Document:

<PAGE>
                                                               [Execution Copy]

                                                                  Exhibit 10.24

             LIMITED WAIVER, CONSENT, RELEASE AND AMENDMENT NO. 4
                                      TO
              SECOND AMENDED AND RESTATED MASTER SHELF AGREEMENT
                         (Western Gas Resources, Inc.)

    This LIMITED WAIVER, CONSENT, RELEASE AND AMENDMENT NO. 4 TO SECOND AMENDED
AND RESTATED MASTER SHELF AGREEMENT (this "Amendment") is entered into as of
August 25, 2000, 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 a 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 and Limited Waiver, Consent, Release and
Amendment No. 3 dated June 1, 1999 (as amended, the "Agreement"). Capitalized
terms not otherwise defined herein shall have the meanings specified in the
Agreement, as amended hereby.

    2.  The Company is the sole shareholder of Western Gas Resources - Oklahoma,
Inc. ("WGRO").

    3.  WGRO has recently acquired all of the partnership interests in Westana
Gathering Company, a general partnership ("Westana"), the primary assets of
which are the Avard/Waynoka gathering system (the "System") and the Chester gas
processing plant (the "Plant").

    4.  Concurrently with the repayment of the notes issued under the 1995
Note Purchase Agreement (the "American General Notes"), the Company desires to
undertake the following transactions with respect to WGRO and Westana
(collectively, the "Westana Transactions"):

        (a)  all of Westana's assets shall be liquidated into WGRO;

        (b)  WGRO shall concurrently therewith create a new subsidiary
    ("Newco"), which shall be capitalized with the System; and

        (c)  subsequent to the creation Newco, WGRO will be merged with and into
    Newco with Newco being the surviving entity, and, as a result of such
    merger, the Company shall become the owner of the Plant.

<PAGE>

    5.  In order to secure the Obligations (as defined in the hereinafter
defined WGRO Guaranty), including the obligations of the Company under the Notes
and the Agreement, the Company caused Western Gas Resources - Oklahoma, Inc., a
Delaware corporation ("WGR Oklahoma"), to execute and deliver to Prudential a
Guaranty in favor of Prudential together with all subsequent holders of the
Obligations (the "WGRO Guaranty").

    6.  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, the common stock of WGRO (the "WGRO Pledged Stock").

    7.  The Company and WGRO have requested that Prudential (i) terminate the
WGRO Guaranty and otherwise grant a general release of WGRO under the WGRO
Guaranty and any other obligations and liabilities arising under all documents
and agreements delivered pursuant to the WGRO Guaranty or in connection
therewith and (ii) release Prudential's security interest in the shares of
capital stock of WGRO under the Pledge Agreement (the "Release").

    8.  The Company and Prudential desire to amend the Agreement for the
purposes described herein and to consent to the consummation of the Westana
Transactions in connection with the repayment of the American General Notes.

    9.  Prudential is willing to grant the Release, subject to the condition
that the lenders parties to the NCNB Agreement grant a similar release of the
guaranty provided by WGRO to such lenders parties to the NCNB Agreement and NCNB
and the respective security interests of such lenders parties to the NCNB
Agreement and NCNB in shares of common stock of WGRO (the "Corresponding
Release").

    10. 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.  Consents, Waivers and Releases.

    (a) Consent to Westana Transactions; Waiver. Prudential hereby consents to
the Westana Transactions.

                                       2
<PAGE>

        (b) Release of WGRO.  Prudential hereby terminates, releases and
discharges the shares of common stock of WGRO from the liens and security
interests granted by the Company pursuant to the Pledge Agreement, in each case
on or after the Westana Effective Date and the satisfaction in full of all
conditions precedent set forth in Section 3 of this Amendment.  The Company
hereby agrees to deliver to Prudential, within 30 days after the Westana
Effective Date, an amendment to the Intercreditor Agreement duly executed by the
parties thereto confirming the matters described in Section 3(h) hereto and the
removal of American General Life Insurance Company as a party thereto due to
payment in full of the American General Notes.

    SECTION 2.  Amendments.

