EXHIBIT 4.51
                                                                    ------------



                          DATED AS OF NOVEMBER 20, 2003





                                   NEXEN INC.

                  (FORMERLY CANADIAN OCCIDENTAL PETROLEUM LTD.)

                                       AND

                            CIBC MELLON TRUST COMPANY





                          FOURTH SUPPLEMENTAL INDENTURE

                                     TO THE

                                 TRUST INDENTURE

                           DATED AS OF APRIL 28, 1998

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                                TABLE OF CONTENTS


1.       INTERPRETATIONS AND AMENDMENTS........................................2

         1.1      SUPPLEMENTAL INDENTURE.......................................2
         1.2      DEFINITIONS IN TRUST INDENTURE...............................2
         1.3      INTERPRETATION NOT AFFECTED BY HEADINGS......................2

2.       NOTES ................................................................2

         2.1      FORM AND TERMS OF NOTES......................................2
         2.2      ISSUANCE OF NOTES............................................4

3.       REDEMPTION OF NOTES...................................................4

         3.1      REDEMPTION OF NOTES..........................................4
         3.2      ADDITIONAL DEFINITIONS.......................................5
         3.3      NOTICE TO TRUSTEE............................................6

4.       DEBT SECURITIES GUARANTEE.............................................6

         4.1      COVENANT TO PROVIDE DEBT SECURITIES GUARANTEE................6
         4.2      OPINIONS.....................................................6
         4.3      AUTOMATIC TERMINATION........................................7

5.       GENERAL ..............................................................7

         5.1      INDENTURE SUPPLEMENTAL TO TRUST INDENTURE....................7
         5.2      ACCEPTANCE OF TRUST..........................................7
         5.3      COUNTERPARTS AND FORMAL DATE.................................8

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THIS FOURTH SUPPLEMENTAL INDENTURE (this "Supplemental Indenture") dated as of
November 20, 2003.

BETWEEN:

         NEXEN INC. (FORMERLY "CANADIAN OCCIDENTAL PETROLEUM LTD."), a
         corporation amalgamated under the Canada Business Corporations Act and
         having its head office in the City of Calgary, in the Province of
         Alberta (the "Corporation")

                                                               OF THE FIRST PART

                                     - and -

         CIBC MELLON TRUST COMPANY, a trust corporation incorporated under the
         laws of Canada, having an office in the City of Calgary, in the
         Province of Alberta (the "Trustee")

                                                              OF THE SECOND PART

         WITNESSETH THAT:

         WHEREAS in and by a trust indenture made as of April 28, 1998 (the
"Original Indenture") between the Corporation and the Trustee (the Original
Indenture, as amended and supplemented by the first supplemental indenture dated
as of April 28, 1998 (the "First Supplemental Indenture"), the second
supplemental indenture dated as of February 4, 1999 (the "Second Supplemental
Indenture"), the third supplemental indenture dated as of March 11, 2002 (the
"Third Supplemental Indenture") and this Supplemental Indenture and as the same
may be further amended or supplemented from time to time, being hereinafter
called the "Trust Indenture," which term includes any and every instrument
supplemental or ancillary thereto or in implementation thereof and the form and
terms of any particular series of Debt Securities (as defined below) established
thereunder), provision was made for the issue, in one or more series, by the
Corporation of debt securities of the Corporation (the "Debt Securities") in an
unlimited aggregate principal amount;

         AND WHEREAS the proper officers of the Corporation have duly authorized
the creation and issuance of a series of Debt Securities to be designated as
5.05% Notes due 2013 (the "Notes"), and to be limited (subject to the exceptions
described herein and in the Trust Indenture) to the aggregate principal amount
of U.S.$500,000,000, the further terms and conditions thereof being hereinafter
set forth, all in accordance with a resolution of the directors of the
Corporation;

         AND WHEREAS all necessary acts and proceedings have been done and taken
to authorize the execution of this Supplemental Indenture, to establish the
terms and form of the Notes pursuant to the Trust Indenture, and to make this
Supplemental Indenture and the said series of Notes, when executed and delivered
by the Corporation and certified and delivered by the Trustee as provided in the
Trust Indenture, legal, valid and binding upon the Corporation;

         AND WHEREAS the foregoing recitals are made as representations and
statements of fact by the Corporation and not by the Trustee;

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         NOW THEREFORE, it is hereby covenanted, agreed and declared as follows:

1.       INTERPRETATIONS AND AMENDMENTS

1.1      SUPPLEMENTAL INDENTURE

         As used herein "Supplemental Indenture", "hereto", "herein", "hereof",
"hereby", "hereunder" and similar expressions refer to this Supplemental
Indenture and not to any particular Article, Section or other portion hereof and
include any and every instrument supplemental or ancillary hereto or in
implementation hereof, and further include the terms of the Notes set forth in
the form of Note annexed as Schedule A hereto.

