<PAGE>

                                                                   EXHIBIT 10.15

                        FIRST AMENDMENT TO LOAN AGREEMENT
                        ---------------------------------

         THIS FIRST AMENDMENT TO LOAN AGREEMENT (herein called this "Amendment")
is made as of the 10th day of June, 1999 by and among Western Gas Resources,
Inc. ("Borrower"), and NationsBank, N.A., as Agent ("Agent"), and the Lenders
under the Loan Agreement referred to below.

                                   WITNESSETH:

         WHEREAS, Borrower, Agent, and Lenders have entered into that certain
Loan Agreement dated as of April 29, 1999 (as amended, restated, or supplemented
to the date hereof, the "Original Agreement"), for the purposes and
consideration therein expressed, pursuant to which Lenders made and became
obligated to make loans to Borrower as therein provided;

         WHEREAS, in order to secure the Obligations, including the obligations
of Borrower under the Notes and the Loan Agreement, Borrower caused (i) WGR
Canada, Inc., a New Brunswick corporation ("WGRC"), to execute and deliver to
the Agent for the benefit of Lenders, that certain Guaranty dated as of April
29, 1999 (the "WGRC Guaranty"), and (ii) MGTC, Inc., a Wyoming corporation
("MGTC"), to execute and deliver to the Agent for the benefit of Lenders, that
certain Guaranty dated as of April 29, 1999 (the "MGTC Guaranty");

         WHEREAS, Borrower has been advised by its Wyoming counsel that the MGTC
Guaranty is void because the approval of the Wyoming Public Service Commission
was not obtained with respect thereto and that the MGTC Guaranty should be
released and replaced by a new guaranty executed by MGTC which shall become
effective only upon obtaining the approval thereof by the Wyoming Public Service
Commission;

         WHEREAS, in order to secure the Obligations, including the obligations
of Borrower under the Notes and the Loan Agreement, Borrower executed and
delivered that certain Pledge Agreement dated as of April 29, 1999 in favor of
Agent for the benefit of Lenders (the "Pledge Agreement"), pursuant to which
Borrower pledged and granted a security interest to Agent, for the benefit of
Lenders, in, among other things, 1,000 shares of common stock of WGRC (the "WGRC
Pledged Stock");

         WHEREAS, Borrower and WGRC have requested that Agent and Lenders (i)
terminate the WGRC Guaranty and otherwise grant a general release of WGRC under
the WGRC Guaranty and any other obligations and liabilities arising under all
documents and agreements delivered pursuant to the WGRC Guaranty or in
connection therewith, (ii) terminate the MGTC Guaranty and otherwise grant a
general release of MGTC under the MGTC Guaranty and any other obligations and
liabilities arising under all documents and agreements delivered pursuant to the
MGTC Guaranty or in connection therewith and (iii) release the Agent's security
interest in 35% of the WGRC Pledged Stock (the "Releases");

         WHEREAS, Borrower and Lenders desire to provide for Lenders' consent
and agreement that certain matters relating to the terms of subordinated debt to
<PAGE>

be offered by Borrower are to Lenders' satisfaction, and the amendment of the
Loan Agreement as specified herein; and

         WHEREAS, Administrative Agent and Lenders are willing to grant the
Releases, subject to the condition that the note purchasers parties to the Note
Purchase Agreement among Borrower and the American General Group, as amended
(the "AG Agreement") and the holders of the notes issued pursuant to the Master
Shelf Agreement dated as of December 19, 1991 between the Borrower and the
Prudential Insurance Company of America, as amended (the "Prudential Agreement")
grant similar releases of the guaranties provided by WGRC and MGTC to such note
purchasers and such holders, and the respective security interests of such note
purchasers and such holders in 350 shares of common stock of WGRC (the
"Corresponding Releases");

         NOW, THEREFORE, in consideration of the premises and the mutual
covenants and agreements contained herein and in the Original Agreement, in
consideration of the loans which may hereafter be made by Lenders to Borrower,
and for other good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the parties hereto do hereby agree as follows:

                                   ARTICLE I.
                           Definitions and References
                           --------------------------

         ss. 1.I. Defined Terms. Unless the context otherwise requires or unless
otherwise expressly defined herein, the terms defined in the Original Agreement
shall have the same meanings whenever used in this Amendment. As used herein,
the term "Loan Agreement" means the Original Agreement as amended by this
Amendment.

