EXHIBIT 8.1
                                                                    -----------



           [LETTERHEAD OF PAUL WEISS RIFKIND WHARTON & GARRISON LLP]



                                           May 7, 2007


Nexen Inc.
801 - 7th Avenue S.W.
Calgary, Alberta
Canada
T2P 3P7


Dear Ladies and Gentlemen:

         In  connection  with  the  Registration  Statement  on Form  F-3  (the
"Registration  Statement")  filed  by  Nexen  Inc.  (the  "Company"),  with the
Securities and Exchange  Commission  pursuant to the Securities Act of 1933, as
amended (the "Act"), and the rules and regulations thereunder (the "Rules"), we
have been  requested  to render our opinion as to the matters  hereinafter  set
forth.  Capitalized  terms used and not otherwise defined herein shall have the
meanings attributed thereto in the Registration Statement.

         In this regard, we have reviewed copies of the Registration  Statement
(including the exhibits and  amendments  thereto) with respect to the Company's
registration  of 2,000,000  common shares (the "Common  Shares") of the Company
that are to be sold from time to time  pursuant  to the  Company's  amended and
restated   Dividend   Reinvestment   Plan.   We  have  also  made  such   other
investigations  of fact and law and have  examined  the  originals,  or  copies
authenticated  to  our   satisfaction,   of  such  other   documents,   record,
certificates  or  other  instruments  as  in  our  judgment  are  necessary  or



ABG Sundal Collier Norge ASA                                                  2
Deutsche Bank AG, London Branch


appropriate to render the opinion  expressed below.  Additionally,  the Company
has made factual  representations  to us with  respect to, among other  things,
their prior,  current and projected  operations,  revenues and assets.  We have
relied   without   independent   investigation   upon  the   accuracy  of  such
representations.

         The opinion set forth below is limited to the Internal Revenue Code of
1986,  as amended (the "Code"),  administrative  rulings,  judicial  decisions,
Treasury regulations and other applicable authorities,  all as in effect on the
date hereof. The statutory  provisions,  regulations,  and interpretations upon
which our opinion is based are subject to change,  and such changes could apply
retroactively.  Any such change  could  affect the  continuing  validity of the
opinion  set forth  below.  We assume no  responsibility  to advise  you of any
subsequent   changes  in  existing   law  or  facts,   nor  do  we  assume  any
responsibility to update this opinion with respect to any matters expressly set
forth herein,  and no opinions are to be implied or may be inferred  beyond the
matters expressly so stated.

         Based upon and subject to the foregoing, in our opinion the statements
of law and legal conclusions as to the United States Federal income tax matters
set forth under the caption  "United States Tax Federal Income  Considerations"
in the  prospectus  are  correct  in all  material  respects.  In this  regard,
however, we note that the statements as to the Company's beliefs and intentions
as to factual  matters  represent the views of the Company's  management and do
not represent legal opinions of its counsel.

         We hereby  consent to the filing of this  opinion as an exhibit to the
Registration  Statement,  or any amendment  pursuant to Rule 462 under the Act,
and to the reference to us under the heading "Legal  Matters" in the Prospectus
included in the Registration  Statement,  or any amendment pursuant to Rule 462
under the Act.  In giving  this  consent,  we do not hereby  agree that we come
within the  category  of persons  whose  consent is  required by the Act or the
Rules.

                               Very truly yours,


                               /s/ PAUL, WEISS, RIFKIND, WHARTON & GARRISON LLP

                               PAUL, WEISS, RIFKIND, WHARTON & GARRISON LLP