<PAGE>

                                                                EXHIBIT 10.5 (e)

                  FOURTH AMENDMENT TO CREDIT AGREEMENT BY AND
                          BETWEEN INDIAN OIL COMPANY
                         AND BANK ONE, OKLAHOMA, N.A.


     THIS FOURTH AMENDMENT TO CREDIT AGREEMENT (the "Amendment") is made
effective the 1st day of November, 1999 by and between INDIAN OIL COMPANY, an
Oklahoma corporation. (the "Borrower") and BANK ONE, OKLAHOMA, N.A. (the
"Lender").


                             W I T N E S S E T H:

     WHEREAS, on December 22, 1997, Borrower and Lender entered into that
certain Credit Agreement (the "Original Agreement") whereby Lender provided
Borrower with a reducing, revolving line of credit in an amount, subject to a
Borrowing Base, which shall not exceed $50,000,000.00 as evidenced by the Note.

     WHEREAS, on August 10, 1998 Borrower and Lender amended the Original
Agreement for the first time (the "First Amendment") in order to: (i) change the
Debt Service Coverage ratio calculation; (ii) waive the occurrence of certain
events of default; and (iii) certain other changes more particularly set forth
therein.

     WHEREAS, on February 26, 1999, Borrower and Lender amended the Original
Agreement for the second time (the "Second Amendment") in order to (i) reduce
the Borrower's Borrowing Base availability from $24,000,000.00 to
$20,000,000.00; (ii) extend the deadline for Borrower to have completed a
$12,000,000.00 equity offering until March 1, 1999; (iii) extend the deadline
for Borrower to provide an audited financial statement for the fiscal year
ending December 31, 1997 until April 10, 1999; and  (iv) waive the occurrence of
certain events of default.

     WHEREAS, on March 31, 1999, Borrower and Lender amended the Agreement for
the third time (the "Third Amendment") in order to: (i) reset the Borrowing
Base, (ii) reset the Monthly Borrowing Base Reduction, (iii) restate the Debt
Service Coverage Ratio, and (iv) document such other amendments and waivers as
set forth therein (the Original Agreement as amended by the First, Second, and
Third Amendments is referred to herein as the "Agreement")

     WHEREAS, the obligations described in the Agreement are secured by, among
other things not specifically set forth herein, certain Oil and Gas Properties;
and

     WHEREAS, all capitalized terms not otherwise defined herein shall have
those meanings assigned to such terms in the Agreement;
<PAGE>

     WHEREAS, Borrower and Lender now desire to amend the Agreement for the
fourth time in order to (i) permit additional subordinated debt, as described
herein,  (ii) to provide for Unscheduled Redeterminations (defined herein),
(iii) to include commodity rate management language and (iv) to document such
other amendments, extensions and waivers as set forth herein.

     NOW THEREFORE, in consideration of the mutual covenants and agreements
herein contained and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the Borrower and the Lender hereby
agree to amend  the agreement as follows:

     A.   CHANGES TO THE AGREEMENT.  The Agreement is hereby amended as follows:

          1.   The definition of "Affiliate" at Section 1.2 of the Agreement,
                                  ---------
Additional Defined Terms, is hereby amended to specifically include "INDCO,
------------------------
L.C.C." by inserting the following sentence at the end of said definition:

     The term "Affiliates" is also understood to include, but is not limited to,
               ----------
     INDCO, L.L.C.

          2.   The definition of "Obligations" at Section 1.2 of the Agreement,

Additional Defined Terms, is hereby amended and restated in its entirety as
------------------------
follows:

          "Obligations" shall mean, without duplication, (a) all
           -----------
     Indebtedness evidenced by the Note, (b) the obligation of the
     Borrower for the payment of any fees required herein, (c) any and
     all obligations, contingent or otherwise, whether now existing or
     hereafter arising, of Borrower to Lender arising under or in
     connection with Rate Management Transactions, and (d) all other
     obligations and liabilities of the Borrower to the Lender, now
     existing or hereafter incurred, under, arising out of or in
     connection with any Loan Document, and with respect to all of the
     foregoing to the extent that any of the same includes or refers
     to the payment of amounts deemed or constituting interest, only
     so much thereof as shall have accrued, been earned and remains
     unpaid at each relevant time of determination.