                                   ARTICLE II.
                         Consent, Release and Amendments
                         -------------------------------

         ss. 2.I. Consent. Subject to the terms and conditions set forth herein,
and in reliance upon the representations and warranties of Borrower contained
herein, pursuant to the definition of the term "Subordinated Debt" in Section
1.1 of the Loan Agreement, Lenders hereby consent and agree that $155,000,000 of
Debt of Borrower proposed to be issued by Borrower on the terms provided in that
certain Indenture, draft dated June 3, 1999, among Borrower, the Guarantors
(other than WGRC) and Chase Bank of Texas, N.A. (the "Draft Indenture") and the
guaranties by such Guarantors 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 Obligations owing by
Borrower and such Guarantors in a manner satisfactory to Lenders, so long as 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.

         Pursuant to Section 8.1(i), each Lender hereby consents to the
execution and delivery of the following by Borrower and Guarantors: (i) an
amendment to the American General Agreement substantially in the form of the
draft thereof dated June 8, 1999 and (ii) an amendment to the Prudential
Agreement substantially in the form of the draft thereof dated June 6, 1999.
<PAGE>

         ss. 2.II. Release.

         (a) WGRC Guaranty.' Agent and each Lender hereby release and discharge,
         effective as of the Effective Date, WGRC and its successors and assigns
         from all obligations and liabilities under the WGRC Guaranty and all
         documents and agreements delivered pursuant thereto and further hereby
         terminates the WGRC Guaranty and deletes WGRC from the definition of
         Guarantor in Section 1.1 of the Loan Agreement; provided that the
         Corresponding Releases are executed and delivered on or prior to the
         Effective Date.

         (b) WGRC Stock. In addition, Agent and each Lender hereby release and
         discharge, effective as of the Effective Date, 350 shares of common
         stock of WGRC from the liens and security interests granted by Borrower
         pursuant to the Pledge agreement, automatically and without further
         action by any party; provided that the Corresponding Releases are
         executed and delivered on or prior to the Effective Date. Borrower
         agrees that it shall, from time to time, execute, acknowledge and
         deliver to Agent instruments, agreements, and other documents as Agent
         shall reasonably request in order to further evidence the liens and
         security interests in the 650 shares of common stock of WGRC remaining
         subject to the Pledge Agreement, including delivering to NationsBank,
         N.A., as Agent and as bailee for the note purchasers under the American
         General Agreement and the holders under the Prudential Agreement, a new
         stock certificate and a new stock power evidencing 650 shares of common
         stock of WGRC.

         (c) MGTC Guaranty. Agent and Lenders hereby release and discharge MGTC,
         effective as of the Effective Date, from all obligations and
         liabilities under the MGTC Guaranty and all documents and agreements
         delivered pursuant thereto, provided that on or prior to the Effective
         Date (i) the Corresponding Releases are executed and delivered, and
         (ii) the conditional Guaranty described in Section 3.1(e) is executed
         and delivered. In addition. Agent and Lenders hereby waive any Default
         or Event of Default arising solely due to the failure of MGTC to obtain
         prior approval of the Wyoming Public Service Commission with respect to
         the MGTC Guaranty.

         ss. 2.III. Definitions. (a) The definition of "Change in Control" in
Section 1.1 of the Loan Agreement is hereby amended by deleting the period at
the end thereof, adding a semicolon and the word "or" in place thereof, and
adding a new clause (f) to read as follows:

                  "(f) the occurrence of a 'Change of Control' as defined in the
                  Indenture."