1.2      DEFINITIONS IN TRUST INDENTURE

         All terms contained in this Supplemental Indenture which are defined in
the Original Indenture and not defined herein shall, for all purposes hereof,
have the meanings given to such terms in the Original Indenture unless the
context otherwise specifies or requires; provided, however, that notwithstanding
the foregoing, the terms "Corporation" and "Trustee" shall have the respective
meanings given to them in the Original Indenture.

1.3      INTERPRETATION NOT AFFECTED BY HEADINGS

         The division of this Supplemental Indenture into Articles and Sections,
the provision of the table of contents hereto and the insertion of headings are
for convenience of reference only and shall not affect the construction or
interpretation of this Supplemental Indenture.

2.       NOTES

2.1      FORM AND TERMS OF NOTES

         There shall be and is hereby created for issuance under the Trust
Indenture a series of Debt Securities which shall consist of and be limited
(subject to the exceptions set forth in Section 2.2(b) of the Original
Indenture) to the aggregate principal amount of U.S.$500,000,000 and shall be
designated as 5.05% Notes due 2013; provided, however, that if the Corporation
shall, at any time after the date hereof, increase the principal amount of Notes
which may be issued and issue such increased principal amount (or any portion
thereof), then any such additional Notes so issued shall have the same form and
terms (other than the date of issuance and the date from which interest thereon
shall begin to accrue and, under certain circumstances, the first interest
payment date), and shall carry the same right to receive accrued and unpaid
interest, as the Notes theretofore issued; and provided, further, that,
notwithstanding the foregoing, the Corporation shall not be entitled to increase
the principal amount of Notes which may be issued or issue any such increased
principal amount if the Corporation has effected satisfaction and discharge of
the Trust Indenture pursuant to Section 7.4 of the Original Indenture or
defeasance or covenant defeasance pursuant to Article 13 of the Original
Indenture.

         The Notes will mature, and the principal of the Notes and accrued and
unpaid interest thereon will be due and payable, on November 20, 2013, or such
earlier date as the principal of any of the Notes may become due and payable in
accordance with the provisions of the Trust Indenture.

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                                       -3-


         The Notes shall bear interest on the principal amount thereof from
November 20, 2003 or from the last date to which interest shall have been paid
or duly made available for payment on the Notes, whichever is later, at the rate
of 5.05% per annum, payable semi-annually in arrears on May 20 and November 20
(each, an "Interest Payment Date") in each year, commencing May 20, 2004, until
the principal of and premium, if any, on the Notes is paid or duly made
available for payment; and should the Corporation at any time default in the
payment of any principal of, or premium, if any, or interest on the Notes when
due, the Corporation shall pay interest (such interest to be payable on demand),
to the extent permitted by law, on the amount in default at the same rate.
Interest on the Notes shall be computed on the basis of a 360-day year comprised
of twelve 30-day months. The interest payable, and punctually paid or duly
provided for, on any Interest Payment Date will, as provided in the Trust
Indenture, be paid to the Persons in whose names the Notes (or one or more
predecessor Notes) are registered at the close of business on May 5 or November
5 (the "Regular Record Dates"), as the case may be, immediately prior to such
Interest Payment Date, regardless of whether any such Regular Record Date is a
business day. Any such interest on the Notes not so punctually paid or duly
provided for on any Interest Payment Date shall be payable as provided in the
form of Note annexed hereto as Schedule A to this Supplemental Indenture.

         For the purposes only of the disclosure required by the INTEREST ACT
(Canada), and without affecting the amount of interest payable to any holder of
a Note or the calculation of interest on any Note, if the rate of interest on
any Note is calculated on the basis of a year (the "deemed year") which contains
fewer days than the actual number of days in the calendar year of calculation,
such rate of interest shall be expressed as a yearly rate for the purposes of
the INTEREST ACT (Canada) by multiplying such rate of interest by the actual
number of days in the calendar year of calculation and dividing it by the number
of days in the deemed year.