          3.   The definition of "Tangible Net Worth" at Section 1.2 of the
Agreement, Additional Defined Terms, is hereby amended and restated in its
           ------------------------
entirety as follows:

          "Tangible Net Worth" shall mean, on any date as of which the
           ------------------
     amount thereof is to be determined, the sum of the following for
     Borrower calculated in accordance with GAAP: (i) the amount of
     stated capital (less cost of treasury shares), plus (ii) the
                                                    ----
     amount of surplus and retained earnings (or in the case of a

                                       2
<PAGE>

     surplus or retained earnings deficit, minus the amount of such
                                           -----
     deficit), plus (iii) all debt owed to shareholders and/or
               ----
     Affiliates subordinated to the Obligations, minus (iv) all debt
                                                 -----
     owed to Borrower by shareholders and/or Affiliates.

          4.   A new definition shall be added to Section 1.2 of the Agreement,
Additional Defined Terms, for the term "Rate Management Transaction" which shall
-------------------------
be inserted in Section 1.2 in its alphabetical location and which shall state as
follows:

          "Rate Management Transaction" shall mean any transaction
           ---------------------------
     (including an agreement with respect thereto) now existing or
     hereafter entered into between Borrower and Lender which is a
     rate swap, basis swap, forward transaction, commodity swap,
     commodity option, equity or equity index swap, equity or equity
     index option, bond option, interest rate option, foreign exchange
     transaction, cap transaction, floor transaction, collar
     transaction, forward transaction, currency swap transaction,
     cross-currency rate swap transaction, currency option or any
     other similar transaction (including any option with respect to
     any of these transactions) or any combination thereof, whether
     linked to one or more interest rates, foreign currencies,
     commodity prices, equity prices or other financial measures.


          5.   A new definition shall be added to Section 1.2 of the Agreement,
Additional Defined Terms, for the term "Unscheduled Redetermination" which shall
-------------------------
be inserted in Section 1.2 in its alphabetical location and which shall state as
follows:

          "Unscheduled Redetermination" shall mean a redetermination
           ---------------------------
     of the Maximum Borrowing Base made at any time other than on the
     dates set for the regular semi-annual redetermination of the
     Maximum Borrowing Base which is made (A) at the reasonable
     request of a Borrower, (B) at any time it appears to the Lender,
     in the exercise of its reasonable discretion, that either (i)
     there has been a material decrease in the value of the Oil and
     Gas Properties, or (ii) an event has occurred which is reasonably
     expected to have a Material Adverse Effect.

          6.   Paragraph (b) of Section 2.9 of the Agreement, Borrowing Base
                                                              --------------
Determinations and Monthly Borrowing Base Reductions, is hereby amended and
----------------------------------------------------
restated in its entirety as follows:

          (b)  Subsequent Borrowing Base Determinations shall be made
     by the Lender at least semi-annually on March 1 and September 1
     (each a "Determination Date") of each year beginning September 1,
     1999 or at any other time as an Unscheduled Borrowing Base
     Determination. In connection with, and as of, each Borrowing Base
     Determination, the Lender shall also determine the Monthly
     Borrowing Base Reduction and Half Life of the Borrower's Oil and
     Gas

