**Exhibit 10.37****PLEDGE AGREEMENT**This **PLEDGE AGREEMENT,**made this 26th day of July, 2007, is by and between:Rio Vista Energy Partners L.P., a Delaware limited partnership, having an office at 820Gessner Road, Suite 1285, Houston, Texas 77024 (hereinafter referred to as the “Pledgor”), and RZBFinance LLC, having an office at 1133 Avenue of the Americas, New York, NY 10036 (hereinafterreferred to as the “Lender”).**W I T N E S S E T H :****WHEREAS:**(A) The Pledgor and the Lender have entered into a Loan Agreement dated as of July 26, 2007(as amended, supplemented or otherwise modified from time to time, the “Loan Agreement”) pursuantto which the Lender will make loans to the Borrower upon and subject to the terms and conditionsset forth in the Loan Agreement;(B) In order to induce the Lender to enter into the Loan Agreement, the Pledgor has agreed togrant a security interest in and pledge to the Lender all of the issued and outstanding stock ofRegional Enterprises, Inc., a Virginia corporation (which is or will be the surviving corporationin the Merger (as defined in the Loan Agreement), hereinafter referred to as “Merger Sub”), whichare now or hereafter owned by it;(C) The execution and delivery by the Pledgor of this Pledge Agreement is a conditionprecedent to the Lender making loans under the Loan Agreement; and(D) All capitalized terms which are not defined herein but which are defined in the LoanAgreement shall have the respective meanings given to them in the Loan Agreement.**NOW, THEREFORE,**in consideration of the foregoing, and for other good and valuableconsideration, the receipt and sufficiency of which are hereby acknowledged, the Pledgor herebyagrees with the Lender, as follows:1. The term “Pledged Stock” as used herein shall mean and include all of the issued andoutstanding shares of stock of Merger Sub, whether now owned or hereafter acquired by the Pledgor,and certificates evidencing such shares, which are set forth in Schedule A annexed hereto, and,also, any shares, stock certificates, options or rights issued by Merger Sub as an addition to, insubstitution of, or in exchange for any such shares, all dividends and distributions thereon andany and all proceeds thereof, now or hereafter owned or acquired by the Pledgor.
 
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2. (a) As collateral security for the due payment and performance of all Obligations, whethernow existing or hereafter arising, the Pledgor hereby pledges, assigns, hypothecates, delivers andsets over to the Lender, all the Pledged Stock, and hereby grants to the Lender, a first prioritysecurity interest in all such Pledged Stock, and in any and all proceeds thereof and substitutionstherefor.(b) If the Pledgor shall become entitled to receive or shall receive any stock certificate(including, without limitation, any certificate representing a stock dividend or a distribution inconnection with any reclassification, increase or reduction of capital), option or rights, whetheras an addition to, in substitution of, or in exchange for any shares of the Pledged Stock, orotherwise, the Pledgor shall accept any such instruments as the Lender’s agent, shall hold them intrust for the Lender, and shall deliver them forthwith to the Lender in the exact form received,with the Pledgor’s endorsement when necessary and/or appropriate stock powers duly executed inblank, to be held by the Lender, subject to the terms hereof, as further collateral security forthe Obligations. All of the foregoing property described in this Section 2(b) shall be deemed tobe Pledged Stock.(c) (i) At any time: (A) any or all shares of the Pledged Stock held by the Lender hereundermay, at the option of the Lender or its nominee, be registered in the name of the Lender or itsnominee, and (B) the Lender or its nominee may, without notice, exercise all voting and corporaterights at any meeting of Merger Sub and exercise any and all rights of conversion, exchange,subscription or any other rights, privileges or options pertaining to any shares of the PledgedStock as if it were the absolute owner thereof, including, without limitation, the right to receivedividends payable thereon, and the right to exchange, at its discretion, any and all of the PledgedStock upon the merger, consolidation, reorganization, recapitalization or other readjustment ofMerger Sub or upon the exercise by Merger Sub of any right, privilege or option pertaining to anyshares of the Pledged Stock, and in connection therewith, to deposit and deliver any and all of thePledged Stock with any committee, depositary, transfer agent, registrar or other designated agencyupon such terms and conditions as it may determine, all without liability except to account forproperty actually received by it, but the Lender shall have no duty to exercise any of theaforesaid rights, privileges or options and shall not be responsible for any failure to do so ordelay in so doing.(ii) Without limitation of the foregoing, prior to the occurrence of any Event of Default,subject to the covenants contained in Section 4, the Pledgor shall be entitled to vote or consentwith respect to the Pledged Stock in a manner not inconsistent with this Pledge Agreement, the LoanAgreement or any other Loan Document.(d) All dividends and all distributions of capital payable with respect to any part of thePledged Stock (including, without limitation, pursuant to the recapitalization or reclassificationof the capital of Merger Sub or pursuant to the reorganization, dissolution or liquidation thereof)shall be paid or delivered to the Lender at the Lender’s account set forth in Section 2.4(A) of theLoan Agreement or any other account designated by Lender to be held by the Lender as additionalsecurity hereunder until applied to the Obligations. The Pledgor shallexecute an instruction letter to Merger Sub in the form of Exhibit B hereto, and deliver acopy to the Lender duly acknowledged and agreed to by Merger Sub.
 
