<PAGE>

                                                                    EXHIBIT 4.10

                          THIRD SUPPLEMENTAL INDENTURE


         THIRD SUPPLEMENTAL INDENTURE (this "SUPPLEMENTAL INDENTURE"), dated as
of December 31, 2001, among Westport Resources Corporation, a company
incorporated under the laws of the State of Nevada, formerly known as Belco Oil
& Gas Corp. (the "COMPANY"), the existing Subsidiary Guarantors under the
indenture referred to below (the "EXISTING SUBSIDIARY GUARANTORS"), the parties
identified as New Subsidiary Guarantors on the signature page hereto (the "NEW
SUBSIDIARY GUARANTORS"), and The Bank of New York, a New York banking
corporation, as trustee under the indenture referred to below (the "TRUSTEE").

                                   WITNESSETH

         WHEREAS, the Company and the Existing Subsidiary Guarantors have
heretofore executed and delivered to the Trustee a certain Indenture (as
previously amended, the "INDENTURE"), dated as of September 23, 1997, providing
for the issuance of an aggregate principal amount at maturity of $150,000,000 of
8-7/8% Senior Subordinated Notes due 2007 (the "NOTES");

         WHEREAS, certain Subsidiary Guarantors have been merged out of
existence and the New Subsidiary Guarantors are being created;

         WHEREAS, Section 4.16 of the Indenture provides that, under certain
circumstances, the Company is required to cause Subsidiary Guarantors to execute
and deliver to the Trustee a Subsidiary Guarantee on the terms and conditions
set forth herein; and

         WHEREAS, pursuant to Section 9.1 of the Indenture, the Company, each
Subsidiary Guarantor and the Trustee are authorized to execute and deliver this
Supplemental Indenture.

         NOW, THEREFORE, in consideration of the foregoing and for other good
and valuable consideration, the receipt of which is hereby acknowledged, the
Company, the Existing Subsidiary Guarantors, the New Subsidiary Guarantors and
the Trustee mutually covenant and agree for the equal and ratable benefit of the
Holders as follows:

         1. CAPITALIZED TERMS.

                  Capitalized terms used herein without definition shall have
the meanings assigned to them in the Indenture.

         2. INDENTURE PROVISIONS PURSUANT TO WHICH GUARANTEE IS GIVEN.

                  This Supplemental Indenture is being executed and delivered
pursuant to Sections 4.16 and 9.1 of the Indenture.



<PAGE>

         3. AGREEMENTS TO GUARANTEE.

                  The New Subsidiary Guarantors hereby agree as follows:

                  (a) The New Subsidiary Guarantors, jointly and severally with
all other Subsidiary Guarantors, if any, unconditionally guarantee to each
Holder of a Note authenticated and delivered by the Trustee and to the Trustee
and its successors and assigns, regardless of the validity and enforceability of
the Indenture, the Notes and the obligations of the Company under the Indenture
and the Notes, that:

                           (i) the principal of, premium, if any, and interest
on the Notes shall be promptly paid in full when due, whether at maturity, by
acceleration, redemption or otherwise, and interest on the overdue principal of,
premium, if any, and interest on the Notes, to the extent lawful, and all other
obligations of the Company to the Holders or the Trustee thereunder shall be
promptly paid in full, all in accordance with the terms thereof; and

                           (ii) in case of any extension of time for payment or
renewal of any Notes or any of such other obligations, that the same shall be
promptly paid in full when due in accordance with the terms of the extension or
renewal, whether at maturity, by acceleration or otherwise.

                  Notwithstanding the foregoing, in the event that the
Subsidiary Guarantee of any New Subsidiary Guarantor would constitute or result
in a violation of any applicable fraudulent conveyance or similar law of any
relevant jurisdiction, the liability of such New Subsidiary Guarantor under this
Supplemental Indenture and its Subsidiary Guarantee shall be limited to such
amount as will not, after giving effect thereto, and to all other liabilities of
such New Subsidiary Guarantor, result in such amount constituting a fraudulent
transfer or conveyance.