        (a) Amendment to Paragraph 6C(4). Sale of Stock and Debt of
Subsidiaries. Paragraph 6C(4) is hereby amended in its entirety to read as
follows:

        "6C(4).  Sale of Stock and Debt of Subsidiaries. Sell or otherwise
    dispose of, or part with control of, any shares of stock or Debt of any
    Subsidiary, except to the Company or another Wholly Owned Subsidiary, and
    except that all shares of stock and Debt of any Subsidiary at the time owned
    by or owed to the Company and all Subsidiaries may be sold as an entirety
    for a cash consideration which represents the fair value (as determined in
    good faith by the Board of Directors of the Company) at the time of sale of
    the shares of stock and Debt so sold, provided that (i) the assets of such
    Subsidiary together with (ii) the assets of all other Subsidiaries the stock
    or Debt of which was sold or otherwise disposed of in the preceding 12-month
    period and (iii) the assets of the Company and its Subsidiaries sold,
    leased, transferred or otherwise disposed of pursuant to clause (v) of
    paragraph 6C(5) in the preceding 12-month period (in each transaction
    measured by the greater of book value or Fair Market Value), do not
    represent more than 15% of Consolidated Net Tangible Assets as reflected on
    the most recent annual or quarterly consolidated balance sheet, and provided
    further that, at the time of such sale, such Subsidiary shall not own,
    directly or indirectly, any shares of stock or Debt of, or any other
    continuing investment in any other Subsidiary (unless all of the shares of
    stock and Debt of such other Subsidiary owned, directly or indirectly, by
    the Company and all Subsidiaries are simultaneously being sold as permitted
    by this paragraph 6C(4)), or any shares of stock or Debt of the Company."

        (b) Amendment to Paragraph 6C(5).  Merger and Sale of Assets.  Clause
(v) of paragraph 6C(5) is amended in its entirety to read as follows:

        "(v)  the Company or any Subsidiary may sell, lease, transfer or
    otherwise dispose of any of its assets to any Person, provided, that (a)
    such assets together with (b) all other assets of the Company and its
    Subsidiaries sold, leased, transferred or otherwise disposed of during the
    preceding 12-month period, and (c) the assets of all

                                       3
<PAGE>

     Subsidiaries the stock or Debt of which has been sold or otherwise disposed
     of during the preceding 12-month period pursuant to the first proviso of
     paragraph 6C(4) (in each transaction measured by the greater of book value
     or Fair Market Value), do not represent more that 15% of Consolidated Net
     Tangible Assets as reflected on the most recent annual or quarterly
     consolidated balance sheet,"

        (c) Amendment to Paragraph 6.  Negative Covenants.  Paragraph 6 is
amended by adding to end thereof a new paragraph 6F to read as follows:

        "6F. Restrictions on Hedging Transactions. The Company will not and will
    not permit any Subsidiary to enter into Hedging Transactions with respect to
    crude oil, natural gas or liquid hydrocarbons other than Hedging
    Transactions (i) that apply to not more than the Adjusted Equity Gas Volume
    in any calendar year and (ii) for the succeeding three calendar years;
    provided, however, that this paragraph 6F shall not prevent the Company and
    its Subsidiaries from entering into Hedging Transactions with respect to
    100% of the natural gas of the Company or its Subsidiaries (a) held in
    storage facilities owned by the Company or its Subsidiaries or (b) purchased
    from third parties and sold to third parties in connection with the
    marketing operations of the Company and its Subsidiaries. 'Adjusted Equity
    Gas Volume' for any calendar year, shall mean the volumes of natural gas and
    liquid hydrocarbons owned by the Company and its Subsidiaries either through
    ownership of interests in oil and gas properties ('ownership interests') or
    pursuant to contractual gathering and processing agreements ('contractual
    arrangements') that, (x) in the case of ownership interests, do not exceed
    75% of proved developed producing reserves to be produced in such calendar
    year and 50% of proved undeveloped reserves projected to be producing during
    such calendar year and, (y) in the case of contractual arrangements, do not
    exceed 75% of the Company's and its Subsidiaries' pro rata share of minimum
    contracted volumes of gas to be collected and/or processed in such calendar
    year. 'Hedging Transactions' shall mean, with respect to the Company and its
    Subsidiaries, any commodity basis swap, forward commodity transaction,
    commodity swap, commodity option, commodity index swap, commodity cap
    transaction, commodity floor transaction, commodity collar transaction, any
    other similar transaction that relates to commodities (including any option
    with respect to any of the foregoing transactions) or any combination of the
    foregoing transactions. The Company will deliver to the holder of each Note
    a statement detailing its hedge position in a form satisfactory to the
    Required Holders of the Notes of each Series at the same time it delivers
    the financial statements contemplated by paragraphs 5A (i) and 5A(ii)."

