<PAGE>   1
                                                                   EXHIBIT 10.20


                       AMENDMENT TO EMPLOYMENT AGREEMENT


         THIS AMENDMENT, effective as of the 26th day of June, 1997, by and
between HUGOTON ENERGY CORPORATION, a Kansas corporation (hereinafter referred
to as the "Company") and JAY W. DECKER (hereinafter referred to as "Employee").

         WITNESSETH:

         WHEREAS, the Company and Employee have entered into and executed an
Employment Agreement, dated September 7, 1995 (the "Agreement").

         WHEREAS, the Board of Directors of the Company has unanimously voted
to amend the Agreement and Employee has agreed to the amendments proposed by
the Board of Directors.

         WHEREAS, the parties desire, at this time, to amend and modify said
Agreement in the manner herein specified.

         NOW THEREFORE, in consideration of the premises herein contained, the
parties do hereby agree that said Agreement shall be modified and amended as
follows:

         1.      The Employment Period referred to in paragraph 1 shall be
extended an additional year to September 7, 1999.

         2.      Paragraph 8 shall be deleted in its entirety and the following
shall be substituted in lieu thereof:

                 8.       Termination By the Company Without Cause or
         Termination By Employee for Good Reason.

                 (a)  The Company may terminate Employee's employment under
         this Agreement without Cause.  The termination shall be evidenced by
         written notice thereof to the Employee and shall specify that the
         termination was without Cause.  An involuntary transfer of Employee
         shall be considered termination without Cause.

                 (b)  If Employee's employment with the Company is terminated
         without Cause or if Employee terminates his employment for Good
         Reason, Employee shall be entitled to receive, within ten (10) days of
         such termination, the amount of his entire salary for the remaining
         term of this Agreement.  Notwithstanding the foregoing, if the payment
         referred to above is not made within ten (10) days of Employee's
         termination, all unpaid amounts shall bear interest at a rate equal to
         the


                                      1
<PAGE>   2
         New York Prime (as published in the Wall Street Journal) on the date
         of such termination.

         3.      The following additional Paragraph 17 shall be added:

                 17.      Termination By Employee for Good Reason.

                 (a)  If a Change of Control (as defined hereafter) in the
         Company has occurred, Employee may terminate his employment during the
         Employment Period for Good Reason (defined hereafter) upon thirty (30)
         days' notice to the Company.  Such notice period may begin prior to
         the date of the Change of Control if an agreement is in place that
         would have the effect of causing a Change of Control.  For purposes of
         this Agreement, the term "Good Reason" shall mean the occurrence,
         without Employee's express written consent, of any one or more of the
         following events:

                 (i)      A change in Employee's duties or a change in the
                 title or offices held by Employee.

                 (ii)     A reduction in Employee's compensation or the failure
                 by the Company to continue to provide prompt payment (or
                 reimbursement to Employee) of all reasonable expenses incurred
                 by Employee in connection with Employee's professional and
                 business activities.

                 (iii)    A failure by the Company to waive any and all
                 restrictions that might exist on the exercise of any stock
                 options held by Employee under the Company's stock option
                 plans as of the date of a Change of Control.

                 (iv)     The failure of the Company to obtain the assumption
                 of this Agreement, without limitation or reduction, by any
                 successor to the Company.

         In addition to the above, Good Reason shall be deemed to exist if a
         Change of Control in the Company has occurred and Employee agrees to
         stay employed by the Company for a reasonable transition period after
         the Change of Control, not to exceed three (3) months.

                 (b)      Change of Control.  A "Change of Control" shall have
         occurred if:

                 (i)      fifty percent (50%) or more of the outstanding common
                 stock of the Company has been acquired by any person or
                 persons (as defined in Section 3(a)(9) of the Securities
                 Exchange Act of 1934 (the





                                       2
<PAGE>   3
                 "Act")), provided such person(s) is not a current
                 stockholder(s) of the Company currently holding ten percent
                 (10%) or more of the outstanding common stock of the Company.
                 For purposes of this paragraph 7, such person shall include
                 affiliated persons (as defined in the Act);

                 (ii)     there has been a merger or equivalent combination
                 involving the Company after which fifty percent (50%) or more
                 of the voting stock of the surviving corporation is held by
                 persons other than those persons who were stockholders holding
                 ten percent (10%) or more of the outstanding stock of the
                 Company immediately prior to the date of such merger or
                 equivalent combination; or

                 (iii)    there has been a merger or equivalent combination or
                 stock sale involving the Company and after such transaction
                 fifty percent (50%) or more of the members of the Board
                 elected by stockholders are persons who were not directors
                 immediately prior to such transaction.

         4.      The parties hereto ratify and confirm all other provisions of
the Employment Agreement.

         IN WITNESS WHEREOF, each of the parties has executed this Agreement as
of the day and year first above written.

"COMPANY"                               "EMPLOYEE"

HUGOTON ENERGY CORPORATION              /s/ JAY W. DECKER
                                        ---------------------------------
                                        Jay W. Decker
By: /s/ FLOYD C. WILSON
    -----------------------------
Name: Floyd C. Wilson
      ---------------------------
Title: President
       --------------------------


ATTEST:

/s/ ILLEGIBLE
---------------------------------



                                       3