         All payments of principal of and premium, if any, and interest on the
Notes will be made in such coin or currency of the United States of America as
at the time of payment is legal tender for payment of public and private debts,
and all references herein to "United States dollars", "U.S.$" or "U.S. dollars"
shall be deemed to refer to such coin or currency of the United States of
America.

         The principal of and premium, if any, and interest on the Notes shall
be payable, and the Notes may be surrendered for exchange, registration,
transfer or discharge from registration, at the office or agency maintained by
the Corporation for such purpose from time to time in the City of Calgary,
Alberta (which shall initially be the office of the Trustee in the City of
Calgary, Alberta) and, under the circumstances described in Section 2.14(j) of
the Original Indenture, in the Borough of Manhattan, The City of New York, and
in such other places as the Corporation may from time to time designate. The
Trustee is hereby appointed as the initial Paying Agent, registrar and transfer
agent for the Notes in the City of Calgary, Alberta.

         The Notes shall be issued only as fully registered Notes, without
coupons, in denominations of U.S.$1,000 and integral multiples thereof. The
Notes initially will be represented by one or more Global Debt Securities
(collectively, the "Global Note") registered in the name of The Depository Trust
Company, as Depositary or its nominee, or a successor depositary or its nominee.

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                                       -4-


         The Notes and the certificate of the Trustee endorsed thereon shall be
substantially in the form set out in Schedule A to this Supplemental Indenture
with such appropriate insertions, omissions, substitutions and variations as the
Trustee may approve and shall be numbered in such manner as the Trustee may
approve, such approvals of the Trustee concerning any Note to be conclusively
evidenced by its certification of such Note.

         The register referred to in Section 3.2 of the Original Indenture
shall, with respect to the Notes, be kept at the office or agency in the City of
Calgary, Alberta that the Corporation may from time to time designate for such
purpose (which shall initially be the office of the Trustee in the City of
Calgary, Alberta), and at such other place or places as the Corporation with the
approval of the Trustee may hereafter designate. Without limitation to the
foregoing, if the Corporation shall at any time be required by the terms of the
Notes or the Trust Indenture to appoint a registrar for the Notes in the Borough
of Manhattan, The City of New York, the Corporation shall cause a register for
the Notes also to be kept at the office of the registrar in the Borough of
Manhattan, The City of New York and the Trustee hereby gives its approval to a
register being kept at such place.

         The Notes shall be subject to redemption at the option of the
Corporation as provided in Article 3 of this Supplemental Indenture. The
Corporation shall not be required to redeem, purchase or repay Notes pursuant to
any mandatory redemption, sinking fund or analogous provision or at the option
of the holders thereof. The Notes will not be convertible into or exchangeable
for securities of any Person. The Notes will be entitled to the benefit of Debt
Securities Guarantees upon the terms and conditions set forth in Article 4 of
this Supplemental Indenture.

         The Notes shall have the other terms and provisions set forth in the
form of Note attached hereto as Schedule A to this Supplemental Indenture, with
the same force and effect as if such terms and provisions were set forth in full
at this place.

2.2      ISSUANCE OF NOTES

         Notes in the aggregate principal amount of U.S.$500,000,000 shall be
executed by the Corporation and delivered by it to the Trustee on the date of
issue for certification and delivery pursuant to and in accordance with the
provisions of Sections 2.4, 2.5 and 2.6 of the Original Indenture and, upon the
requirements of such provisions being complied with, the Notes shall be
certified by or on behalf of the Trustee and delivered by it to or upon the
written order of the Corporation without any further act or formality on the
part of the Corporation. The Trustee shall have no duty or responsibility with
respect to the use or application of any of the Notes so certified and delivered
or the proceeds thereof.

3.       REDEMPTION OF NOTES

3.1      REDEMPTION OF NOTES

         The Notes will be redeemable at any time, in whole or from time to time
in part, at the option of the Corporation (in the manner and in accordance with
and subject to the terms and provisions set forth in the Trust Indenture), at a
redemption price equal to the greater of:

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                                       -5-


         (a)      100% of the principal amount of the Notes to be redeemed; and

         (b)      the sum of the present values of the remaining scheduled
                  payments of principal and interest thereon (exclusive of
                  interest accrued to the date of redemption) discounted to the
                  redemption date on a semi-annual basis (assuming a 360-day
                  year consisting of twelve 30-day months) at the Treasury Rate,
                  plus 20.0 basis points;

plus in each case accrued interest to the date of redemption; provided that
installments of interest on Notes which are due and payable on any date falling
on or prior to a redemption date will be payable to the registered holders of
such Notes (or one or more predecessor Notes), registered as such as of the
close of business on the relevant record dates.

