Exhibit 10.27

LETTER AMENDMENT NO. 6
TO
SECOND AMENDED AND RESTATED MASTER SHELF AGREEMENT

March 1,
2002

The
Prudential Insurance Company of America
Pruco Life Insurance Company
c/o Prudential Capital Group
2200 Ross Avenue, Suite 4200E
Dallas, Texas 75201

Ladies
and Gentlemen:

We
refer to the Second Amended and Restated Master Shelf Agreement dated as of December 19, 1991 (effective January 31, 1996), as amended by Letter Amendment No. 1
dated November 21, 1997, Letter Amendment No. 2 dated March 31, 1999, Limited Waiver, Consent, Release and Amendment No. 3 dated June 1, 1999, Limited Waiver,
Consent, Release and Amendment No. 4 dated August 25, 2000 and Letter Amendment No. 5 dated March 30, 2001 (as amended, the
"Agreement") among the undersigned, Western Gas Resources, Inc. (the "Company") and you. Unless
otherwise defined herein, the terms defined in the Agreement shall be used herein as therein defined.

It
is hereby agreed by you and us as follows:

The
Agreement is, effective the date first above written, hereby amended as follows:

(a)Paragraph 6A(3). Total Debt Maintenance.Paragraph 6A(3) of the Agreement is amended in its entirety to
read as follows:



(b)Paragraph 6A(4). Senior Debt Maintenance.Paragraph 6A(4) of the Agreement is amended in its entirety to
read as follows:



(c)Paragraph 6A(7). Senior Debt to EBITDA.Paragraph 6A(7) of the Agreement is amended in its entirety to read
as follows:



(d)Paragraph 6C(6). Lease Rentals.Paragraph 6C(6) of the Agreement is amended in its entirety to read
as follows:



(e)Paragraph 10B. Other Terms.Paragraph 10B of the Agreement is amended by adding thereto in alphabetical
order the following definitions:



(f)Schedule 6C(3). Joint Ventures.Schedule 6C(3) of the Agreement is hereby amended in its entirety to
read as set forth on Schedule 6C(3) attached hereto:



This
Letter Amendment may be executed in any number of counterparts and by any combination of the parties hereto in separate counterparts, each of which counterparts shall be an original
and all of which taken together shall constitute one and the same Letter Amendment.

If
you agree to the terms and provisions hereof, please evidence your agreement by executing and returning at least a counterpart of this Letter Amendment to the Company at its address
at 12200 N. Pecos Street, Denver, CO 80234, Attention of Chief Financial Officer. This Letter Amendment shall become effective as of the date first above written when and if counterparts of this
Letter Amendment shall have been executed by us and you and the consent attached hereto shall have been executed by the Guarantors.

| |  | Very truly yours,          
| |  | ---------------------------
| |  | WESTERN GAS RESOURCES, INC.
| |  | By:                        



Agreed
as of the date first above written:

|THE PRUDENTIAL INSURANCE COMPANY OF AMERICA |  |               
|------------------------------------------- |  | --------------
|By:                                         |  | Vice President
|PRUCO LIFE INSURANCE COMPANY                |  |               
|By:                                         |  | Vice President



CONSENT

Each of the undersigned is a Guarantor ("Guarantor" and, collectively
"Guarantors") under separate guaranties (each being a "Guaranty") in favor of The Prudential Insurance
Company of America and Pruco Life Insurance Company (together, "Prudential") with respect to the obligations of Western Gas Resources, Inc. (the
"Company") under that certain Second Amended and Restated Master Shelf Agreement dated as of December 19, 1991 (effective January 31,
1996) as amended by Letter Amendment No. 1 dated November 21, 1997, Letter Amendment No. 2 dated March 31, 1999, Limited Waiver, Consent, Release and Amendment No. 3
dated June 1, 1999, Limited Waiver, Consent, Release and Amendment No. 4 dated August 25, 2000 and Letter Amendment No. 5 dated March 30, 2001 (as amended, the
"Agreement"). Prudential and the Company are entering into that certain Letter Amendment No. 6 to Second Amended and Restated Master Shelf
Agreement, dated as of March 1, 2002 (the "Amendment"). Each of the undersigned hereby consents to the Amendment and each hereby confirms and
agrees that its Guaranty is, and shall continue to be, in full force and effect and is hereby confirmed and ratified in all respects except that, upon the effectiveness of, and on and after the date
of this consent, all words of like import referring to the Agreement shall mean the Agreement as amended by the Amendment, as the same may be further amended or modified from time to time.

Dated
as of March 1, 2002

LANCE OIL & GAS COMPANY, INC.
MGTC, INC.
MIGC, INC.
MOUNTAIN GAS RESOURCES, INC.
WESTERN GAS RESOURCESTEXAS, INC.
WESTERN GAS WYOMING, L.L.C.

|By: |  | William J. Krysiak, as Chief Financial Officer of each of the above-named companies. |  | 
|--- |  | ------------------------------------------------------------------------------------ |  | 



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