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                                                                  EXHIBIT 10.18



                                                                 EXECUTION COPY


FOURTH AMENDMENT, dated as of December 21, 1998 (this "Fourth Amendment"), to
the CREDIT AGREEMENT, dated as of September 23, 1997, as amended by the First
Amendment and Waiver thereto, dated as of November 25, 1997, the Second
Amendment and Consent thereto, dated as of February 25, 1998, and the Third
Amendment, dated as of May 29, 1998 (as the same may be further amended,
modified or supplemented from time to time, the "Credit Agreement"), among
BELCO OIL & GAS CORP., a Nevada corporation (the "Borrower"), the several
banks, financial institutions and other entities from time to time parties to
the Credit Agreement (the "Lenders") and THE CHASE MANHATTAN BANK, as
administrative agent (in such capacity, the "Administrative Agent").

                             W I T N E S S E T H :

                  WHEREAS, pursuant to the Credit Agreement, the Lenders have
agreed to make, and have made, certain loans and extensions of credit to the
Borrower; and

                  WHEREAS, the Borrower has requested that the Administrative
Agent and the Lenders enter into this Fourth Amendment.

                  NOW, THEREFORE, in consideration of the premises and of the
mutual agreements herein contained, the parties hereto hereby agree as follows:

                  1. Definitions. Unless otherwise defined herein, terms
defined in the Credit Agreement shall be used as so defined.

                  2. Amendments to the Credit Agreement.

                  (a) Subsection 1.1. (i) Subsection 1.1 of the Credit
Agreement is hereby amended to add the following definition in appropriate
alphabetical order:

                  'Fourth Amendment': the Fourth Amendment, dated as of
December 17, 1998, to this Agreement.

                  (ii) Subsection 1.1 of the Credit Agreement is hereby amended
by deleting the definitions of "Big Bear Acquisition" and "Consolidated Net
Income" in their entireties and substituting, in lieu thereof, the following:

                  "'Big Bear Acquisition': the acquisition by Belco Canada of
up to C $15,000,000 of 5% Convertible Preferred Stock of Big Bear in accordance
with the terms set forth in Schedule 8.8(i) to the Third Amendment.

                  'Consolidated Net Income': for any period, net income of the
Borrower and its Restricted Subsidiaries determined on a consolidated basis in
accordance with GAAP, minus (i) non-cash gains resulting from the net change in
value of the Borrower's mark-to-market portfolio of price risk management
activities for that period to the extent such gains are included in the net
income of the

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Borrower for such period and (ii) any extraordinary or nonrecurring gain or
loss, together with any related provision for taxes on such extraordinary or
nonrecurring gain or loss."

                  (b) Subsection 3.1. Subsection 3.1(a) of the Credit Agreement
is hereby amended by deleting the amount "$65,000,000" appearing in clause (i)
thereof and inserting, in lieu thereof, "$25,000,000."

                  (c) Subsection 8.7. Subsection 8.7(b) of the Credit Agreement
is hereby amended by deleting the amount "$25,000,000" and inserting, in lieu
thereof, "$50,000,000."

                  (d) Subsection 8.8. Subsection 8.8(i) of the Credit Agreement
is hereby amended by deleting such subsection in its entirety and substituting,
in lieu thereof, the following:

                  "(i) in addition to Investments permitted by subsection
8.8(g) above (and without Investments under this subsection 8.8(i) being
included in any calculations under subsection 8.8(g) above), Investments by the
Borrower in Belco Canada, to permit Belco Canada to complete the Big Bear
Acquisition; provided that (i) the aggregate amount of such Investments (in any
form other than shares of common stock of the Borrower or the proceeds thereof)
shall not exceed C $15,000,000 for the purchase of Big Bear's 5% Convertible
Preferred Stock, (ii) Belco Canada and Big Bear and its Subsidiaries are
Unrestricted Subsidiaries, and (iii) Belco Canada shall acquire such
Convertible Preferred Stock substantially in accordance with the terms attached
as Schedule 8.8(i) to the Third Amendment without material amendment, waiver or
modification without the prior written consent of the Administrative Agent."

                  3. Effective Date. This Fourth Amendment shall become
effective upon the Administrative Agent receiving this Fourth Amendment,
executed and delivered by a duly authorized officer of each of the
Administrative Agent, the Borrower and the Required Lenders, and consented to
by each Guarantor, with a counterpart for each Lender. This Fourth Amendment
may be delivered by facsimile transmission of the relevant signature pages
hereof.

                  4. Representations and Warranties. The Borrower represents
and warrants to each Lender that: (a) this Fourth Amendment constitutes the
legal, valid and binding obligation of the Borrower, enforceable against the
Borrower in accordance with its terms, subject to the effects of bankruptcy,
insolvency, fraudulent transfer or conveyance, reorganization, moratorium and
other similar laws relating to or affecting creditors' rights generally,
general equitable principles (whether considered in a proceeding in equity or
at law) and an implied covenant of good faith and fair dealing, (b) the
representations and warranties made by the Borrower and the other Loan Parties
in or pursuant to the Loan Documents are true and correct on and as of the date
hereof (except to the extent that such representations and warranties are
expressly made only as of an earlier date, in which case such representations
and warranties shall have been true and correct on and as of such earlier
date), and (c) after giving effect to the amendments and consents contained
herein, no Default or Event of Default has occurred and is continuing.