3.2      ADDITIONAL DEFINITIONS

         For the purposes of this Supplemental Indenture, the following
expressions shall have the following meanings:

"Treasury Rate" means, with respect to any redemption date, the rate per year
equal to the semiannual equivalent yield to maturity of the Comparable Treasury
Issue, assuming a price for the Comparable Treasury Issue (expressed as a
percentage of its principal amount) equal to the Comparable Treasury Price for
such redemption date;

"Comparable Treasury Issue" means the U.S. Treasury security selected by an
Independent Investment Banker as having a maturity comparable to the remaining
term of the Notes to be redeemed that would be utilized, at the time of
selection and in accordance with customary financial practice, in pricing new
issues of corporate debt securities of comparable maturity to the remaining term
of such Notes;

"Independent Investment Banker" means one of the Reference Dealers selected by
the Corporation;

"Comparable Treasury Price" means, with respect to any redemption date, (i) the
average of the bid and asked prices for the Comparable Treasury Issue (expressed
in each case as a percentage of its principal amount) on the third business day
preceding such redemption date, as set forth in the daily statistical release
(or any successor release) published by the Federal Reserve Bank of New York and
designated "Composite 3:30 p.m. Quotations for U.S. Government Securities" or
(ii) if such release (or any successor release) is not published or does not
contain such prices on such business day, (A) the average of the Reference
Treasury Dealer Quotations for such redemption date, after excluding the highest
and lowest such Reference Treasury Dealer Quotations, or (B) if the Corporation
is unable to obtain four such Reference Treasury Dealer Quotations, the average
of all such Quotations;

"Reference Treasury Dealer Quotations" means, with respect to each Reference
Dealer and any redemption date, the average, as determined by the Corporation,
of the bid and asked prices for the Comparable Treasury Issue (expressed in each
case as a percentage of its principal amount) quoted in writing to the
Corporation by such Reference Dealer at 5:00 p.m. (New York City time) on the
third business day preceding such redemption date; and

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                                       -6-


"Reference Dealer" means each of Banc of America Securities LLC and Deutsche
Bank Securities Inc. and their respective successors, and two other firms that
are primary U.S. government securities dealers (each a "Primary Treasury
Dealer") which we specify from time to time; provided, however, that if any of
the foregoing Reference Dealers shall cease to be a Primary Treasury Dealer, the
Corporation shall substitute therefor another Primary Treasury Dealer.

3.3      NOTICE TO TRUSTEE

         The Corporation will provide notice to the Trustee prior to the
redemption date of the calculation of the redemption price.

4.       DEBT SECURITIES GUARANTEE

4.1      COVENANT TO PROVIDE DEBT SECURITIES GUARANTEE

         The Corporation covenants and agrees that, so long as any of the Notes
remains outstanding: (a) if any Subsidiary of the Corporation shall at any time
guarantee (a "Subsidiary Guarantee") any Indebtedness of the Corporation
pursuant to Article 4 of the fourth supplemental indenture (the "Fourth
Supplemental Indenture") made as of the 2nd day of July, 1997 to the trust
indenture dated as of June 7, 1991 between the Corporation and CIBC Mellon Trust
Company, as successor trustee, then the Corporation will cause such Subsidiary
(a "Guarantor") to execute a Debt Securities Guarantee pursuant to which it will
guarantee the Debt Securities (including the Notes) on substantially the same
terms as are set forth in such Subsidiary Guarantee with such changes as may be
necessary to conform to local law or practice of the jurisdiction in which the
Guarantor is organized (which, in the case of a Subsidiary which is organized
and existing under the laws of any State of the United States or the District of
Columbia, may include a customary provision limiting amounts payable under such
Debt Securities Guarantee to such amount as will not violate applicable
fraudulent conveyance or other similar laws and a customary provision providing
for contribution from other Guarantors, if any) or as may be recommended by
independent legal counsel to the Trustee and incorporating provisions pursuant
to which the Guarantor shall submit to jurisdiction, waive immunity, and appoint
an agent for service of process to the same extent done by the Corporation
pursuant to the Trust Indenture; and (b) the Corporation will comply, and will
cause such Guarantor to comply, with all applicable securities laws and
regulations, including, without limitation, the Securities Act of 1933, as
amended, the Securities Exchange Act of 1934, as amended, and the Trust
Indenture Act of 1939, as amended, (and in each of the foregoing cases all
applicable rules and regulations thereunder) and with state securities and blue
sky laws and regulations of the United States in connection with any Debt
Securities Guarantee. Any such Debt Securities Guarantee shall be subject to the
terms and provisions of the Trust Indenture, including, without limitation,
Article 12 of the Original Indenture. Any such Debt Securities Guarantee shall
be duly executed and delivered to the Trustee by the applicable Guarantor, and
shall be effective as of a date not later than 90 days after the date on which
the Corporation becomes obligated to provide such Subsidiary Guarantee.

