EXHIBIT 3.97

                            CERTIFICATE OF FORMATION
                                       OF
                          PG INVESTMENTS SEVEN, L.L.C.

         The undersigned, an authorized natural person, for the purpose of
forming a limited liability company, under the provisions and subject to the
requirements of the State of Delaware (particularly Chapter 18, Title 6 of the
Delaware Code and the acts amendatory thereof and supplemental thereto, and
known, identified and referred to as the "Delaware Limited Liability Company
Act"), hereby certifies that:

         FIRST: The name of the limited liability company (hereinafter the
"limited liability company") is PG Investments Seven, L.L.C.

         SECOND: The address of its registered office in the State of Delaware
is Corporation Trust Center, 1209 Orange Street, in the City of Wilmington,
County of New Castle. The name of its registered agent at such address is The
Corporation Trust Company.

         IN WITNESS WHEREOF, the undersigned has executed this Certificate of
Formation of PG Investments Seven, L.L.C. this 25th day of August, 2000.

                                                           /s/ David F. Hannan
                                                           ---------------------
                                                               David F. Hannan
                                                               Authorized Person

                                                            STATE OF DELAWARE
                                                           SECRETARY OF STATE
                                                        DIVISION OF CORPORATIONS
                                                       FILED 03:00 PM 08/15/2000
                                                           001433086 - 3280065



                                                           STATE OF DELAWARE
                                                           SECRETARY OF STATE
                                                        DIVISION OF CORPORATIONS
                                                       FILED 01:30 PM 07/18/2001
                                                           010346576 - 3280065

                            CERTIFICATE OF AMENDMENT

                                       OF

                          PG INVESTMENTS SEVEN, L.L.C.

1.       The name of the limited liability company is PG INVESTMENTS SEVEN,
         L.L.C.

2.       The Certificate of Formation of the limited liability company is hereby
         amended as follows:

         Please amend Article 1 to read as follows:

         "1.      Name of LLC

         The name of the LLC as of the date of this Agreement is and shall
         continue to be MUSTANG ENERGY COMPANY, L.L.C."

         IN WITNESS WHEREOF, the undersigned has executed this Certificate of
         Amendment on this 19th day of July, 2001.

                                                        /s/  James C. Sevem
                                                        -------------------
                                                             James C. Sevem
                                                             Assistant Secretary