                                       3
<PAGE>

     Properties. Borrower shall provide Lender with engineering
     information pursuant to the terms of Section 6.4 below. Lender
     shall, by notice to the Borrower by March 1 and September 1 of
     each year, or within a reasonable time thereafter (herein called
     the "Determination Date"), notify the Borrower of the designation
     by the Lender of the new Borrowing Base, Monthly Borrowing Base
     Reduction and Half Life of the Borrower's Oil and Gas Properties
     for the period beginning on such Determination Date and
     continuing until, but not including, the next Determination Date.
     If an Unscheduled Borrowing Base Determination is made by the
     Lender, the Lender shall notify the Borrower within a reasonable
     time after receipt of all requested information of the new
     Borrowing Base, Monthly Borrowing Base Reduction, and Half Life
     of the Borrower's Oil and Gas Properties and such new Borrowing
     Base, Monthly Borrowing Base Reduction, and Half Life of the
     Borrower's Oil and Gas Properties shall continue until the next
     Borrowing Base Determination. If the Borrower does not furnish
     all such information, reports and data by any date specified in
     this Section 2.9.2 or Section 6.4, the Lender may nonetheless
     designate the Borrowing Base, Monthly Borrowing Base Reduction,
     and Half Life of the Borrower's Oil and Gas Properties at any
     amounts which the Lender in its discretion determines and may
     redesignate the Borrowing Base, Monthly Borrowing Base Reduction,
     and Half Life of the Borrower's Oil and Gas Properties from time
     to time thereafter until the Lender receives all such
     information, reports and data, whereupon the Lender shall
     designate a new Borrowing Base, Monthly Borrowing Base Reduction,
     and Half Life of the Borrower's Oil and Gas Properties as
     described above. At each Borrowing Base Determination Lender
     shall determine the amount of the Borrowing Base, Monthly
     Borrowing Base Reduction, and Half Life of the Borrower's Oil and
     Gas Properties based upon the Lender's evaluation of the
     Collateral which Lender in its discretion (using such
     methodology, assumptions and discounts rates as Lender
     customarily uses in assigning collateral value to oil and gas
     properties, oil and gas gathering systems, gas processing and
     plant operations) assigns to such Oil and Gas Properties and
     other Collateral of the Borrower at the time in question and
     based upon such other credit factors consistently applied
     (including, without limitation, the assets, liabilities, cash
     flow, business, properties, prospects, management and ownership
     of the Borrower and its Affiliates) as Lender customarily
     considers in evaluating similar oil and gas loans. If at any time
     any of the Oil and Gas Properties are sold, the Borrowing Base
     then in effect shall automatically be reduced by a sum equal to
     the amount of prepayment required to be made pursuant to Section
     8.4 hereof. It is expressly understood that the Lender has no
     obligation to designate the Borrowing Base, Monthly Borrowing
     Base Reduction or Half Life of the Borrower's Oil and Gas
     Properties at any particular amounts, except in the exercise of
     its discretion, whether in relation to the Commitment or
     otherwise. Provided, however, that the Lender shall not have the
     obligation to designate a

                                       4
<PAGE>

     Borrowing Base in an amount in excess of the Commitment or its
     legal or internal lending limits.

          7.   Article III of the Agreement, COLLATERAL AND OTHER FORMS OF
                                             -----------------------------
CREDIT ENHANCEMENT, is hereby amended to include a new section at Section 3.7
------------------
entitled, INDCO, L.L.C. Collateral, which shall state as follows:
          ------------------------

          3.7  INDCO, L.L.C. Collateral. As further security for the
               ------------------------
     Obligations, Borrower shall pledge to Lender its $960,000.00 note
     from INDCO, L.L.C. and upon request by Lender, INDCO, L.L.C.
     shall pledge and/or grant Lender a security interest in and to
     all assets transferred to INDCO, L.L.C. by Borrower, and any
     consideration received by INDCO, L.L.C. for the sale or transfer
     of the aforementioned assets.

          8.   Section 6.5 of the Agreement, Reserve Reports, is hereby  amended
                                             ---------------
and restated in its entirety to allow for Unscheduled Redeterminations as
follows:

          6.4  Reserve Reports.
               ---------------

          (a)  Deliver to the Lender no later than February 1, 2000,
     and each February 1 thereafter during the term of this Agreement
     and, upon request, in conjunction with any Unscheduled
     Redetermination, engineering reports prepared by an independent
     third party engineer approved by Lender (and/or such other
     appropriate information acceptable to Lender) covering or
     pertaining to Borrower's Oil and Gas Properties in form and
     substance acceptable to Lender setting forth (i) the proven
     producing and proven non-producing oil and gas reserves
     (separately classified as such) attributable to the Oil and Gas
     Properties as of each January 1 of each year for which the
     Reserve Reports are furnished, (ii) the aggregate present value
     determined on the basis of stated pricing assumptions, of the
     future net income with respect to such Oil and Gas Properties,
     discounted at a stated per annum discount rate of proven and
     producing reserves, and (iii) projections of the annual rate of
     production, gross income and net income with respect to such
     proven and producing reserves.

