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                                                                    EXHIBIT 99.4


                             JOINT FILING AGREEMENT

         In accordance with Rule 13d-1(f) under the Securities Exchange Act of
1934, as amended, the undersigned hereby agree to the joint filing with each
other of the attached statement on Schedule 13D and to all amendments to such
statement and that such statement and all amendments to such statement are made
on behalf of each of them.

         IN WITNESS WHEREOF, the undersigned hereby execute this agreement on
June 27, 2000.



                           Chesapeake Energy Corporation, an Oklahoma
                           corporation


                           By /s/ Aubrey K. McClendon
                              -------------------------------------------------
                              Aubrey K. McClendon, Chief Executive Officer


                           Chesapeake Exploration Limited Partnership, an
                           Oklahoma limited partnership

                           By       Chesapeake Operating, Inc., an Oklahoma
                                    corporation, as general partner

                                    By /s/ Aubrey K. McClendon
                                       ----------------------------------------
                                       Aubrey K.McClendon
                                       Chief Executive Officer


                           Chesapeake Energy Marketing, Inc., an Oklahoma
                           corporation


                           By /s/ Aubrey K. McClendon
                              -------------------------------------------------
                              Aubrey K. McClendon, Chief Executive Officer