London, Dec 23, IRNA -- The National Iranian Oil Company (NIOC) has retained its position as the world's fourth top oil company, according to the latest annual ranking by the Petroleum Intelligence Weekly (PIW).

The rankings for the first time show private sector oil firms dominating the top 10 positions following a series of mergers that began building up in the late 1990s.

State-owned Saudi Aramco remains in first place, but Exxon Mobil of the US moved up to second, overtaking Petroleos de Venezuela (PDV), which falls back to third position.

The rankings, based on six operational criteria, place NIOC third among the top upstream companies. For oil and gas reserves, it is listed as fourth and second, respectively, but for output, second and sixth, respectively.

In downstream measures, NIOC is ranked 14th for refining capacity and 12th for oil product sales. Its 114,602 employees is the seventh highest, far above Saudi Aramco, Exxon Mobil and PDV, but well below the 510,000 employed by Sinopec, 4222,554 by PetroChina and 306,300 by Gazprom of Russia.

As a result of the mega-mergers, the most significant changes in the top 50 oil companies are by ConocoPhillips, up eight places to 13th, ChevronTexaco, up six places to eighth, and EnCana, up 22 to 41st.

Private sector firms also dominate downstream activities, led respectively by Exxon Mobil, BP and Royal Dutch Shell in the top three positions for both refinery capacity and oil product sales.