EXHIBIT 4.2
                                                                    -----------



===============================================================================




                            DATED AS OF MAY 4, 2007



                                   NEXEN INC.

                                      AND

                      DEUTSCHE BANK TRUST COMPANY AMERICAS








                          FIRST SUPPLEMENTAL INDENTURE

                                     TO THE

                                TRUST INDENTURE

                            DATED AS OF MAY 4, 2007


===============================================================================

<PAGE>

THIS FIRST SUPPLEMENTAL  INDENTURE (this "FIRST SUPPLEMENTAL  INDENTURE") dated
as of May 4, 2007 between  NEXEN INC.,  a  corporation  incorporated  under the
Canada  Business  Corporations  Act and having  its head  office in the City of
Calgary,  in the Province of Alberta  (the  "ISSUER")  and DEUTSCHE  BANK TRUST
COMPANY AMERICAS,  a banking  corporation duly organized and existing under the
laws of the State of New York, having an office in the City of New York, in the
State of New York (the "Trustee")


                            RECITALS OF THE COMPANY

WHEREAS, the Issuer and the Trustee entered into an Indenture,  dated as of May
4, 2007 (the  "ORIGINAL  INDENTURE").  Section 9.01 of the  Original  Indenture
provides  that the Issuer  and the  Trustee  may,  without  the  consent of any
Holder,  enter into a supplemental  indenture to establish the form or terms of
Securities of any series as permitted by Section 2.01 and 3.01 thereof.

WHEREAS,  pursuant to Sections  2.01 and 3.01 of the  Original  Indenture,  the
Issuer desires to provide for the establishment of a series of Securities under
the Original  Indenture,  and the form and terms thereof,  as  hereinafter  set
forth.

WHEREAS,  the Issuer has  requested  that the Trustee  execute and deliver this
First  Supplemental  Indenture.  The Issuer  has  delivered  to the  Trustee an
Opinion of Counsel and an Officers' Certificate pursuant to Sections 1.02, 3.03
and 9.03 of the Original Indenture to the effect,  among other things, that all
conditions  precedent provided for in the Indenture to the Trustee's  execution
and delivery of this First Supplemental  Indenture have been complied with. All
acts and  things  necessary  have been done and  performed  to make this  First
Supplemental  Indenture  enforceable  in  accordance  with its  terms,  and the
execution  and  delivery  of this First  Supplemental  Indenture  has been duly
authorized in all respects.

         WHEREAS the proper  officers of the Issuer  have duly  authorized  the
creation and issuance of: (i) a series of  Securities to be designated as 5.65%
Notes due 2017 (the "2017 NOTES") and to be initially  limited  (subject to the
exceptions  described  herein and in the Original  Indenture)  to the aggregate
principal  amount of  U.S.$250,000,000;  and (ii) a series of  Securities to be
designated  as 6.40% Notes due 2037 (the "2037  NOTES" and,  together  with the
2017 Notes, the "NOTES") and to be initially limited (subject to the exceptions
described  herein and in the Original  Indenture)  to the  aggregate  principal
amount of  U.S.$1,250,000,000;  the further terms and conditions  thereof being
hereinafter set forth,  all in accordance with a resolution of the directors of
the Issuer;

         NOW, THEREFORE, THIS FIRST SUPPLEMENTAL INDENTURE WITNESSETH:  For and
in  consideration of the premises and the purchase of the Securities (as herein
defined) by the holders thereof,  it is mutually covenanted and agreed, for the
equal and proportionate benefit of all holders of the Securities, as follows:

<PAGE>
                                      -2-


1.       INTERPRETATIONS AND AMENDMENTS

1.1      FIRST SUPPLEMENTAL INDENTURE

         As used herein "FIRST  SUPPLEMENTAL  INDENTURE",  "HERETO",  "HEREIN",
"HEREOF",  "HEREBY",  "HEREUNDER" and similar  expressions  refer to this First
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 2017 Notes  annexed as  Schedule A hereto and the form
of 2037 Notes annexed as Schedule B hereto.