        (d) Amendment to Paragraph 10B.  Other Terms.  Paragraph 10B of the
Agreement is amended by amending the definition of "Consolidated Net Earnings"
in its entirety to read as follows:

            "Consolidated Net Earnings' shall mean consolidated gross revenues
    of the Company and its Subsidiaries excluding gains resulting from the sale,
    conversion or other disposition of capital assets (including capital stock
    of Subsidiaries and other

                                       4
<PAGE>

     assets not constituting current assets) and other non-cash gains, less all
     operating and non-operating expenses of the Company and its Subsidiaries
     (other than losses resulting from the sale, conversion or other disposition
     of capital assets, including capital stock of Subsidiaries and other assets
     not constituting current assets and other non-cash losses) and all charges
     of a proper character (including current and deferred taxes on income,
     provision for taxes on unremitted foreign earnings that are included in
     gross revenues, and current additions to reserves), but not including in
     gross revenues any gains resulting from the write-up of assets, any equity
     of the Company or any Subsidiary in the unremitted earnings of any Person
     that is not a Subsidiary, any earnings of any Person acquired by the
     Company or any Subsidiary through purchase, merger or consolidation or
     otherwise for any period prior to the time of acquisition, or any deferred
     credit representing the excess of equity in any Subsidiary at the date of
     acquisition over the cost of the investment in such Subsidiary, all
     determined in accordance with generally accepted accounting principles."

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

            SECTION 3.  Conditions of Effectiveness.

        Except for the Westana Amendments, this Amendment shall become
effective as of the date first above written (the "Amendment No. 4 Effective
Date") when, and only when, Prudential shall have received all of the following
and the Westana Amendments shall become effective as the Westana Effective Date
when and only when, Prudential shall have received all of the following:

        (a) duly executed counterparts of this Amendment;

        (b) copies of an amendment in similar form and substance to this
    Amendment to the NCNB Agreement certified as true and correct copies by the
    Company;

        (c) copies of the Corresponding Release;

        (d) the Consent attached hereto, duly executed by each Guarantor except
    WGRO;

        (e) a certificate of a duly authorized officer of the Company dated the
    date of this Amendment certifying: (i) that all of the representations and
    warranties set forth in Section 4 hereof are true and correct at and as of
    the time of such effectiveness; and (ii) as to such other corporate matters
    as Prudential shall deem necessary;

        (f) a written legal opinion of in-house counsel for the Company, dated
    as of the date of this Amendment, addressed to Prudential, to the effect
    that this Amendment

                                       5
<PAGE>

    has been duly authorized, executed and delivered by the Company and that the
    Agreement and each other document, as affected hereby, to which any Related
    Person is a party constitutes the legal, valid and binding obligation of
    each such Related Person, enforceable in accordance with their terms
    (subject, as to enforcement of remedies, to applicable bankruptcy,
    reorganization, insolvency and similar laws and to general principles of
    equity) and such other matters as Prudential may require;

        (g)  payment of $2,700.00 to compensate Prudential for its allocable
    overhead for in-house legal support; and

        (h)  The Westana Amendments shall become effective as of the date on
    which the American General Notes are repaid in full (the "Westana Effective
    Date") when and only when, Prudential shall have received evidence
    satisfactory to Prudential, in its sole and absolute discretion, that each
    other Person then a party to the Intercreditor Agreement has released its
    guaranty from WGRO and lien and security interest in the common stock of
    WGRO.

    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)  Representations. The representations and warranties contained in
    paragraph 8 of the 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).

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

        (c)  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.

        (d)  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

                                       6
<PAGE>

    (including any agreement with stockholders), instrument, order, judgment,
    decree, statute, law, rule or regulation to which the Company is subject.

        (e)  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.

        (f)  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 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.

        (g)  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, 1999.

        (h)  No Event of Default or Default. As of the date of this Amendment,
    no Event of Default or Default exists.

