<PAGE>   1



                                                                  EXHIBIT 10.16

         SECOND AMENDMENT AND CONSENT, dated as of February 25, 1998 (this
"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 (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, the Borrower intends to enter into a merger with and
into Coda Energy, Inc. ("Coda"), a wholly-owned subsidiary of the Borrower (the
"Coda Merger");

                  WHEREAS, upon the consummation of the Coda Merger, the
Borrower shall be the surviving corporation and shall assume all of the
liabilities and obligations of Coda and the Restricted Subsidiaries of the
Borrower will guarantee, on a subordinated basis, obligations under the Coda
Indenture (the "Coda Guarantees");

                  WHEREAS, the Borrower has requested that the Administrative
Agent and the Lenders enter into this Amendment in connection with the
assumption, by the Borrower, of the obligations of Coda under the Coda
Indenture pursuant to a Supplemental Indenture, and in connection with the
Subsidiary Guarantees, each of which will be entered into upon the consummation
of the Coda Merger;

                  WHEREAS, pursuant to subsections 8.2(k), 8.2(h) and 8.4(c) of
the Credit Agreement, certain terms of the Coda Indenture, as assumed by the
Borrower, and the Coda Guarantees must be reasonably satisfactory to the
Administrative Agent and the Required Lenders;

                  WHEREAS, the parties hereto wish to amend certain provisions
of the Credit Agreement and to consent to certain actions of the Borrower on
the terms set forth herein;

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

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

                  (b) Amendments and Waiver to the Credit Agreement.

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

                  "`Coda Guarantees': the senior subordinated guarantees, by the
         Restricted Subsidiaries, of the Coda Notes pursuant to Article 11 of
         the Coda Indenture and the Third Supplemental Coda Indenture.

                  `Coda Indenture': that certain Indenture, dated as of March
         18, 1996, among the Borrower (as successor to Coda by the Coda
         Merger), as issuer, its Subsidiaries, as subsidiary guarantors, and
         Texas Commerce Bank National Association, as trustee, relating to the
         Coda Notes, as supplemented by the Second Supplemental Coda Indenture
         and the Third Supplemental Coda Indenture.

                  `Coda Merger':  the merger of the Borrower with and into Coda,
         with the Borrower to be the surviving corporation.

                  `Coda Notes': the 10-1/2% Senior Subordinated Notes of the 
         Borrower (as successor to Coda by the Coda Merger) due 2006, issued
         pursuant to the Coda Indenture. `Guarantors': the Restricted
         Subsidiaries party to the Subsidiary Guarantee.

                  `Subsidiary Guarantee': any guarantee agreement, in form and 
         substance satisfactory to the Administrative Agent, entered into by
         Restricted Subsidiaries of the Borrower pursuant to which such


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         Restricted Subsidiaries shall guarantee the Borrower's obligations
         under the Credit Agreement and the Loan Documents.

                  `Second Supplemental Coda Indenture': the Second Supplemental 
         Indenture, to be dated as of the consummation of the Coda Merger, to
         the Coda Indenture, containing terms substantially similar to those
         contained in the draft distributed to the Lenders on February 20,
         1998.

                  `Third Supplemental Coda Indenture': the Third Supplemental
         Indenture, to be dated as of the consummation of the Coda Merger, to
         the Coda Indenture, containing terms substantially similar to those
         contained in the draft distributed to the Lenders on February 20,
         1998."

                  (ii) Subsection 1.1 of the Credit Agreement is hereby amended
by deleting the definitions of "Loan Documents" and "Loan Parties" in their
entireties and substituting in lieu thereof the following in their appropriate
alphabetical order:

                  "`Loan Documents': this Agreement, any Notes, the Pledge 
         Agreement, the Subsidiary Guarantee and the L/C Applications.

                  `Loan Parties':  the Borrower, the Guarantors and the 

         Pledgors."

                  (B) Section 5.22. Section 5 of the Credit Agreement is hereby
amended by adding the following subsection 5.22 at the end of such Section:

                  "5.22 Coda Indenture. The execution, delivery and performance
         of the Loan Documents, the Extensions of Credit and the incurrence of
         Indebtedness hereunder and the use of the proceeds of such Extensions
         of Credit have not violated and will not violate any applicable
         provision of the Coda Indenture. The Notes and the Indebtedness
         evidenced hereby and thereby are "Senior Debt" and "Designated Senior
         Debt" as such terms are defined in the Coda Indenture, and the
         Guarantee Agreement is a "Senior Guarantee" as such term is defined in
         the Coda Indenture."

