Exhibit 10.1





Exhibit 10.1




TCW ASSET MANAGEMENT COMPANY

865 South Figueroa Street, Suite 1800

Los Angeles, Califorina 90017



April 13, 2009

Rio Vista Penny LLC
 2601 Northwest
Expressway #902E
Oklahoma City, Oklahoma
93112
 Attention: Ian Bothwell





	 	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.






Gentlemen:



Reference is made to the Note Purchase Agreement. Terms that are defined in the Note Purchase
Agreement and not otherwise defined herein are used herein with the meanings given them in the
Note Purchase Agreement.



Company has requested that Holders extend the required payments of principal and accrued
interest that are due and payable on April 13, 2009 and also to extend the date to deliver the
Engineering Report effective as of November 1, 2008, in each case to May 20, 2009. Accordingly, in
reliance upon the representations, warranties, covenants, and waivers of the Restricted Persons
contained in this Letter, and subject to the terms and conditions of this Letter, Administrative
Agent and Holders hereby agree that (i) the mandatory principal payment in respect of the Notes due
and payable on the December 2008 Quarterly Payment Date and the March 2009 Quarterly Payment Date
pursuant Section 2.8(a) of the Note Purchase Agreement is hereby extended to May 20, 2009 (which
was previously extended to April 13, 2009 by letter agreement dated March 23, 2009 among the
parties hereto with respect to the mandatory principal payment due on the December 2008 Quarterly
Payment Date), (ii) the required payment of accrued interest in respect of the Notes due and
payable on the December 2008 Quarterly Payment Date and March 2009 Quarterly Payment Date is hereby
extended to May 20, 2009 (which was previously expended to April 13, 2009 by letter agreement dated
March 23, 2009 among the parties hereto with respect to the required payment of accrued interest
due on the December 2008 Quarterly Payment Date), and (iii) the delivery of the Engineering Report
to be effective as of November 1, 2008 that was due prior to December 1, 2008 pursuant to Section
7.2(i) of the Note Purchase Agreement is hereby extended to May 20, 2009 (which was previously
extended to April 13, 2009 by letter agreement dated March 23, 2009 among the parties hereto).



[Letter Agreement]



 


 









 





Rio Vista Penny LLC

April 13, 2009

Page 2




In order to induce Holder Parties to enter into this Letter, Company represents and warrants
to each Holder Party that the representations and warranties contained in Article V of the Note
Purchase Agreement are true and correct at and as of the time of the effectiveness hereof, except
to the extent such representations and warranties specifically refer to an earlier date, in which
case they are true and correct as of such earlier date. Company hereby further acknowledges,
confirms and agrees (i) that as of the close of business on April 13, 2009, Company is indebted to
Holders in respect of the Loans in the aggregate principal amount of $24,700,000.00 and in respect
of accrued but unpaid interest related thereto in the amount of $1,689,548,61 and (ii) that the
amount of such accrued interest shall be recalculated on all outstanding Loans at the Default Rate
from December 30, 2008 if such accrued interest is not paid in full on or before May 20, 2009. All
such Loans, together with interest accrued and accruing thereon, and fees, costs, expenses and
other charges now or hereafter payable by Company to Administrative Agent and Holders under the
Note Documents, are unconditionally owing by Company to Administrative Agent and Holders, without
offset, defense or counterclaim of any kind, nature or description whatsoever. Company (and each
other Restricted Person by execution of the attached Consent and Reaffirmation) hereby
acknowledges, confirms and agrees that Administrative Agent and Holders have and shall continue to
have valid, enforceable and perfected first-priority Liens in the Collateral heretofore granted to
Administrative Agent and Holders pursuant to the Note Documents or otherwise granted to or held by
such Persons.


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. As used in this paragraph, “Released
Claims” and “Released Persons” mean:




“Released Claims” means any and all actions, causes of action, judgments,
executions, suits, debts, claims, demands, controversies, liabilities, obligations,
damages and expenses of any and every character (whether known or unknown, liquidated or
unliquidated, absolute or contingent, acknowledged or disputed, direct or indirect), at
law or in equity, of whatsoever kind or nature (including claims of usury), whether
heretofore or hereafter accruing, for or because of any matter or things done, omitted or
suffered to be done by any of the Released Persons prior to and including the date hereof
that in any way directly or indirectly arise out of or in any way are connected to (a) any
of the Note Documents or any default or event of default thereunder, (b) any negotiation,
discussion, enforcement action, agreement or failure to agree related to any Note Document
or any default or event of default thereunder, or (c) any action, event, occurrence, or
omission otherwise related to the rights, duties, obligations and relationships among the
various Restricted Persons and Holder Parties.