          (b)  The report provided pursuant to this Section 6.4 shall
                                                    -----------
     be submitted to the Lender together with additional data as the
     Lender may reasonably request concerning pricing, quantities of
     production from the Oil and Gas Properties, purchasers of
     production and engineering and geological data. By August 1 of
     each year and, upon request, in conjunction with Unscheduled
     Redeterminations, Borrower shall provide Lender with such other
     reserve information as Lender may reasonably request to complete
     its semi annual redetermination of the Borrowing Base, Monthly
     Borrowing Base Reduction, and Half Life of the Borrower's Oil and
     Gas Properties.

                                       5
<PAGE>

          (c)  In conjunction with the Reserve Report, Borrower shall furnish
     Lender a report on the status of all gas balancing affecting any of the Oil
     and Gas Properties.

          9.   Section 8.1 of the Agreement, Indebtedness, is hereby amended and
                                             ------------
restated in its entirety in order to permit certain of Borrower's obligations to
INDCO, L.L.C. as a permitted indebtedness as follows:

          8.1  Indebtedness. Create, incur, assume or suffer to exist
               ------------
     any Indebtedness that exceed, in the aggregate, the sum of
     $100,000.00, whether by way of loan or otherwise; provided
                                                       --------
     however, the foregoing restriction shall not apply to (a) the
     -------
     Obligations; (b) unsecured current accounts payable incurred in
     he ordinary course of business; (c) the MidFirst Debt; or (d)
     that certain $500,000.00 note payable to INDCO, L.L.C., which
     such note shall be expressly subordinate to the Obligations.

          10.  Section 8.13 of the Agreement, Limitation of Hedging Activities,
                                              --------------------------------
is hereby amended and restated in its entirety, in order to provide for Rate
Management Transactions as follows:

          8.13  Limitation of Hedging Activities. The Borrower will
                --------------------------------
     not enter into any transaction providing (i) for the hedging,
     forward sale, swap or any derivation thereof of crude oil or
     natural gas or other commodities; or (ii) for a swap, collar,
     floor, cap, option, corridor or other contract which is intended
     to reduce or eliminate risk of fluctuation of interest rates, as
     such terms are referred in the capital markets, except the
     foregoing prohibitions shall not apply to: (a) transactions
     consented to in writing by the Lender which are on terms
     acceptable to the Lender; or (b) Pre-Approved Contracts. The term
     "Pre-Approved Contracts" as used herein shall mean (i) any
     physical delivery contract or agreement with an oil and gas
     purchaser under which the Borrower agrees to sell its oil and gas
     at an agreed upon price for an agreed upon period of time and for
     volume amounts which do not exceed the monthly production
     forecast for all of the Borrower's proved developed producing
     reserves; and/or (ii) any financial contract or agreement entered
     into between the Borrower and an affiliate of the Lender or such
     other counterparty as is acceptable to Lender and which is
     consistent with the following: (a) the sales price is at or above
     the Bank's oil and gas pricing then in effect; (b) the volume
     amounts do not exceed 75% of the Borrower's monthly production
     forecast for all of the Borrower's proved developed producing
     reserves; (c) the term does not extend beyond an twelve month
     period. Upon entering into any such contract allowed hereunder,
     Borrower agrees to notify Bank so as to allow for an adjustment
     to the Maximum Borrowing Base if and when deemed appropriate by
     the Bank in its sole discretion.

                                       6
<PAGE>

          11.  The Event of Default set forth at Section 9.1 of the Agreement,
Enumeration of Events of Default, at paragraph (c) is hereby amended and
---------------------------------
restated in its entirety in order to include Rate Management Transactions as
follows:

          (c) default shall be made by Borrower in the due observance
     or performance of any of its obligations, covenants or agreements
     (for payment or otherwise) contained in any of the Loan Documents
     or pursuant to any Rate Management Transaction and such default
     could be expected to have a Material Adverse Effect;

     B.   NOTICES.  Borrower is hereby notified of the following occurrences:

          1.   Borrowing Base Determinations.  Pursuant to Section 2.9 and
               -----------------------------
Section 7.2 of the Agreement, Borrower and Lender hereby affirm the Borrowing
Base at $18,292,000.00 until December 1, 1999 at which time the Monthly
Borrowing Base Reduction shall resume at $244,000.00.  Borrower and Bank
acknowledge that the aforementioned sentence effectively provides a waiver of
the $244,000.00 Monthly Borrowing Base reduction otherwise scheduled for
November 1, 1999. Borrower and Bank further acknowledge that the regularly
scheduled Borrowing Base Determination dates are March 1 and September 1.  The
Borrowing Base Determination Dates set forth in Section 2.9 of the Agreement
notwithstanding, the September 1, 1999 Borrowing Base Determination Date is
hereby extended until December 15, 1999.

