Leading oil distributor Nippon Oil Corp. and two other Japanese firms likely will conclude a basic contract with the Iraqi government as early as this month to obtain the right to develop the massive Nasiriyah oil field in southern Iraq, it has been learned.

Negotiations on final details may begin as early as Sunday.

The other Japanese companies involved are energy supply firm Inpex Corp. and plant engineering firm JGC Corp. Talks are expected to focus on the purchase price of crude oil from the field.

The Japanese side is proposing a system through which the Japan Bank for International Cooperation would lend the Iraqi government funding to develop the field, and Iraq would use money from sales of crude oil to repay the debt.

The negotiations also are expected to touch on such subjects as making it possible to extend the initial two-year period of the contract in certain cases, so as to ensure a certain level of crude oil imports to Japan. Participants also are expected to discuss the question of responsibility for losses should development of the field not proceed according to plan.

The Italian firm that Iraq had drawn to the negotiations over developing the Nasiriyah field effectively has withdrawn, and the Japan side felt it should be able to achieve a contract unless there was some considerable conflict over terms.

After the basic contract is concluded, leaders of the Japanese firms will travel to Iraq to sign the final contract.

The area scheduled to be developed by Japan is expected to produce a maximum of 600,000 barrels a day in the future. This would cover more than 10 percent of Japan's consumption of crude oil.

The Nasiriyah field is likely to become the largest independently developed oil field in this country's history and a turning point for its energy security.