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(e) At any time, the Lender without demand of performance or other demand, advertisement ornotice of any kind (except the notice specified below of time and place of public or private sale)to or upon the Pledgor or any other person or entity (all and each of which demands, advertisementsand/or notices are, to the extent permitted by law, hereby expressly waived) may forthwith collect,receive, appropriate and realize upon the Pledged Stock, or any part thereof, and/or may forthwithsell, assign, give an option or options to purchase, contract to sell or otherwise dispose of anddeliver said Pledged Stock, or any part thereof, in one or more parcels at public or private saleor sales, at any exchange, broker’s board or at any of the Lender’s offices or elsewhere at suchprices and on such terms (including, without limitation, a requirement that any purchaser of all orany part of the Pledged Stock shall be required to purchase the shares constituting the PledgedStock for investment and without any intention to make a distribution thereof) as it may deem best,for cash or on credit or for future delivery without assumption of any credit risk, with the rightof the Lender or any purchaser upon any such sale or sales, whether public or private, to purchasethe whole or any part of the Pledged Stock so sold, free of any right or equity of redemption inthe Pledgor, which right or equity is hereby expressly waived and released.(f) The proceeds of any collection, recovery, receipt, appropriation, realization or sale asset forth in Section 2(d) or 2(e), shall be applied as follows:First, to the costs and expenses of every kind incurred in connection therewith or incidentalto the care, safekeeping or otherwise of any and all of the Pledged Stock or in any way relating tothe rights of the Lender hereunder or under the other Loan Documents, including reasonableattorneys’ fees and legal expenses;Second, to the satisfaction of the Obligations in such order as the Lender shall determine;Third, to the payment of any other amounts required by applicable law (including, withoutlimitation, Section 9-608(a)(1)(C) of the New York Uniform Commercial Code); andFourth, to the Pledgor to the extent of the surplus proceeds, if any.(g) The Lender need not give more than ten days’ notice of the time and place of any publicsale or of the time after which a private sale may take place and such notice shall be deemed to bereasonable notification of such matters.(h) In the event that the proceeds of any collection, recovery, receipt, appropriation,realization, or sale as aforesaid are insufficient to pay all amounts to which the Lender islegally entitled, the Pledgor will be liable for the deficiency, together with interestthereon, at the Default Rate provided for in the Loan Agreement, and the reasonable fees andexpenses of any attorneys employed by the Lender to collect such deficiency.
 
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3. The Pledgor represents and warrants that:(a) The Pledged Stock owned by the Pledgor is owned directly and beneficially and of record bythe Pledgor in the respective amounts set forth on Schedule A annexed hereto;(b) The shares of the Pledged Stock constitute the percentage of all of the issued andoutstanding shares of capital stock of Merger Sub as is set forth in Schedule A annexed hereto;Schedule A accurately lists the total authorized shares and treasury shares of the Pledged Stock;(c) All of the shares of the Pledged Stock have been duly and validly issued, are fully paidand non-assessable and are owned by the Pledgor free and clear of any pledge, mortgage,hypothecation, Lien, charge, encumbrance or any security interest in such shares or the proceedsthereof except for the security interest granted to the Lender hereunder; and(d) Upon delivery of the Pledged Stock to the Lender or an agent for the Lender, this PledgeAgreement creates and grants a valid first priority lien on and perfected security interest in theshares of the Pledged Stock and the proceeds thereof, subject to no prior or equal securityinterest, Lien, charge or encumbrance or to any agreement purporting to grant to any third party asecurity interest in the property or assets of the Pledgor which would include the Pledged Stock.4. (a) The Pledgor hereby covenants that so long as any Obligations shall be outstanding andunpaid in whole or in part, or the Lender has any obligation to make credit extensions under theLoan Agreement, the Pledgor will not:(i) sell, convey or otherwise dispose of any shares of the Pledged Stock or any interesttherein, nor will the Pledgor create, incur or permit to exist any pledge, mortgage, Lien, charge,encumbrance or any security interest whatsoever with respect to any of such Pledged Stock or theproceeds thereof, other than that created hereby; or(ii) consent to or approve the issuance of any additional shares of any class of capital stockof Merger Sub or any option, warrant or other agreement with respect to such stock; or(iii) vote to enable or otherwise permit Merger Sub to dissolve, liquidate, reduce its capitalor merge or consolidate with any Person.(b) The Pledgor warrants and will defend the Lender’s right, title, special property andsecurity interest in and to the Pledged Stock against the claims of any person, firm, corporationor other entity.
 
