<PAGE>   1
                                                                  EXHIBIT 10.2.1

                  FIRST AMENDMENT TO CANADIAN CREDIT AGREEMENT

         THIS FIRST AMENDMENT TO CANADIAN CREDIT AGREEMENT (herein called the
"Amendment") made as of March 1, 2001 by and among Northstar Energy Corporation,
an Alberta corporation, "Canadian Borrower"), Bank of America Canada,
individually and as administrative agent ("Canadian Agent"), and the Canadian
Lenders party to the Original Agreement defined below ("Canadian Lenders").

                                   WITNESSETH:

         WHEREAS, Canadian Borrowers, Canadian Agent and Canadian Lenders
entered into that certain Canadian Credit Agreement dated as of August 29, 2000
(as amended, supplemented, or restated to the date hereof, the "Original
Agreement"), for the purpose and consideration therein expressed, whereby
Canadian Lenders became obligated to make loans to Canadian Borrowers as therein
provided;

         WHEREAS, on January 1, 2001 Northstar Energy, Devon Energy Canada
Holding Corporation (the successor by amalgamation to Devon Energy Canada) and
certain other Alberta corporations, all of which were Subsidiaries of US
Borrower, amalgamated under the name Northstar Energy Corporation (defined above
as the "Canadian Borrower") which is now the sole Canadian Borrower;

         WHEREAS, US Borrower has filed a Registration Statement on Form S-3
under the Securities Act of 1933 on November 16, 2000, with respect to the
issuance by US Borrower of Common Stock, Preferred Stock, Debt Securities, Stock
Purchase Agreements and Stock Purchase Units and the issuance by Devon Financing
Trust II of Trust Preferred Securities guaranteed by US Borrower;

         WHEREAS, in connection with the issuance of such securities, Canadian
Borrower, Canadian Agent and Canadian Lenders desire to amend the Original
Agreement;

         NOW, THEREFORE, in consideration of the premises and the mutual
covenants and agreements contained herein and in the Original Agreement, in
consideration of the loans which may hereafter be made by Canadian Lenders to
Canadian Borrower, and for other good and valuable consideration, the receipt
and sufficiency of which are hereby acknowledged, the parties hereto do hereby
agree as follows:

<PAGE>   2

                                   ARTICLE I.

                           Definitions and References

         Section 1.1. Terms Defined in the Original Agreement. Unless the
context otherwise requires or unless otherwise expressly defined herein, the
terms defined in the Original Agreement shall have the same meanings whenever
used in this Amendment.

         Section 1.2. Other Defined Terms. Unless the context otherwise
requires, the following terms when used in this Amendment shall have the
meanings assigned to them in this Section 1.2.

                  "Amendment" means this First Amendment to Canadian Credit
         Agreement.

                  "Amendment Documents" means this Amendment and any other
         documents delivered to Canadian Agent pursuant to Article III.

                  "Canadian Agreement" means the Original Agreement as amended
         hereby.

                                   ARTICLE II.

                        Amendments to Original Agreement

         Section 2.1. Defined Terms. (a) The following defined terms in Annex I
of the Original Agreement are hereby amended in their entirety to read as
follows:

                  "Canadian Borrower(s)" means Northstar Energy Corporation, an
         amalgamated Alberta corporation resulting from the amalgamation on
         January 1, 2001 of Northstar Energy, Devon Energy Canada Holding
         Corporation (which in turn was formed from the prior amalgamation of
         Devon Energy Canada and Devon Energy Canada Holding Corporation on
         January 1, 2001) and other Alberta corporations, all of which were
         wholly-owned Subsidiaries of US Borrower.

                  "Total Capitalization" means the sum (without duplication) of
         (i) US Borrower's Consolidated Total Funded Debt plus (ii) US
         Borrower's Consolidated shareholder's equity plus (iii) 60% of the
         outstanding balance of the Devon Trust Securities.

                  "Total Funded Debt" means (i) Liabilities referred to in
         clauses (a), (b), (c), (d), and (e) of the definition of
         "Indebtedness", plus (ii) 40% of the outstanding balance of the Devon
         Trust Securities. Total Funded Debt shall not include the PennzEnergy
         Exchangeable Debentures.

