From the US Federal Register, Executive Order 13303:

"I, GEORGE W. BUSH, President of the United States of America, find that the threat of attachment or other judicial process against the Development Fund for Iraq, Iraqi petroleum and petroleum products, and interests therein, and proceeds, obligations, or any financial instruments of any nature whatsoever arising from or related to the sale or marketing thereof, and interests therein, obstructs the orderly reconstruction of Iraq, the restoration and maintenance of peace and security in the country, and the development of political, administrative, and economic institutions in Iraq. This situation constitutes an unusual and extraordinary threat to the national security and foreign policy of the United States and I hereby declare a national emergency to deal with that threat.
I hereby order:

Section 1. Unless licensed or otherwise authorized pursuant to this order, any attachment, judgment, decree, lien, execution, garnishment, or other judicial process is prohibited, and shall be deemed null and void, with respect to the following:

(a) the Development Fund for Iraq, and

(b) all Iraqi petroleum and petroleum products..."

As the clever folks at UnderReported.com point out, the separation of the Development fund and the oil products in the above list is deliberate.