Document:

<PAGE>

                                                                    EXHIBIT 4.12

                         WESTERN GAS RESOURCES, INC.,
                                  As Issuer,

                           THE SUBSIDIARY GUARANTORS
                          Named on Schedule I hereto

                                      AND

                  CHASE BANK OF TEXAS, NATIONAL ASSOCIATION,
                                  As Trustee

                         THIRD SUPPLEMENTAL INDENTURE

                          Dated as of January 3, 2001

                         Supplementing the Indenture,
                       Dated as of June 15, 1999, among
                       Western Gas Resources, Inc., the
                         Guarantors named therein and
                  Chase Bank of Texas, National Association,
                        as Trustee, as supplemented by
                       the First Supplemental Indenture,
                      Dated as of October 19, 1999, among
                       Western Gas Resources, Inc., the
                         Guarantors named therein and
                  Chase Bank of Texas, National Association,
                                  as Trustee
                          and as supplemented by the
                        Second Supplemental Indenture,
                     Dated as of September 29, 2000, among
                         Western Gas Resources, Inc.,
                       the Guarantors named therein and
                   The Chase Manhattan Bank, as successor to
                The Chase Bank of Texas, National Association,
                                  as Trustee

<PAGE>

                                 $155,000,000

                    10% SENIOR SUBORDINATED NOTES DUE 2009

     THIS THIRD SUPPLEMENTAL INDENTURE, dated as of January 3, 2001 (the "Third
Supplemental Indenture"), is by and among (i) Western Gas Resources, Inc., a
Delaware corporation (the "Company"), as issuer of the 10% Senior Subordinated
Notes due 2009 (the "Notes"), which were exchanged by the Company for 10% Senior
Subordinated Notes due 2009 registered under the Securities Act of 1933 (the
"Exchange Notes"), (ii) each of the Subsidiaries of the Company set forth on
Schedule I hereto and each of the Company's Subsidiaries which becomes a
guarantor of the Exchange Notes in compliance with Section 9.16 of the Original
Indenture referred to herein in which such Subsidiary agrees to be bound by the
terms of the Original Indenture, as guarantors of the Company's obligations
under the Original Indenture and the Exchange Notes (each, a "Guarantor"), and
(iii} The Chase Manhattan Bank, as successor to the Chase Bank of Texas,
National Association, as trustee (the "Trustee").

     WHEREAS, the Company, the Guarantors as of June 15, 1999 and the Trustee
heretofore executed and delivered an Indenture, dated as of June 15, 1999 (the
"Original Indenture"); and

     WHEREAS, pursuant to the Original indenture the Company issued and the
Trustee authenticated and delivered $155 million aggregate principal amount of
the Notes, which Notes were guaranteed by each of the Company's Subsidiaries set
forth in clause (1) of the definition of "Guarantors" in the Original Indenture;
and

     WHEREAS, the Company, the Guarantors as of October 19, 1999 and the Trustee
heretofore executed and delivered the First Supplemental Indenture, dated as of
October 19, 1999 (the "First Supplemental Indenture"), pursuant to which the
Company's subsidiary, MGTC, Inc., a Wyoming corporation ("MGTC"), expressly
agreed to issue a Subsidiary Guarantee (as defined in the Original Indenture)
and to be bound as, and assume the obligations of, a Guarantor under the
Original Indenture; and

     WHEREAS, on November 23, 1999, the Company consummated an exchange offer
through which the Notes were exchanged for the Exchange Notes, which Exchange
Notes were guaranteed by each of the Company's Subsidiaries set forth in clause
(1) of the definition of "Guarantors" in the Original Indenture and by MGTC; and

     WHEREAS, on September 28, 2000, the Company repaid in full all amounts due
in respect of the notes (the "American General Notes") issued under the Amended
and Restated Note Purchase Agreement, dated as of April 28, 1999, by and among
the Company, American General Life Insurance Company and the other note
purchasers party thereto, as amended and restated from time to time; and

     WHEREAS, the Company, the Guarantors as of September 29, 2000 and the
Trustee heretofore executed and delivered the Second Supplemental Indenture,
dated as of September 29, 2000 (the "Second Supplemental Indenture"), pursuant
to which the Subsidiary Guarantee issued by the Company's subsidiary, Western
Gas Resources - Oklahoma, Inc., a Delaware corporation, under the Original
Indenture was discharged and released; and

     WHEREAS, the Company and The Prudential Insurance Company of America and
Pruco Life Insurance Company (together, "Prudential") entered into, as of
November 22, 2000, a Waiver and Consent (the "Waiver") to the Second Amended and
Restated Master Shelf Agreement, dated as of December 19, 1991 (effective
January 31, 1996), as amended and restated from time to time (the "Master Shelf
Agreement"), pursuant to which Prudential agreed to, subject to certain
conditions that were satisfied as of January 3, 2001, terminate the guaranty by
the Company's subsidiary, Pinnacle Gas Treating, Inc., a Texas corporation
("PGT"), of the Company's obligations arising under the Master Shelf Agreement,
to otherwise grant a general release of PGT under the PGT Guaranty (as defined
in the Waiver) and to release Prudential's lien and security interest in the
shares of capital stock of PGT; and
<PAGE>

