<PAGE>

                                                                     Exhibit 2.2


                      AMENDMENT NO. 1 TO MERGER AGREEMENT

     This Amendment No. 1 to Merger Agreement (the "Amendment") is made and
effective as of this 14th day of April 2000.

     WHEREAS, the parties hereto are all of the parties to the Agreement and
Plan of Merger, dated as of December 13, 1999, among Union Oil Company of
California, Titan Resources Holdings, Inc. (now named Pure Resources, Inc.),
TRH, Inc. and Titan Exploration, Inc. (the "Merger Agreement"); and

     WHEREAS, in accordance with Section 10.3 of the Merger Agreement,  the
parties hereto wish to amend the Merger Agreement on the terms set forth herein;

     NOW THEREFORE, in consideration of the respective agreements contained
herein, the parties hereto agree as follows (capitalized terms used but not
defined herein have the meaning set forth in the Merger Agreement):

     Section 1.  Amendments.

          (a) For purposes of clarifying Sections 2.2(f) and 2.3(d) of the
     Merger Agreement, Exhibit A to the Merger Agreement shall be deemed to
     include the gas plants in the Dollarhide, Massey, Sacrock and Coyonosa
     Fields, so that such plants shall be considered Assets and not Excluded
     Assets.

          (b) The reference to "the Confidentiality Agreement dated October 4,
     1999 between Union Oil and Titan" at the end of Section 6.1(a) of the
     Merger Agreement is amended to refer to "the Confidentiality Agreement
     dated October 11, 1999 between Union Oil and Titan".

          (c) The reference to "the Confidentiality Agreement dated October 4,
     1999 between Union Oil and Titan" at the end of Section 6.1(b) of the
     Merger Agreement is amended to refer to "the Confidentiality Agreement
     dated August 23, 1999 between Union Oil and Titan".

          (d) The second sentence of Section 6.4(a) of the Merger Agreement is
     amended to read in its entirety as follows:

          The stockholder vote required for the adoption and approval of this
          Agreement and the Transactions contemplated hereby shall be (i) the
          affirmative vote of the holders of a majority of the outstanding
          shares of Titan Common Stock, and (ii) the affirmative vote of a
          majority of the shares of Titan Common Stock, other than shares held
          by officers of Titan, who are present at the Special Meeting in person
          or by proxy and vote for or against adoption and approval of this
          Agreement and the transactions contemplated hereby.
<PAGE>

          (e) Exhibit A of Exhibit 6.17(a) to the Merger Agreement is amended to
     read in its entirety as set forth on Annex A hereto.

     Section 2.  Remainder of Agreement Not Affected.  Except set forth in
Section 1 hereof, the terms and provisions of the Merger Agreement remain in
full force and effect.

     Section 3.  Authority.   Each party represents that such party has full
corporate power and authority to enter into this Amendment, and that this
Amendment constitutes a legal, valid and binding obligation of such party,
enforceable against such party in accordance with its terms.

     Section 4.  Counterparts. This Amendment may be executed in one or more
counterparts, each of which shall be deemed to be an original, but all of which
shall constitute one and the same agreement.

     Section 5.  Governing Law.   This Amendment shall be governed by Delaware
law, without regard to the conflicts of laws principles thereof.
<PAGE>

     The parties have caused this Amendment to be duly executed individually or
by their authorized representatives on the day and year first above written.


     UNION OIL COMPANY OF CALIFORNIA



     By: /s/ Timothy H. Ling
        ----------------------------



     PURE RESOURCES, INC.



     By: /s/ Phillip Ballard
        ----------------------------

     TRH, INC.



     By: /s/ Phillip Ballard
        ----------------------------

     TITAN EXPLORATION, INC.