“Released Persons” means Administrative Agent, Holders, and Royalty Owner, together
with their respective employees, agents, attorneys, officers, partners, shareholders,
accountants, consultants, and directors, and their respective successors and assigns.



[Letter Agreement]



 


 









 





Rio Vista Penny LLC

April 13, 2009

Page 3




The Note Purchase Agreement and the other Note Documents are hereby ratified and confirmed in
all respects. Except as expressly set forth above, the execution, delivery and effectiveness of
this letter shall not operate as a waiver of any right, power or remedy of Administrative Agent or
Holders under the Note Purchase Agreement or any other Note Document, nor constitute a waiver of
any provision of the Note Purchase Agreement or any other Note Document.



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.



This Letter may be executed in any number of counterparts, each of which when so executed and
delivered shall be deemed an original, but all such counterparts together shall constitute but one
and the same instrument. This Letter may be executed by facsimile signature and all such
signatures shall be effective as originals.



This Letter is a “Note Document”, as defined in the Note Purchase Agreement, and, except as
expressly provided herein to the contrary, this Letter is subject to all provisions of the Note
Purchase Agreement governing such Note Documents.



Please execute a counterpart of this Letter in the place provided below to evidence your
agreement to the foregoing and your continuing ratification of the Note Purchase Agreement and the
other Note Documents in consideration hereof, whereupon this Letter shall become effective as of
the date first written above.



[Letter Agreement]



 


 









 





Rio Vista Penny LLC

April 13, 2009

Page 4




	 	 	 	 	 
	 	Yours truly,



TCW ASSET MANAGEMENT COMPANY, as

Administrative Agent

 	 
	 	By:  	/s/ Patrick Hickey
 	 
	 	 	Name:  	Patrick Hickey 	 
	 	 	Title:  	SVP 	 
	 	 	 
	 	By:  	
/s/ Curt S. Taylor
 	 
	 	 	Name:  	Curt S. Taylor 	 
	 	 	Title:  	SVP 	 



[Letter Agreement]









 


 









 







Rio Vista Penny LLC

April 13, 2009

Page 5




HOLDERS:




TCW ENERGY FUND X — NL, L.P.,

a California limited partnership




By: TCW (ENERGY X), LLC, its General Partner:




	 	 	 	 	 
	 	By:  	TCW Asset Management Company,
 	 
	 	 	 its Managing Member 	 



	 	 	 	 	 
	 	By:  	/s/ Patrick Hickey
 	 
	 	 	Name:  	Patrick Hickey 	 
	 	 	Title:  	SVP 	 



	 	 	 	 	 
	 	By:  	/s/  Curt S. Taylor
 	 
	 	 	Name:  	Curt S. Taylor 	 
	 	 	Title:  	SVP 	 



TCW ENERGY FUND XB — NL, L.P.,

a California limited partnership




By: TCW (ENERGY X), LLC, its General Partner:




	 	 	 	 	 
	 	By:  	 TCW Asset Management Company,
 	 
	 	 	 its Managing Member 	 



	 	 	 	 	 
	 	By:  	
/s/ Patrick Hickey
 	 
	 	 	Name:  	Patrick Hickey 	 
	 	 	Title:  	SVP 	 



	 	 	 	 	 
	 	By:  	
/s/ Curt S. Taylor
 	 
	 	 	Name:  	Curt S. Taylor 	 
	 	 	Title:  	SVP 	 



[Letter Agreement]










 


 









 







Rio Vista Penny LLC

April 13, 2009

Page 7




	 	 	 	 	 
	 	

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-Custodian
 

	 
	 	By:  	/s/ Patrick Hickey
 	 
	 	 	Name:  	Patrick Hickey 	 
	 	 	Title:  	SVP 	 



	 	 	 	 	 
	 	By:  	
/s/ Curt S. Taylor
 	 
	 	 	Name:  	Curt S. Taylor 	 
	 	 	Title:  	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-Custodian
 

	 
	 	By:  	/s/ Patrick Hickey
 	 
	 	 	Name:  	Patrick Hickey 	 
	 	 	Title:  	SVP 	 



	 	 	 	 	 
	 	By:  	
/s/ Curt S. Taylor
 	 
	 	 	Name:  	Curt S. Taylor 	 
	 	 	Title:  	SVP 	 



[Letter Agreement]









 


 









 







Rio Vista Penny LLC

April 13, 2009

Page 6




TCW ENERGY FUND XC — NL, L.P.,

a California limited partnership




By: TCW (ENERGY X), LLC, its General Partner:




