EXHIBIT 10.4

                            VASTAR RESOURCES, INC.
                              FIRST AMENDMENT TO
                     COMPREHENSIVE MANAGEMENT MEDICAL PLAN

     Pursuant to the power of amendment reserved therein, Vastar Resources, Inc.
Comprehensive Management Medical Plan (the "Plan") is hereby amended as follows:

     1.   Paragraph 1(c) is amended in its entirety, effective as of the
Effective Time (as defined in Annex A to the Plan as amended hereby), to read as
follows:

          "(c)  "Benefit Trigger Window" means the 24-month period commencing on
the date immediately following the Effective Time and means the 24-month period
commencing on the date that a Subsequent Change of Control occurs.

     2.   Paragraph 1 is amended to delete the last sentence of subparagraph
(o).

     3.   Paragraph 1 is amended to renumber subparagraph (g) as (i), to
renumber subparagraph (h) as (k), to renumber subparagraphs (i) through (l) as
(m) through (p), and to renumber subparagraphs (m) through (o) as (r) through
(t), and to add new subparagraphs (g), (h),(j), (l), and (q) to read as follows:

          "(g)  "Company" means Vastar Resources, Inc.

          (h)   "Effective Time" shall be ascribed the meaning set forth for
such term on Annex A attached hereto.

          (j)   "Merger" shall be ascribed the meaning set forth for such term
on Annex A attached hereto.

          (l)   "Parent Company" shall be ascribed the meaning set forth for
such term on Annex A attached hereto.

          (q)   "Subsequent Change of Control" shall be ascribed the meaning set
forth for such term on Annex A attached hereto."

     When amending sections or subsections currently set forth in the Plan, the
section or subsection references below refer to the sections and subsections as
they existed prior to the renumbering as provided for above. Moreover, all
current references in the Plan to other sections and subsections of the Plan
shall be deemed to refer to the applicable section or subsection as renumbered
herein.

     4.   Paragraph 2 is amended to add "; or" at the end of subparagraph
(b)(i)(B)(II) and to add a new subparagraph (b)(i)(B)(III) which shall read as
follows:

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          "(III) with respect to Tier 1 and 2 Employees of the Company, a
demotion to a lesser job."

     5.   Paragraph 2 is amended to eliminate subparagraph (b)(ii).

     6.   Paragraph 2 is amended to eliminate the parenthetical "(excluding an
ARCO Acquisition)" from subparagraph (d).

     7.   Paragraph 3 is amended to delete the second sentence of subparagraph
(c).

     8.   Paragraph 3 is amended to eliminate subparagraph (d).

     9.   Paragraph 4 is amended in its entirety and shall now read as follows:


          "4.  AUTHORIZED OFFICER

               Both the Chief Financial Officer and the Vice President, Human
Resources of the Company whose duties with respect to this Plan are provided
herein, will be those persons who have been employed in such positions by the
Company since April 1, 1999. Any action required to be taken under this Plan by
the Chief Financial Officer and the Vice President, Human Resources of the
Company may be taken by either individual if the other position is vacant. If
the positions of Chief Financial Officer and Vice President, Human Resources of
the Company are both vacant, actions required to be taken under the Plan by such
officers may only be taken by one of the officers of the Company set forth below
(in the order stated) who was in such position on April 1, 1999:

                    (a)  Controller
                    (b)  General Counsel
                    (c)  Senior Vice President of Production
                    (d)  Senior Vice President of Exploration
                    (e)  Vice President of Business Development.

If none of such persons is in such position, the Special Plan Administrator
shall take actions required to be taken under the Plan."

     10.  Annex A is amended, with respect to the definition of "Change of
Control" and the definition of "Outside Director", to replace the terms
"Atlantic Richfield Company" and "ARCO" with the term "Parent Company". This
amendment shall be construed consistently with the intent that the Merger, if it
occurs, is a Change of Control.

     11.  Annex A, with respect to the definition of "Change of Control," is
amended, effective as of the Effective Time, to add "; or" at the end of
Paragraph (6) and to add new Paragraph (7) which shall read as follows:

          "(7) A Subsequent Change of Control."

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     12.  Annex A is amended to add the following definitions:

          ""Effective Time" means, with respect to the Merger, such time as the
Certificate of Merger is duly filed with the Secretary of State of the State of
Delaware or at such later time as is specified in the Certificate of Merger in
accordance with the Delaware General Corporation Law.

          "Merger" shall mean the merger between Atlantic Richfield Company and
Prairie Holdings, Inc., a subsidiary of BP Amoco p.l.c.

          "Parent Company" shall mean, until the Effective Time of the Merger,
Atlantic Richfield Company and, after the Effective Time of the Merger, BP Amoco
p.l.c. or any of its subsidiaries or affiliates and any of their respective
successors.

          "Subsequent Change of Control" shall mean each Change of Control which
occurs after the consummation of a transaction constituting a Change of
Control."

     13.  The Plan is amended to replace (except in Paragraph 4(b) of the Plan
as amended above) the term "General Counsel" with the term "Vice President,
Human Resources."

     14.  All amendments shall be effective as of July 21, 1999 unless otherwise
noted.

     Executed as of the 12th day of August, 1999.



     ATTEST:                              VASTAR RESOURCES, INC.


     By: /s/ Johnathan D. Edelfelt        By:  /s/ Jeffrey M. Bender
         ---------------------------          ----------------------------------
          Jonathan D. Edelfelt                 Jeffrey M. Bender
          Associate Secretary                  Vice President, Human Resources


APPROVED as to form this 12th day
of August, 1999

WACHOVIA BANK, N.A.,


By:  /s/ Peter D. Quinn
    -------------------------------

Name:  Peter D. Quinn
      -----------------------------

Title:  Senior Vice President
       ----------------------------

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