         4. SUBORDINATION.

                  Each Subsidiary Guarantor agrees, and each Holder by accepting
a Note agrees, that: (a) the obligations of each Subsidiary Guarantor under its
Subsidiary Guarantee are subordinated in right of payment to the prior payment
in full (when due) of all existing and future Guarantor Senior Indebtedness of
such Subsidiary Guarantor, including without limitation any guarantee by such
Subsidiary Guarantor of the Indebtedness under the New Credit Facility or of any
Senior Debt of the Company or of any Guarantor Senior Indebtedness of any other
Subsidiary Guarantor, to the extent and in the matter provided in Article 10
(including, without limitation, Section 10.12) of the Indenture (as if the
Subsidiary Guarantor were the Company for purposes of such Article 10 and all
defined terms used therein, and the Guarantor Senior Indebtedness of the
Guarantor were Senior Debt), and this Subsidiary Guarantee is made subject to
such provisions (which are hereby incorporated herein by reference); and (b)
such subordination is for the benefit of and enforceable by the holders of
Guarantor Senior Indebtedness of the Subsidiary Guarantor.



                                       2
<PAGE>

         5. EXECUTION AND DELIVERY OF SUBSIDIARY GUARANTEE.

                  (a) To evidence its Subsidiary Guarantee set forth in this
Supplemental Indenture, each New Subsidiary Guarantor hereby agrees that a
notation of such Subsidiary Guarantee substantially in the form of Annex A
hereto shall be endorsed by an officer of such New Subsidiary Guarantor on each
Note authenticated and delivered by the Trustee after the date hereof.

                  (b) Notwithstanding the foregoing, each New Subsidiary
Guarantor hereby agrees that its Subsidiary Guarantee set forth herein shall
remain in full force and effect notwithstanding any failure to endorse on each
Note a notation of such Subsidiary Guarantee.

                  (c) If an officer whose signature is on this Supplemental
Indenture or on the Subsidiary Guarantee no longer holds that office at the time
the Trustee authenticates the Note on which a Subsidiary Guarantee is endorsed,
the Subsidiary Guarantee shall be valid nevertheless.

                  (d) The delivery of a Note by the Trustee, after the
authentication thereof under the Indenture, shall constitute due delivery of the
Subsidiary Guarantee set forth in this Supplemental Indenture on behalf of each
New Subsidiary Guarantor.

                  (e) Each New Subsidiary Guarantor hereby agrees that its
obligations hereunder shall be unconditional, regardless of the validity,
regularity or enforceability of the Notes or the Indenture, the absence of any
action to enforce the same, any waiver or consent by any Holder of the Notes
with respect to any provisions hereof or thereof, the recovery of any judgment
against the Company, any action to enforce the same or any other circumstance
which might otherwise constitute a legal or equitable discharge or defense of a
guarantor.

                  (f) Each New Subsidiary Guarantor hereby waives diligence,
presentment, demand of payment, filing of claims with a court in the event of
insolvency or bankruptcy of the Company, any right to require a proceeding first
against the Company, protest, notice and all demands whatsoever and covenants
that its Subsidiary Guarantee made pursuant to this Supplemental Indenture will
not be discharged except by complete performance of the obligations contained in
the Notes and the Indenture or pursuant to Section 6(b) of this Supplemental
Indenture.

                  (g) If the Trustee or any Holder has instituted any proceeding
to enforce any right or remedy under this Supplemental Indenture or under the
Subsidiary Guarantee made pursuant to this Supplemental Indenture and such
proceeding has been discontinued or abandoned for any reason, or has been
determined adversely to the Trustee or to such Holder, then, and in every such
case, subject to any determination in such proceeding, the Subsidiary Guarantor,
the Trustee and the Holders shall be restored severally and respectively to
their former positions hereunder and thereafter all rights and remedies of each
Subsidiary Guarantor, the Trustee and the Holders shall continue as though no
such proceeding had been instituted.