                  5. Continuing Effect. Except as expressly amended hereby, the
Credit Agreement and the other Loan Documents shall continue to be and shall
remain in full force and effect in accordance



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with their terms. Except as expressly set forth herein, this Fourth Amendment
shall not by implication or otherwise limit, impair, constitute a waiver of, or
otherwise affect the rights and remedies of the Lenders under the Credit
Agreement or any Loan Document, and shall not alter, modify, amend or in any
way affect any of the terms, conditions, obligations, covenants or agreements
contained in the Credit Agreement or any other Loan Document.

                  6. GOVERNING LAW. THIS FOURTH AMENDMENT SHALL BE GOVERNED BY,
AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

                  7. Counterparts. This Fourth Amendment may be executed in any
number of counterparts and by different parties hereto in separate
counterparts, each of which when so executed shall be deemed to be an original
and all of which taken together shall constitute one and the same agreement.
Delivery of an executed counterpart of a signature page to this Fourth
Amendment by telecopier shall be effective as delivery of a manually executed
counterpart of this Fourth Amendment.

                  8. Costs and Expenses. The Borrower shall pay on demand all
reasonable out-of-pocket costs and expenses of the Administrative Agent in
connection with the preparation, execution and delivery of this Amendment,
including the reasonable fees and disbursements of counsel for the
Administrative Agent with respect hereto.

                  9. Miscellaneous. Each of the parties signatories hereto
acknowledges that, based on the Reserve Reports dated June 30, 1998 and
submitted to the Administrative Agent pursuant to subsection 4.9(b), the
Borrowing Base is $150,000,000.

                  [Remainder of Page Intentionally Left Blank]

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                  IN WITNESS WHEREOF, the parties hereto have caused this
Fourth Amendment to be executed by their respective officers thereunto duly
authorized, as of the date first above written.

                                              BELCO OIL & GAS CORP.


                                              By:
                                                 -------------------------------
                                                 Title:


Each Guarantor listed below hereby consents to the foregoing Fourth Amendment.

                                              BELCO ENERGY CORP. (formerly Coda
                                              Energy, Inc.)
                                              
                                              
                                              
                                              By:
                                                 -------------------------------
                                                 Name:
                                                 Title:
                                              
                                              ELECTRA RESOURCES, INC.
                                              
                                              
                                              By:
                                                 -------------------------------
                                                 Name:
                                                 Title:
                                              
                                              GIN LANE COMPANY &
                                              FORTUNE CORP. &
                                              BELCO FINANCE CO.
                                              
                                              
                                              By:
                                                 -------------------------------
                                                 Name:
                                                 Title:

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                                  BELCO ENERGY I, L.P.
                                  
                                  By:      Belco Energy Corp.,
                                           its sole general partner
                                  
                                  
                                  By:
                                     -------------------------------------------
                                     Name:  Dominick J. Golio
                                     Title: Senior Vice President - Finance
                                            and Chief Financial Officer
                                  
                                  BOG WYOMING L.L.C.
                                  
                                  By:      Belco Energy Corp.,
                                           its managing member
                                  
                                  
                                  By:
                                     -------------------------------------------
                                     Name:  Dominick J. Golio
                                     Title: Senior Vice President - Finance
                                            and Chief Financial Officer

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                                       THE CHASE MANHATTAN BANK, as            
                                       Administrative Agent and as a Lender
                                       
                                       
                                       By:
                                          ----------------------------------
                                          Name:
                                          Title:
                                       
                                       
                                       
                                       CHRISTIANIA BANK OG
                                       KREDITKASSE ASA
                                       
                                       
                                       
                                       By:
                                          ----------------------------------
                                          Name:
                                          Title:
                                       
                                       
                                       By:
                                          ----------------------------------
                                          Name:
                                          Title:
                                       
                                       
                                       CREDIT LYONNAIS
                                       NEW YORK BRANCH
                                       
                                       
                                       By:
                                          ----------------------------------
                                          Name:
                                          Title:
                                       
                                       
                                       DEN NORSKE BANK ASA
                                       
                                       
                                       
                                       By:
                                          ----------------------------------
                                          Name:
                                          Title:
                                       
                                       
                                       
                                       By:
                                          ----------------------------------
                                          Name:
                                          Title:




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                                               THE FUJI BANK, LIMITED
                                               
                                               
                                               
                                               By:
                                                  ---------------------------
                                                  Name:
                                                  Title:
                                               
                                               THE SANWA BANK LIMITED
                                               
                                               
                                               
                                               By:
                                                  ---------------------------
                                                  Name:
                                                  Title:
                                               
                                               SOCIETE GENERALE, SOUTHWEST
                                               AGENCY
                                               
                                               
                                               
                                               By:
                                                  ---------------------------
                                                  Name:
                                                  Title:
                                               
                                               
                                               BANK OF SCOTLAND
                                               
                                               
                                               
                                               By:
                                                  ---------------------------
                                                  Name:
                                                  Title: