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                                                                     EXHIBIT 5.1


                                  July 30, 2001

Tri-Union Development Corporation
530 Lovett Boulevard
Houston, Texas 77006

Tri-Union Operating Company
530 Lovett Boulevard
Houston, Texas 77006

     Re: Registration Statement on Form S-4
         $130,000,000 aggregate principal amount of 12.5% Senior Secured Notes
         due 2006

Gentlemen:

         We have acted as counsel to Tri-Union Development Corporation, a Texas
corporation ("Tri-Union"), and Tri-Union Operating Company, a Delaware
corporation and wholly owned subsidiary of Tri-Union ("Tri-Union Operating")
(Tri-Union and Tri-Union Operating are collectively referred to herein as the
"Companies"), with respect to the preparation of the Registration Statement on
Form S-4 (the "Registration Statement") filed with the Securities and Exchange
Commission (the "Commission") under the Securities Act of 1933, as amended, in
connection with the registration of (i) the offer and exchange by Tri-Union (the
"Exchange Offer") of up to $130,000,000 aggregate principal amount of its 12.5%
Senior Secured Notes due 2006 (the "Old Notes") for new notes of Tri-Union
containing substantially identical terms and in like principal amount (the "New
Notes") and (ii) the guarantee (the "Guarantee") of Tri-Union Operating. The New
Notes will be offered under an Indenture dated as of June 18, 2001 (the
"Indenture"), among Tri-Union, as issuer, Tribo Petroleum Corporation, as parent
guarantor, and Firstar Bank, National Association, as trustee (the "Trustee").

         In so acting, we have examined originals, or photostatic or certified
copies, of the Indenture, the form of the New Notes and such records of the
Companies, certificates of officers of the Companies and of public officials,
and such other documents as we have deemed relevant. In such examination, we
have assumed the genuineness of all signatures, the authenticity of all
documents submitted to us as originals, the conformity to original documents of
all documents

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Tri-Union Development Corporation
Tri-Union Operating Company
July 30, 2001
Page 2

submitted to us as certified or photostatic copies and the authenticity of the
originals of such documents. We have also assumed that the Registration
Statement, and any amendments thereto (including post-effective amendments),
will have become effective and the New Notes will be issued and sold in
compliance with applicable federal and state securities laws and in the manner
described in the Registration Statement.

         Based upon the foregoing, and subject to the qualifications stated
herein, we are of the opinion that:

         Upon the New Notes being duly executed by Tri-Union and authenticated
by the Trustee therefor in accordance with the terms of the Indenture, and duly
issued and delivered against the receipt of Old Notes surrendered in exchange
therefor, and assuming a court of appropriate jurisdiction would hold that the
New Notes are governed by and are to be construed under the laws of the State of
Texas (notwithstanding the selection contained in the New Notes and the
Indenture of the laws of New York as the governing laws), (i) the New Notes will
constitute the legal, valid and binding obligations of Tri-Union, enforceable
against Tri-Union in accordance with their terms, and (ii) the Guarantee remains
a valid and binding obligation of Tri-Union Operating, enforceable against
Tri-Union Operating in accordance with its terms, except in each case as the
enforcement thereof may be limited by bankruptcy, insolvency, reorganization,
moratorium or similar laws, court decisions relating to or affecting the
enforcement of creditors' rights generally and except as enforcement thereof is
subject to general principles of equity (regardless of whether enforcement is
considered in a proceeding in equity or at law).

         We are further of the opinion that the statements contained in the
prospectus constituting a part of the Registration Statement under the caption
"CERTAIN UNITED STATES FEDERAL INCOME TAX CONSIDERATIONS," as qualified therein,
constitute an accurate description, in general terms of the indicated United
States federal income tax consequences to a U.S. holder of the purchase,
ownership and disposition of the New Notes.

         This opinion is limited in all respects to the laws of the State of
Texas and applicable federal laws, and we express no opinion herein as to the
effect that the laws and decisions of courts of any jurisdiction other than the
United States and the State of Texas may have upon such opinion.

         We consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to us in the prospectus included
therein under the caption "Legal Matters." In giving this consent, we do not
admit that we are within the category of persons whose consent is required under

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Tri-Union Development Corporation
Tri-Union Operating Company
July 30, 2001
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Section 7 of the Securities Act of 1933, as amended, or the general rules and
regulations of the Commission promulgated thereunder.

                                       Very truly yours,



                                       THOMPSON & KNIGHT LLP