<PAGE>

                              FOURTH AMENDMENT OF
                     AMENDED AND RESTATED CREDIT AGREEMENT

     THIS FOURTH AMENDMENT OF AMENDED AND RESTATED CREDIT AGREEMENT (this 
"Amendment"), dated as of August 20, 1998, is by and among BASIN EXPLORATION, 
INC., a Delaware corporation ("Borrower"), U.S. BANK NATIONAL ASSOCIATION 
f/k/a COLORADO NATIONAL BANK (USB"), UNION BANK OF CALIFORNIA, N.A. 
("Union"), and NATIONSBANK, N.A. f/k/a NATIONSBANK OF TEXAS, N.A. ("NBT"), in 
its capacity as a Lender and as Agent for Lenders.  USB, Union and NBT are 
herein collectively referred to as "Lenders."

                                    RECITALS

     A. Borrower and Lenders entered into an Amended and Restated Credit 
Agreement dated as of August 6, 1996 (the "Original Credit Agreement"), as 
amended by a First Amendment of Amended and Restated Credit Agreement dated 
as of June 11, 1997, a Second Amendment of Amended and Restated Credit 
Agreement dated as of November 1, 1997, and a Third Amendment of Amended and 
Restated Credit Agreement dated as of April 30, 1998, in order to set forth 
the terms upon which Lenders would make loans to Borrower and issue letters 
of credit at the request of Borrower and by which such loans and letters of 
credit would be governed.  Capitalized terms used herein without definition 
shall have the same meanings as set forth in the Original Credit Agreement, 
as amended as set forth above (the "Credit Agreement").

     B. The parties hereto wish to enter into this Amendment in order to 
amend certain terms and provisions of the Credit Agreement.

                                   AGREEMENT

     NOW, THEREFORE, in consideration of $10.00 and other good and valuable 
consideration, the receipt and sufficiency of which are hereby acknowledged, 
the parties hereby agree as follows:

     1.   CREDIT AGREEMENT. Effective as of the date of this Amendment, the 
Credit Agreement shall be, and hereby is, amended as follows:

          (a)  The following new definitions shall be inserted in proper 
alphabetical order in Section 1.1 of the Credit Agreement:

               "REGULAR BORROWING BASE"means,at any time during the time 
     period from August 20, 1998 to the date as of which the November 1, 1998 


                                      -1-
<PAGE>

     redetermination of the Borrowing Base becomes effective, $85,000,000, 
     unless Borrower and Lenders hereafter mutually agree upon a different 
     amount.

               "SUPPLEMENTAL BORROWING BASE" means, at any time during the 
     time period from August 20, 1998 to the date as of which the November 1, 
     1998 redetermination of the Borrowing Base becomes effective, the excess 
     of the Borrowing Base over the Regular Borrowing Base.

          (b)  The definition of "Base Rate Spread" in Section 1.1 on page 1 
of the Credit Agreement shall be deleted, and the following shall be 
substituted therefor:

               "BASE RATE SPREAD" means: (a) for any and all calendar months 
     that the Capitalization Ratio is greater than or equal to 50 percent, 
     0.25 percentage points per annum; and (b) for any and all calendar 
     months that the Capitalization Ratio is less than 50 percent, 0.00 
     percentage points per annum; provided that, as to any and all amounts by 
     which the outstanding principal balance of the Loan plus the face amount 
     of all Letters of Credit outstanding hereunder exceeds the Regular 
     Borrowing Base, the amounts set forth in clauses (a) and (b) above shall 
     be increased to 1.00 percentage points per annum.

         (c)  The definition of Borrowing Base" in Section 1.1 on page 3 of 
the Credit Agreement shall be deleted, and the following shall be substituted 
therefor:

               "BORROWING BASE" means, at any time, the aggregate loan value 
     of the Borrowing Base Properties, as determined by Lenders in accordance 
     with the provisions of Section 3.2 below; provided that, for the time 
     period from August 20, 1998 to the date as of which the November 1, 1998 
     redetermination of the Borrowing Base becomes effective, the Borrowing 
     Base shall be $90,000,000, unless Borrower and Lenders hereafter 
     mutually agree upon a different amount or unless the Borrowing Base is 
     redetermined pursuant to Section 3.2 below prior to such redetermination 
     date.

          (d)  The definition of "Fixed Rate Spread" in Section 1.1 on page 7 
of the Credit Agreement shall be deleted, and the following shall be 
substituted therefor:


                                      -2-
<PAGE>

               "FIXED RATE SPREAD" means: (a)  for any and all calendar 
     months that the Capitalization Ratio is greater than or equal to 50 
     percent, 1.25 percentage points per annum; (b)  for any and all calendar 
     months that the Capitalization Ratio is less than 50 percent but greater 
     than or equal to 40 percent, 1.00 percentage point per annum; (c) for 
     any and all calendar months that the Capitalization Ratio is less than 
     40 percent but greater than or equal to 30 percent, 0.75 percentage 
     point per annum; and (d) for any and all calendar months that the 
     Capitalization Ratio is less than 30 percent, 0.625 percentage point per 
     annum; provided that, as to any and all amounts by which the outstanding 
     principal balance of the Loan plus the face amount of all Letters of 
     Credit outstanding hereunder exceeds the Regular Borrowing Base, the 
     amounts set forth in clauses (a) through (d) above shall be increased to 
     2.00 percentage points per annum.

