Exhibit 5.1 OPINION OF FRIED, FRANK, HARRIS, SHRIVER & JACOBSON October 28, 1999 Burlington Resources Inc. 5051 Westheimer, Suite 1400 Houston, TX 77056 Ladies and Gentlemen: We are acting as special counsel to Burlington Resources Inc., a Delaware corporation ("BR"), in connection with the Registration Statement on Form S-3 (the "Registration Statement"), under the Securities Act of 1933, as amended (the "Securities Act"), relating to the issuance by BR of up to 38,500,000 shares of common stock of BR ("Common Stock") to be issuable upon exchange of the exchangeable shares (the "Exchangeable Shares") of Burlington Resources Canada Inc., an Alberta corporation and a subsidiary of BR, in connection with the acquisition of Poco Petroleums Ltd. by BR. Capitalized terms used herein have the meanings set forth in the Registration Statement, unless otherwise defined herein. With your permission, all assumptions and statements of reliance herein have been made without any independent investigation or verification on our part except to the extent otherwise expressly stated, and we express no opinion with respect to the subject matter or accuracy of such assumptions or items relied upon. In connection with this opinion, we have (i) investigated such questions of law, (ii) examined originals, or certified, conformed or reproduction copies of such agreements, instruments, documents and records of BR, such certificates of public officials and such other documents, and (iii) received such information from officers and representatives of BR as we have deemed necessary or appropriate for the purposes of this opinion. In all such examinations, we have assumed the legal capacity of all natural persons, the genuineness of all signatures, the authenticity of original and certified documents and the conformity to original or certified documents of all copies submitted to us as conformed or reproduction copies. As to various questions of fact relevant to the opinion expressed herein, we have relied upon, and assume the accuracy of, representations and warranties contained in the documents relevant hereto and certificates and oral or written statements and other information of or from representatives of BR and others and assume compliance on the part of all parties to such documents with their covenants and agreements contained therein. We have assumed that the certificate of incorporation and by-laws of BR at the time of issuance of the shares of Common Stock will be identical to the certificate of incorporation and by-laws of BR as in effect at the date hereof. We also have assumed, with respect to all parties to the documents relevant hereto other than BR, that such parties have the power and authority to enter into and perform such documents and to consummate the transactions contemplated thereby, that such documents have been duly authorized, executed and delivered by such parties and constitute legal, valid and binding obligations of such parties enforceable against such parties in accordance with the terms, and that such parties will comply with all of their obligations under such documents and all laws applicable thereto. As to various questions of fact relevant to such opinions, we have relied upon, and have assumed the accuracy of, certificates and oral or written statements and other information of or from public officials, officers or representatives of BR and others. Based upon the foregoing and assuming that all of the Exchangeable Shares are validly issued, fully paid and non-assessable, it is our opinion that, if shares of Common Stock are issued as described in the Registration Statement, at such time of issuance such shares of Common Stock will be validly issued, fully paid and non-assessable. We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the reference to this firm under the caption "Legal Matters" in the Registration Statement as having passed upon the validity of the issuance of the Common Stock. In giving this consent, we do not hereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act. Very truly yours, FRIED, FRANK, HARRIS SHRIVER & JACOBSON By: /s/ Warren de Wied ------------------ Warren de Wied