1.2      DEFINITIONS IN FIRST SUPPLEMENTAL INDENTURE

         All terms  contained in this First  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 "ISSUER" 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 First  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 First Supplemental Indenture.

2.       NOTES

2.1      FORM AND TERMS OF NOTES

         There  shall be and there is hereby  created  for  issuance  under the
Original Indenture, as supplemented by this First Supplemental Indenture; (i) a
series of Securities  which shall consist of an aggregate  principal  amount of
U.S.$250,000,000  2017  Notes;  and (ii) a series  of  Securities  which  shall
consist  of an  aggregate  principal  amount of  U.S.$1,250,000,000  2037 Notes
provided, however, that if the Issuer shall, at any time after the date hereof,
increase  the  principal  amount of either or both series 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 Issuer shall not be entitled to increase the principal amount of
Notes which may be issued or issue any such increased  principal  amount if the
Issuer has effected  satisfaction  and discharge of the  Indenture  pursuant to
Section 4.01 of the Original  Indenture or  defeasance  or covenant  defeasance
pursuant to Article 13 of the Original Indenture.

         The 2017 Notes will  mature,  and the  principal of the 2017 Notes and
accrued and unpaid interest  thereon will be due and payable,  on May 15, 2017,
or such earlier date as the principal of

<PAGE>
                                      -3-


any of the 2017  Notes  may  become  due and  payable  in  accordance  with the
provisions of the Original Indenture and this First Supplemental Indenture.

         The 2037 Notes will  mature,  and the  principal of the 2037 Notes and
accrued and unpaid interest  thereon will be due and payable,  on May 15, 2037,
or such earlier  date as the  principal of any of the 2037 Notes may become due
and payable in accordance with the provisions of the Original Indenture and the
First Supplemental Indenture.

         The Notes shall bear interest on the principal amount thereof from May
4, 2007 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
5.65%  per  annum in the case of the 2017  Notes  and at the rate of 6.40%  per
annum in the case of the 2037  Notes,  in each case  payable  semi-annually  in
arrears on May 15 and November 15 (each,  an "INTEREST  PAYMENT  DATE") in each
year, commencing November 15, 2007, until the principal of and premium, if any,
on the  applicable  series of Notes is paid or duly made available for payment;
and should the Issuer at any time default in the payment of any  principal  of,
or premium,  if any, or  interest on either  series of the Notes when due,  the
Issuer  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  applicable
to such  series of Notes.  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 Original  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 1 or November 1 (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  applicable,  as provided in the forms of Notes annexed  hereto as
Schedule A and Schedule B to this First 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 Corporate Trust Office of the
Trustee  in the City of New York,  New York,  and in such  other  places as the
Issuer  may  from  time to time  designate  in  accordance  with  the  Original
Indenture.

<PAGE>
                                      -4-


The Trustee is hereby  appointed as the initial  Paying  Agent,  registrar  and
transfer agent for the Notes in the City of New York, New York.

         The Notes  shall be issued  only as fully  registered  Notes,  without
coupons,  in  denominations  of  U.S.$2,000  and  integral  multiples of $1,000
thereafter.  Each series of Notes  initially will be represented by one or more
global Securities (collectively,  the "GLOBAL NOTES") registered in the name of
The  Depository  Trust  Company,  as Depositary or its nominee,  or a successor
depositary or its nominee.

         The  certificates  representing  the Notes  shall  bear the  following
legend:

