EXHIBIT 5.1
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                                                   Blake, Cassels & Graydon LLP
                                                        Barristers & Solicitors
                                                     Patent & Trade-mark Agents
                                                          855 - 2nd Street S.W.
                                            Suite 3500, Bankers Hall East Tower
                                                      Calgary AB T2P 4J8 Canada
                                           Tel: 403-260-9600  Fax: 403-260-9700

                                                             Reference: 80545/7


May 7, 2007


Securities and Exchange Commission
450 - Fifth Street NW
Washington, DC 20549

Re: Nexen Inc. Dividend Reinvestment Plan

Dear Sirs:

Nexen Inc. (the "COMPANY") has filed a Registration  Statement on Form F-3 (the
"REGISTRATION  STATEMENT")  under the  United  States  Securities  Act of 1933,
including  a form of  Prospectus  dated May 7, 2007  (the  "PROSPECTUS"),  with
respect to 2,000,000 additional common shares, without nominal or par value, of
the  Company  ("COMMON  SHARES")  to be  offered to  registered  holders of the
Company's Common Shares pursuant to the Company's  Dividend  Reinvestment  Plan
(the "PLAN").

We have  examined  all such  corporate  records and other  documents as we have
deemed  relevant  and  necessary in order to give this  opinion.  As to various
questions  of fact  material to such  opinion and which were not  independently
established,  we have relied upon certificates of public officials and officers
of the  Company.  In reviewing  the  foregoing  documents,  we have assumed the
genuineness of all signatures and the  authenticity of all documents  submitted
to us as originals and the conformity to authentic or original documents of all
documents submitted to us as certified, true or photostatic copies.

We are  members  of the Law  Society of Alberta  and this  opinion is  rendered
solely with respect to the laws of the Province of Alberta and the federal laws
of Canada applicable therein as at the date hereof.

Based upon the foregoing and subject to the qualifications herein expressed, we
are of the opinion that the 2,000,000  additional  Common Shares of the Company
in respect  of which the  Registration  Statement  has been  filed  will,  when
properly  issued in  accordance  with the terms and  conditions of the Plan, be
validly issued.

In  our  opinion,   the  summary  set  forth  under  the  caption  "Income  Tax
Considerations   Relating   to  the  Plan  -   Canadian   Federal   Income  Tax
Considerations"  is a fair  and  accurate  summary  of the  principal  Canadian
federal income tax considerations  generally applicable to a Participant in the
Plan.

We  consent to the  filing of this  opinion  as an exhibit to the  Registration
Statement  and to the use of our  name in the  Registration  Statement  and the
Prospectus under the captions "Enforceability of Certain Civil Liabilities" and
"Legal  Matters".  In giving this consent,  we do not hereby admit that we come
within the category of persons  whose  consent is required by the United States
Securities Act of 1933 or the rules and regulations promulgated thereunder.




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In giving the above opinions,  we have assumed that certificates for any of the
said Common Shares will,  before their  issuance,  have been duly signed by the
Company, countersigned by the transfer agent and registered by the registrar.

Yours truly,

/s/ BLAKE, CASSELS & GRAYDON LLP