FIRST AMENDMENT TO THE FIRST AMENDED AND RESTATED AGREEMENT OF LIMITED PARTNERSHIP OF RIO VISTA OPERATING PARTNERSHIP L.P.

This First Amendment to the First Amended and Restated Agreement of Limited Partnership of Rio Vista Operating Partnership L.P., dated as of October 26, 2005 (this “First Amendment”), is entered into by and between Rio Vista Operating GP LLC, a Delaware limited liability company, as the General Partner, and Rio Vista Energy Partners L.P., a Delaware limited partnership, as the sole Limited Partner, of Rio Vista Operating Partnership L.P., a Delaware limited partnership (the “Operating Partnership”). Capitalized terms used, and not otherwise defined, herein shall have the same meaning as set forth in the First Amended and Restated Agreement of Limited Partnership of the Operating Partnership dated as of September 16, 2004 (the “Operating Partnership Agreement”).

WHEREAS, the General Partner and the Limited Partner are party to the Operating Partnership Agreement; and

WHEREAS, in accordance with Section 13.2 of the Operating Partnership Agreement, the General Partner and the Limited