          2.   Debt Service Coverage Ratio.  For the quarterly reporting period
               ---------------------------
ending September 30, 1999 and only for the quarterly reporting period ending
September 30, 1999, the denominator of the Debt Service Coverage Ratio shall be
$488,000.00 (which such amount equates to the Monthly Borrowing Base Reduction
for two months - not three- in order to offset the $250,000.00 payment to Reid
Investments incurred during the month of September, 1999) plus interest expense
                                                          ----
(less any accrued and unpaid interest on subordinated debt ) for the quarter
then ended, plus any other current maturities of long term debt.   The
            ----
calculation of the Debt Service Coverage Ratio as set forth in the Agreement
shall continue and be reinstated for all quarters subsequent to September 30,
1999.

          3.   Tangible Net Worth.  Lender hereby waives the default created by
               ------------------
Borrower's failure to maintain the level of Tangible Net Worth required pursuant
to Section 7.3 of the Agreement, Tangible Net Worth, for the quarterly reporting
                                 ------------------
periods ending March 31, 1999 and June 30, 1999. This waiver is on a one-time
basis and is specifically not intended to waive any future violations of Section
7.3 of the Agreement or any other terms, covenants or conditions of the
Agreement.


     C.   REPRESENTATIONS AND WARRANTIES

                                       7
<PAGE>

     Borrower hereby represents and warrants to Lender that:

          1.   Borrower is a corporation, duly organized, legally existing, and
in good standing under the laws of the State of Oklahoma, and is duly qualified
as a foreign corporation and in good standing in all other states wherein the
nature of its business or its assets make such qualification necessary.

          2.   Borrower's execution and delivery of this Amendment and
performance of its obligations hereunder: (a) are and will be within its
corporate powers; (b) are duly authorized by its board of directors; (c) are not
and will not be in contravention of any law, statute, rule or regulation, the
terms of its articles or certificates of incorporation and bylaws, nor of any
preferred stock provision, indenture, agreement or undertaking to which
Corporation or any of its properties are bound; (d) do not require any consent
or approval (including governmental) which has not been given; and (e) will not
result in the imposition of Liens, charges or encumbrances on any of its
properties or assets, except those in favor of Lender hereunder.

          3.   This Amendment, when duly executed and delivered, will constitute
the legal, valid and binding obligations of each Borrower, enforceable in
accordance with its terms.

          4.   All financial statements, balance sheets, income statements and
other financial data which have been or are hereafter furnished to Lender by
each Borrower to induce Lender to make the loans hereunder due, and as to
subsequent financial statements will, fairly represent each Borrower's financial
condition as of the dates for which the same are furnished. All such financial
statements, reports, papers and other data furnished to Lender are and will be,
when furnished: prepared in accordance with generally accepted accounting
principles consistently applied; accurate and correct in all material respects;
and complete insofar as completeness may be necessary to give Lender a true and
accurate knowledge of the subject matter.  Since the date of the last such
financial statements, no material adverse change has occurred in the operations
or condition, financial or otherwise and other financial data provided to
Lender; of either Borrower, nor, to the best of their knowledge, has either
Borrower incurred, any material liabilities or made any material investment or
guarantees, direct or contingent, in any single case or in the aggregate, which
has not been disclosed to Lender.

          5.   The Borrower is the sole and lawful owner of the Collateral,
pledged, mortgaged or assigned by it, and Borrower has, and as to after acquired
property or New Properties will have, good right to cause the Collateral to be
hypothecated to Lender as security for the Obligations.

                                       8
<PAGE>

          6.   All of Borrower's other representations and warranties set forth
in Sections 5.1 through 5.22 of the Agreement are true and correct on and as of
the date hereof with the same effect as though made and repeated by Borrower as
of the date hereof.


     D.   CONDITIONS

     Lender's obligations under the Agreement, as hereby amended, is subject to
the following conditions:

          1.   Lender and Borrower shall have executed and delivered this
Amendment.

          2.   Lender shall have executed and delivered such endorsements and
assignments necessary, in the discretion of Bank, to pledge the $960,000.00
promissory note from INDCO, L.L.C. to Borrower.

