1
                                                                     EXHIBIT 3.6

                                                                  PAGE 1

                                STATE OF DELAWARE
                        OFFICE OF THE SECRETARY OF STATE
                      ------------------------------------


         I, EDWARD J. FREEL, SECRETARY OF STATE OF THE STATE OF DELAWARE, DO
HEREBY CERTIFY THE ATTACHED IS A TRUE AND CORRECT COPY OF THE CERTIFICATE OF
AGREEMENT OF MERGER, WHICH MERGES:

         "MW PETROLEUM CORPORATION", A COLORADO CORPORATION,

         WITH AND INTO "APACHE CORPORATION" UNDER THE NAME OF" APACHE
CORPORATION", A CORPORATION ORGANIZED AND EXISTING UNDER THE LAWS OF THE STATE
OF DELAWARE, AS RECEIVED AND FILED IN THIS OFFICE THE NINETEENTH DAY OF AUGUST,
A.D. 1998, AT 8:31 O'CLOCK A.M.

         AND I DO HEREBY FURTHER CERTIFY THAT THE EFFECTIVE DATE OF THE
AFORESAID CERTIFICATE OF AGREEMENT OF MERGER IS THE FIRST DAY OF SEPTEMBER, A.D.
1998.

         A FILED COPY OF THIS CERTIFICATE HAS BEEN FORWARDED TO THE NEW CASTLE
COUNTY RECORDER OF DEEDS.


                                       /s/ Edward J. Freel                  
                                       -----------------------------------------
                                       Edward J. Freel, Secretary of State

                                     [SEAL]


     0482215  8100M                            AUTHENTICATION:    9261680
     981324886                                           DATE:    8-19-98



   2


                       CERTIFICATE OF OWNERSHIP AND MERGER


                                     MERGING
                            MW PETROLEUM CORPORATION
                                      INTO
                               APACHE CORPORATION


         Apache Corporation, a corporation organized and existing under the laws
of the State of Delaware, DOES HEREBY CERTIFY:

         FIRST: That Apache Corporation was incorporated on the 6th day of
December, 1954, pursuant to the General Corporation Law of the State of
Delaware.

         SECOND: That Apache Corporation owns all of the issued and outstanding
shares of the capital stock of MW Petroleum Corporation, a corporation formerly
known as MW Acquisition Corporation and which was incorporated on the 20th day
of December, 1991, pursuant to the Revised Statutes of the State of Colorado.

         THIRD: That Apache Corporation, by the following resolutions of its
Board of Directors, duly adopted at a meeting held on the 30th day of April,
1998, determined to and did merge into itself said MW Petroleum Corporation:

         RESOLVED: That MW Petroleum Corporation ("MW"), a wholly owned
         subsidiary of Apache Corporation, be merged with and into Apache
         Corporation ("Apache"), with Apache being the surviving corporation.

         FURTHER RESOLVED: That the merger shall be effective on September 1,
         1998.

         FURTHER RESOLVED: That the Restated Certificate of Incorporation of
         Apache, as heretofore amended and as in effect on the date of the
         merger, shall continue in full force and effect as the Restated
         Certificate of Incorporation of the surviving corporation.

         FURTHER RESOLVED: That each share of the capital stock of Apache, the
         surviving corporation, issued and outstanding on the effective date of
         the merger shall remain issued and outstanding.

         FURTHER RESOLVED: That each share of the capital stock of MW issued and
         outstanding on the effective date of the merger shall be cancelled
         without consideration, and no shares of the capital stock of Apache,
         the surviving corporation, shall be issued in exchange therefore.

                                       1

   3


         FURTHER RESOLVED: That the bylaws of Apache, as they exist on the
         effective date of the merger, shall be and remain the bylaws of Apache,
         the surviving corporation, until the same shall be altered, amended or
         repealed as therein provided.

         FURTHER RESOLVED: That the directors and officers of Apache on the
         effective date of the merger shall be the directors and officers of the
         surviving corporation and shall continue in office until the next
         annual meeting of stockholders and until their successors have been
         elected and qualified.

         FURTHER RESOLVED: That, upon the merger taking effect, Apache shall
         thereupon and thereafter possess all the rights, privileges,
         immunities, and franchises, of a public as well as a private nature, of
         each of Apache and MW; that all property, real, personal, and mixed,
         and all debts due on whatever account, including subscriptions to
         shares, and all other chooses in action, and every other interest of or
         belonging to or due to MW shall be deemed to be transferred to and
         vested in Apache without further act or deed; that the title to any
         real estate, or any interest therein vested in either of Apache or MW
         shall not revert or be in any way impaired by reason of the merger; and
         that such transfer to and vesting in Apache shall be deemed to occur by
         operation of law, and no consent or approval of any other person shall
         be required in connection with any such transfer or vesting unless such
         consent or approval is specifically required in the event of merger by
         law or by express provision in any contract, agreement, decree, order,
         or other instrument to which either of Apache or MW is a party or by
         which either is bound.

         FURTHER RESOLVED: That, upon the merger taking effect, Apache shall be
         responsible and liable for all the liabilities and obligations of MW;
         that any claim existing or action or proceeding, whether civil or
         criminal, pending by or against MW may be prosecuted as if the merger
         had not taken place; and that neither the rights of creditors nor any
         liens upon the property of either of Apache or MW shall be impaired by
         such merger.

         FURTHER RESOLVED: That the proper officers of Apache are hereby
         authorized and directed, in the name and on behalf of Apache, to
         prepare and execute a Certificate of Ownership and Merger setting forth
         a copy of the resolutions authorizing the merger of MW with and into
         Apache and the assumption by Apache of the liabilities and obligations
         of MW, and the date of adoption thereof, and to cause such Certificate
         of Ownership and Merger to be filed with the Delaware Secretary of
         State and a certified copy of same recorded in the office of the
         Recorder of Deeds of New Castle County.

                                       2

   4


         FURTHER RESOLVED: That the proper officers of Apache be, and they
         hereby are, authorized and directed to take such further action and to
         execute and file such certificates and other documents as they, in
         their discretion, shall deem necessary or advisable to consummate the
         merger and effect the foregoing resolutions including, without
         limitation, articles of merger or share exchange for filing with the
         Colorado Secretary of State.

         FURTHER RESOLVED: This merger may be amended or terminated and
         abandoned by the Board of Directors of Apache at any time prior to the
         date of filing the merger with the Secretary of State of Delaware and
         of Colorado.

         IN WITNESS WHEREOF, Apache Corporation has caused this Certificate of
Ownership and Merger to be executed by its duly authorized officers as of this
7th day of August, 1998.

                                       APACHE CORPORATION


                                       By: /s/ G. Steven Farris              
                                           -------------------------------------
                                           G. Steven Farris
                                           President and Chief Operating Officer

ATTEST:


/s/ Cheri L. Peper                          
- -----------------------------------
Cheri L. Peper
Corporate Secretary

                                       3