         (b) The following additional defined terms are hereby added to Annex I
of the Original Agreement in appropriate alphabetical order to read as follows:

                                       2


<PAGE>   3

                  "Devon Trust" means Devon Financing Trust II, a statutory
         business trust formed under the laws of the State of Delaware.

                  "Devon Trust Registration Statement" means the Registration
         Statement on Form S-3 filed by US Borrower under the Securities Act of
         1933 on November 16, 2000 with respect to the issuance by US Borrower
         of Common Stock, Preferred Stock, Debt Securities, Stock Purchase
         Agreements and Stock Purchase Units, and the issuance by Devon
         Financing Trust II of Trust Preferred Securities guaranteed by US
         Borrower, as amended and supplemented from time to time.

                  "Devon Trust Securities" means those certain Trust Preferred
         Securities, which may be issued by Devon Trust pursuant to the
         Registration Statement in an aggregate face amount not to exceed
         $447,261,200.

                  "Subordinated US Borrower Debentures" means those certain
         Convertible Junior Subordinated Debentures which may be issued by US
         Borrower to Devon Trust pursuant to the Registration Statement in an
         aggregate amount not to exceed $447,261,200, which will be subordinate
         to the Obligations.

         (c) Section 7.4(a) of the Original Agreement is deleted and replaced
with the following:

                  Section 7.4. Limitation on Issuance of Securities by
         Subsidiaries of US Borrower; Ownership of certain Restricted
         Subsidiaries by US Borrower.

                  (a) Neither Canadian Borrower nor any Subsidiary of Canadian
         Borrower that is a Restricted Person will issue any additional shares
         of its capital stock, additional partnership interests or other
         securities or any options, warrants or other rights to acquire such
         additional shares, partnership interests or other securities except to
         another Restricted Person which is a wholly-owned direct or indirect
         Subsidiary of US Borrower unless (i) such securities are being issued
         to acquire a business, directly or indirectly through the use of the
         proceeds of such issuance, and (ii) such securities are convertible
         into the common shares or similar securities of US Borrower and/or can
         be redeemed in cash at the option of the Restricted Person that issued
         such securities. In addition, (A) Canadian Borrower may issue
         "Exchangeable Shares" (as defined in the Articles of Amalgamation of
         Canadian Borrower and in this section called "Exchangeable Shares")
         upon the terms specified in the Articles of Amalgamation of Canadian
         Borrower as in effect on January 1, 2001, which terms are substantially
         the same as those set forth in the Restated Articles of Incorporation
         of Northstar Energy Corporation immediately prior to the amalgamation
         of Canadian Borrower, and (B) Canadian Borrower may issue stock options
         to its employees from time to time to acquire such Exchangeable Shares,
         provided that such options are granted under a stock option plan of
         Canadian Borrower and/or US Borrower.

                                       3

<PAGE>   4

         (d) Section 10.22 of the Original Agreement is deleted and replaced
with the following:

                  Section 10.22. Canadian Borrower. It is acknowledged that on
         January 1, 2001 Northstar Energy, Devon Energy Canada Holding
         Corporation (which in turn was formed from the prior amalgamation of
         Devon Energy Canada and Devon Energy Canada Holding Corporation on
         January 1, 2001) and other Alberta corporations, all of which were
         wholly-owned Subsidiaries of US Borrower, amalgamated under the name
         Northstar Energy Corporation and that effective January 1, 2001
         Northstar Energy Corporation is the only Canadian Borrower. Effective
         January 1, 2001, all references in the Original Agreement to Northstar
         Energy or to Canadian Borrower(s) shall be construed as a reference to
         such amalgamated corporation Northstar Energy Corporation. Northstar
         Energy Corporation shall continue to be liable for all of the
         obligations of Northstar Energy and Devon Energy Canada under the
         Original Agreement and the other Canadian Loan Documents.

         Section 2.2. Disclosure Schedule. Paragraph 6 of the Disclosure
Schedule to the Original Agreement is hereby deleted and replaced by the list
set forth in Schedule 1.

                                  ARTICLE III.