                                       3
<PAGE>

                  (h) Each Subsidiary Guarantor hereby waives and will not in
any manner whatsoever claim or take the benefit or advantage of, any rights of
reimbursement, indemnity or subrogation or any other rights against the Company
or any other Subsidiary Guarantor as a result of any payment by such Subsidiary
Guarantor under its Subsidiary Guarantee. Each Subsidiary Guarantor further
agrees that, as between the Subsidiary Guarantors, on the one hand, and the
Holders and the Trustee, on the other hand:

                           (i) the maturity of the obligations guaranteed hereby
may be accelerated as provided in Article 6 of the Indenture for the purposes of
the Subsidiary Guarantee made pursuant to this Supplemental Indenture,
notwithstanding any stay, injunction or other prohibition preventing such
acceleration in respect of the obligations guaranteed hereby; and

                           (ii) in the event of any declaration of acceleration
of such obligations as provided in such Article 6, such obligations (whether or
not due and payable) shall forthwith become due and payable by each Subsidiary
Guarantor for the purpose of the Subsidiary Guarantee made pursuant to this
Supplemental Indenture.

                  (i) Each Subsidiary Guarantor shall have the right to seek
contribution from any other non-paying Subsidiary Guarantor, if any, so long as
the exercise of such right does not impair the rights of the Holders under the
Subsidiary Guarantee made pursuant to this Supplemental Indenture.

                  (j) Each New Subsidiary Guarantor covenants (to the extent
that it may lawfully do so) that it will not at any time insist upon, or plead,
or in any manner whatsoever claim or take the benefit or advantage of, any stay,
extension or usury law wherever enacted, now or at any time hereafter in force,
which may affect the covenants or the performance of the Indenture or this
Subsidiary Guarantee; and each New Subsidiary Guarantor (to the extent that it
may lawfully do so) hereby expressly waives all benefit or advantage of any such
law, and covenants that it will not hinder, delay or impede the execution of any
power herein granted to the Trustee, but will suffer and permit the execution of
every such power as though no such law had been enacted.

         6. SUBSIDIARY GUARANTORS MAY CONSOLIDATE, ETC. ON CERTAIN TERMS.

                  (a) Except as set forth in Articles 4 and 5 of the Indenture,
nothing contained in the Indenture, this Supplemental Indenture or in the Notes
shall prevent any consolidation or merger of a Subsidiary Guarantor with or into
the Company or any other Subsidiary Guarantor or shall prevent any transfer,
sale or conveyance of the property of the Subsidiary Guarantor as an entirety or
substantially as an entirety, to the Company or any other Subsidiary Guarantor.

                  (b) Except as set forth in Article 5 of the Indenture, upon
the sale or disposition of all of the Capital Stock of a Subsidiary Guarantor by
the Company or the Subsidiary of the Company, or upon the consolidation or
merger of a Subsidiary Guarantor with or into any Person, or the sale of all or
substantially all of the assets of a Subsidiary Guarantor (in each case, other
than to an Affiliate of the Company), such Subsidiary Guarantor shall be



                                       4
<PAGE>

deemed automatically and unconditionally released and discharged from all
obligations under this Subsidiary Guarantee without any further action required
on the part of the Trustee or any Holder if no Default shall have occurred and
be continuing; provided, that in the event of an Asset Sale, the Net Proceeds
therefrom are treated in accordance with Section 4.10 of the Indenture. Except
with respect to transactions set forth in the preceding sentence, the Company
and each Subsidiary Guarantor covenant and agree that upon any such
consolidation, merger or transfer of assets, the performance of all covenants
and conditions of this Supplemental Indenture to be performed by such Subsidiary
Guarantor shall be expressly assumed by supplemental indenture satisfactory in
form to the Trustee, by the corporation formed by such consolidation, or into
which the Subsidiary Guarantor shall have merged, or by the corporation which
shall have acquired such property. Upon receipt of an Officer's Certificate of
the Company or any Subsidiary Guarantor, as the case may be, to the effect that
the Company or such Subsidiary Guarantor has complied with the first sentence of
this Section 6(b), the Trustee shall execute any documents reasonably requested
by the Company or such Subsidiary Guarantor, at the cost of the Company or such
Subsidiary Guarantor, as the case may be, in order to evidence the release of
such Subsidiary Guarantor from its obligations under its Guarantee endorsed on
the Notes and under the Indenture and this Supplemental Indenture.