          3.   INDCO, L.L.C. and Borrower shall have executed and delivered to
Lender for its execution an agreement, satisfactory to Lender, subordinating
INDCO, L.L.C.'s $500,000.00 promissory note from the Borrower (principal and
interest) to the Obligations.

          4.   Borrower's representations and warranties set forth in Section
C hereof shall be true and correct on and as of the date hereof, and the date of
any subsequent advance with the same effect as though such representation and
warranty had been on and as of such date.

          5.   Borrower shall have satisfied all conditions set forth in Section
4.1 of the Agreement.

          6.   Except for the default created by Borrower's failure to provide
Bank with a copy of its audited financial statements for year ending December
31, 1998, as of the date hereof, and the date of any subsequent Advance, no
Event of Default nor any event which, with the giving of notice or lapse of
time, would constitute an Event of Default shall have occurred and be
continuing.


     E.   OTHER COVENANTS AND MISCELLANEOUS TERMS

          1.   Except as expressly amended and supplemented hereby, the
Agreement shall remain unchanged and in full force and effect, and the same is
hereby ratified and extended.

                                       9
<PAGE>

          2.   The Obligations, including but not limited to the indebtedness
evidenced by the Note executed in conjunction with the Agreement, shall continue
to be secured by the Collateral, without interruption or impairment of any kind.

          3.   Borrower shall make, execute or endorse, and acknowledge and
deliver or file or cause the same to be done, all such vouchers, invoices,
notices, certifications and additional agreements, undertakings, conveyances,
deeds of trusts, mortgages, transfers, assignments, financing statements or
other assurances, and take any and all such other action, as Lender may, from
time to time, deem reasonably necessary or proper in connection with any of the
Loan Documents and the obligations of Borrower assuring and confirming unto
Lender all or any part of the security for any such obligation.  In connection
herewith, Lender may require Borrower to execute additional mortgages or deeds
of trust pursuant to the terms of the Agreement.

          4.   The Borrower hereby agrees to pay all reasonable attorney fees
and legal expenses incurred by Lender in preparation, execution and
implementation of this Amendment and any mortgages, deeds of trust, security
agreements, pledge agreements or any amendments thereto.

          5.   This Amendment shall be construed in accordance with and governed
by the laws of the State of Oklahoma, and shall be binding on and inure to the
benefit of the Borrower and Lender, and their respective successors and assigns.
All obligations of the Borrower under the Agreement and all rights of Lender and
any other holder of the Notes, whether expressed herein or in any Note, shall be
in addition to and not in limitation of those provided by applicable law.
Borrower irrevocably agrees that, subject to Lender's sole election, all suits
or proceedings arising from or related to the Agreement, as amended, or the
Notes may be litigated in courts (whether State or Federal) sitting in Oklahoma
City, Oklahoma, and the Borrower hereby irrevocably waives any objection to such
jurisdiction and venue.

          6.   This Amendment may be executed in as many counterparts as are
deemed necessary or convenient, and it shall not be necessary for the signature
of more than any one party to appear on any single counterpart.  Each
counterpart shall be deemed an original, but all shall be construed together as
one and the same instrument.  The failure of any party to sign shall not affect
or limit the liability of any party executing any such counterpart.

     BORROWER:                INDIAN OIL COMPANY,
                              an Oklahoma corporation

                              __________________________________________
                              By:  Richard R. Dunning
                              Title:  Chief Executive Officer

                                       10
<PAGE>

     LENDER:                  BANK ONE, OKLAHOMA, N.A.

                              __________________________________________
                              By: John K. Slay, Jr.
                              Title: Vice President

                                       11
<PAGE>

                        INDCO, L.L.C. JOINDER

     INDCO, L.L.C., for good and valuable consideration the receipt and
sufficiency of which are hereby acknowledged, certify that it is familiar with,
has read, and is bound by applicable provisions concerning it contained therein.
Further, INDCO, L.L.C. hereby acknowledges and ratifies its agreements set forth
that certain letter agreement with Bank One, Oklahoma, N.A. and Indian Oil
Company dated as of September 20, 1999.


                                    INDCO, L.L.C.,


                                    ____________________________________
                                    By:  Richard R. Dunning
                                    Title:  Manager

                                       12