RIO VISTA ENERGY PARTNERS L.P. COMMON UNIT PURCHASE WARRANT

Exhibit 10.73

THIS WARRANT HAS NOT BEEN REGISTERED OR QUALIFIED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”), OR THE SECURITIES LAWS OF ANY STATE (THE “STATE LAWS”). THIS WARRANT MAY NOT BE SOLD, ASSIGNED, TRANSFERRED, ENCUMBERED OR OTHERWISE DISPOSED OF, IN WHOLE OR IN PART, IN THE ABSENCE OF SUCH REGISTRATION OR QUALIFICATION OR THE AVAILABILITY OF AN APPLICABLE EXEMPTION FROM THE REGISTRATION AND QUALIFICATION REQUIREMENTS OF SUCH ACT AND STATE LAWS EVIDENCED BY AN OPINION OF LEGAL COUNSEL, WHICH OPINION AND LEGAL COUNSEL ARE SATISFACTORY TO THE PARTNERSHIP.

RIO VISTA ENERGY PARTNERS L.P.

COMMON UNIT PURCHASE WARRANT

This Warrant certifies that, for value received, TCW Energy Funds X Holdings, L.P., a California limited partnership, or its registered assigns (“Holder”) , is entitled to purchase from Rio Vista Energy Partners L.P., a Delaware limited partnership (the “Partnership” ):

(a) at any time or from time to time after May 19, 2008 until such time as the Demand Loan (the “Demand Loan” ) under the Note Purchase Agreement dated as of November 19, 2007 (the “Note Purchase Agreement”) by and among Rio Vista Penny LLC (“Rio Vista Penny”) , TCW Asset Management Company, a California corporation ( “TAMCO” ), TCW Energy Fund X-NL, L.P., a California limited partnership, TCW Energy Fund XB-NL, L.P., a California limited partnership, TCW Energy Fund XC-NL, L.P., a California limited partnership, TCW Energy Fund XD-NL, L.P., a California limited partnership, TAMCO, as Investment Manager under the Amended and Restated Investment Management and Custody Agreement dated as of December 3, 2003 among Ensign Peak Advisors, Inc., TAMCO, and Trust Company of the West ( “TCW” ), a California trust company, as Sub-Custodian; TAMCO as Investment Manager under the Amended and Restated Investment Management and Custody Agreement dated as of March 18, 2004 among ING Life Insurance and Annuity Company, TAMCO, and TCW as Sub Custodian; and TAMCO as Investment Manager under the Amended and Restated Investment Management and Custody Agreement dated as of December 11, 2003 among Harry L. Bradley, Jr. Partition Trust, Harry L. Bradley, Jr. Trust, Jane Bradley Uihlien Pettit Partition Trust, Jane Bradley Uihlien Trust, TAMCO, and TCW as Sub-Custodian, (such entities, the “Note Holders” ) is paid in full (the “Tranche 1 Exercise Period”) , that number of fully paid and non-assessable common units of the Partnership derived by dividing (i) the amount of outstanding principal owed by Rio Vista Penny to the Note Holders pursuant to the Demand Loan, by (ii) the Tranche 1 Exercise Price (defined below), at an exercise price equal to the lower of (A) $13.33 per common unit (the “Fixed Tranche 1 Price” ) and (B) (x) 90% multiplied by (y) the average Market Price for the 20 trading days preceding the date of Holder’s election to exercise this Warrant for such Tranche 1 Warrant Units pursuant to Section 1 (such lower price, the “Tranche 1 Exercise Price”) , and

(b) at any time or from time to time after November 19, 2008 until such time as the outstanding amounts under the Note Purchase Agreement (other than the Demand Loan) are paid in full (the “Tranche 2 Exercise Period”), that number of fully paid and non-assessable common units of the Partnership derived by dividing (i) the amount of outstanding principal, accrued interest, and expenses owed by Rio Vista Penny to Holder pursuant to the Note Purchase Agreement, less the amount of outstanding principal under the Demand Loan, on the date of Holder’s election to exercise this Warrant for such Tranche 2 Warrant Units pursuant to Section 1, by (ii) the Tranche 2 Exercise Price (defined below) for an exercise price (the “Tranche 2 Exercise Price” ) equal to (x) 90% multiplied by (y) the average Market Price for the 20 trading days preceding the date of Holder’s election to exercise this Warrant for such Tranche 2 Warrant Units pursuant to Section 1 . As used herein, the term “Warrant Units” refers to the common units of the Partnership issuable upon exercise of this Warrant, the term “Tranche 1 Warrant Units” refers to the common units of the Partnership issuable upon the exercise of this Warrant during the Tranche 1 Exercise Period, and the term “Tranche 2 Warrant Units” refers to the common units of the Partnership issuable upon the exercise of this Warrant during the Tranche 2 Exercise Period. The Fixed Tranche 1 Price and number of Tranche 1 Warrant Units is subject to adjustment as provided in Section 3 hereof.