         7. RELEASES UPON RELEASE OF GUARANTEE OF GUARANTEED INDEBTEDNESS.

                  Concurrently with the release or discharge of any Subsidiary
Guarantor's guarantee of the payment of: (i) notes and other obligations under
an indenture, dated as of November 5, 2001, between the Company and the Trustee
(as trustee); and (ii) obligations under a credit agreement, dated as of August
21, 2001, among the Lender, the several lenders and agents party thereto and The
Chase Manhattan Bank, as issuing bank and administrative agent (other than a
release or discharge by or as a result of payment under such guarantee of
Guaranteed Indebtedness), the Subsidiary Guarantor shall be automatically and
unconditionally released and relieved of its obligations under this Supplemental
Indenture and its Subsidiary Guarantee made pursuant to Section 5 of this
Supplemental Indenture. Upon delivery by the Company to the Trustee of an
Officers' Certificate to the effect that such release or discharge has occurred,
the Trustee shall execute any documents reasonably required in order to evidence
the release of the relevant Subsidiary Guarantor from its obligations under this
Supplemental Indenture and such Subsidiary Guarantor's Subsidiary Guarantee made
pursuant hereto; provided, that such documents shall not affect or impair the
rights of the Trustee and Paying Agent under Section 7.7 of the Indenture.

         8. EXISTING SUBSIDIARY GUARANTORS.

                  Each Existing Subsidiary Guarantor executes this Supplemental
Indenture to: (i) ratify and confirm its existing Subsidiary Guaranty as
continuing to be in full force and effect; and (ii) evidence its agreement to
the terms of this Supplemental Indenture.



                                       5
<PAGE>

         9. NEW YORK LAW TO GOVERN.

                  The substantive law of the State of New York shall govern and
be used to construe this Supplemental Indenture.

         10. COUNTERPARTS.

                  The parties may sign any number of copies of this Supplemental
Indenture. Each signed copy shall be an original, but all of them together
represent the same agreement.

         11. RECITALS, ETC.

                  The recitals herein contained are made by the Company and the
Subsidiary Guarantors and not by the Trustee, and the Trustee assumes no
responsibility for the correctness thereof. The Trustee makes no representations
as to the validity or sufficiency of this Supplemental Indenture.

         12. EFFECT OF HEADINGS.

                  The Section headings herein are for convenience only and shall
not effect the construction hereof.




                            [SIGNATURE PAGE FOLLOWS]



                                       6
<PAGE>

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


                                       WESTPORT RESOURCES CORPORATION



                                       By: /s/ BARTH E. WHITHAM
                                          --------------------------------------
                                       Name:  Barth E. Whitham
                                            ------------------------------------
                                       Title: President and Chief
                                             -----------------------------------
                                               Operating Officer


                                       EXISTING SUBSIDIARY GUARANTORS:

                                       WESTPORT CANADA LLC

                                       By:  WESTPORT OIL AND GAS COMPANY, L.P.

                                            By: WHG, INC.

                                                By: /s/ BARTH E. WHITHAM
                                                   -----------------------------
                                                Name:  Barth E. Whitham
                                                     ---------------------------
                                                Title: President and Chief
                                                      --------------------------
                                                        Operating Officer


                                       JERRY CHAMBERS EXPLORATION COMPANY

                                       By:  WESTPORT OIL AND GAS COMPANY, L.P.

                                            By: WHG, INC.

                                                By: /s/ BARTH E. WHITHAM
                                                   -----------------------------
                                                Name:  Barth E. Whitham
                                                     ---------------------------
                                                Title: President and Chief
                                                      --------------------------
                                                        Operating Officer


                                       WESTPORT FINANCE CO.,
                                       formerly, Belco Finance Co.