	 	 	 	 	 
	 	By:  	                                                  TCW Asset Management Company,
 	 
	 	 	its Managing Member 	 



	 	 	 	 	 
	 	By:  	
/s/ Patrick Hickey
 	 
	 	 	Name:  	Patrick Hickey 	 
	 	 	Title:  	SVP 	 



	 	 	 	 	 
	 	By:  	
/s/ Curt S. Taylor
 	 
	 	 	Name:  	Curt S. Taylor 	 
	 	 	Title:  	SVP 	 



TCW ENERGY FUND XD — NL, L.P.,

a California limited partnership




By: TCW (ENERGY X), LLC, its General Partner:




	 	 	 	 	 
	 	By:  	                                                   TCW Asset Management Company,
 	 
	 	 	its Managing Member 	 



	 	 	 	 	 
	 	By:  	
/s/ Patrick Hickey
 	 
	 	 	Name:  	Patrick Hickey 	 
	 	 	Title:  	SVP 	 



	 	 	 	 	 
	 	By:  	
/s/ Curt S. Taylor
 	 
	 	 	Name:  	Curt S. Taylor 	 
	 	 	Title:  	SVP 	 



[Letter Agreement]





 


 









 







Rio Vista Penny LLC

April 13, 2009

Page 8




	 	 	 	 	 
	 	

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-Custodian
 

	 
	 	By:  	/s/ Patrick Hickey
 	 
	 	 	Name:  	Patrick Hickey 	 
	 	 	Title:  	SVP 	 



	 	 	 	 	 
	 	By:  	/s/ Curt S. Taylor
 	 
	 	 	Name:  	Curt S. Taylor 	 
	 	 	Title:  	SVP 	 



WARRANT OWNER:




ENERGY FUNDS X HOLDINGS, L.P.,
a California limited
partnership




By: TCW (ENERGY X), LLC, its General Partner:




	 	 	 	 	 
	 	By:  	                                                  TCW Asset Management Company,
 	 
	 	 	its   Managing Member 	 



	 	 	 	 	 
	 	By:  	/s/ Patrick Hickey
 	 
	 	 	Name:  	Patrick Hickey 	 
	 	 	Title:  	SVP 	 



	 	 	 	 	 
	 	By:  	/s/ Curt S. Taylor
 	 
	 	 	Name:  	Curt S. Taylor 	 
	 	 	Title:  	SVP 	 



[Letter Agreement]








 


 









 







Rio Vista Penny LLC

April 13, 2009

Page 9




Accepted and agreed to as of the dale first written above:




RIO VISTA PENNY LLC




	 	 	 	 	 
	By:

	 	/s/ Ian Bothwell	 	 
	 

	 	 

Ian Bothwell
	 	 
	 

	 	Manager	 	 






[Letter Agreement]





 


 









 







Rio Vista Penny LLC

April 13, 2009

Page 10




CONSENT AND REAFFIRMATION




Each
of the undersigned hereby (i) consents to the provisions of this
Letter Agreement and the
transactions contemplated herein and agrees to the provisions therein related to it as a Restricted
Person, (ii) ratifies and confirms the Management Services Agreement, the Guaranty dated November
19, 2007, and the other Note Documents, in each case to which it is a party, (iii) agrees that all
of its respective obligations and covenants thereunder shall remain unimpaired by the execution and
delivery of this Letter Agreement, and (iv) agrees that the Management Services Agreement, such
Guaranty, and such other Note Documents shall remain in full force and effect.



	 	 	 	 	 
	 	
RIO VISTA ECO LLC

 	 
	 	By:  	/s/ Ian Bothwell
 	 
	 	 	Ian Bothwell 	 
	 	 	Manager 	 
	 	

RIO VISTA GO LLC

 	 
	 	By:  	/s/ Ian Bothwell
 	 
	 	 	Ian Bothwell 	 
	 	 	Manager 	 
	 
	 	GO, LLC

 	 
	 	By:  	/s/ Ian Bothwell
 	 
	 	 	Ian Bothwell 	 
	 	 	Manager 	 
	 
	 	MV PIPELINE COMPANY

 	 
	 	By:  	/s/ Ian Bothwell
 	 
	 	 	Ian Bothwell 	 
	 	 	President	 



[Letter Agreement]







 


 









 







Rio Vista Penny LLC

April 13, 2009

Page 11




	 	 	 	 	 
	 	

RIO VISTA OPERATING LLC

 	 
	 	By:  	/s/ Ian Bothwell
 	 
	 	 	Ian Bothwell 	 
	 	 	Manager	 



RIO VISTA ENERGY PARTNERS L.P.




By: Rio Vista GP LLC, its general partner




	 	 	 	 	 
	 	By:  	/s/ Ian Bothwell
 	 
	 	 	Ian Bothwell 	 
	 	 	Acting CEO	 



[Letter Agreement]