        (i)  Newco Guaranty Not Required. Upon the Westana Effective Date, Newco
shall not be required to execute and deliver to Agent under the NCNB Agreement a
guaranty of the obligations under Section 7.3 of the NCNB Agreement.

    SECTION 5.  Miscellaneous.

        (a)  Upon and after the Amendment No. 4 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

                                       7
<PAGE>

    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.

        (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:  __________________________________
                              Vice President-Finance

THE PRUDENTIAL INSURANCE
 COMPANY OF AMERICA

By: __________________________________
    Vice President

PRUCO LIFE INSURANCE COMPANY

By: __________________________________
    Vice President

                                       8
<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, 1991 (effective as of January
31, 1996) as amended by Letter Amendment No. 1 dated November 21, 1997, Letter
Amendment No. 2 dated March 31, 1999 and Limited Waiver, Consent, Release and
Amendment No. 3 dated June 1, 1999 (as amended, the "Agreement").  Prudential
and the Company are entering into that certain Limited Waiver, Consent, Release
and Amendment No. 4 to Second Amended and Restated Master Shelf Agreement, dated
as of August 25, 2000 (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 August 25, 2000.

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

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

<PAGE>

                                                                       EXHIBIT B

                                    Issuers
                                    -------

<TABLE>
<CAPTION>

Corporations
Issuer                                                 Certificate No.      No. of Shares          Class
------                                                 ---------------      -------------        --------
<S>                                                    <C>                  <C>                  <C>
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             1,000,834            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

</TABLE>

Limited Liability Companies

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

 Western Gas Wyoming, L.L.C.                 100%<PAGE>
                                                                  Exhibit 10.25

                      FOURTH AMENDMENT TO LOAN AGREEMENT
                      ----------------------------------

     THIS FOURTH AMENDMENT TO LOAN AGREEMENT (herein called this "Amendment") is
made as of the 25th day of August, 2000 by and among Western Gas Resources, Inc.
("Borrower"), and Bank of America, N.A. ("Agent"), and the Lenders under the
Loan Agreement referred to below.

                             W I T N E S S E T H:

     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, Borrower is the sole shareholder of Western Gas Resources -
Oklahoma, Inc. ("WGRO");

     WHEREAS, WGRO has recently acquired all of the partnership interests in
Westana Gathering Company, a general partnership ("Westana"), the primary assets
of which are the Avard/Waynoka gathering system (the "System") and the Chester
gas processing plant (the "Plant");

     WHEREAS, concurrently with the repayment of the AG Debt (as defined below),
Borrower desires to undertake the following transactions with respect to WGRO
and Westana (collectively, the "Westana Transactions"):

          (a) all of Westana's assets shall be liquidated into WGRO;

          (b) WGRO shall concurrently therewith create a new subsidiary
     ("Newco"), which shall be capitalized with the System; and

          (c) subsequent to the creation of Newco, WGRO will be merged with and
     into the Company with the Company being the surviving entity, and, as a
     result of such merger, Borrower shall become the owner of the Plant;

     WHEREAS, Borrower, Agent, and Lenders desire to amend the Original
Agreement for the purposes described herein, including, but limited to,
consenting to Borrower's incurrence of indebtedness under a commercial paper
program in an aggregate face amount not to exceed $50,000,000 at any time
outstanding, and  to the consummation of the Westana Transactions in connection
with the repayment of the AG Debt;

<PAGE>

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

     (S) 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 following terms shall have the following meanings:

     "AG Debt" means all of the Debt owing by Borrower and the other Related
      -------
Persons to the American General Group under the Debt Securities described in
part (ii) of the definition of Debt Securities contained in Section 1.1 of the
Original Agreement.

     "Amendment" means this Fourth Amendment to Loan Agreement.
      ---------

     "Current Amendments" means the amendments described in Section 2.1 of this
      ------------------
Amendment.

     "Effective Date" has the meaning give it in Section 3.1 of this Agreement.
      --------------

     "Intercreditor Agreement" means the Intercreditor Agreement dated as of
      -----------------------
April 29, 1999, by and among the Lenders named therein, as amended,
supplemented, or restated to the date hereof.

     "Loan Agreement" means the Original Agreement as amended by this Amendment.
      --------------

     "Pledge Agreement" means the Pledge Agreement dated as of April 29, 1999 by
      ----------------
Borrower in favor of Agent for the benefit of Lenders, as amended, supplemented,
or restated to the date hereof.