                           Conditions of Effectiveness

         Section 3.1. Effective Date. This Amendment shall become effective as
of the date first above written when and only when:

         (a) Canadian Agent shall have received all of the following, at
Canadian Agent's office, duly executed and delivered and in form and substance
satisfactory to Canadian Agent, all of the following:

                  (i) the Amendment;

                  (ii) the written opinion of Canadian Borrower's counsel,
         addressed to Canadian Agent, to the effect that this Amendment and the
         other Amendment Documents have been duly authorized, executed and
         delivered by Canadian Borrower and that the Canadian Agreement and the
         other Amendment Documents constitute the legal, valid and binding
         obligations of Canadian Borrower, enforceable in accordance with their
         terms (subject, as to enforcement of remedies, to applicable
         bankruptcy, reorganization, insolvency and similar laws and to
         moratorium laws and other laws affecting creditors' rights generally
         from time to time in effect);

                  (iii) a certificate of the Secretary or Assistant Secretary
         and of the Chairman of the Board, President, or Vice President -
         Finance of Canadian Borrower dated the date of this Amendment
         certifying: (i) that resolutions adopted by the Board of Directors of

                                       4

<PAGE>   5

         Canadian Borrower authorize the execution, delivery and performance of
         this Amendment and the other Amendment Documents by Canadian Borrower;
         (ii) the names and true signatures of the officers of Canadian Borrower
         authorized to sign this Amendment and the other Amendment Documents;
         and (iii) that all of the representations and warranties set forth in
         Article IV hereof are true and correct at and as of the time of such
         effectiveness; and

                  (iv) such other supporting documents as Canadian Agent may
         reasonably request.

         (b) Canadian Borrower shall have paid, in connection with such Canadian
Loan Documents, all fees and reimbursements to be paid to Canadian Agent
pursuant to any Canadian Loan Documents, or otherwise due Canadian Agent and
including fees and disbursements of Canadian Agent's attorneys.

                                   ARTICLE IV.

                         Representations and Warranties

         Section 4.1. Representations and Warranties of Canadian Borrower. In
order to induce each Canadian Lender to enter into this Amendment, Canadian
Borrower represents and warrants to each Canadian Lender that:

         (a) The representations and warranties contained in Article V of the
Original Agreement are true and correct at and as of the time of the
effectiveness hereof, except to the extent that the facts on which such
representations and warranties are based have been changed by the extension of
credit under the Canadian Agreement.

         (b) Canadian Borrower is duly authorized to execute and deliver this
Amendment and the other Amendment Documents and is and will continue to be duly
authorized to borrow monies and to perform its obligations under the Canadian
Agreement. Canadian Borrower has duly taken all corporate action necessary to
authorize the execution and delivery of this Amendment and the other Amendment
Documents and to authorize the performance of its obligations hereunder and
thereunder.

         (c) The execution and delivery by Canadian Borrower of this Amendment
and the other Amendment Documents, the performance by Canadian Borrower of its
obligations hereunder and thereunder and the consummation of the transactions
contemplated hereby and thereby do not and will not conflict with any provision
of law, statute, rule or regulation or of the organizational documents and any
unanimous shareholders agreement of Canadian Borrower, or of any material
agreement, judgment, license, order or permit applicable to or binding upon
Canadian Borrower, or result in the creation of any lien, charge or encumbrance
upon any assets or properties of Canadian Borrower. Except for those which have
been obtained, no consent, approval, authorization or order of any court or
governmental authority or third party is required

                                        5

<PAGE>   6

in connection with the execution and delivery by Canadian Borrower of this
Amendment and the other Amendment Documents or to consummate the transactions
contemplated hereby and thereby.

         (d) When duly executed and delivered, each of this Amendment, the
Canadian Agreement and the other Amendment Documents will be a legal and binding
obligation of Canadian Borrower, enforceable in accordance with its terms,
except as limited by bankruptcy, insolvency or similar laws of general
application relating to the enforcement of creditors' rights and by equitable
principles of general application.

