EXHIBIT 4.50
                                                                    ------------



                            FOURTH AMENDING AGREEMENT


         THIS AGREEMENT is made as of November 4, 2003

BETWEEN:

         NEXEN INC., a corporation subsisting under the laws of Canada,
         (hereinafter referred to as the "Borrower"),

                                                              OF THE FIRST PART,

                                     - and -

         THE FINANCIAL INSTITUTIONS SET FORTH ON THE SIGNATURE PAGES HEREOF
         UNDER THE HEADING "LENDERS:" (hereinafter sometimes collectively
         referred to as the "Lenders" and sometimes individually referred to as
         a "Lender"),

                                                             OF THE SECOND PART,

                                     - and -

         THE TORONTO-DOMINION BANK, a Canadian chartered bank, as agent of the
         Lenders (hereinafter referred to as the "Agent"),

                                                              OF THE THIRD PART.

         WHEREAS the parties hereto entered into the Restated Credit Agreement;

         AND WHEREAS the parties hereto have agreed to amend and supplement
certain provisions of the Restated Credit Agreement as hereinafter set forth;

         NOW THEREFORE THIS AGREEMENT WITNESSES that in consideration of the
covenants and agreements herein contained and other good and valuable
consideration, the receipt and sufficiency of which are hereby conclusively
acknowledged by each of the parties hereto, the parties hereto covenant and
agree as follows:

1.       INTERPRETATION

1.1.     In this Agreement and the recitals hereto, unless something in the
subject matter or context is inconsistent therewith:

         "AGREEMENT" means this agreement, as amended, modified, supplemented or
         restated from time to time.

<PAGE>
                                       -2-


         "RESTATED CREDIT AGREEMENT" means the restated credit agreement made as
         of December 29, 1988 and amended and restated as of November 17, 2000
         between the Borrower, as borrower, the Lenders and the Agent, as
         amended to the date hereof.

1.2.     Capitalized terms used herein without express definition shall have the
same meanings herein as are ascribed thereto in the Restated Credit Agreement.

1.3.     The division of this Agreement into Sections and the insertion of
headings are for convenience of reference only and shall not affect the
construction or interpretation of this Agreement. The terms "this Agreement",
"hereof", "hereunder" and similar expressions refer to this Agreement and not to
any particular Section or other portion hereof and include any agreements
supplemental hereto.

1.4.     Schedule A attached to this Agreement is incorporated by reference and
shall be deemed to be a part hereof.

1.5.     This Agreement shall be governed by and construed in accordance with
the laws of the Province of Alberta and the federal laws of Canada applicable
therein.

2.       AMENDMENT TO DEFINITION OF CAPITAL SECURITIES

2.1.     Section 1.1 of the Restated Credit Agreement is hereby amended by
deleting the existing definition of "Capital Securities" in its entirety and
substituting therefor the definition of "Capital Securities" set forth in
Schedule A attached hereto.

3.       REPRESENTATIONS AND WARRANTIES

3.1.     The Borrower hereby represents and warrants as follows to each Lender
and the Agent and acknowledges and confirms that each Lender and the Agent is
relying upon such representations and warranties:

         (a)      CAPACITY, POWER AND AUTHORITY

                  (i)      It is duly incorporated and is validly subsisting
                           under the laws of its jurisdiction of incorporation
                           or creation and has all the requisite corporate
                           capacity, power and authority to carry on its
                           business as presently conducted and to own its
                           property; and

                  (ii)     It has the requisite corporate capacity, power and
                           authority to execute and deliver this Agreement.

         (b)      AUTHORIZATION; ENFORCEABILITY

                  It has taken or caused to be taken all necessary action to
                  authorize, and has duly executed and delivered, this
                  Agreement, and this Agreement is a legal, valid and binding
                  obligation of it, enforceable against it in accordance with
                  its terms, subject to applicable bankruptcy, reorganization,
                  winding up, insolvency, moratorium or other laws of general
                  application affecting the enforcement of

<PAGE>
                                       -3-


                  creditors' rights generally and to the equitable and statutory
                  powers of the courts having jurisdiction with respect thereto.

3.2.     The representations and warranties set out in this Agreement shall
survive the execution and delivery of this Agreement and the making of each
Drawdown, notwithstanding any investigations or examinations which may be made
by the Agent, the Lenders or Lenders' Counsel. Such representations and
warranties shall survive until the Restated Credit Agreement has been
terminated.

4.       CONFIRMATION OF RESTATED CREDIT AGREEMENT AND OTHER DOCUMENTS

         The Restated Credit Agreement and the other Documents and all
covenants, terms and provisions thereof, except as expressly amended and
supplemented by this Agreement, shall be and continue to be in full force and
effect and the Restated Credit Agreement, as amended and supplemented by this
Agreement, and each of the other Documents is hereby ratified and confirmed and
shall from and after the date hereof continue in full force and effect, in the
case of the Restated Credit Agreement as herein amended and supplemented, with
such amendments and supplements being effective as of the date hereof.

5.       FURTHER ASSURANCES

         The parties hereto shall from time to time do all such further acts and
things and execute and deliver all such documents as are required in order to
effect the full intent of and fully perform and carry out the terms of this
Agreement.

