Exhibit 10.5

Assumption Agreement

ASSUMPTION AGREEMENT, dated as of December 18, 2009, made by R.E. GAS
DEVELOPMENT, LLC, a Delaware limited liability company (the “Additional
Grantor”), in favor of KeyBank National Association, as administrative agent (in
such capacity, the “Administrative Agent”) for the Lenders party to the Credit
Agreement referred to below. All capitalized terms not defined herein shall have
the meaning ascribed to them in the Guaranty and Collateral Agreement referred
to below.

W I T N E S E T H:

WHEREAS, Rex Energy Corporation, a corporation duly formed and existing under
the laws of the State of Delaware (the “Borrower”), the Administrative Agent,
and certain financial institutions (the “Lenders”) have entered into that
certain Credit Agreement, dated as of September 28, 2007 (as amended, restated,
supplemented or otherwise modified from time to time, the “Credit Agreement”);

WHEREAS, in connection with the Credit Agreement, the Borrower and certain of
its affiliates (other than the Additional Grantor) have entered into that
certain Guaranty and Collateral Agreement, dated as of September 28, 2007 (as
amended, restated, supplemented or otherwise modified from time to time, the
“Guaranty and Collateral Agreement”) in favor of the Administrative Agent for
the ratable benefit of the Secured Parties;

WHEREAS, the Credit Agreement requires the Additional Grantor to become a party
to the Guaranty and Collateral Agreement; and

WHEREAS, the Additional Grantor has agreed to execute and deliver this
Assumption Agreement in order to become a party to the Guaranty and Collateral
Agreement;

NOW, THEREFORE, IT IS AGREED:

1. Guaranty and Collateral Agreement. By executing and delivering this
Assumption Agreement, the Additional Grantor, as provided in Section 9.13 of the
Guaranty and Collateral Agreement, hereby becomes a party to the Guaranty and
Collateral Agreement as a Grantor thereunder with the same force and effect as
if originally named therein as a Grantor and, without limiting the generality of
the foregoing, hereby expressly assumes all obligations and liabilities of a
Grantor and a Guarantor thereunder. The information set forth in Annex 1-A
hereto is hereby added to the information set forth in the Schedules to the
Guaranty and Collateral Agreement. The Additional Grantor hereby represents and
warrants that, with respect to itself and as applicable, each of the
representations and warranties contained in Article IV of the Guaranty and
Collateral Agreement is true and correct on and as the date hereof (after giving
effect to this Assumption Agreement) as if made on and as of such date.

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2. Governing Law. This Assumption Agreement shall be governed by, and construed
in accordance with, the laws of the State of Texas.

3. Miscellaneous. This Assumption Agreement may be executed in counterparts (and
by different parties hereto on different counterparts), each of which shall
constitute an original, but all of which when taken together shall constitute a
single contract. Any provision of this Assumption Agreement held to be invalid,
illegal or unenforceable in any jurisdiction shall, as to such jurisdiction, be
ineffective to the extent of such invalidity, illegality or unenforceability
without affecting the validity, legality and enforceability of the remaining
provisions hereof; and the invalidity of a particular provision in a particular
jurisdiction shall not invalidate such provision in any other jurisdiction.

IN WITNESS WHEREOF, the undersigned has caused this Assumption Agreement to be
duly executed and delivered as of the date first above written.

 

R.E. GAS DEVELOPMENT, LLC By:   /s/ Benjamin W. Hulburt Name:   Benjamin W.
Hulburt Title:   President and Chief Executive Officer

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ANNEX 1-A

NOTICE ADDRESSES OF GRANTOR

 

Grantor

 

Notice Address

R.E. Gas Development, LLC

 

R.E. Gas Development, LLC

c/o Rex Energy Corporation

476 Rolling Ridge Drive, Suite 300

State College, PA 16801

Attn: Thomas Stabley, Chief Financial Officer

Fax No. 814.278.7286

INVESTMENT PROPERTY

Description of Pledged Securities

 

Owner/Grantor

  

Issuer

   Percentage
Owned    Percentage
Pledged    Class of
Stock or other
Equity Interest    No. of
Shares    Certificate
No.

None

                 

Description of Pledged Notes

None.

FILINGS AND OTHER ACTIONS

REQUIRED TO PERFECT SECURITY INTERESTS

Uniform Commercial Code Filings

 

  1. None

Description of Commercial Tort Claims With An Asserted Value in Excess of
$1,000,000

None.

 

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JURISDICTION OF ORGANIZATION AND CHIEF EXECUTIVE OFFICE

 

Legal Name/Address

   Trade Names
Used in Past
5 Years    Current
Jurisdiction of
Organization    Jurisdiction of
Organizations in
Past 5 Years    Organizational
No.    Taxpayer
Identification
No.   

Chief Executive Office or Sole
Place of Business
over the last 5 years

R. E. Gas Development, LLC

1975 Waddle Road

State College, PA 16803

   None    Delaware    Not Applicable       20-8814402   

476 Rolling Ridge Drive, Suite 300

State College, PA 16801

LOCATIONS OF INVENTORY AND EQUIPMENT

 

Grantor

 

Locations

R. E. Gas Development, LLC

 

1. 476 Rolling Ridge Drive, Suite 300 State College, PA 16801

 

2. 400 Southpointe Blvd, Suite 410, Canonsburg, Pennsylvania 15317

INTELLECTUAL PROPERTY

Copyrights and Copyright Licenses

None.

Patents and Patent Licenses

None.

Trademarks and Trademark Licenses

None.

 

RECEIVABLES WITH GOVERNMENTAL AUTHORITY AS OBLIGOR

None.