                  (C) Subsection 7.2(g). Subsection 7.2 of the Credit Agreement
is hereby amended by adding the following paragraph (g) at the end of such
subsection and changing the punctuation in such subsection accordingly:

                  "(g) concurrently with the delivery of the financial
         statements referred to in subsections 7.1(a) and (b), a certificate of
         a Responsible Officer stating that, to the best of such officer's
         knowledge, all Extensions of Credit made during such period were
         permitted to be incurred under Section 4.09 of the Coda Indenture, and
         setting forth in reasonable detail any calculations necessary to
         determine compliance with such covenant."


                  (D) Subsection 8.9(c). Subsection 8.9(c) of the Credit
Agreement is hereby amended by deleting such subsection in its entirety.

                  (E) Section 9(i). Section 9(i) of the Credit Agreement is
hereby amended by deleting such subsection in its entirety and by substituting
in lieu thereof the following:

         "(i) (i) The Pledge Agreement or the Guarantee Agreement shall cease,
         for any reason, to be in full force and effect or any Pledgor or
         Guarantor party to such Loan Document shall so assert in writing or
         (ii) the Lien created by the Pledge Agreement shall cease to be
         enforceable and of the same effect and priority purported to be
         created thereby; or"

                  (F) Subsection 11.13(d). Subsection 11.13(d) of the Credit
Agreement is hereby amended by deleting such subsection in its entirety and by
substituting in lieu thereof the following:

         "(d) the Notes and the Indebtedness evidenced hereby and thereby are
         "Designated Senior Debt" as such term is defined in the Senior
         Subordinated Indenture and as such term is defined in the Coda
         Indenture, and the Guarantee Agreement is a "Senior Guarantee" as such
         term is defined in the Coda Indenture."

                  (G) Schedule. Schedule 5.15 attached to the Credit Agreement
is hereby deleted and the Schedule attached hereto as Schedule 5.15 to this
Amendment is inserted in lieu thereof.



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                  c. Consents pursuant to Subsections 8.2(k), 8.2(h) and 8.4(c)
                  of the Credit Agreement. 
                  (a) The Administrative Agent and the Required Lenders hereby 
consent, pursuant to subsections 8.2(k) and 8.2(h) of the Credit Agreement, to
the terms of the Coda Notes and the Coda Indenture.

                  (b) The Administrative Agent and the Required Lenders hereby
consent, pursuant to subsection 8.4(c) of the Credit Agreement, to the
subordination provisions of the Coda Guarantees.

                  d. Effective Date. This Amendment shall become effective (the
"Amendment Effective Date") upon the satisfaction of the following conditions
precedent: (a) the Administrative Agent shall have received this Amendment,
executed and delivered by a duly authorized officer of each of the
Administrative Agent, the Borrower and the Required Lenders, with a counterpart
for each Lender; (b) the Administrative Agent shall have received a certificate
of the Borrower attesting to the consummation of the Coda Merger and the
execution of the Second Supplemental Coda Indenture; (c) the Administrative
Agent shall have received the Subsidiary Guarantee, executed and delivered by a
duly authorized officer of each of the Restricted Subsidiaries; (d) the
Administrative Agent shall have received such legal opinions, in form and
substance satisfactory to the Administrative Agent, in respect of any aspect or
consequence of the transactions contemplated hereby as it shall reasonably
request; and (e) the Administrative Agent shall have received such other
documents, including certificates executed by a Secretary or an Assistant
Secretary of the Loan Parties in form and substance satisfactory to the
Administrative Agent, in respect of any aspect or consequence of the
transactions contemplated hereby as the Administrative Agent shall reasonably
request.

                  e. Representations and Warranties. The Borrower represents
and warrants to each Lender that as of the date hereof and as of the Amendment
Effective Date: (a) this 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 (as amended on the date hereof) are true and correct on and
as of the date hereof and as of the Amendment Effective Date (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 on the date hereof or on the Amendment
Effective Date.

                  f. 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 with their terms. Except as
expressly set forth herein, this 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.

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

                  h. Counterparts. This 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 Amendment by telecopier shall
be effective as delivery of a manually executed counterpart of this Amendment.

                  i. 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.

                  [Remainder of Page Intentionally Left Blank]



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





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                                           BELCO OIL & GAS CORP.


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



                                           THE CHASE MANHATTAN BANK, as
                                           Administrative Agent and as a Lender


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


                                           CHRISTIANIA BANK OG KREDITKASSE ASA



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



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


                                           CREDIT LYONNAIS NEW YORK BRANCH



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




<PAGE>   5



                                           DEN NORSKE BANK ASA



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



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



                                           THE FUJI BANK, LIMITED



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



                                           ROYAL BANK OF CANADA



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


                                           THE SANWA BANK LIMITED



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





<PAGE>   6



                                           SOCIETE GENERALE, SOUTHWEST AGENCY



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