Exhibit 5.1
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                     [Opinion of Thompson & Knight L.L.P.]

                              September 19, 2001

Pure Resources, Inc.
500 West Illinois Avenue
Midland, Texas 79701

          Re:  $350,000,000 Aggregate Principal Amount of
            7 1/8% Senior Notes Due 2011 of Pure Resources, Inc.

Ladies and Gentlemen:

     We have acted as counsel to Pure Resources, Inc., a Delaware corporation
(the "Company") and the Guarantors, as defined below, in connection with the
registration of $350,000,000 aggregate principal amount of Series B 7 1/8%
Senior Notes due 2011 (the "New Notes") by the Company on Form S-4 filed with
the Securities and Exchange Commission (the "Commission") on September 14, 2001
(the "Registration Statement").  The New Notes will be issued pursuant to an
indenture (the "Indenture"), dated June 8, 2001, among the Company, First Union
National Bank, as trustee (the "Trustee") and Titan Exploration, Inc., a
Delaware corporation, Pure Resources Holdings, Inc., a Nevada corporation, Pure
Resources, L.P., a Texas limited partnership, Pure Resources I, Inc., a Delaware
corporation, HEC Petroleum, Inc., a Delaware corporation, PK I, L.P., PK II,
L.P., PK III, L.P., PK IV, L.P., HEP Pure, LP, and HEP Partners, L.P., each of
which is a Delaware limited partnership, Pure GP, LLC and HEP Pure Acquisition,
LLC, each of which is a Delaware limited liability company, Pure Resources
Holdings, LLC, a Nevada limited liability company, and La Plata Associates, LLC
and HEC La Plata, LLC, each of which is a Colorado limited liability company
(collectively, the "Guarantors"). The New Notes will be issued in exchange for
the Company's outstanding Series A 7 1/8% Senior Notes due 2011 (the "Old
Notes") on the terms set forth in the prospectus contained in the Registration
Statement and the Letter of Transmittal filed as an exhibit thereto (the
"Exchange Offer"). References herein to the "Guarantees" refer to (a) the
provisions of Article 9 of the Indenture and (b) the Notation attached to the
New Notes, under which the Guarantors have guaranteed the payment of the New
Notes.

     For purposes of this opinion, we have made such legal and factual
examinations and inquiries, including an examination of originals or copies
certified or otherwise identified to our satisfaction, of such documents,
corporate records and other instruments as we have deemed necessary or
appropriate for purposes of this opinion.  In our examination, we have assumed
the genuineness of all signatures, the authenticity of all documents submitted
to us as originals, and the conformity to authentic original documents of all
documents submitted to us as copies.  We have obtained and relied upon such
certificates and assurances from public officials and we have deemed necessary.

     Subject to the foregoing and the other matters set forth herein, it is our
opinion that, as of the date hereof:

     1.   The New Notes have been duly authorized by all necessary action of the
Company, and when duly executed, issued, authenticated and delivered in
accordance with the terms of the Exchange Offer and the Indenture, will be
legally valid and binding obligations of the Company, enforceable against the
Company in accordance with their terms.

     2.   The Guarantees have been duly authorized by all necessary action of
the applicable Guarantors, and when duly executed and delivered and when the New
Notes are duly executed, issued, authenticated and delivered in accordance with
the terms of the Exchange Offer and the Indenture, will be legally valid and
binding obligations of the applicable Guarantors, enforceable against the
applicable Guarantors in accordance with their terms.

     This opinion is limited by, subject to and based on the following:

     (a)  The opinion relating to the enforceability of the New Notes and the
          Guarantees is subject to bankruptcy, insolvency, fraudulent transfer,
          reorganization, moratorium and similar laws of general applicability
          relating to or affecting creditors' rights and to general equity
          principles.

     (b)  We express no opinion as to the enforceability of any provisions of
          the New Notes or the Indenture


          providing for (i) the waiver of a right of immunity, stay, extension,
          or usury laws, or (ii) any party's consent to jurisdiction or venue.

     (c)  This opinion is limited in all respects to the laws of the State of
          Texas, the General Corporation Law of the State of Delaware, the
          General Corporation Law of the State of Nevada, Chapter 86 of the
          Nevada Revised Statutes (which relates to limited liability
          companies), the Colorado Limited Liability Company Act, and the
          federal laws of the United States, and, only as to the enforceability
          of the New Notes and the Guarantees, the laws of the State of New
          York.

     To the extent that the obligations of the Company and the Guarantors under
the New Notes and the Guarantees may be dependent upon such matters, we have
assumed for purposes of this opinion that (i) the Trustee is validly existing
and in good standing under the laws of its jurisdiction of organization; (ii)
the Trustee has been duly qualified to engage in the activities contemplated by
the Indenture; (iii) the Trustee is in compliance generally, and with respect to
acting as Trustee under the Indenture, with all applicable laws and regulations;
(iv) the Indenture has been duly authorized, executed and delivered by the
Trustee and constitutes the legally valid and binding obligation of the Trustee,
enforceable against the Trustee in accordance with its terms; and (v) the
Trustee has the requisite organizational and other power and authority to
perform its obligations under the Indenture.

     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to our firm's name under the heading
"Legal Matters" in the prospectus contained therein.

                                   Very truly yours,

                                   /s/ Thompson & Knight L.L.P.
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