         (b) Section 1.1 of the Loan Agreement is hereby amended by adding the
following definition of "Indenture" immediately after the definition of "Highest
Lawful Rate" and by adding the following definition of "Required Holders"
immediately after the definition of "Related Person:"
<PAGE>

                  "Indenture" means the Indenture dated June 15, 1999 among
         Borrower, Guarantors and Chase Bank of Texas National Association, as
         trustee, relating to the issuance of up to $225,000,000 aggregate
         principal amount of Borrower's senior subordinated notes, as such
         Indenture is in effect on June 15, 1999."

                  "Required Holders" means Majority Lenders for purposes of the
         Indenture."

         ss. 2.IV. Limitation on Prepayments of Debt Securities and Subordinated
Debt. Section 6.2(e) of the Loan Agreement is hereby amended by replacing the
period at the end of subsection (iii) with a semicolon and adding thereafter a
new subsection (iv) to read as follows:

                  "(iv) purchase, repurchase, defease or make any pre-payments
                  on the Subordinated Debt."

         ss. 2.V. Guaranties of Borrower's Subsidiaries. Section 7.3 of the Loan
Agreement is hereby amended in its entirety to read as follows:

                  "Section 7.3. Guaranties of Borrower's Subsidiaries. Borrower
                  shall require each of the following Subsidiaries (other than
                  WGRC) to immediately execute and deliver to Agent an absolute
                  and unconditional guaranty of the timely repayment of the
                  Obligations and the due and punctual performance of the
                  obligations of Borrower hereunder, which guaranty shall be
                  satisfactory to Agent in form and substance:

                  (a)      Each Subsidiary of Borrower which has EBITDA in any
                           Fiscal Quarter which constitutes ten percent (10%) or
                           more of Borrower's Consolidated EBITDA for such
                           Fiscal Quarter or which has assets at any time with a
                           book value equal to or exceeding ten percent (10%) of
                           the book value of Borrower's Consolidated assets at
                           such time;

                  (b)      If the aggregate amount of Borrower's unconsolidated
                           EBITDA for any Fiscal Quarter plus the aggregate
                           EBITDA of Guarantors during such Fiscal Quarter does
                           not constitute eighty- five percent (85%) or more of
                           Borrower's Consolidated EBITDA for such Fiscal
                           Quarter or if the book value of Borrower's individual
                           assets at any time plus the aggregate book value of
                           the assets of Guarantors at such time does not exceed
                           eight-five percent (85%) of
<PAGE>

                           the book value of Borrower's Consolidated assets at
                           such time, then Subsidiaries of Borrower with
                           aggregate assets and/or EBITDA necessary to comply
                           with the eighty-five percent (85%) tests contained in
                           this subsection;

                  (c)      each Subsidiary which guaranties the Debt Securities
                           or the Subordinated Debt; and

                  (d)      Upon request by Agent on behalf of Majority Lenders,
                           any other Subsidiary of Borrower."

         ss. 2.VI. Events of Default. Section 8.1(b) is hereby amended in its
entirety to read as follows:

                  "(b)     Any default occurs under any Loan Document, any
                           document governing or evidencing the Debt Securities
                           or the Subordinated Debt, or evidencing any interest
                           therein, or any event of default or termination event
                           occurs under any Hedging Contract to which a Lender
                           is a party, and such default, event of default or
                           termination event is not remedied within the
                           applicable period of grace (if any) provided for in
                           such document; or"

                                  ARTICLE III.
                           Conditions of Effectiveness
                           ---------------------------

         ss. 3.I. Effective Date. This Amendment shall become effective (the
"Effective Date") as of the date first above written when, and only when, Agent
shall have received all of the following:

         (1) This Amendment, duly authorized, executed and delivered by Borrower
         and Majority Lenders, and in form and substance satisfactory to Agent.