     WHEREAS, Prudential executed, as of January 3, 2001, a Release of Liens and
Termination of Guaranty (the "Prudential Release") pursuant to which Prudential
fully released and discharged all security interests and liens existing in favor
of Prudential with respect to the PGT Stock (as defined in the Prudential
Release) and terminated the guaranty by PGT of the Company's obligations arising
under the Master Shelf Agreement; and

     WHEREAS, the Company, Bank of America, N.A., as agent (the "Agent"), and
the lenders party thereto (collectively, the "Lenders") entered into, as of
November 22, 2000, a Fifth Amendment (the "Amendment") to the Loan Agreement
among the Company, the Agent and the Lenders, dated as of April 29, 1999, as
amended and restated from time to time (the "Loan Agreement"), pursuant to which
the Lenders consented to the termination of the Guaranty, dated as of April 29,
1999, executed by PGT in favor of the Agent, and to the release of the Agent's
lien and security interest in the common stock of PGT, subject to certain
conditions that were satisfied as of January 3, 2001; and

     WHEREAS, the Agent executed, as of January 3, 2001, a Release of Lien and
Termination of Guaranty (the "Agent Release") pursuant to which the Agent, on
behalf of itself and each Lender, fully released and discharged all security
interests and liens existing in favor of the Agent with respect to the common
stock of PGT and terminated the guaranty by PGT of the Company's obligations
arising under the Loan Agreement; and

     WHEREAS, the Company and Anadarko Petroleum Corporation, a Delaware
corporation ("Anadarko"), entered into, as of November 30, 2000, a Stock
Purchase Agreement pursuant to which the Company agreed to sell and Anadarko
agreed to purchase all the capital stock of PGT (the "Sale"); and

     WHEREAS, with respect to the Sale, the Company has delivered to the Trustee
an officers' certificate in compliance with Section 9.14(b) of the Original
Indenture and is otherwise in compliance with Section 9.14 of the Original
Indenture; and

     WHEREAS, the Sale is being consummated as of January 3, 2001; and

     WHEREAS, Section 9.16 of the Original Indenture provides that each
Restricted Subsidiary (as defined in the Original Indenture) of the Company be a
Guarantor for so long as such Restricted Subsidiary has outstanding any
Guarantees (as defined in the Original Indenture) with respect to Senior Debt
(as defined in the Original Indenture); and

     WHEREAS, as a result of the Waiver, the Amendment, the Prudential Release,
the Agent Release and the repayment in full of all amounts due in respect of the
American General Notes, PGT, which is a Restricted Subsidiary, has no
outstanding Guarantees with respect to any Senior Debt; and

     WHEREAS, Section 12.07 of the Original Indenture provides that (a) any
Subsidiary Guarantee of a Guarantor will be released and discharged upon the
release or discharge of all Guarantees by such Guarantor of all Senior Debt of
the Company and (b) that a Guarantor released upon such occurrence shall be
released from and relieved of its obligations under its Subsidiary Guarantee
upon execution and delivery of a supplemental indenture satisfactory to the
Trustee; and

     WHEREAS, this Third Supplemental Indenture has been duly authorized by all
necessary corporate action on the part of the Company and the Guarantors.

     NOW, THEREFORE, the Company, the Guarantors listed on Schedule I hereto and
the Trustee agree as follows for the equal and ratable benefit of each other and
the Holders of the Exchange Notes:

                                   ARTICLE I
                             RELEASE OF GUARANTOR
                             --------------------

     SECTION 1.1.  Release of Guarantor.  In accordance with Section 12.07 of
                   ---------------------
the Original Indenture, the Subsidiary Guarantee issued by PGT under the
Original Indenture is hereby discharged and released.
<PAGE>

     SECTION 1.2.  Trustee's Acceptance.  The Trustee hereby accepts this Third
                   ---------------------
Supplemental Indenture and agrees to perform the same under the terms and
conditions set forth in the Original Indenture.

                                  ARTICLE II
                                 MISCELLANEOUS
                                 -------------

     SECTION 2.1.  Effect of Supplemental Indenture.  Upon the execution and
                   ---------------------------------
delivery of this Third Supplemental Indenture by the Company, the Guarantors
listed in Schedule I hereto and the Trustee, the Original Indenture, as
supplemented by the First Supplemental Indenture and the Second Supplemental
Indenture, shall be supplemented in accordance herewith, and this Third
Supplemental Indenture shall form a part of the Original Indenture, as
supplemented by the First Supplemental Indenture and the Second Supplemental
Indenture, for all purposes, and every Holder of Exchange Notes heretofore or
hereafter authenticated and delivered under the Original Indenture, as
supplemented by the First Supplemental Indenture and the Second Supplemental
Indenture, shall be bound thereby.

