeria would have earned $2.5 billion.

The average price of the Nigerian oil during the three-year period was $13.61 a barrel, but the report states that nigeria should be compensated at the price of $35.90 a barrel. The current cost of Nigerian crude is $37 a barrel.