         (2) A certificate of a duly authorized officer of Borrower to the
         effect that all of the representations and warranties set forth in
         Article IV hereof are true and correct at and as of the time of such
         effectiveness.

         (3) A certificate of the Secretary of Borrower dated the date of this
         Amendment certifying that attached thereto is a true and complete copy
         of resolutions adopted by the Board of Directors of Guarantor
         authorizing the execution, delivery and performance of this Amendment
         by Borrower and certifying the names and true signatures of the
         officers of Borrower authorized to sign this Amendment and the other
         Loan Documents on behalf of Borrower.
<PAGE>

         (4) A Guaranty executed by MGTC, which shall be in the form attached to
         this Agreement expressly stating that the effectiveness thereof is
         conditioned upon MGTC's obtaining the approval of the Public Service
         Commission of the State of Wyoming.

         (5) The Corresponding Releases.

                  (f)      Such supporting documents as Agent may reasonably
                           request.

                  (g)      Payment of all fees and expenses owing to Agent and
                           Lenders in connection with this Amendment and payment
                           of fees and disbursements of Thompson & Knight, P.C.
                           relating to this Amendment and the Loan Agreement as
                           provided in the Loan Agreement.

                                   ARTICLE IV.
                         Representations and Warranties
                         ------------------------------

         ss. 4.I. Representations and Warranties of Borrower. In order to induce
each Lender to enter into this Amendment, Borrower represents and warrants to
each Lender that:

         (1) The representations and warranties contained in Article V of the
         Original Agreement are true and correct at and as of the time of the
         effectiveness hereof except as such representations and warranties have
         been modified by the transactions contemplated herein).

         (2) Borrower is duly authorized to execute and deliver this Amendment
         and Borrower is and will continue to be duly authorized to borrow
         monies and to perform its obligations under the Loan Agreement.
         Borrower has duly taken all corporate action necessary to authorize the
         execution and delivery of this Amendment.

         (3) The execution and delivery by Borrower of this Amendment, the
         performance of its obligations hereunder and the consummation of the
         transactions contemplated hereby do not and will not conflict with any
         provision of law, statute, rule or regulation or of the certificate of
         incorporation and bylaws of Borrower or of any material agreement,
         judgment, license, order or permit applicable to or binding upon
         Borrower or result in the creation of any lien, charge or encumbrance
         upon any assets or properties of Borrower. Except for those which have
         been obtained, no consent, approval, authorization or order of any
         court or governmental authority or third party is required in
         connection with the execution and delivery by Borrower of this
         Amendment.

         (4) When duly executed and delivered, this Amendment and the Loan
         Agreement will be a legal and binding obligation of Borrower
         enforceable against Borrower in accordance with its terms, except as
         limited by bankruptcy, insolvency or similar laws of general
         application relating to the enforcement of creditors' rights and by
         equitable principles of general application.
<PAGE>

         (5) The unaudited Consolidated quarterly financial statements of
         Borrower dated as of March 31, 1999 fairly present the Consolidated
         financial position at such date of Borrower and the Consolidated
         statement of operations and the changes in Consolidated financial
         position for the periods ending on such date for Borrower. Copies of
         such financial statements have heretofore been delivered to Agent.
         Since March 31, 1999, no material adverse change has occurred in the
         financial condition or business or in the Consolidated financial
         condition or business of Borrower.

         (f)      the 650 shares of common stock of WGRC remaining subject to
                  the liens and security interests granted by Borrower under the
                  Pledge Agreement, after giving effect to the release provided
                  for in Section 2.2(b) of this Amendment, constitute 65% of the
                  total number of issued and outstanding shares of capital stock
                  of WGRC.