     SECTION 2.2. Original Indenture Remains in Full Force and Effect.  Except
                  ----------------------------------------------------
as supplemented hereby and by the First Supplemental Indenture and the Second
Supplemental Indenture, all provisions in the Original Indenture shall remain in
full force and effect.

     SECTION 2.3. Original Indenture and Supplemental Indentures Construed
                  --------------------------------------------------------
Together.  This Third Supplemental Indenture is an indenture supplemental to and
---------
in implementation of the Original Indenture, and the Original Indenture, this
Third Supplemental Indenture, the First Supplemental Indenture and the Second
Supplemental Indenture shall henceforth be read and construed together.

     SECTION 2.4. Confirmation and Preservation of Original Indenture.  The
                  ----------------------------------------------------
Original Indenture, as supplemented by the First Supplemental Indenture, the
Second Supplemental Indenture and this Third Supplemental Indenture, is in all
respects confirmed and preserved.

     SECTION 2.5. Conflict with Trust Indenture Act.  If any provision of this
                  ----------------------------------
Third Supplemental Indenture limits, qualifies or conflicts with a provision
which is required to be included in this Third Supplemental Indenture by the
Trust Indenture Act of 1939 (the "TIA"), the latter provision shall control. If
any provision of this Third Supplemental Indenture modifies or excludes any
provision of the TIA that may be so modified or excluded, the latter provision
shall be deemed to apply to the Original Indenture (as supplemented by the First
Supplemental Indenture and the Second Supplemental Indenture) as so modified or
to be excluded by this Third Supplemental Indenture, as the case may be.

     SECTION 2.6. Severability. In case any provision in this Third Supplemental
                  -------------
Indenture shall be invalid, illegal or unenforceable, the validity, legality and
enforceability of the remaining provisions shall not in any way be affected or
impaired thereby.

     SECTION 2.7. Terms Defined in the Original Indenture.  All capitalized
                  ----------------------------------------
terms not otherwise defined herein shall have the meanings ascribed to them in
the Original Indenture.

     SECTION 2.8. Headings.  The Article and Section headings of this Third
                  ---------
Supplemental Indenture are for convenience only and shall not affect the
construction hereof.

     SECTION 2.9. Benefits of Third Supplemental Indenture, etc.  Nothing in
                  ----------------------------------------------
this Third Supplemental Indenture or the Exchange Notes, express or implied,
shall give to any Person, other than the parties hereto and thereto and their
successors hereunder and thereunder and the Holders, any benefit or any legal or
equitable right, remedy or claim under the Original Indenture, the First
Supplemental Indenture, the Second Supplemental Indenture or this Third
Supplemental Indenture.

     SECTION 2.10. Successors and Assigns.  All covenants and agreements in this
                   -----------------------
Third Supplemental Indenture by the Company and the Guarantors in this Third
Supplemental Indenture and the
<PAGE>

Exchange Notes, as applicable, shall bind their successors and assigns, whether
so expressed or not. All agreements of the Trustee in this Third Supplemental
Indenture shall bind its successors.

     SECTION 2.11. Trustee Not Responsible for Recitals.  The recitals contained
                   -------------------------------------
herein shall be taken as the statements of the Company and the Guarantors, and
the Trustee assumes no responsibility for their correctness. The Trustee shall
have no liability for the validity or sufficiency of this Third Supplemental
Indenture.

     SECTION 2.12. Certain Duties and Responsibilities of the Trustee. In
                   ---------------------------------------------------
entering into this Third Supplemental Indenture, the Trustee shall be entitled
to the benefit of every provision of the Original Indenture, the First
Supplemental Indenture the Second Supplemental Indenture relating to the conduct
or affecting the liability or affording protection to the Trustee, whether or
not elsewhere herein so provided.

     SECTION 2.13. Governing Law.  THIS THIRD SUPPLEMENTAL INDENTURE SHALL BE
                   --------------
GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK
(WITHOUT GIVING EFFECT TO THE CONFLICTS OF LAW PRINCIPLES THEREOF).

     SECTION 2.14. No Recourse Against Others.  No director, officer, employee,
                   ---------------------------
incorporator or stockholder of the Company or any Guarantor, as such, shall have
any liability for any obligations of the Company or the Guarantors under the
Exchange Notes, the Original Indenture, the First Supplemental Indenture, the
Second Supplemental Indenture, this Third Supplemental Indenture, the Subsidiary
Guarantees, or for any claim based on, in respect of,  or by reason of, such
obligations or their creation. Each Holder of Exchange Notes by accepting an
Exchange Note waives and releases all such liability. The waiver and release are
part of the consideration for issuance of the Exchange Notes. The waiver may not
be effective to waive liabilities under the federal securities laws, and it is
the view of the Commission that such a waiver is against public policy.

     SECTION 2.15. Multiple Originals.  The parties may sign any number of
                   -------------------
copies of this Third Supplemental Indenture. Each signed copy shall be an
original, but all of them together represent the same agreement. One signed copy
is enough to prove this Third Supplemental Indenture. The Third Supplemental
Indenture may be executed in any number of counterparts, each of which shall be
deemed an original, but all such counterparts shall together constitute but one
and the same instrument.