         "THIS  SECURITY  IS IN GLOBAL  FORM WITHIN THE MEANING OF THE
         TRUST INDENTURE  HEREINAFTER REFERRED TO AND IS REGISTERED IN
         THE NAME OF THE  DEPOSITARY  OR A NOMINEE OF THE  DEPOSITARY.
         UNLESS  AND  UNTIL  IT IS  EXCHANGED  IN WHOLE OR IN PART FOR
         SECURITIES IN CERTIFICATED FORM IN THE LIMITED  CIRCUMSTANCES
         DESCRIBED  IN  THE  INDENTURE,   THIS  SECURITY  MAY  NOT  BE
         TRANSFERRED  EXCEPT AS A WHOLE BY THE DEPOSITARY TO A NOMINEE
         OF THE  DEPOSITARY  OR BY A NOMINEE OF THE  DEPOSITARY TO THE
         DEPOSITARY  OR ANOTHER  NOMINEE OF THE  DEPOSITARY  OR BY THE
         DEPOSITARY OR ANY SUCH NOMINEE TO A SUCCESSOR DEPOSITARY OR A
         NOMINEE OF SUCH SUCCESSOR DEPOSITARY."

         The Notes and the certificate of the Trustee endorsed thereon shall be
in the form set out in Schedule A, in the case of the 2017 Notes,  and Schedule
B, in the case of the 2037 Notes,  to this First  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  Security  Register  referred to in Section  3.05 of the  Original
Indenture shall,  with respect to the Notes, be kept at the office or agency in
the City of New York,  New York that the Issuer may from time to time designate
for such purpose  (which shall  initially be the Corporate  Trust Office of the
Trustee in the City of New York,  New York),  and at such other place or places
as the Issuer with the approval of the Trustee may hereafter designate.

         The Notes shall be subject to  redemption  at the option of the Issuer
as provided in Article 3 of this First Supplemental Indenture. The Issuer 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 shall have the other terms and  provisions  set forth in the
forms of  Notes  attached  hereto  as  Schedule  A to this  First  Supplemental
Indenture,  in the  case of the  2017  Notes,  and  Schedule  B to  this  First
Supplemental  Indenture, in the case of the 2037 Notes, with the same force and
effect as if such terms and provisions were set forth in full herein.

<PAGE>
                                      -5-


2.2      ISSUANCE OF NOTES

         2017 Notes in the aggregate principal amount of  U.S.$250,000,000  and
2037 Notes in the aggregate  principal  amount of  U.S.$1,250,000,000  shall be
executed by the Designated Officer of the Issuer and delivered by the Issuer to
the Trustee on the date of issue for  authentication  and delivery  pursuant to
and in accordance with the provisions of Section 3.03 of the Original Indenture
and, upon the  requirements of such  provisions  being complied with, the Notes
shall be authenticated by or on behalf of the Trustee and delivered by it to or
upon the Issuer Order of the Issuer without any further act or formality on the
part of the  Issuer.  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

         Each series of the Notes will be  redeemable  at any time, in whole or
from time to time in part,  at the  option of the  Issuer (in the manner and in
accordance with and subject to the terms and provisions set forth in Article 11
of the Original Indenture), at a Redemption Price equal to the greater of:

         (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 basis points in the case of the 2017 Notes, and
                  plus 35 basis points in the case of the 2037 Notes;

plus in each case  accrued  interest  to the  Redemption  Date;  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 Regular Record Dates.

         The Issuer will provide  notice to the Trustee prior to the Redemption
Date of the calculation of the Redemption Price.

3.2      ADDITIONAL DEFINITIONS

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

         "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  applicable  series of 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;

<PAGE>
                                      -6-


         "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  Issuer  is unable  to  obtain  four such  Reference
Treasury Dealer Quotations, the average of all such Quotations;

         "INDEPENDENT  INVESTMENT  BANKER" means one of the  Reference  Dealers
selected by the Issuer;

         "REFERENCE  DEALER"  means  each of Banc of  America  Securities  LLC,
Citigroup  Global  Markets  Inc. and Deutsche  Bank  Securities  Inc. and their
respective  successors,  and two other firms that are primary  U.S.  government
securities  dealers in the City of New York (each a "PRIMARY  TREASURY Dealer")
which the Issuer specifies from time to time; provided, however, that if any of
the foregoing  Reference  Dealers shall cease to be a Primary  Treasury Dealer,
the Issuer shall substitute therefor another Primary Treasury Dealer;

         "REFERENCE  TREASURY DEALER  QUOTATIONS"  means,  with respect to each
Reference  Dealer and any redemption  date,  the average,  as determined by the
Issuer,  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  Issuer by such  Reference  Dealer at 3:30 p.m.  (New York City
time) on the third business day preceding such redemption date; and

         "TREASURY RATE" means,  with respect to any redemption  date, the rate
per  year  equal  to  the  semi-annual  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.