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5. (a) If the Lender shall determine to exercise its right to sell all or any part of thePledged Stock, and if in the opinion of counsel for the Lender it is necessary to have the PledgedStock, or that portion thereof to be sold, registered under the provisions of theSecurities Act of 1933, as amended (the “Securities Act”), the Pledgor will use its best efforts to cause Merger Subto execute and deliver, and cause the directors and officers of Merger Sub to execute and deliver,all at the Pledgor’s expense, all such instruments and documents, and to do or cause to be done allsuch other acts and things as may be necessary to register the Pledged Stock, or that portionthereof to be sold, under the provisions of the Securities Act and to cause the registrationstatement relating thereto to become effective and to remain effective for a period of one yearfrom the date of the first public offering of the Pledged Stock, or that portion thereof so to besold, and to make all amendments thereto and/or to the related prospectus which, in the opinion ofthe Lender or its counsel, are necessary or advisable, all in conformity with the requirements ofthe Securities Act and the rules and regulations of the Securities and Exchange Commissionapplicable thereto; to cause Merger Sub to comply with the provisions of the “Blue Sky” law of anyjurisdiction which the Lender shall designate; and to cause Merger Sub to make available to itssecurity holders, as soon as practicable, an earnings statement (which need not be audited)covering a period of twelve months, but not more than eighteen months, beginning with the firstmonth after the effective date of any such registration statement, which earnings statement willsatisfy the provisions of Section 11(a) of the Securities Act.(b) The Pledgor acknowledges that a breach of any of the covenants contained in Section 5(a)above will cause irreparable injury to the Lender, that the Lender shall have no adequate remedy atlaw in respect of such breach and, as a consequence, the covenants of the Pledgor contained inSection 5(a) shall be specifically enforceable against the Pledgor, and the Pledgor hereby waives,and shall not assert, any defenses against an action for specific performance of such covenants.(c) Notwithstanding the foregoing, the Pledgor recognizes that the Lender may be unable toeffect a public sale of all or a part of the Pledged Stock, and may be compelled to resort to oneor more private sales to a restricted group of purchasers who will be obligated to agree, amongother things, to acquire such securities for their own account, for investment and not with a viewto the distribution or resale thereof. The Pledgor acknowledges that any such private sales may beat prices and on terms less favorable to the seller than if sold at public sales and agrees thatsuch private sales shall be deemed to have been made in a commercially reasonable manner, and thatthe Lender has no obligation to delay sale of any such securities for the period of time necessaryto permit Merger Sub to register such securities for public sale under the Securities Act.6. The Pledgor shall at any time and from time to time upon the written request of theLender, execute and deliver such further documents and do such further acts and things as theLender may reasonably request in order to effect the purposes of this PledgeAgreement, including, without limitation, delivering to the Lender on the date hereof or atany time hereafter irrevocable proxies in respect of the Pledged Stock in the form of Exhibit Aannexed hereto.7. (a) Beyond the exercise of reasonable care to assure the safe custody of the Pledged Stockwhile held hereunder, the Lender shall have no duty or liability to preserve rights pertainingthereto, and shall be relieved of all responsibility for the Pledged Stock upon surrendering it tothe Pledgor or in accordance with the Pledgor’s instructions.
 