     IN WITNESS WHEREOF, the parties have caused this Third Supplemental
Indenture to be duly executed as of the date first written above.

                              WESTERN GAS RESOURCES, INC.
                              By: __________________________________
                              Name:  William J. Krysiak
                              Title: Vice President-Finance
                                        CHASE BANK OF TEXAS, NATIONAL
                              ASSOCIATION, as Trustee

                              By: __________________________________
                              Name:  Mauri J. Cowen
                              Title: Vice President & Trust Officer

                              PINNACLE GAS TREATING, INC.

                              By: __________________________________
                              Name:
                              Title:
                              (Representative of Anadarko to
<PAGE>

                              sign here)

                              GUARANTORS:
                              -----------

                         LANCE OIL & GAS COMPANY, INC.
                         MGTC, INC.
                         MIGC, INC.
                         MOUNTAIN GAS RESOURCES, INC.
                         PINNACLE GAS TREATING, INC.
                         WESTERN GAS RESOURCES-TEXAS, INC.
                         WESTERN GAS RESOURCES-OKLAHOMA, INC.
                         WESTERN GAS WYOMING, L.L.C.

                         By: ______________________________
                         Name:  John C. Walter
                         Title: Executive Vice President

                                  Schedule I
                                  Guarantors
                                  ----------

                         Name of Subsidiary
-------------------------------------------

                         LANCE OIL & GAS COMPANY, INC.
                         MGTC, INC.
                         MIGC, INC.
                         MOUNTAIN GAS RESOURCES, INC.
                         PINNACLE GAS TREATING, INC./1/
                         WESTERN GAS RESOURCES - TEXAS, INC.
                         WESTERN GAS RESOURCES - OKLAHOMA, INC.
                         WESTERN GAS WYOMING, L.L.C.

_________________________

/1/ As a result of this Third Supplemental Indenture, the Subsidiary Guarantee
issued by this entity under the Original Indenture is discharged and released.<PAGE>

                                                                   EXHIBIT 10.14

             LIMITED WAIVER, CONSENT, RELEASE AND AMENDMENT NO. 3 TO
               SECOND AMENDED AND RESTATED MASTER SHELF AGREEMENT

         This LIMITED WAIVER, CONSENT, RELEASE AND AMENDMENT NO. 3 TO SECOND
AMENDED AND RESTATED MASTER SHELF AGREEMENT (this "Amendment") is entered into
as of June 1, 1999, by and among Western Gas Resources, Inc., a Delaware
corporation (the "Company"), and The Prudential Insurance Company of America and
Pruco Life Insurance Company (together, "Prudential").

                             PRELIMINARY STATEMENTS

         1. The Company and Prudential entered into 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
and Letter Amendment No. 2 ("Letter Amendment No. 2") dated March 31, 1999 (as
amended, the "Agreement"). Capitalized terms not otherwise defined herein shall
have the meanings specified in the Agreement, as amended hereby.

         2. In order to secure the Obligations (as defined in the hereinafter
defined WGR Canada Guaranty), including the obligations of the Company under the
Notes and the Agreement, the Company caused WGR Canada, Inc., a New Brunswick
corporation (WGR Canada"), to execute and deliver to Prudential a Guaranty,
dated as of May 30, 1997 in favor of Prudential together with all subsequent
holders of the Obligations (the "WGR Canada Guaranty").

         3. In order to secure the Secured Obligations (as defined in the Pledge
Agreement), including the obligations of the Company under the Notes and the
Agreement, the Company executed and delivered the Pledge Agreement pursuant to
which the Company pledged and granted a security interest to Prudential in,
among other things, 1,000 shares of common stock of WGR Canada (the "WGR Canada
Pledged Stock").

         4. The Company and WGR Canada have requested that Prudential (i)
terminate the WGR Canada Guaranty and otherwise grant a general release of WGR
Canada under the WGR Canada Guaranty and any other obligations and liabilities
arising under all documents and agreements delivered pursuant to the WGR Canada
Guaranty or in connection therewith and (ii) release Prudential's security
interest in 35% of the total number of issued and outstanding shares of capital
stock of WGR Canada under the Pledge Agreement (the "Releases").

         5. The Company and Prudential desire to provide for (a) the limited
waiver of certain provisions of the Agreement, (b) Prudential's consent and
agreement that certain matters relating to the terms of subordinated debt to be
offered by Company are to Prudential's satisfaction, and (c) the amendment of
the Agreement as specified herein.

         6. Prudential is willing to grant the Releases, subject to the
condition that the lenders parties to the NCNB Agreement and the holders of the
notes issued pursuant to the 1995 Note Purchase Agreement grant similar releases
of
<PAGE>

the guaranties provided by WGR Canada to such lenders parties to the NCNB
Agreement, NCNB and such holders of the notes issued pursuant to the 1995 Note
Purchase Agreement and the respective security interests of such lenders parties
to the NCNB Agreement, NCNB and such holders of the notes issued pursuant to the
1995 Note Purchase Agreement in 350 shares of common stock of WGR Canada (the
"Corresponding Releases").