     "Westana Amendments" means the amendments described in Section 2.2 and the
      ------------------
waivers and consents described in Section 2.3.

     "Westana Effective Date" has the meaning given it in Section 3.2 of this
      ----------------------
Agreement.

                                       2
<PAGE>

                                  ARTICLE II.

                   Amendments, Waivers, and Other Agreements
                   -----------------------------------------

     (S) 2.I. Current Amendments.  Effective as of the Effective Date (as
              ------------------
defined in Article III), the Original Agreement is hereby amended as follows:

     (a)      Definitions.
              -----------

              (i)  The definition of "EBITDA" in Section 1.1 of the Original
     Agreement is hereby amended in its entirety to read as follows:

              "EBITDA" means with respect to any Person, for any period, the
               ------
     amount equal to (i) such Person's net income (or net loss), plus (ii) such
     Person's Consolidated taxes, interest, depreciation, amortization and
     depletion expenses taken into account in determining such net income (or
     net loss) for such period, plus (iii) other non-cash items deducted in
     determining such net income (or net loss), minus (iv) other non-cash items
                                                -----
     added in determining such net income (or net loss), determined without
     duplication on a Consolidated basis and in accordance with GAAP.

              (ii) The following definitions are hereby added to Section 1.1 of
     the Original Agreement in proper alphabetical order:

              "CP Debt" means unsecured Debt in the form of commercial paper
               -------
     issued by Borrower which meets the following requirements: (i) such Debt
     has a maturity of not more than 270 days after the date of issuance
     thereof, (ii) the offering of such Debt is not required to be registered
     under the Securities Act of 1933, as amended, (iii) such Debt is not the
     subject of a Guarantee of any Related Person, (iv) at the time Borrower
     incurs such Debt, no Default or Event of Default shall have occurred and be
     continuing hereunder, and (v) the documentation evidencing such Debt shall
     contain no terms, conditions or defaults (other than pricing and back-up
     availability under this Agreement) which are more favorable to the third
     party creditor than those contained in this Agreement are to Lenders, as
     determined by Majority Lenders in their discretion (provided that Majority
     Lenders shall make any such determination at the time the initial
     documentation covering the issuance of CP Debt is executed and delivered
     and each time such documentation is modified, taking into consideration any
     amendments or modifications to this Agreement then in effect), and shall
     not contain any provision which attempts to modify, amend or restrict any
     of the rights or remedies of Agent or Lenders hereunder or under any of the
     other Loan Documents.

              "Guarantee" means as applied to any Debt, (i) a guarantee (other
               ---------
     than by enforcement of negotiable instruments of collection in the ordinary
     course of business), direct or indirect, in any manner all or any part of
     such Debt and (ii) any obligation to purchase or acquire or to otherwise
     protect or insure a creditor against

                                       3
<PAGE>

     loss in respect of such Debt (such as obligations under working capital
     maintenance agreements, agreements to keep-well, or agreements to purchase
     such Debt, assets, goods, securities or services, or payment of amounts
     drawn under letters of credit).

     (b)       Use of Proceeds. The first sentence of Section 2.5 of the
               ---------------
Original Agreement is hereby amended in its entirety to read as follows:

               "Borrower shall use all funds from the Borrowings under the Loans
     to make capital expenditures and provide working capital for its
     operations, to meet its reimbursement obligations under LCs, and for other
     general business purposes (including but not limited to the repayment from
     time to time of CP Debt)."

     (c)       Debt. Section 6.2(a) of the Original Agreement is hereby amended
               ----
by amending subsection (x) thereof to read as follows and by adding thereto the
following subsection (xi):

               "(x) CP Debt, the aggregate face amount of which shall not exceed
     $50,000,000 at any one time outstanding; and

               (xi) miscellaneous items of Debt not described in subsections (i)
     through (x) of thus subsection (a) which do not in the aggregate (taking
     into account all Debt of all Related Persons) exceed $5,000,000 at any one
     time outstanding."

     (d)       Limitation on Sales of Property. Subsection (iv) of Section
               -------------------------------
6.2(d) of the Original Agreement is hereby amended in its entirety to read as
follows:

               "(iv) so long as no Default or Event of Default has occurred,
     other assets or property which are sold in arm's length transactions to
     third parties that are not Affiliates of Borrower and are sold for fair
     consideration not in the aggregate in excess of $35,000,000 during any
     Fiscal Year."