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(b) No course of dealing between the Pledgor or the Lender, nor any failure to exercise, norany delay in exercising, on the part of the Lender, any right, power or privilege hereunder orunder the Loan Agreement or other Loan Documents shall operate as a waiver thereof; nor shall anysingle or partial exercise of any right, power or privilege hereunder or thereunder preclude anyother or further exercise thereof or the exercise of any other right, power or privilege.(c) The rights and remedies herein provided, and provided in the Loan Agreement, the LoanDocuments and in all other agreements, instruments and documents delivered pursuant to the LoanAgreement, are cumulative and are in addition to, and not exclusive of, any rights or remediesprovided by law including, without limitation, the rights and remedies of a secured party under theNew York Uniform Commercial Code as in effect from time to time.8. All notices and other communications to the Pledgor pursuant to this Agreement shall begiven by sending the same to the Pledgor in the manner provided in the Loan Agreement.9. This Pledge Agreement shall inure to the benefit of, and be binding upon, the successorsand assigns of the parties hereto, provided that the Pledgor may not assign any of its rights andobligations hereunder without the prior written consent of the Lender.10. The invalidity, illegality or unenforceability in any jurisdiction of any provision in orobligation under this Pledge Agreement shall not affect or impair the validity, legality orenforceability of the remaining provisions or obligations under this Pledge Agreement or of suchprovision or obligation in any other jurisdiction.11. THIS PLEDGE AGREEMENT SHALL BE GOVERNED BY, AND SHALL BE CONSTRUED AND ENFORCED INACCORDANCE WITH, THE INTERNAL LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD TO CONFLICTS OF LAWSPRINCIPLES**.**12. THE PLEDGOR HEREBY CONSENTS TO THE JURISDICTION OF ANY STATE OR FEDERAL COURT LOCATEDWITHIN THE BOROUGH OF MANHATTAN OF THE STATE OF NEW YORK AND IRREVOCABLY AGREES THAT, UNLESSWAIVED BY THE LENDER IN WRITING, ALLACTIONS OR PROCEEDINGS ARISING OUT OF OR RELATING TO THIS PLEDGE AGREEMENT OR THE OTHER LOANDOCUMENTS, INCLUDING ALL CLAIMS AND ACTIONS AGAINST THE LENDER, SHALL BE LITIGATED IN SUCH COURTS.THE PLEDGOR ACCEPTS FOR ITSELF AND IN CONNECTION WITH ITS PROPERTIES, GENERALLY ANDUNCONDITIONALLY, THE NON-EXCLUSIVE JURISDICTION OF THE AFORESAID COURTS AND WAIVES ANY DEFENSE OFFORUMNONCONVENIENS, AND IRREVOCABLY AGREES TO BE BOUND BY ANY JUDGMENTRENDERED THEREBY IN CONNECTION WITH THIS PLEDGE AGREEMENT, THE OTHER LOAN DOCUMENTS OR THEOBLIGATIONS**.**
 
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13. EACH OF THE PLEDGOR AND THE LENDER HEREBY WAIVES ITS RIGHTS TO A JURY TRIAL OF ANY CLAIMOR CAUSE OF ACTION BASED UPON OR ARISING OUT OF THIS PLEDGE AGREEMENT OR THE OTHER LOAN DOCUMENTS.EACH OF THE PLEDGOR AND THE LENDER ACKNOWLEDGES THAT THIS WAIVER IS A MATERIAL INDUCEMENT TO ENTERINTO A BUSINESS RELATIONSHIP, THAT EACH HAS ALREADY RELIED ON THE WAIVER IN ENTERING INTO THISPLEDGE AGREEMENT AND THE OTHER LOAN DOCUMENTS AND THAT EACH WILL CONTINUE TO RELY ON THE WAIVER INTHEIR RELATED FUTURE DEALINGS. EACH OF THE PLEDGOR AND THE LENDER FURTHER WARRANTS AND REPRESENTSTHAT EACH HAS REVIEWED THIS WAIVER WITH ITS LEGAL COUNSEL, AND THAT EACH KNOWINGLY AND VOLUNTARILYWAIVES ITS JURY TRIAL RIGHTS FOLLOWING CONSULTATION WITH LEGAL COUNSEL.14. This Pledge Agreement and any amendments, waivers, consents, or supplements may beexecuted in any number of counterparts and by different parties hereto in separate counterparts,each of which when so executed and delivered shall be deemed an original, but all of whichcounterparts together shall constitute but one and the same instrument. This Pledge Agreement shallbecome effective upon the execution of a counterpart hereof by each of the parties hereto.Delivery of an executed counterpart of a signature page to this Pledge Agreement, to anyamendments, waivers, consents or supplements, or to any other Loan Document by telecopier shall beas effective as delivery of a manually executed counterpart thereof.
 
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IN WITNESS WHEREOF, the parties have caused this Pledge Agreement to be duly executed anddelivered the day and year first above written.      PLEDGOR