<PAGE>

                                                                    EXHIBIT 4.11

                         WESTERN GAS RESOURCES, INC.,
                                  As Issuer,

                           THE SUBSIDIARY GUARANTORS
                          Named on Schedule I hereto

                                      AND

                  CHASE BANK OF TEXAS, NATIONAL ASSOCIATION,
                                  As Trustee



                         SECOND SUPPLEMENTAL INDENTURE


                        Dated as of September 29, 2000



                         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

<PAGE>

                                 $155,000,000

                    10% SENIOR SUBORDINATED NOTES DUE 2009

     THIS SECOND SUPPLEMENTAL INDENTURE, dated as of September 29, 2000 (the
"Second 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. {"MGTC"], a Wyoming corporation, 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, the Company and The Prudential Insurance Company of America and
Pruco Life Insurance Company (together, "Prudential") entered into, as of
Augusta25, 2000, a Limited Waiver, Consent, Release and Amendment No. 4 (the
"Limited 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
September 29, 2000, terminate the guaranty by the Company's subsidiary, Western
Gas Resources - Oklahoma, Inc. ("WGRO"), a Delaware corporation, of the
Company's obligations arising under the Master Shelf Agreement, to otherwise
grant a general release of WGRO under the WGRO Guaranty (as defined in the
Limited Waiver) and to release Prudential's lien and security interest in the
shares of capital stock of WGRO; 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
August 25, 2000, a Fourth 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 WGRO in favor of the Agent and to the release of the Agent's
lien and security interest in the Company's common stock of WGRO, subject to
certain conditions that were satisfied as of September 29, 2000; and

     WHEREAS, pursuant to the Limited Waiver and the Amendment, Prudential and
the Agent and Lenders, respectively, have consented to the following
transactions with respect to WGRO and the Westana
<PAGE>

Gathering Company ("Westana"), a general partnership, all of the partnership
interests of which WGRO recently acquired (the "Westana Transactions"):

     (a)  the liquidation of all of Westana's assets
          into WGRO;

     (b)  the creation, concurrent with the transaction
          described in (a) above, of a new subsidiary
          ("Newco") to hold certain assets of Westana; and

     (c)  the merger of WGRO with and into Newco,
          with Newco being the surviving entity
          (the "Merger"); and

     WHEREAS, as of September 29, 2000, the Merger had not yet taken place; 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, 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 the Senior
Debt (as defined in the Original Indenture); and

     WHEREAS, as a result of the Limited Waiver, the Amendment and the repayment
in full of all amounts due in respect of the American General Notes, neither
WGR0, which is a Restricted Subsidiary, nor Newco, which became a Restricted
Subsidiary upon its creation, have outstanding any 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 Second 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 GUARANTORS
                             ---------------------

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

     SECTION 1.2. Trustee's Acceptance.  The Trustee hereby accepts this Second
                  ---------------------
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 Second Supplemental Indenture by the Company, the Guarantors
listed in Schedule I hereto and the Trustee, the
<PAGE>

Original Indenture, as supplemented by the First Supplemental Indenture, shall
be supplemented in accordance herewith, and this Second Supplemental Indenture
shall form a part of the Original Indenture, as supplemented by the First
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, 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, all provisions
in the Original Indenture shall remain in full force and effect.

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

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

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

     SECTION 2.6. Severability.  In case any provision in this Second
                  -------------
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 Oriainal 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 Second
                  ---------
Supplemental Indenture are for convenience only and shall not affect the
construction hereof.

     SECTION 2.9. Benefits of Second Supplemental Indenture.  Nothing in this
                  ------------------------------------------
Second 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 or this Second Supplemental Indenture.

     SECTION 2.10. Successors and Assiqns.  All covenants and agreements in this
                   -----------------------
Second Supplemental Indenture by the Company and the Guarantors in this Second
Supplemental Indenture and the Exchange Notes, as applicable, shall bind their
successors and assigns, whether so expressed or not. All agreements of the
Trustee in this Second 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 Second Supplemental
Indenture.

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

     SECTION 2.13. Governing Law.  THIS SECOND 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, this
Second 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 Oriqinals.  The parties may sign any number of
                   -------------------
copies of this Second 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 Second Supplemental Indenture. The Second 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 Second 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
<PAGE>

                               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.
                         WESTERN GAS RESOURCES - TEXAS, INC.
                         WESTERN GAS RESOURCES - OKLAHOMA, INC.
                         WESTERN GAS WYOMING, L.L.C.