         7. The Company and Prudential wish to amend the Agreement in certain
other respects.

         8. Prudential is the holder of 100% of the outstanding principal amount
of the Notes issued under the Agreement.

         In consideration of the foregoing and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
parties hereto agree as follows:

SECTION 1. Waivers, Consents and Releases.

         (a) Consent to Certain Subordinated Debt. Subject to the terms and
         conditions set forth herein, and in reliance upon the representations
         and warranties of the Company contained herein, pursuant to the
         definition of the term "Subordinated Debt" and "Subordinated Debt
         Guaranties" in paragraph 10B of the Agreement, Prudential hereby
         consents and agrees that $155,000,000 of Debt of the Company proposed
         to be issued by the Company on the terms provided in that certain
         Indenture, draft dated June 3, 1999, among the Company, the Guarantors
         and Chase Bank of Texas, N.A. (the "Draft Indenture") and the
         guaranties by certain Subsidiaries of the Company of such Debt to be
         entered into in connection with the offering of such Debt and on the
         terms provided the Draft Indenture will be subordinate in right of
         payment to the payment of the Notes and the Guaranties, respectively,
         in a manner satisfactory to Prudential, and therefore will constitute
         Subordinated Debt and Subordinated Debt Guaranties, provided that the
         terms of the final indenture entered into relating to the offering of
         such Debt conform in all material respects to the terms provided in the
         Draft Indenture.

         (b) Releases of WGR Canada and WGRS; Release of Existing MGTC Guaranty.
         Prudential hereby releases and discharges, effective as of the
         Effective Date, WGR Canada and its successors and assigns from all
         obligations and liabilities under the WGR Canada Guaranty and all
         documents and agreements delivered pursuant to the WGR Canada Guaranty
         or in connection therewith and further hereby terminates the WGR Canada
         Guaranty. In addition, Prudential hereby authorizes releases and
         discharges 350 shares of common stock of WGR Canada from the liens and
         security interests granted by the Company pursuant to the Pledge
         Agreement, automatically and without further action by any party. The
         Company agrees that it shall, from time to time, execute, acknowledge
         and deliver to Prudential instruments, agreements, and other documents
         as Prudential shall reasonably request in order to further evidence the
         liens and security interests in the 650 shares of common
<PAGE>

         stock of WGR Canada remaining subject to tho Pledge Agreement,
         including delivering to NCNB. as bailee on behalf of the holders of the
         Notes, a new certificate and a new stock power evidencing 650 shares of
         common stock of WGR Canada.

                  By its execution of this Amendment, Prudential confirms that,
         effective as of the Amendment No. 3 Effective Date, it has released
         WGRS from all obligations and liabilities under the Guaranty executed
         by WGRS and all documents and agreements delivered pursuant thereto,
         provided that each other holder of Senior Debt has also released WGRS
         from all obligations and liabilities under all Existing Guaranties
         executed by WGRS and all documents and agreements delivered pursuant
         thereto.

                  Prudential hereby releases and discharges, effective as of the
         Amendment No. 3 Effective Date MGTC from all obligations and
         liabilities under the Existing Guaranty executed by MGTC and all
         documents and agreements delivered pursuant thereto, provided that (a)
         each other holder of Senior Debt also releases MGTC from all Guaranties
         executed by MGTC and all documents and agreements delivered pursuant
         thereto, and (b) MGTC delivers the conditional Guaranty described in
         Section 17(e) of this Amendment.

         (c) Waiver. Prudential hereby waives any Default or Event of Default
         arising under either clause (xvii) or clause (xviii) of paragraph 7A of
         the Agreement to the extent that any such Default or Event of Default
         may be deemed to have arisen out of the releases provided for in this
         Section 1 or, in the case of the existing MGTC Guaranty, due to the
         failure of MGTC to obtain prior approval of the Wyoming Public Service
         Commission with respect to the existing MGTC Guaranty.

         (d) Certain Other Documents Satisfactory. Prudential hereby confirms
         that the documents delivered pursuant to clauses (x), (xii), (xiii) and
         (xiv) of Section IID of Letter Amendment No. 2 are satisfactory to
         Prudential in form and substance.

SECTION 2. Amendments.