6.       COUNTERPARTS

         This Agreement may be executed in any number of counterparts, each of
which shall be deemed to be an original and all of which taken together shall be
deemed to constitute one and the same instrument, and it shall not be necessary
in making proof of this Agreement to produce or account for more than one such
counterpart. This Agreement may be executed and delivered by facsimile, which
when so executed and delivered shall constitute a binding agreement of the
parties hereto.


         [THE REMAINDER OF THIS PAGE HAS BEEN LEFT BLANK INTENTIONALLY]

<PAGE>
                                       -4-


         IN WITNESS WHEREOF the parties hereto have executed this Agreement as
of the date first above written.

BORROWER:                               AGENT:

NEXEN INC.                              THE TORONTO-DOMINION BANK,
                                        AS AGENT

By:  /s/ Una Power                      By:  /s/ Michael A. Freeman
     ----------------------------            ----------------------------
     Name:  Una Power                        Name:  Michael A. Freeman
     Title: Treasurer                        Title: Vice President, Loan
                                                    Syndications - Agency

By:  /s/ Rick C. Beingessner            By:
     ----------------------------            ----------------------------
     Name:  Rick C. Beingessner              Name:
     Title: Assistant Secretary              Title:

LENDERS:

THE TORONTO-DOMINION BANK               ROYAL BANK OF CANADA

By:  /s/ Dean Ariss                     By:  /s/ David Foltz
     ----------------------------            ----------------------------
     Name:  Dean Ariss                       Name:  David Foltz
     Title: Managing Director                Title: Senior Manager

By:  /s/ Kevin Kynoch                   By:
     ----------------------------            ----------------------------
     Name:  Kevin Kynoch                     Name:
     Title: Associate -                      Title:
            Corporate Credit

THE BANK OF NOVA SCOTIA                 CANADIAN IMPERIAL BANK OF
                                        COMMERCE

By:  /s/ Dan W. Lindquist               By:  /s/ Chris A. Perks
     ----------------------------            ----------------------------
     Name:  Dan W. Lindquist                 Name:  Chris A. Perks
     Title: Director                         Title: Executive Director

By:  /s/ R.L. Mack                      By:  /s/ David J. Swain
     ----------------------------            ----------------------------
     Name:  R.L. Mack                        Name:  David J. Swain
     Title: Associate                        Title: Managing Director

BANK OF AMERICA, N.A., CANADA BRANCH    BNP PARIBAS (CANADA)

By:  /s/ Nelson Lam                     By:  /s/ Jean-Phillippe Cadot
     ----------------------------            ----------------------------
     Name:  Nelson Lam                       Name:  Jean-Phillipe Cadot
     Title: Vice President                   Title: Vice President
                                                    Energy & Project Finance

By:                                     By:  /s/ Michael Gosselin
     ----------------------------            ----------------------------
     Name:                                   Name:  Michael Gosselin
     Title:                                  Title: Director

<PAGE>

                                   SCHEDULE A
                        TO THE FOURTH AMENDING AGREEMENT

"CAPITAL SECURITIES" means securities, including debt securities, issued by the
Borrower which at all times have the following characteristics:

         (a)      an initial final maturity extending beyond the last Maturity
                  Date applicable to any Lender or Lenders;

         (b)      no scheduled payments of principal thereunder prior to the
                  last Maturity Date applicable to any Lender or Lenders;

         (c)      a Default, Event of Default, acceleration of the payment of
                  the Obligations or enforcement of the rights and remedies of
                  the Lenders under the Documents or a default, event of
                  default, acceleration or similar circumstance under any other
                  Senior Debt shall not (i) cause a default or event of default
                  (with the passage of time or otherwise) under such securities
                  or the indenture governing the same, or (ii) cause or permit
                  the obligations thereunder to be due and payable prior to the
                  stated maturity thereof;

         (d)      payments of principal due and payable thereunder can be
                  satisfied by delivering common shares in the capital of the
                  Borrower in accordance with the indenture governing such
                  securities;

         (e)      all amounts payable in respect to such securities are
                  subordinate and junior in right of payment to the prior
                  payment in full of all Obligations and any other Senior Debt
                  upon a payment default on any Senior Debt in respect of which
                  any applicable grace period has ended and such default has not
                  been cured or waived or ceased to exist or the acceleration of
                  any Senior Debt which has not been rescinded; and

         (f)      such securities shall not be entitled to any distribution upon
                  the distribution of assets of the Borrower to creditors for
                  any reason (except in connection with any proceeding permitted
                  under Section 9.2(d)), including dissolution, bankruptcy or
                  any such similar proceedings, until all Obligations and all
                  other Senior Debt have been paid in full;

provided that, for certainty, Capital Securities shall include the Borrower's
9.75% Junior Subordinated Debentures due October 30, 2047 issued pursuant to an
Indenture dated as of October 30, 1998 between the Borrower and IBJ Schroder
Bank & Trust Company, the Borrower's 9.375% Junior Subordinated Debentures due
March 31, 2048 issued pursuant to an Indenture dated as of February 9, 1999
between the Borrower and IBJ Whitehall Bank and Trust Company and the Borrower's
7.35% Subordinated Notes due November 1, 2043 issued pursuant to a Subordinated
Debt Indenture dated as of November 4, 2003 between the Borrower and Deutsche
Bank Trust Company Americas, provided that such Debentures and Notes at all
times have all of the characteristics described in subparagraphs (a) to (f),
inclusive, above of this definition.