Exhibit 10.6

Supplement

SUPPLEMENT, dated as of December 18, 2009, made by REX ENERGY CORPORATION, a
Delaware corporation (the “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 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 (including the 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 Grantor to pledge the Equity
Interests described in Schedule 2-S hereto; and

WHEREAS, the Grantor has agreed to execute and deliver this Supplement in order
to pledge such Equity Interests;

NOW, THEREFORE, IT IS AGREED:

1. Guaranty and Collateral Agreement. By executing and delivering this
Supplement, the information set forth in Schedule 2-S hereto is hereby added to
the information set forth in Schedule 2 to the Guaranty and Collateral
Agreement. The 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 Supplement) as if made on
and as of such date.

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

3. Miscellaneous. This Supplement 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 Supplement 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 Supplement to be duly
executed and delivered as of the date first above written.

REX ENERGY CORPORATION

By: /s/ Benjamin W. Hulburt

Name: Benjamin W. Hulburt

Title: President and Chief Executive Officer

--------------------------------------------------------------------------------

SCHEDULE 2-S

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.

Rex Energy

Corporation

   R.E. Gas Development, LLC   100%   100%  

    Membership

    Interest

 

        Not

        Applicable

 

        Not

        Applicable