         (a) Amendment to Paragraph SM. Guaranties. Paragraph 5M of the
         Agreement is amended by deleting the text reading "the NCNB Agreement,
         the Bridge Facility or the 1995 Note Purchase Agreement" and replacing
         it with "any Debt" and adding immediately after the words "execute and
         deliver a Guaranty" the words and punctuation ", or if such Subsidiary
         or entity shall have previously executed and delivered a Guaranty which
         has been subsequently released by the Required Holder(s), to execute
         and deliver a new Guaranty,"

         (b) Amendment to Paragraph 5N. Pledge of Subsidiary Stock. The last
         sentence of paragraph 5N of the Agreement is amended and restated in
         its entirety to read as follows:
<PAGE>

                  "If, however, after any release described in the preceding
                  sentence the Company is downgraded below the Minimum Rating,
                  the Company shall immediately pledge, and cause its
                  Subsidiaries to pledge, all stock or other equity interests in
                  all Guarantors, and 65 % (or if the Company shall pledge any
                  higher percentage of the issued and outstanding capital stock
                  of WGR Canada to any other Person, then such higher
                  percentage) of the issued and outstanding capital stock of WGR
                  Canada, to the holders of the Notes under one or more Pledge
                  Agreements."

         (c) Amendment to Paragraph 6C(5). Merger and Sale of Assets. Paragraph
         6C(5) of the Agreement is amended by inserting the word "and" at the
         end of clause (vi) thereof, deleting the comma and the word "and" at
         the end of clause (vii) thereof, adding a period at the end of clause
         (vii) thereof and deleting clause (viii) thereof.

         (d) Amendment to Paragraph 6C(7). Limitation on Credit Extension.
         Paragraph 6C(7) of the Agreement is amended by deleting the words "have
         not executed a Guaranty" and replacing them with the words "are not
         Guarantors" and by deleting the words "has executed a Guaranty" and
         replacing them with the words "is a Guarantor".

         (e) Amendment to Paragraph 6C(10). Guaranties. Paragraph 6C(10) of the
         Agreement is hereby amended by inserting the phrase "or such
         Subsidiary, as the case may be," after the word "Company" in the
         proviso at the end of such paragraph.

         (f) Amendment to Paragraph 6E(1). Modifications. Paragraph 6E(1) of the
         Agreement is amended, effective of the Amendment No. 3 Effective Date,
         in its entirety to read as follows:

                  "6E(1) Modifications. The Company will not amend or modify (i)
                  any term or provision of the NCNB Agreement or the 1995 Note
                  Purchase Agreement so as to change to an earlier date the date
                  on which any payment of principal is to be made thereunder,
                  (ii) any term or provision of the NCNB Agreement so as to
                  shorten the duration or increase the amount of any commitment
                  thereunder, or (iii) any term or provision of the 1995 Note
                  Purchase Agreement so as to increase the principal amount
                  outstanding thereunder or to change to an earlier date the
                  date on which any payment of principal is to be made
                  thereunder; provided, that the Company may increase the
                  interest rate or fees payable under or with respect to the
                  1995 Note Purchase Agreement or the NCNB Agreement if the
                  Company complies with the other provisions of
<PAGE>

                  this Agreement, including, without limitation, paragraph
                  6E(3)."

         (g) Waiver Under Paragraph 6E(3). To the extent that clause (v) of
         Section 6.2(a) of the NCNB Agreement could be interpreted to require
         the execution of an intercreditor agreement due to the issuance of the
         Subordinated Debt described in Section 1(a) of this Amendment,
         Prudential hereby waives any requirement under the final grammatical
         paragraph of paragraph 6E(3) that the holders of such Subordinated Debt
         (or the trustee under the indenture described in Section 1(a) of this
         Amendment) enter into an intercreditor agreement with the holders of
         the Notes.

         (h) Amendment to Paragraph 7A. Acceleration. Paragraph 7A of the
         Agreement is amended by (I) amending clause (iii) by deleting the
         dollar amount "$10,000,000" and replacing it with the dollar amount
         "$5,000,000" and (II) adding immediately after the word "Company" in
         the clause reading "then (a) if such event is a Event of Default
         specified in clause (viii), (ix) or (x) of this paragraph 7A with
         respect to the Company" words and punctuation reading ", any Restricted
         Subsidiary that is a Significant Subsidiary or a group of Restricted
         Subsidiaries that, taken together, would constitute a Significant
         Subsidiary".

         (i) Amendment of Paragraph 10B. Other Terms. Paragraph 10B of the
         Agreement is amended by (I) amending the following definitions:

                  (1) the definition of "Guarantor" is amended by deleting the
                  text and punctuation reading "WGR Canada, Inc., a New
                  Brunswick corporation;" and adding the words "after the
                  Amendment No. 3 Effective Date" to the end thereof; and

                  (2) the definition of "Restricted Payment" is amended by
                  adding immediately after the text and punctuation reading "any
                  payment of principal of," the following text and punctuation:

                           "or any deposit of any cash, securities or other
                           property in defeasance of,"

         and (II) adding the following defined terms in their respective
         alphabetical order:

                  "Amendment No. 3 Effective Date" shall mean the date on which
                  the Limited Waiver, Consent, Release and Amendment No. 3 to
                  Second Amended and Restated Master Shelf Agreement shall
                  become effective.
<PAGE>

                  "Existing Guaranty" shall mean each guaranty of a Subsidiary
                  of the Company existing immediately prior to the Amendment No.
                  3 Effective Date.