For the purposes of this Warrant, the “Market Price” on any day shall be the last sale price on such day on the NASDAQ Stock Market, or, if the common units are not then listed or admitted to trading on the NASDAQ Stock Market, on such other principal stock exchange on which such units are then listed or admitted to trading, or, if no sale takes place on such day on any such exchange, the average of the closing bid or asked prices on such day as officially quoted on any such exchange, or, if the common units are not then listed or admitted to trading on any stock exchange, the average of the reported closing bid and asked prices on such day in the over-the-counter market as quoted on the National Association of Securities Dealers Automated Quotation System or, if not so quoted, then as furnished by any member of the National Association of Securities Dealers, Inc. selected by the Partnership. If there shall be no meaningful over-the-counter market, the Market Price shall be the fair market value as determined solely by the general partner of the Partnership (with such fair market value being equal to or in excess of the book value).

This Warrant, together with all warrants issued upon transfer, exchange or in replacement hereof pursuant to Section 7 hereof (collectively, the “Warrants”), is subject to the following additional terms, provisions, and conditions:

Section 1. Manner of Exercise; Issuance of Certificates; Payment for Warrant Units . Subject to the provisions hereof, this Warrant may be exercised by Holder, in whole or in part, by the surrender of this Warrant, together with a completed Exercise Agreement in the form attached hereto, to the Partnership during normal business hours on any business day at the Partnership’s office in El Segundo, California (or such other office or agency of the Partnership as it may designate by notice to Holder), during, with respect to the Tranche 1 Warrant Units, the Tranche 1 Exercise Period, and with respect to the Tranche 2 Warrant Units, the Tranche 2 Exercise Period, and upon payment to the Partnership of the Tranche 1 Exercise Price for the Tranche 1 Warrant Units or the Tranche 2 Exercise Price for the Tranche 2 Warrant Units specified in said Exercise Agreement. Payment of the Tranche 1 Exercise Price shall be made by Holder by reducing the outstanding principal owed by Rio Vista Penny to the Note Holders under the Demand Loan by the amount of such payment. Payment of the Tranche 2 Exercise Price shall be made by Holder by reducing the outstanding principal (other than pursuant to the Demand Loan), accrued interest, and expenses owed by Rio Vista Penny to the Note Holders pursuant to the Note Purchase Agreement by the amount of such payment.

The Partnership shall not be required to issue fractional Warrant Units upon any exercise of this Warrant, but instead the value of any such fractional Warrant Units shall remain outstanding under the Note Purchase Agreement. The Warrant Units so purchased shall be deemed to be issued to Holder or its designees as the record owner of such shares as of the close of business on the date or dates on which this Warrant shall have been surrendered, the completed Exercise Agreement delivered, and payment made for such Warrant Units as aforesaid. Certificates for the Warrant Units so purchased, representing the aggregate number of common units specified in said Exercise Agreement, shall be delivered to Holder within a reasonable time, not exceeding ten business days, after this Warrant shall have been so exercised.

The Warrant Units so delivered shall be in such denominations as may be reasonably requested by Holder, shall be listed for trading on any NASDAQ Stock Market (or such other exchange as the common units are then listed or traded), and shall be registered in the name of Holder or such other name as shall be designated by Holder. The Partnership will file a registration statement on Form S-3 (the “Registration Statement”) under the Securities Act of 1933, as amended (the “Securities Act” ) covering the Warrant Units within 90 days following the first exercise of this Warrant and will use commercially reasonable efforts to cause the Registration Statement to be declared effective as soon as practicable thereafter. The Partnership will maintain the effectiveness of the Registration Statement until the Warrant Units have been resold pursuant to the Registration Statement or are eligible for resale, without restriction, pursuant to Rule 144 of the Securities Act.

Holder will cooperate with the Partnership as reasonably requested by the Partnership in connection with the preparation and filing of the Registration Statement. Holder will furnish to the Partnership such information regarding itself, the common units held by it, and the intended method of disposition of such common units as shall be reasonably required to cause the effectiveness of the Registration Statement and will execute and deliver such documents in connection with the Registration Statement as the Partnership may reasonably request. Holder will, upon receipt of notice from the Partnership of any event requiring suspension of the use of the prospectus included as part of the Registration Statement, immediately discontinue disposition of common units pursuant to the Registration Statement until Holder’s receipt of the copies of the supplemented or amended prospectus or receipt of notice that no supplement or amendment is required. Holder covenants and agrees that it will comply with the prospectus delivery requirements of the Securities Act as applicable to it or comply with the provisions of an exemption from such prospectus delivery requirements in connection with sales of common units pursuant to the Registration Statement.