                                       By: /s/ BARTH E. WHITHAM
                                          --------------------------------------
                                       Name:  Barth E. Whitham
                                            ------------------------------------
                                       Title: President and Chief
                                             -----------------------------------
                                               Operating Officer


<PAGE>

                                       WESTPORT ARGENTINA LLC


                                       By: /s/ BARTH E. WHITHAM
                                          --------------------------------------
                                       Name:  Barth E. Whitham
                                            ------------------------------------
                                       Title: President and Chief
                                             -----------------------------------
                                               Operating Officer

                                       WESTPORT OVERRIDING ROYALTY LLC


                                       By: /s/ BARTH E. WHITHAM
                                          --------------------------------------
                                       Name:  Barth E. Whitham
                                            ------------------------------------
                                       Title: President and Chief
                                             -----------------------------------
                                               Operating Officer



                                       NEW SUBSIDIARY GUARANTORS:

                                       WESTPORT OIL AND GAS COMPANY, L.P.

                                       By: WHG, INC.


                                       By: /s/ BARTH E. WHITHAM
                                          --------------------------------------
                                       Name:  Barth E. Whitham
                                            ------------------------------------
                                       Title: President and Chief
                                             -----------------------------------
                                               Operating Officer


                                       WHG, INC.


                                       By: /s/ BARTH E. WHITHAM
                                          --------------------------------------
                                       Name:  Barth E. Whitham
                                            ------------------------------------
                                       Title: President and Chief
                                             -----------------------------------
                                               Operating Officer


                                       WHL, INC.


                                       By: /s/ BARTH E. WHITHAM
                                          --------------------------------------
                                       Name:  Barth E. Whitham
                                            ------------------------------------
                                       Title: President and Chief
                                             -----------------------------------
                                               Operating Officer



<PAGE>

                                       THE BANK OF NEW YORK,
                                       as Trustee



                                       By:  /s/ MARIE E. TRIMBOLI
                                          --------------------------------------
                                       Name:  Marie E. Trimboli
                                            ------------------------------------
                                       Title: Assistant Vice President
                                             -----------------------------------



<PAGE>

                        ANNEX A TO SUPPLEMENTAL INDENTURE

                FORM OF NOTATION OF SUBSIDIARY GUARANTEE ON NOTE


         Each Subsidiary Guarantor (as defined in the Indenture) has jointly and
severally unconditionally guaranteed: (a) the due and punctual payment of the
principal of, premium, if any, and interest on the Notes, whether at Stated
Maturity or an Interest Payment Date, by acceleration, call for redemption or
otherwise; (b) the due and punctual payment of interest on the overdue principal
and premium of, and interest, to the extent lawful, on the Notes; and (c) that
in case of any extension of time of payment or renewal of any Notes or any of
such other obligations, the same will be promptly paid in full when due in
accordance with the terms of the extension of renewal, whether at stated
maturity, by acceleration or otherwise.

         Notwithstanding the foregoing, in the event that the Subsidiary
Guarantee would constitute or result in a violation of any applicable fraudulent
conveyance or similar law of any relevant jurisdiction, the liability of the
Subsidiary Guarantor under its Subsidiary Guarantee shall be limited to such
amount as will not, after giving effect thereto, and to all other liabilities of
the Subsidiary Guarantor, result in such amount constituting a fraudulent
transfer or conveyance.

         This Subsidiary Guarantee shall not be valid or obligatory for any
purpose until the certificate of authentication on the Note upon which the
Subsidiary Guarantee is noted shall have been executed by the Trustee under the
Indenture by the manual signature of one of its authorized signatories.



Dated:                ,     .
      ----------------  ----

                                       [SUBSIDIARY GUARANTOR]


                                       By:
                                          --------------------------------------
                                       Name:
                                            ------------------------------------
                                       Title:
                                             -----------------------------------