4.2      OPINIONS

         The Trustee will, at the expense of the Corporation, engage independent
legal counsel of recognized standing (in the jurisdiction in which the Guarantor
is organized) with respect to

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                                       -7-


matters relating to publicly issued debt securities to review the form of Debt
Securities Guarantee, and both counsel to the Corporation and such independent
legal counsel shall deliver legal opinions to the Trustee to the effect that
such Debt Securities Guarantee has been duly authorized, executed and delivered
by, and is a valid, binding and enforceable obligation of, the Guarantor
(subject to customary exceptions). In addition, the Trustee will, at the expense
of the Corporation, engage independent legal counsel of recognized standing with
respect to matters of United States federal securities laws (which, if possible,
shall be the same independent legal counsel referred to in the first sentence of
this paragraph) and such independent legal counsel shall deliver legal opinions
to the Trustee to the effect that such Debt Securities Guarantee has been
registered under the Securities Act of 1933, as amended, and that the Trust
Indenture has been duly qualified under the Trust Indenture Act of 1939, as
amended, or that no such registration or qualification is required. All such
opinions shall be in form and substance satisfactory to the Trustee, acting
reasonably, and shall be delivered to the Trustee not later than the time of
delivery to the Trustee of the corresponding Debt Securities Guarantee.

4.3      AUTOMATIC TERMINATION

         In the event a Subsidiary Guarantee is discharged or released, then,
upon Written Order of the Corporation to the Trustee accompanied by a
Certificate of the Corporation confirming that the corresponding Subsidiary
Guarantee has been discharged or released, such Debt Securities Guarantee shall
automatically terminate and be released and the Trustee shall return to the
Corporation such Debt Securities Guarantee.

5.       GENERAL

5.1      INDENTURE SUPPLEMENTAL TO TRUST INDENTURE

         This Supplemental Indenture is supplemental to the Original Indenture
and the Original Indenture, the First Supplemental Indenture, the Second
Supplemental Indenture and the Third Supplemental Indenture shall, from this
date forward, be read in conjunction with this Supplemental Indenture. The
Original Indenture, the First Supplemental Indenture, the Second Supplemental
Indenture, the Third Supplemental Indenture and this Supplemental Indenture
shall, from this date forward, have effect so far as practicable as if all the
provisions of the Original Indenture, the First Supplemental Indenture, the
Second Supplemental Indenture, the Third Supplemental Indenture and this
Supplemental Indenture were contained in one instrument.

5.2      ACCEPTANCE OF TRUST

         The Trustee hereby accepts the trusts in this Supplemental Indenture
declared and provided for and agrees to perform the same upon the terms and
conditions herein before set forth in trust for the various Persons who shall
from time to time be Note holders subject to all the terms and conditions herein
set forth.

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                                       -8-


5.3      COUNTERPARTS AND FORMAL DATE

         This Supplemental Indenture may be executed in several counterparts,
each of which so executed shall be deemed to be an original, and such
counterparts together shall constitute one and the same instrument and
notwithstanding their date of execution shall be deemed to bear the date first
above written.



         IN WITNESS WHEREOF the parties hereto have executed this Supplemental
Indenture under their respective corporate seals on the date first above
written.

                                NEXEN INC.


                                    /s/ Marvin F. Romanow
                                    -------------------------------------------
                                    Marvin F. Romanow
                                    Executive Vice President and
                                    Chief Financial Officer


                                                                          [Seal]
                                    /s/ Una M. Power
                                    -------------------------------------------
                                    Una M. Power
                                    Treasurer


                                CIBC MELLON TRUST COMPANY


                                Per:    /s/ Jacquie Fisher
                                        ---------------------------------------
                                Name:   Jacquie Fisher
                                Title:  Manager, Client Relations


                                                                          [Seal]
                                Per:    /s/ Robert Solis
                                        ---------------------------------------
                                Name:   Robert Solis
                                Title:  Account Manager
                                        Corporate Trust Service