4.       GENERAL

4.1      EFFECTIVENESS

         This First  Supplemental  Indenture  will  become  effective  upon its
execution and delivery.

4.2      EFFECT OF RECITALS

         The  recitals  contained  herein  and in the  Securities,  except  the
Trustee's  certificates of authentication,  shall be taken as the statements of
the Issuer,  and neither the Trustee nor any  Authenticating  Agent assumes any
responsibility   for  their   correctness.   Neither   the   Trustee   nor  any
Authenticating  Agent shall be  accountable  for the use or  application by the
Issuer  of the  Securities  or the  proceeds  thereof.  The  Trustee  makes  no
representations  as to the validity or sufficiency  of this First  Supplemental
Indenture or of the Securities  except that the Trustee  represents  that it is
duly  authorized  to execute and  deliver  this First  Supplemental  Indenture,
authenticate  the  Securities  and perform its  obligations  under the Original
Indenture and

<PAGE>
                                      -7-


hereunder,  and  that  the  statements  made  by it or  to be  made  by it in a
Statement of Eligibility and  Qualification  on Form T-1 supplied to the Issuer
are true and accurate.


4.3      RATIFICATION OF ORIGINAL INDENTURE

         The Original  Indenture  as  supplemented  by this First  Supplemental
Indenture  is  in  all  respects   ratified  and  confirmed,   and  this  First
Supplemental  Indenture  shall be deemed part of the Original  Indenture in the
manner and to the extent herein and therein provided.

4.4      GOVERNING LAW

         This  First   Supplemental   Indenture,   the  Original  Indenture  as
supplemented  hereby and the  Securities  shall be governed by and construed in
accordance with the laws of the State of New York.

4.5      SEVERABILITY

         In case  any  provision  in this  First  Supplemental  Indenture,  the
Original  Indenture  as  supplemented  hereby  or in the  Securities  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.

4.6      ACCEPTANCE OF TRUST

         The  Trustee  hereby  accepts  the trusts in this  First  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.

4.7      COUNTERPARTS AND FORMAL DATE

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



<PAGE>


         IN WITNESS WHEREOF the parties hereto have executed this  Supplemental
Indenture on the date first above written.

                                      NEXEN INC.


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


                                      Per:   /s/ Una M. Power
                                             ---------------------------------
                                             Una M. Power
                                             Treasurer


                                      DEUTSCHE BANK TRUST COMPANY
                                      AMERICAS, as trustee

                                      By Deutsche Bank National Trust Company


                                      Per:   /s/ Yana Kalachikova
                                             ---------------------------------
                                             Name: Yana Kalachikova
                                             Title: Assistant Vice President



                                      Per:   /s/ Rodney Gaughan
                                             ---------------------------------
                                             Name: Rodney Gaughan
                                             Title: Vice President


<PAGE>

                               TABLE OF CONTENTS


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

      1.1   FIRST SUPPLEMENTAL INDENTURE......................................2
      1.2   DEFINITIONS IN FIRST SUPPLEMENTAL 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.................................................5

3.    REDEMPTION OF NOTES.....................................................5

      3.1   REDEMPTION OF NOTES...............................................5
      3.2   ADDITIONAL DEFINITIONS............................................5

4.    GENERAL.................................................................6

      4.1   EFFECTIVENESS.....................................................6
      4.2   EFFECT OF RECITALS................................................6
      4.3   RATIFICATION OF ORIGINAL INDENTURE................................7
      4.4   GOVERNING LAW.....................................................7
      4.5   SEVERABILITY......................................................7
      4.6   ACCEPTANCE OF TRUST...............................................7
      4.7   COUNTERPARTS AND FORMAL DATE......................................7