         (e) The audited annual Consolidated financial statements of US Borrower
dated as of December 31, 1999 and the unaudited quarterly Consolidated financial
statements of US Borrower dated as of September 30, 2000 fairly present the
Consolidated financial position at such dates and the Consolidated statement of
operations and the changes in Consolidated financial position for the periods
ending on such dates for US Borrower. Copies of such financial statements have
heretofore been delivered to each Canadian Lender. Since such dates no material
adverse change has occurred in the financial condition or businesses or in the
Consolidated financial condition or businesses of US Borrower.

                                   ARTICLE V.

                                  Miscellaneous

         Section 5.1. Ratification of Agreements. The Original Agreement as
hereby amended is hereby ratified and confirmed in all respects. The Canadian
Loan Documents, as they may be amended or affected by the various Amendment
Documents, are hereby ratified and confirmed in all respects. Any reference to
the Canadian Agreement in any Loan Document shall be deemed to be a reference to
the Original Agreement as hereby amended. The execution, delivery and
effectiveness of this Amendment and the other Amendment Documents shall not,
except as expressly provided herein or therein, operate as a waiver of any
right, power or remedy of Canadian Lenders under the Canadian Agreement, or any
other Canadian Loan Document nor constitute a waiver of any provision of the
Canadian Agreement, or any other Canadian Loan Document.

         Section 5.2. Survival of Agreements. All representations, warranties,
covenants and agreements of Canadian Borrower herein shall survive the execution
and delivery of this Amendment and the performance hereof, including without
limitation the making or granting of the Loans, and shall further survive until
all of the Obligations are paid in full. All statements and agreements contained
in any certificate or instrument delivered by Canadian Borrower or any
Restricted Person hereunder or under the Canadian Agreement to any Canadian
Lender shall be deemed to constitute representations and warranties by, and/or
agreements and covenants of, Canadian Borrower under this Amendment and under
the Canadian Agreement.

                                       6

<PAGE>   7

         Section 5.3. Canadian Loan Documents. This Amendment is a Canadian Loan
Document, and all provisions in the Canadian Agreement pertaining to Canadian
Loan Documents apply hereto and thereto.

         Section 5.4. Governing Law. This Amendment shall be governed by and
construed in accordance the laws of the Province of Alberta and any applicable
laws of Canada in all respects, including construction, validity and
performance.

         Section 5.5. Counterparts; Fax. This Amendment may be separately
executed in counterparts and by the different parties hereto in separate
counterparts, each of which when so executed shall be deemed to constitute one
and the same Amendment. This Amendment and the other Amendment Documents may be
validly executed by facsimile or other electronic transmission.

         THIS AMENDMENT AND THE OTHER CANADIAN LOAN DOCUMENTS REPRESENT THE
FINAL AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF
PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE ARE
NO UNWRITTEN ORAL AGREEMENTS OF THE PARTIES.

         [THE REMAINDER OF THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK.]

                                       7

<PAGE>   8

         IN WITNESS WHEREOF, this Amendment is executed as of the date first
above written.


                                       NORTHSTAR ENERGY CORPORATION
                                       Canadian Borrower



                                       By: /s/ Paul Brereton
                                           -------------------------------------
                                           Paul Brereton
                                           Vice President - Finance


<PAGE>   9

                                       BANK OF AMERICA CANADA
                                       Administrative Agent, Canadian LC Issuer
                                       and Lender


                                       By: /s/ Donald R. Chung
                                           -------------------------------------
                                           Name: Donald R. Chung
                                           Title: Vice President
                                                  Corporate Investment Banking
<PAGE>   10

                                       BANK ONE CANADA
                                       Lender


                                       By: /s/ Jeanie Harman
                                           -------------------------------------
                                           Name: Jeanie Harman
                                           Title: First Vice President
<PAGE>   11

                                       THE CHASE MANHATTAN BANK
                                       Lender


                                       By: /s/ Russell A. Johnson
                                           -------------------------------------
                                           Name: Russell A. Johnson
                                           Title: Vice President
<PAGE>   12

                                       UMB BANK
                                       Lender



                                       By: /s/ Richard J. Lehrter
                                           -------------------------------------
                                           Name: Richard J. Lehrter
                                           Title: Community Bank President
<PAGE>   13