                                   ARTICLE V.
                                  Miscellaneous
                                  -------------

         ss. 5.I. Ratification of Agreements. The Original Agreement as hereby
amended is hereby ratified and confirmed in all respects. Any reference to the
Loan Agreement in any Loan Document shall be deemed to be a reference to the
Original Agreement as hereby amended. The Loan Documents, as they may be amended
or affected by this Amendment, are hereby ratified and confirmed in all
respects. The execution, delivery and effectiveness of this Amendment shall not,
except as expressly provided herein, operate as a waiver of any right, power or
remedy of Lenders under the Loan Agreement, the Notes, or any other Loan
Document nor constitute a waiver of any provision of the Loan Agreement, the
Notes, or any other Loan Document.

         ss. 5.11. Survival Agreements. All representations, warranties,
covenants and agreements of Borrower herein shall survive the execution and
delivery of this Amendment and the performance hereof, including without
limitation the making or granting of the Loans, and shall further survive until
all of the Obligations are paid in full. All statements and agreements contained
in any certificate or instrument delivered by Borrower hereunder or under the
Loan Agreement to any Lender shall be deemed to constitute representations and
warranties by, and/or agreements and covenants of, Borrower under this Amendment
and under the Loan Agreement.

         ss. 5.III. Loan Documents. This Amendment is a Loan Document, and all
provisions in the Loan Agreement pertaining to Loan Documents apply hereto.

         ss. 5.IV. Governing Law. This Amendment shall be governed by and
construed in accordance the laws of the State of Texas and any applicable laws
of the United States of America in all respects, including construction,
validity and performance.

         ss. 5.V. Counterparts. This Amendment may be separately executed in
counterparts and by the different parties hereto in separate counterparts, each
of
<PAGE>

which when so executed shall be deemed to constitute one and the same Amendment.

         THIS AMENDMENT AND THE OTHER LOAN DOCUMENTS REPRESENT THE FINAL
AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR,
CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREE-MENTS OF THE PARTIES. THERE ARE NO
UNWRITTEN ORAL AGREEMENTS OF THE PARTIES.

IN WITNESS WHEREOF, this Amendment is executed as of the date first above
written

                                   WESTERN GAS RESOURCES, INC.

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


NATIONSBANK, N.A., as Agent and Lender
By:
   -------------------------------------------
Name:  Ilene S. Larimore
Title: Senior Vice President

SOCIETE GENERALE SOUTHWEST AGENCY, a Lender
By:
   -------------------------------------------
Name:  Richard A. Erbert
Title: Vice President

ABN AMRO BANK, a Lender
By:
   -------------------------------------------
Name:  Robert J. Cunningham
Title: Group Vice President

By:
   -------------------------------------------
Name:  Michael A. Tribolet
Title: Senior Vice President

CREDIT LYONNAIS, a Lender
By:
   -------------------------------------------
Name:  Philippe Soustra
Title: Senior Vice President

BANKBOSTON, N.A., a Lender
By:
   -------------------------------------------
Name:  Terrence Ronan
Title: Director

THE FIRST NATIONAL BANK OF CHICAGO, a Lender
By:
   -------------------------------------------
Name:  Susan Stiernberg
Title: Authorized Officer
<PAGE>

UNION BANK OF CALIFORNIA, N.A., a Lender
By:
   -------------------------------------------
Name:  Gary Shekerjian
Title: Assistant Vice President

By:
   -------------------------------------------
Name:  Randy Osterberg
Title: Vice President

U.S. BANK NATIONAL ASSOCIATION, a Lender
By:
   -------------------------------------------
Name:  Charles S. Searle
Title: Senior Vice President
<PAGE>

                              CONSENT AND AGREEMENT
                              ---------------------

         Each of the undersigned hereby (i) consents to the provisions of this
Amendment and the transactions contemplated herein, and (ii) ratifies and
confirms its respective Guaranty dated as of April 29, 1999 made by it in favor
of Agent for the Benefit of each Lender, and agrees that its obligations and
covenants thereunder are unimpaired hereby and shall remain in full force and
effect.

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

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