          (e)  Section 3.6(a) on pages 22 and 23 of the Credit Agreement 
shall be deleted,and the following shall be substituted therefor:

               Section 3.6.  FEES.  (a) Borrower shall pay to Agent, on 
     behalf of Lenders (and Agent shall pay each Lender its respective 
     Proportionate Share thereof on the Business Day that any such payment is 
     deemed to be received from Borrower), within 30 days after the end of 
     each three-month period ending on the last day of January, April, July 
     or October during the Revolving Period, commencing with the three-month 
     period ending October 31, 1996, a commitment fee, computed on a daily 
     basis for such three-month period, in an amount equal to : (i) the 
     Commitment Fee Rate, times (ii) the excess of the Commitment Amount over 
     the sum of the outstanding principal balance of the Loan plus the face 
     amount of all Letters of Credit outstanding hereunder; provided that, 
     for the time period from August 20, 1998 to the date as of which the 
     November 1, 1998 redetermination of the Borrowing Base becomes 
     effective, such fee shall be calculated as follows: (1) (A) the 
     Commitment Fee Rate, times (B) the excess of the Regular Borrowing Base 
     over the sum of the outstanding principal balance of the Loan plus the 
     face amount of all Letters of Credit outstanding hereunder; plus (2) (A) 
     0.625 percentage points per annum, times (B) the excess of the 
     Commitment Amount over the greater of: (I) the sum of the outstanding 
     principal balance of the Loan plus the face amount of all Letters of 
     Credit outstanding hereunder; or (II) the Regular Borrowing Base.


                                      -3-
<PAGE>

     2.   LOAN DOCUMENTS.  All references in any document to the Credit 
Agreement shall refer to the Credit Agreement, as amended and supplemented 
pursuant to this Amendment.

     3.   CONDITIONS PRECEDENT.  The obligations of the parties under this 
Amendment are subject, at the option of Lenders, to the prior satisfaction of 
the condition that Borrower shall have executed and/or delivered, or caused 
to have been executed and/or delivered, to or for the benefit of Lenders, the 
following (all documents to be satisfactory in form and substance to Lenders):

          (a)  This Amendment.

          (b)  Such certificates of officers of Borrower as may be required 
by Lenders.

          (c)  Any and all other Loan Documents required by Lenders, 
including without limitation any and all Security Documents required by 
Lenders.

     4.   REPRESENTATIONS AND WARRANTIES.  Borrower hereby certifies to 
Lenders that as of the date of (and after giving effect to) this Amendment, 
except as heretofore disclosed to and waived by Lenders: (a) all of 
Borrower's representations and warranties contained in the Credit Agreement 
are true, accurate and complete in all material respects, and (b) no Default 
or Event of Default has occurred and is continuing under the Credit Agreement.

     5.   CONTINUATION OF THE CREDIT AGREEMENT.  Except as specified in this 
Amendment , the provisions of the Credit Agreement shall remain in full force 
and effect, and if there is a conflict between the terms of this Amendment 
and those of the Credit Agreement, the terms of this Amendment shall control. 
Borrower hereby ratifies, confirms and adopts the Credit Agreement, as 
amended hereby.

     6.   EXPENSES.  Borrower shall pay all reasonable expenses incurred in 
connection with the transactions contemplated by this Amendment, including 
without limitation all reasonable fees and reasonable expenses of Lenders' 
attorneys and all recording and filing fees, charges and expenses.

     7.   MISCELLANEOUS.  This Amendment shall be governed by and construed 
under the laws of the State of Colorado and shall be binding upon and inure 
to the benefit of the parties hereto and their successors and assigns.  This 
Amendment may be executed in any number of counterparts, each of which shall 
be an original, but all of which together shall constitute one instrument.  
Delivery of this Amendment and any and all documents 


                                      -4-
<PAGE>

to be delivered in connection herewith by any party may be effected, without 
limitation, by faxing a signed counterpart of this Amendment to NBT (any 
party that effects delivery in such manner hereby agreeing to transmit 
promptly to NBT an actual signed counterpart).

     EXECUTED as of the date first above written.

                                       BASIN EXPLORATION, INC.

                                       By: /s/ NEIL L. STENBUCK
                                          ---------------------------
                                          Vice President/Chief Financial 
                                          Officer

                                       U.S. BANK NATIONAL ASSOCIATION f/k/a 
                                       COLORADO NATIONAL BANK

                                       By: /s/ KATHRYN A. GAITER
                                          ---------------------------
                                          Vice President

                                       NATIONSBANK, N.A. f/k/a NATIONSBANK 
                                       OF TEXAS, N.A., in its capacity as  
                                       a Lender and as Agent for the Lenders

                                       By: /s/ DAVID C. RUBENKING
                                          ---------------------------
                                          Senior Vice President

                                       UNION BANK OF CALIFORNIA, N.A.

                                       By: /s/ RANDALL L. OSTERBERG
                                          ---------------------------
                                          Vice President

                                       By: /s/
                                          ---------------------------
                                          Senior Vice President


                                      -5-