                                       FIRST UNION NATIONAL BANK
                                       Lender



                                       By: /s/ David Humphreys
                                           -------------------------------------
                                           Name: David Humphreys
                                           Title: Vice President
<PAGE>   14

                                       WESTDEUTSCHE LANDESBANK GIROZENTRALE
                                       Lender


                                       By: /s/ Duncan M. Robertson
                                           -------------------------------------
                                           Name: Duncan M. Robertson
                                           Title: Director


                                       By: /s/ Thomas Lee
                                           -------------------------------------
                                           Name: Thomas Lee
                                           Title: Associate
<PAGE>   15

                                       THE BANK OF NEW YORK
                                       Lender

                                       By: /s/ Raymond J. Palmer
                                           -------------------------------------
                                           Name: Raymond J. Palmer
                                           Title: Vice President
<PAGE>   16

                                       ROYAL BANK OF CANADA
                                       Lender

                                       By: /s/ S. G. Tibbatts
                                           -------------------------------------
                                           Name: S. G. Tibbatts
                                           Title: Senior Manager

<PAGE>   17

                                       SUNTRUST BANK, ATLANTA
                                       Lender


                                       By: /s/ David J. Edge
                                           -------------------------------------
                                           Name: David J. Edge
                                           Title: Director
<PAGE>   18

                                       J.P. MORGAN CANADA
                                       Lender

                                       By: /s/ Kenneth Knowles
                                           -------------------------------------
                                           Name: Kenneth Knowles
                                           Title: Vice President
<PAGE>   19

                                       CITIBANK CANADA
                                       Lender

                                       By: /s/ James K. G. Campbell
                                           -------------------------------------
                                           Name: James K. G. Campbell
                                           Title: Vice President

<PAGE>   20

                                       DEUTSCHE BANK AG NEW YORK
                                       AND/OR CAYMAN ISLANDS
                                       BRANCHES
                                       Lender

                                       By: /s/ Joel Makowsky
                                           -------------------------------------
                                           Name: Joel Makowsky
                                           Title: Vice President

                                       By: /s/ Hans C. Narberhaus
                                           -------------------------------------
                                           Name: Hans C. Narberhaus
                                           Title: Vice President

<PAGE>   21

                                       CANADIAN IMPERIAL BANK OF
                                       COMMERCE
                                       Lender

                                       By: /s/ Joelle Schellenberg
                                           -------------------------------------
                                           Name: Joelle Schellenberg
                                           Title: Director

                                       By: /s/ Chris A. Perks
                                           -------------------------------------
                                           Name: Chris A. Perks
                                           Title: Executive Director

<PAGE>   22

                                       ABN AMRO BANK CANADA
                                       Lender


                                       By: /s/ Mark Bohn
                                           -------------------------------------
                                           Name: Mark Bohn
                                           Title: Group Vice President

                                       By: /s/ Teresa Wu
                                           -------------------------------------
                                           Name: Teresa Wu
                                           Title: Vice President

<PAGE>   23

                                       BAYERISCHE LANDESBANK
                                       GIROZENTRALE, CAYMAN ISLANDS
                                       BRANCH
                                       Lender

                                       By: /s/ Peter Obermann
                                           -------------------------------------
                                           Name: Peter Obermann
                                           Title: Senior Vice President

                                       By: /s/ James H. Boyle
                                           -------------------------------------
                                           Name: James H. Boyle
                                           Title: Vice President

<PAGE>   24

                                       THE FUJI BANK, LIMITED
                                       Lender

                                       By: /s/ Jacques Azagury
                                           -------------------------------------
                                           Name: Jacques Azagury
                                           Title: Senior Vice President &
                                                  Manager

<PAGE>   25

                                       CREDIT LYONNAIS (New York Branch)
                                       Lender


                                       By: /s/ Philippe Soustra
                                           -------------------------------------
                                           Name: Philippe Soustra
                                           Title: Senior Vice President

<PAGE>   26

                                       TOKYO - MITSUBISHI
                                       (CANADA)
                                       Lender

                                       By: /s/ Davis J. Stewart
                                           -------------------------------------
                                           Name: Davis J. Stewart
                                           Title: Vice President