press release

Unocal says Mad Dog appraisal well encounters significant oil pay and extends field

El Segundo, Calif., March 22, 2005 - Unocal Corporation (NYSE: UCL) today said that an appraisal well drilled on the deepwater Mad Dog Southwest Ridge in the Gulf of Mexico encountered significant hydrocarbon pay. The discovery was further delineated by three sidetracks.

The original hole was drilled to a total measured depth of 22,890 feet from Green Canyon 826 to downdip targets in the adjacent Green Canyon 825 block. The blocks are located in approximately 5,000 feet of water.

The well and sidetracks were designed to test the previously undrilled southwest flank of the field. The penetrations individually encountered up to approximately 300 feet of net oil pay in the early Miocene section and found hydrocarbons some 700 feet deeper on the west flank of the structure than previously encountered.

The resources associated with these appraisal wells could either be produced through spar development wells or future subsea tie-backs.

"We are pleased with the results of this appraisal program which has extended the limits of the Mad Dog field," said Joseph H. Bryant, Unocal president and chief operating officer.

The Mad Dog field began production in January 2005 from Green Canyon block 782. Gross field production is currently about 30,000 barrels of oil per day from two wells.

Unocal owns a 15.6 percent working interest in the Mad Dog unit, with net interest after royalty of 13.3 percent. BP is the operator with a 60.5 percent working interest, and BHP Billiton owns a 23.9 percent working interest.

The Mad Dog appraisal was drilled from the Discoverer Spirit drillship, which is under contract to Unocal. The ship has moved to drill the Knotty Head prospect on Green Canyon block 512. The ship is then scheduled to drill the St. Malo #3 appraisal well on Walker Ridge block 677.

At the completion of the St. Malo well, the ship will be subleased to another company for the remainder of its lease term. Unocal's commitment to the Discoverer Spirit terminates in September 2005.

About Unocal Corporation

Unocal is one of the world's leading independent natural gas and crude oil exploration and production companies. The company's principal oil and gas activities are in Asia and North America.

This news release contains forward-looking statements about matters such as drilling, development and other plans for future operations. Although these statements are based upon Unocal's current expectations and beliefs, they are subject to known and unknown risks and uncertainties that could cause actual results and outcomes to differ materially from those described in, or implied by, the forward-looking statements, including volatility in commodity prices; the accuracy of Unocal's estimates and judgments regarding hydrocarbon resources and formations and reservoir performance; and other factors discussed in Unocal's 2004 Annual Report on Form 10-K and subsequent reports filed by Unocal with the U.S. Securities and Exchange Commission (SEC).

Copies of Unocal's SEC filings are available from Unocal by calling 800-252-2233 or from the SEC by calling 800-SEC-0330. The reports are also available on the Unocal web site, Unocal undertakes no obligation to update the forward-looking statements in this news release to reflect future events or circumstances. All such statements are expressly qualified by this cautionary statement, which is provided pursuant to the safe harbor provisions of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934.

Updated: March 2005