                  "Indenture" shall mean that certain Indenture, dated as of
                  June 15, 1999, among the Company, the Guarantors and Chase
                  Bank of Texas, N.A., as trustee, relating to the issuance of
                  up to $225,000,000 aggregate principal amount of senior
                  subordinated notes, as such Indenture is in effect on the date
                  noted above.

                  "Restricted Subsidiary" shall have the meaning specified in
                  the Indenture.

                  "Significant Subsidiary" shall have the meaning specified in
                  the Indenture.

                  "Unrestricted Subsidiary" shall have the meaning specified in
                  the Indenture.

                  "WGR Canada" shall mean WGR Canada, Inc., a New Brunswick
                  corporation.

         (j) Amendment of Exhibits. Exhibit B of the Pledge Agreement is hereby
         deleted and replaced with Exhibit B attached hereto.

SECTION 3. Conditions of Effectiveness.

         Each of the foregoing sections of this Amendment shall become effective
on the date hereof (the "Amendment No. 3 Effective Date"), subject in all cases
to the following having been received by, and being satisfactory to, Prudential:

         (a) duly executed counterparts of this Amendment;

         (b) copies of amendments in similar form and substance to this
         Amendment to the NCNB Agreement and the 1995 Note Purchase Agreement
         certified as true and correct copies by the Company;

         (c) copies of the Corresponding Releases;

         (d) a Consent, in the form attached as Exhibit A hereto, duly executed
         by each Guarantor except WGR Canada; and

         (e) a Guaranty executed by MGTC, which shall be substantially in the
         form of Exhibit E attached to the Agreement, except that it shall
         expressly state that the effectiveness thereof is conditioned upon
         MGTC's obtaining the approval of the Public Service Commission of the
         State of Wyoming; and
<PAGE>

         (f) payment of $6,000.00 to compensate Prudential for its allocable
         overhead for in-house legal support.

SECTION 4. Representations and Warranties of Company.

         As an inducement to Prudential to enter into this Amendment, the
Company represents and warrants as follows:

         (a) Organization. The Company is a corporation duly organized and
         existing in good standing under the laws of the State of Delaware.

         (b) Power and Authority. The Company has all requisite corporate power
         to execute, deliver and perform its obligations under this Amendment.
         The execution, delivery and performance by the Company of this
         Amendment have been duly authorized by all requisite corporate action
         on the part of the Company. The Company has duly executed and delivered
         this Amendment and this Amendment constitutes the legal, valid and
         binding obligation of the Company, enforceable against the Company in
         accordance with its terms.

         (c) No Conflicts. Neither the execution and delivery of this Amendment
         by the Company, nor the consummation of the transactions contemplated
         hereby, nor fulfillment of nor compliance with the terms and provisions
         hereof will conflict with, or result in a breach of the terms,
         conditions or provisions of, or constitute a default under, or result
         in any violation of, or result in the creation of any security
         interest, lien or other encumbrance upon any of the properties or
         assets of the Company pursuant to, its charter or by-laws, any award of
         any arbitrator or any agreement (including any agreement with
         stockholders), instrument, order, judgment, decree, statute, law, rule
         or regulation to which the Company is subject.

         (d) Consents. Neither the nature of the business conducted by the
         Company, nor any of its properties, nor any relationship between the
         Company and any other Person, nor any circumstance in connection with
         the transactions contemplated by this Amendment is such as to require
         any authorization, consent, approval, exemption or other action by or
         notice to or filing with any court or administrative or governmental
         body or any other Person in connection with the execution and delivery
         of this Amendment or fulfillment of or compliance with the terms and
         provisions hereof.

         (e) Additional Documentation. No documents or instruments, including
         consents, authorizations and filings, are required under the
         certificate of incorporation and bylaws of the Company, or any
         applicable law with respect to the Company or any of its property or to
         which the Company or any of its property is subject, or by any material
         provision of any security issued by the Company or of any agreement,
         instrument or undertaking under which the Company is obligated or by
         which it or any of the property
<PAGE>

         owned by it is bound, in connection with the execution, delivery,
         performance, validity and enforceability of this Amendment and the
         other documents to be executed and delivered hereunder.

         (f) No Material Adverse Change. Except as previously disclosed to
         Prudential in writing, there has been no material adverse change in the
         business, property or assets, condition (financial or otherwise) or
         operations of the Company and its Subsidiaries taken as a whole since
         December 31, 1998.

         (g) No Event of Default or Default. Immediately following the
         effectiveness of this Amendment, no Event of Default or Default exists.

         (h) WGR Canada. The 650 shares of common stock of WGR Canada remaining
         subject to the liens and security interests granted by the Company
         under the Pledge Agreement, after giving effect to the release provided
         for in the first paragraph of Section 1 of this Amendment, constitute
         65% of the total number of issued and outstanding shares of capital
         stock of WGR Canada.

         (i) Bridge Facility. All amounts owing under the Bridge Facility have
         been paid in full and the Bridge Facility has been terminated.

         (j) WGRS Existing Guaranties. All Existing Guaranties executed by WGRS
         have been terminated.

