Mitsui & Co. and Mitsui Oil Exploration Co. said Wednesday they have agreed with Finnish energy company Fortum Oyj to acquire its subsidiary for about 20 billion yen to obtain concession rights in Oman's second-largest oil field.

The two firms said they signed a contract Tuesday to acquire Fortum (E&P) BV. Fortum's wholly owned subsidiary holds a nonoperating 35 percent participating interest in the oil-producing Block 9 of the Middle Eastern country.

Mitsui will acquire 60 percent and Mitsui Oil 40 percent of the Dutch subsidiary's shares. They will jointly run the subsidiary and plan to export its equity oil mainly to Japan.

Block 9, a 3,909-sq.-km oil field some 300 km west of Muscat, is operated by Occidental Petroleum Corp. of the United States, which holds the remaining 65 percent interest.

With the agreement, the amount of Mitsui's oil and gas reserves will top 1.5 billion barrels, making it one of the largest oil and gas holders among Japanese trading houses.

The agreement is contingent on various factors, including the Omani government's approval, the two companies said.