     By: /s/ Jack D. Hightower
        ----------------------------

<PAGE>

                                    Annex A



                      See attached replacement Exhibit A.
<PAGE>

                                                                       EXHIBIT A
                                                                       ---------

                           GENERAL RELEASE AGREEMENT
                           -------------------------

1.   GENERAL RELEASE OF CLAIMS UNDER AGE DISCRIMINATION IN EMPLOYMENT ACT.  In
consideration of the Severance or Retention Bonus Payment ("Payment") to be made
to me under the Officer Severance and Retention Bonus Agreement (the
"Agreement"), I hereby release, acquit, and forever discharge (i) Titan
Resources I, Inc. and any parent, subsidiary, affiliated entity, successors or
assigns (the "Company"), and (ii) the stockholders, officers, directors,
employees, agents, representatives, and fiduciaries of the Company (collectively
the "Released Parties"), from any and all claims, liabilities, demands, and
causes of action of whatever kind or character, whether vicarious, derivative,
or direct, that I now have or claim against them arising under the Age
Discrimination in Employment Act.  This General Release does not waive rights or
claims that may arise after the date this General Release is executed. Further
in consideration of the Payment to be made to me under the Agreement, I
acknowledge and agree that the Released Parties may recover from me any loss,
including attorney's fees and costs of defending against any claim brought by
me, they may incur arising out of my breach of this General Release.

     I understand that I may revoke my acceptance of paragraph 1 of this General
Release by so notifying the Company within seven days of the date I execute this
General Release. I further understand that if I do not timely revoke my
acceptance of paragraph 1, paragraph 1 of this General Release is final and
binding, and I agree not to challenge its enforceability. If I do challenge its
enforceability, I agree initially to tender to the Company the Payment made
under the Agreement, and invite the Company to retain such money and agree with
me to cancel this General Release. In the event the Company accepts my offer,
the Company shall retain such money and paragraph 1 of this General Release will
be void. In the event the Company does not accept my offer, the Company shall
place such money in an escrow account pending the resolution of any dispute as
to whether paragraph 1 of this General Release shall be set aside and/or
otherwise rendered unenforceable.

2.   GENERAL RELEASE OF CLAIMS OTHER THAN UNDER AGE DISCRIMINATION IN EMPLOYMENT
ACT.  In consideration of the Payment to be made to me under the Agreement, I
hereby release, acquit, and forever discharge (i) the Company, and (ii) the
Released Parties, from any and all claims, liabilities, demands, and causes of
action of whatever kind or character, whether vicarious, derivative, or direct,
that I now have or claim against them connected in any way to the Agreement or
any claim for benefits under the Agreement (other than as described in Paragraph
3 below), or my employment, continuation of employment, or, if applicable,
termination of employment with any of the Released Parties, or with any other
act, conduct, or omission of any of the Released Parties, including but not
limited to claims arising under any federal, state, or local laws relating to
the employment relationship, other than claims, liabilities, demands, and causes
of action under the Age Discrimination in Employment Act.  This General Release
does not waive rights or claims that may arise after the date this General
Release is executed. Further in consideration of the Payment to be made to me
under the Agreement, I acknowledge and agree that the Released Parties may
recover from me any loss, including attorney's fees and costs of defending
against any claim brought by me, they may incur arising out of my breach of this
General Release.

3.   NO RELEASE OF COMPANY'S OBLIGATION TO MAKE TAX GROSS-UP PAYMENT UNDER THE
AGREEMENT.  Any provision of this General Release or the Letter Agreement
between the Company and myself dated of even date herewith to the contrary
notwithstanding, my execution of this General Release does not constitute a
release of the
<PAGE>

Company's obligation under the Agreement to make additional payments to me if it
shall be determined that I am subject to additional excise tax under Section
4999 of the Internal Revenue Code of 1986, as amended, as a result of any
payments made or benefits provided to me pursuant to the Agreement or otherwise
in connection with a Change of Control (as defined in the Agreement).

     I understand that I may not revoke my acceptance of paragraph 2 of this
General Release and that it is binding upon me whether or not I revoke my
acceptance of paragraph 1 of this General Release.

     I have read and fully understand all of the provisions of this General
Release.  I acknowledge that none of the Released Parties has made any promise
or representation to me in consideration for my agreement to execute this
General Release that is not set out in this General Release, and that in
executing this General Release I am not relying on any such promise or
representation but instead am relying solely on my own judgment.  I further
acknowledge that my execution of this General Release is knowing and voluntary,
that I have had a reasonable time to consider its terms, and that I have been
advised to consult with an attorney about this General Release.


Date signed:________________________     _______________________________________
                                         Signature of Employee

Date signed:________________________     _______________________________________
                                         Witness