     (e)       Events of Default. The parenthetical clause in Section 8.1(f)(i)
               -----------------
of the Original Agreement is hereby amended in its entirety to read as follows:

               "(including, but not limited to, the Debt under the Debt
     Securities or the Subordinated Debt, or the CP Debt)"

     (S) 2.2.  Westana Amendments. On and as of the Westana Effective Date, the
               ------------------
definitions of "Westana" and "WGRO" in Section 1.1 of the Original Agreement and
all references to "Westana" and "WGRO" in the Original Agreement shall be
deleted from the Original Agreement with no further action needed on behalf of
Agent, Lenders, Borrower, or the other Related Persons.  Borrower shall not be
required to pledge any capital stock of Newco to Agent and Lenders.

                                       4
<PAGE>

     (S) 2.3.  Consents and Waiver.  Agent and each Lender hereby consent to the
               -------------------
Westana Transactions and waive any Default or Event of Default arising therefrom
under Section 8.1 of the Original Agreement.  In addition, each Lender hereby
consents to (a) the termination of the Guaranty dated as of April 29, 1999
executed by WGRO in favor of Agent (in this section called the "WGRO Guaranty")
and (b) the release of  Agent's lien and security interest in Borrower's common
stock of WGRO arising under the Pledge Agreement (in this section called the
"Lien Release"), in each case on or after the Westana Effective Date and the
satisfaction in full of all conditions precedent set forth in Article III of
this Amendment. Thereafter Agent shall execute and deliver to Borrower (i) a
document evidencing the termination of the WGRO Guaranty and the Lien Release
and (ii) return to Borrower all stock certificates issued by WGRO which it
possesses under the Pledge Agreement.  Borrower hereby agrees to deliver to
Agent, within 30 days after the Westana Effective Date, an amendment to the
Intercreditor Agreement duly executed by the parties thereto confirming the
matters described in Section 3.2(a) hereto and the removal of American General
as a party thereto due to payment in full of the AG Debt.

                                 ARTICLE III.

                          Conditions of Effectiveness
                          ---------------------------

     (S) 3.I.  Effective Date.  Except for the Westana Amendments, this
               --------------
Amendment shall become effective as of the date first above written (the
"Effective Date") when, and only when, Agent shall have received all of the
                  -------------------
following and the Westana Amendments shall become effective as the Westana
Effective Date when and only when, Agent shall have received all of the
               -------------------
following and the documents described in Section 3.2:

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

               (2)  A certificate of a duly authorized officer of Borrower dated
     the date of this Amendment certifying: (i) 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; and (ii) as to such other corporate
     matters as Agent shall deem necessary.

               (c)  A written legal opinion of in-house counsel for Borrower,
     dated as of the date of this Amendment, addressed to Agent, to the effect
     that this Amendment has been duly authorized, executed and delivered by
     Borrower and that the Loan Agreement and each other Loan Document, as
     affected hereby, to which any Related Person is a party constitutes the
     legal, valid and binding obligation of each such Related Person,
     enforceable in accordance with their terms (subject, as to enforcement of
     remedies, to applicable bankruptcy, reorganization, insolvency and similar
     laws and to general principles of equity) and such other matters of Agent
     may require.

                                       5
<PAGE>

                (d) Payment of fees and disbursements of Thompson & Knight
     L.L.P. relating to this Amendment and the Loan Agreement as provided in the
     Loan Agreement.

                (e) Agent shall have additionally received such other documents
     as Agent may reasonably request.

     (S) 3.II.  Westana Effective Date.  The Westana Amendments shall become
                ----------------------
effective as of the date on which the AG Debt is repaid in full (the "Westana
Effective Date") when and only when, Agent shall have received evidence
                 -------------------
satisfactory to Agent, in its sole and absolute discretion, that each other
Person then a party to the Intercreditor Agreement has released its guaranty
from WGRO and lien and security interest in the common stock of WGRO.

                                  ARTICLE IV.