With respect to the Tranche 1 Warrant Units, if, on the date the Registration Statement is declared effective by the Securities and Exchange Commission and the Warrant Units are not subject to contractual restriction on resale (the “Registration Date” ), the Market Price (the “Registration Date Price” ) is less than the Market Price on the date of such exercise of this Warrant for such Warrant Units (the “Exercise Date Price” ), the Partnership and Rio Vista Penny jointly agree to deliver to Holder either (i) additional common units of the Partnership (the “Additional Units” ) in such number as necessary so that the aggregate value (based on the Registration Date Price) of (A) the Tranche 1 Warrant Units issued to Holder in connection with such exercise plus (B) the Additional Units, equals the value (based on the Exercise Date Price) of the Tranche 1 Warrant Units issued to Holder in connection with such exercise, or (ii) cash (the “Additional Cash” ) in such amount as necessary so that the aggregate value of (A) the Tranche 1 Warrant Units (based on the Registration Date Price) issued to Holder in connection with such exercise plus (B) the Additional Cash, equals at least the value (based on the Exercise Date Price) of the Tranche 1 Warrant Units issued to Holder in connection with such exercise. In lieu of delivery of Additional Units or Additional Cash as provided above, the Partnership and Rio Vista Penny may elect to pay cash to Holder equal to the aggregate value (based on the Exercise Date Price) of the Tranche 1 Warrant Units issued to Holder in connection with such exercise (the “All Cash Payment” ). Upon delivery of the All Cash Payment to Holder, all Tranche 1 Warrant Units issued to Holder in connection with such exercise shall be returned to the Partnership and/or cancelled by the Partnership.

With respect to the Tranche 2 Warrant Units, if, on the Registration Date, the Registration Date Price is less than eighty percent (80%) of the Exercise Date Price, the Partnership and Rio Vista Penny jointly agree to deliver to Holder either (i) Additional Units in such number as necessary so that the aggregate value (based on the Registration Date Price) of (A) the Tranche 2 Warrant Units issued to Holder in connection with such exercise plus (B) the Additional Units, equals at least eighty percent (80%) of the value (based on the Exercise Date Price) of the Warrant Units issued to Holder in connection with such exercise, or (ii) Additional Cash in such amount as necessary so that the aggregate value of (A) the Tranche 2 Warrant Units (based on the Registration Date Price) issued to Holder in connection with such exercise plus (B) the Additional Cash, equals at least eighty percent (80%) of the value (based on the Exercise Date Price) of the Tranche 2 Warrant Units issued to Holder in connection with such exercise. In lieu of delivery of Additional Units or Additional Cash as provided above, the Partnership and Rio Vista Penny may elect to make an All Cash Payment to Holder equal to the aggregate value (based on the Exercise Date Price) of the Tranche 2 Warrant Units issued to Holder in connection with such exercise. Upon delivery of the All Cash Payment to Holder, all Tranche 2 Warrant Units issued to Holder in connection with such exercise shall be returned to the Partnership and/or cancelled by the Partnership.

The Partnership shall pay all taxes and other expenses and charges payable in connection with the preparation, execution, and delivery of unit certificates pursuant to this Section 1 except that, in case such unit certificates shall be registered in a name or names other than Holder of this Warrant, funds sufficient to pay all transfer taxes which shall be payable in connection with the execution and delivery of such unit certificates shall be paid by Holder to the Partnership at the time of the delivery of such unit certificates by the Partnership as mentioned above.

Section 2. Certain Actions Prohibited . The Partnership will not, by amendment of certificate of limited partnership or through any reorganization, transfer of assets, consolidation, merger, dissolution, issue or sale of securities, or any other voluntary action, avoid or seek to avoid the observance or performance of any of the terms to be observed or performed by it hereunder, but will at all times in good faith assist in the carrying out of all the provisions of this Warrant and in the taking of all such action as may reasonably be requested by Holder of this Warrant in order to protect the exercise privilege of Holder of this Warrant against dilution or other impairment, consistent with the tenor and purpose of this Warrant.

Section 3. Anti-dilution Provisions . The Fixed Tranche 1 Price and the number of Tranche 1 Warrant Units shall be subject to adjustment from time to time as provided in this Section 3 .

(a)  Adjustment for Change in Capital . If at any time after the date hereof, the Partnership (i) pays a dividend or makes a distribution on it common units in common units; (ii) subdivides its outstanding common units into a greater number of common units, (iii) combines its outstanding common units into a smaller number of common units, (iv) makes a distribution on its common units in ownership interests other than common units, or (v) issues by reclassification of its common units any ownership interests of the Partnership, then the Fixed Tranche 1 Price in effect immediately prior to such action shall be adjusted so that Holder may receive upon exercise or exchange of this Warrant and payment of the same aggregate consideration, the number of common units of the Partnership which Holder would have owned immediately following such action if Holder had exercised or exchange the Warrant for the Tranche 1