SECTION 5. Miscellaneous.

         (a) Upon and after the Amendment No. 3 Effective Date, each reference
         to the Agreement or "this Agreement" in the Agreement and each Note
         shall mean and be a reference to the Agreement as amended by this
         Amendment.

         (b) Except as specifically amended herein, the Agreement shall remain
         in full force and effect, and is hereby ratified and confirmed.

         (c) Other than as expressly set forth herein, the execution, delivery
         and effectiveness of this Amendment shall not operate as a waiver of
         any right, power or remedy of Prudential, nor constitute a waiver of
         any provision of the Agreement, the Notes, the Guaranties, the Pledge
         Agreement or any other document, instrument or agreement executed and
         delivered in connection with the Agreement. (d) The Company confirms
         its agreement, pursuant to paragraph 11B of the Agreement, to pay
         promptly all expenses of Prudential related to this Amendment and all
         matters contemplated hereby.

         (e) GOVERNING LAW. THIS AMENDMENT SHALL BE CONSTRUED AND ENFORCED IN
         ACCORDANCE WITH, AND THE RIGHTS OF THE PARTIES SHALL BE GOVERNED BY,
         THE LAW OF THE STATE OF NEW YORK.
<PAGE>

         (f) This Amendment may be executed in counterparts (including those
         transmitted by facsimile), each of which shall be deemed an original
         and all of which taken together shall constitute one and the same
         document. Delivery of this Amendment may be made by telecopy of a duly
         executed counterpart copy hereof.

         IN WITNESS WHEREOF, the parties hereto have caused their duly
authorized officers to execute this Amendment as of the day and year first above
written.

                                  WESTERN GAS RESOURCES, INC.

                                  By:
                                     -------------------------------------------
                                      William J. Krysiak, Vice President-Finance

THE PRUDENTIAL INSURANCE
  COMPANY OF AMERICA

By:
   -------------------------------
   Vice President

PRUCO LIFE INSURANCE COMPANY

By:
   -------------------------------
   Vice President
<PAGE>

                                                                       EXHIBIT A
                                                                       ---------

                              CONSENT TO AMENDMENT

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 of 2001 as amended by Letter
Amendment No. 1 dated November 21, 1997 and Letter Amendment No. 2 dated March
31, 1999 (as amended, the "Agreement"). Prudential and the Company are entering
into that certain Limited Waiver, Consent, Release and Amendment No. 3 to Second
Amended and Restated Master Shelf Agreement, dated as of June 1, 1999 (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 references in the Guaranty of the undersigned to the "Agreement,"
"thereunder," "thereof," or 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 June 1, 1999.

LANCE OIL & GAS COMPANY, INC.
MGTC, INC.
MIGC, INC.
MOUNTAIN GAS RESOURCES, INC.
PINNACLE GAS TREATING, INC.
WESTERN GAS RESOURCES - TEXAS, INC.
WESTERN GAS RESOURCES - OKLAHOMA, INC.
WESTERN GAS WYOMING, L.L.C.

By:
   -------------------------------------------------
     William J. Krysiak, Vice President-Finance
     of each of the above-name companies.
<PAGE>

                                     Issuers
                                     -------

CORPORATIONS                           Certificate   No. of
    Issuer                                 No.       Shares        Class
---------------------------------------------------------------------------

MIGC, Inc...............................    3        100,000       common

Western Gas Resources-Texas, Inc........    3            990       common
Western Gas Resources-Texas, Inc........    4             10       common
Mountain Gas Resources, Inc.............  A-3      1,000,843       common
Western Gas Resources-Oklahoma, Inc.        1          1,000       common
Western Power Services, Inc.............    1          1,000       common
Pinnacle Gas Treating, Inc..............    1          1,000       common

Lance Oil & Gas Company, Inc............    1          1,000       common

LIMITED LIABILITY COMPANIES
---------------------------

Issuer                                    Membership Interest
------                                    -------------------

Western Gas Wyoming, L.L.C..........             100%
<PAGE>

                              CONSENT TO AMENDMENT

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 of 2001as amended by Letter
Amendment No. 1 dated November 21, 1997 and Letter Amendment No. 2 dated March
31, 1999 (as amended, the "Agreement"). Prudential and the Company are entering
into that certain Limited Waiver, Consent, Release and Amendment No. 3 to Second
Amended and Restated Master Shelf Agreement, dated as of June 1, 1999 (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 references in the Guaranty of the undersigned to the "Agreement,"
"thereunder," "thereof," or 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 June 1, 1999.

LANCE OIL & GAS COMPANY, INC.
MGTC, INC.
MIGC, INC.
MOUNTAIN GAS RESOURCES, INC.
PINNACLE GAS TREATING, INC.
WESTERN GAS RESOURCES - TEXAS, INC.
WESTERN GAS RESOURCES - OKLAHOMA, INC.
WESTERN GAS WYOMING, L.L.C.

By:
   -----------------------------------------------
     William J. Krysiak, Vice President-Finance
     of each of the above-name companies.

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