                        Representations and Warranties
                        ------------------------------

     (S) 4.I.  Representations and Warranties of Borrower.  In order to induce
               ------------------------------------------
each Lender to enter into this Amendment, Borrower represents and warrants on
the date hereof and as of the Effective Date 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, the Loan
     Agreement, and each other Loan Document, as affected hereby, will be a
     legal and binding obligation of each Related Person that is a party hereto
     and thereto enforceable against such Related Person in accordance with its
     terms, except as limited by bankruptcy, insolvency or

                                       6
<PAGE>

     similar laws of general application relating to the enforcement of
     creditors' rights and by equitable principles of general application.

               (5)  The audited Consolidated financial statements of Borrower
     dated as of December 31, 1999 and the unaudited Consolidated financial
     statements of Borrower dated as of June 30, 2000 fairly present the
     Consolidated financial position at such dates of Borrower and the
     Consolidated statement of operations and the changes in Consolidated
     financial position for the periods ending on such dates for Borrower.
     Copies of such financial statements have heretofore been delivered to
     Agent. Since June 30, 2000, no material adverse change has occurred in the
     financial condition or business or in the Consolidated financial condition
     or business of Borrower.

               (f)  Upon the Westana Effective Date, Newco shall not be required
     to execute and deliver to Agent a guaranty of the Obligations under Section
     7.3 of the Original Agreement.

                                  ARTICLE V.

                                 Miscellaneous
                                 -------------

     (S) 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.

     (S) 5.II.  Survival of 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.

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

                                       7

<PAGE>

     (S) 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.

     (S) 5.V.  Counterparts.  This Amendment may be separately executed in
               ------------
counterparts and by the different parties hereto in separate counterparts, each
of 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 AGREEMENTS OF THE PARTIES.  THERE ARE NO
UNWRITTEN ORAL AGREEMENTS OF THE PARTIES.

                                       8
<PAGE>

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

                              WESTERN GAS RESOURCES, INC.

                              By:_________________________________
                                 Name:
                                 Title:
<PAGE>

                              BANK OF AMERICA, N.A., as Agent and Lender

                              By:__________________________________
                                 Name:
                                 Title:
<PAGE>

                              SOCIETE GENERALE SOUTHWEST AGENCY,
                              a Lender

                              By:__________________________________
                                 Name:
                                 Title:
<PAGE>

                              ABN AMRO BANK N.V., a Lender

                              By:____________________________________
                                 Name:
                                 Title:

                              By:____________________________________
                                 Name:
                                 Title:
<PAGE>

                              CREDIT LYONNAIS, a Lender

                              By:_________________________________
                                 Name:
                                 Title:
<PAGE>

                              FLEET NATIONAL BANK, a Lender

                              By:__________________________________
                                 Name:
                                 Title:
<PAGE>

                              UNION BANK OF CALIFORNIA, N.A., a Lender

                              By:_______________________________
                                 Name:
                                 Title:

                              By:_______________________________
                                 Name:
                                 Title:
<PAGE>

                              BANK ONE, NA, a Lender

                              By:________________________________
                                 Name:
                                 Title:

<PAGE>

                              U.S. BANK NATIONAL ASSOCIATION,
                              a Lender

                              By:____________________________________
                                 Name:
                                 Title:
<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.

Date:  August 25, 2000

                              MIGC, INC.

                              By:____________________________________
                                 Name:
                                 Title:

                              WESTERN GAS RESOURCES TEXAS, INC.

                              By:____________________________________
                                 Name:
                                 Title:

                              MOUNTAIN GAS RESOURCES, INC.

                              By:____________________________________
                                 Name:
                                 Title:

                              WESTERN GAS RESOURCES - OKLAHOMA,
                              INC.

                              By:____________________________________
                                 Name:
                                 Title:

<PAGE>

                              LANCE OIL & GAS COMPANY, INC.

                              By:____________________________________
                                 Name:
                                 Title:

                              PINNACLE GAS TREATING, INC.

                              By:____________________________________
                                 Name:
                                 Title:

                              WESTERN GAS WYOMING, L.L.C.

                              By:____________________________________
                                 Name:
                                 Title:

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

    The undersigned hereby (i) consents to the provisions of this Amendment and
the transactions contemplated herein, and (ii) ratifies and confirms its
Guaranty dated as of October 14, 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.

Date:  August 25, 2000

                              MGTC, INC.

                              By:____________________________________
                                 Name:
                                 Title:

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