Exhibit 10.1
 TCW ASSET MANAGEMENT COMPANY  865 South Figueroa Street, Suite 1800  Los Angeles, Califorina 90017
  
 Re:
    
    Note Purchase Agreement, dated as of November 19, 2007 (as amended,
supplemented or otherwise modified, the Note Purchase Agreement), by and among RIO
VISTA PENNY LLC, an Oklahoma limited liability company (Company), the Holders party
thereto, and TCW ASSET MANAGEMENT COMPANY, as Administrative Agent.
    [Letter Agreement]




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To induce Holder Parties to enter into this Letter, Company (and each other Restricted Person
by execution of the attached Consent and Reaffirmation) hereby (a) represents and warrants that
as of the date of this Letter there are no claims or offsets against or defenses or counterclaims
to its obligations under the Note Documents, and waives any and all such claims, offsets,
defenses, or counterclaims, whether known or unknown, arising prior to the date of this Letter,
(b) releases and forever discharges the Released Persons from any and all Released Claims, and (c)
covenants not to assert (and not to assist or enable any other Person to assert) any Released Claim
against any Released Person. The Restricted Persons acknowledge and agree that such release is a
general release of any and all Released Claims that constitutes a full and complete satisfaction
for all or any alleged injuries or damages arising out of or in connection with the Released
Claims, all of which are herein compromised and settled.
 Released Claims
 Released Persons
 [Letter Agreement]




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THIS LETTER AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES HEREUNDER SHALL BE GOVERNED BY, AND
SHALL BE CONSTRUED AND ENFORCED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK WITHOUT
REGARD TO CONFLICT OF LAWS PRINCIPLES THEREOF.
    [Letter Agreement]




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Yours truly,TCW ASSET MANAGEMENT COMPANY,as
Administrative AgentBy:
    /s/ Patrick HickeyName:
    Patrick HickeyTitle:
    SVPBy:
    /s/ Curt S. TaylorName:
    Curt S. TaylorTitle:
    SVP [Letter Agreement]




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HOLDERS:
 TCW ENERGY FUND X  NL, L.P.,
  By:
   TCW Asset Management Company,its Managing Member By:
    /s/ Patrick HickeyName:
    Patrick HickeyTitle:
    SVP By:
    /s/  Curt S. TaylorName:
    Curt S. TaylorTitle:
    SVP TCW ENERGY FUND XB  NL, L.P.,
  By:
    TCW Asset Management Company,its Managing Member By:
    /s/ Patrick HickeyName:
    Patrick HickeyTitle:
    SVP By:
    /s/ Curt S. TaylorName:
    Curt S. TaylorTitle:
    SVP [Letter Agreement]




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TCW ASSET MANAGEMENT COMPANY,a
California corporation, as Investment Manager under
the Amended and Restated Investment Management and
Custody Agreement dated as of December 3, 2003 among
Ensign Peak Advisors, Inc., TCW Asset Management
Company and Trust Company of the West, a California
trust company, as Sub-CustodianBy:
    /s/ Patrick HickeyName:
    Patrick HickeyTitle:
    SVP By:
    /s/ Curt S. TaylorName:
    Curt S. TaylorTitle:
    SVP TCW ASSET MANAGEMENT COMPANY,a
California corporation, 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, TCW
Asset Management Company and Trust Company of the
West, a California trust company, as Sub-CustodianBy:
    /s/ Patrick HickeyName:
    Patrick HickeyTitle:
    SVP By:
    /s/ Curt S. TaylorName:
    Curt S. TaylorTitle:
    SVP [Letter Agreement]




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TCW ENERGY FUND XC  NL, L.P.,
  By:
    TCW Asset Management Company,its Managing Member By:
    /s/ Patrick HickeyName:
    Patrick HickeyTitle:
    SVP By:
    /s/ Curt S. TaylorName:
    Curt S. TaylorTitle:
    SVP TCW ENERGY FUND XD  NL, L.P.,
  By:
    TCW Asset Management Company,its Managing Member By:
    /s/ Patrick HickeyName:
    Patrick HickeyTitle:
    SVP By:
    /s/ Curt S. TaylorName:
    Curt S. TaylorTitle:
    SVP [Letter Agreement]




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TCW ASSET MANAGEMENT COMPANY, a California
corporation, 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, TCW Asset Management
Company and Trust Company of the West, a California
trust company, as Sub-CustodianBy:
    /s/ Patrick HickeyName:
    Patrick HickeyTitle:
    SVP By:
    /s/ Curt S. TaylorName:
    Curt S. TaylorTitle:
    SVP WARRANT OWNER:
 ENERGY FUNDS X HOLDINGS, L.P.,
  By:
    TCW Asset Management Company,its   Managing Member By:
    /s/ Patrick HickeyName:
    Patrick HickeyTitle:
    SVP By:
    /s/ Curt S. TaylorName:
    Curt S. TaylorTitle:
    SVP [Letter Agreement]




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RIO VISTA PENNY LLC
 By:
    
    /s/ Ian BothwellIan BothwellManager
 [Letter Agreement]




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CONSENT AND REAFFIRMATION
  RIO VISTA ECO LLCBy:
    /s/ Ian BothwellIan BothwellManagerRIO VISTA GO LLCBy:
    /s/ Ian BothwellIan BothwellManagerGO, LLCBy:
    /s/ Ian BothwellIan BothwellManagerMV PIPELINE COMPANYBy:
    /s/ Ian BothwellIan BothwellPresident [Letter Agreement]




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RIO VISTA OPERATING LLCBy:
    /s/ Ian BothwellIan BothwellManager RIO VISTA ENERGY PARTNERS L.P.
  By:

/s/ Ian BothwellIan BothwellActing CEO [Letter Agreement]