Document:

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                                                                     EXHIBIT 4.1

INCORPORATED UNDER THE LAWS                               COMMON STOCK
 OF THE STATE OF DELAWARE                           PAR VALUE $.01 PER SHARE

                             [PICTURE APPEARS HERE]

THIS CERTIFICATE IS TRANSFERABLE IN                    CUSIP 84855W 10 9
CHICAGO, ILLINOIS AND NEW YORK, NEW YORK     SEE REVERSE FOR CERTAIN DEFINITIONS

                         SPINNAKER EXPLORATION COMPANY

This Certifies that

is the owner of

          FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK OF

Spinnaker Exploration Company, transferable on the books of the Company by the
holder hereof in person or by duly authorized attorney, upon surrender of this
Certificate properly endorsed. This Certificate is not valid until countersigned
by the Transfer Agent and registered by the Registrar.

        Witness the facsimile corporate seal and the facsimile signatures of its
duly authorized officers.

/s/  R. JARVIS                           DATED
          PRESIDENT
                                         COUNTERSIGNED AND REGISTERED
                                           COMPUTERSHARE INVESTOR SERVICES, LLC.
                                                                  TRANSFER AGENT
                                                                   AND REGISTRAR

/s/  J.M. ALEXANDER                      BY
          SECRETARY                                         AUTHORIZED SIGNATURE

NUMBER

SE

[LOGO OF
SPINNAKER
APPEARS
HERE]

SHARES<PAGE>

                                                                    EXHIBIT 10.5

                                                                 August   , 2000

Spinnaker Exploration Company
Two Allen Center
1200 Smith Street, Suite 800
Houston, TX  77002

Credit Suisse First Boston Corporation
Donaldson, Lufkin & Jenrette Securities Corporation
Banc of America Securities LLC
Dain Rauscher Incorporated
Jefferies & Company, Inc.
c/o Credit Suisse First Boston Corporation
600 Travis, Suite 3030
Houston, TX  77002

Dear Sirs:

          As an inducement to the Underwriters to execute the Underwriting
Agreement, pursuant to which an offering will be made of shares of common stock,
par value $.01 per share (the "Securities"), of Spinnaker Exploration Company
(the "Company"), the undersigned hereby agrees that, for a period of 90 days
after the date of the prospectus (the "Commencement Date"), the undersigned will
not offer, sell, contract to sell, pledge or otherwise dispose of, directly or
indirectly, any shares of Securities or securities convertible into or
exchangeable or exercisable for any shares of Securities, or publicly disclose
the intention to make any such offer, sale, pledge or disposal, without the
prior written consent of Credit Suisse First Boston Corporation.

          In furtherance of the foregoing, the Company and its transfer agent
and registrar are hereby authorized to decline to make any transfer of shares of
Securities if such transfer would constitute a violation or breach of this
agreement.

          This agreement shall be binding on the undersigned and the respective
successors, heirs, personal representatives and assigns of the undersigned.
This agreement shall lapse and become null and void if the Commencement Date
shall not have occurred on or before October 31, 2000.

                                    Very truly yours,

                                    ------------------------------------<PAGE>

                                                                     EXHIBIT 4.1

================================================================================

                             POGO PRODUCING COMPANY

                             ______________________

                                Fourth Amendment

                            Dated as of May 3, 2000

                                       to

                     Amended and Restated Credit Agreement

                           Dated as of August 1, 1997

================================================================================
<PAGE>

           FOURTH AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT

     THIS FOURTH AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT, dated as of
May 3, 2000 (the "Amendment"), among POGO PRODUCING COMPANY, a Delaware
                  ---------
corporation (the "Borrower"), the various financial institutions which are or
                  --------
may become parties to the Credit Agreement, as amended hereby (collectively, the
"Lenders"), BANK OF MONTREAL, acting through its Chicago, Illinois branch, (the
 -------
"Bank"), as administrative agent (the "Agent") for the Lenders, and PARIBAS,
 ----                                  -----
formerly known as Banque Paribas, as documentation agent (either the
"Documentation Agent" or "Co-Agent", and together with the Agent, the "Agents"),
--------------------      --------                                     ------
for the Lenders,

                              W I T N E S S E T H

     WHEREAS the Borrower, the Lenders and the Agents are parties to a certain
Amended and Restated Credit Agreement, dated as of August 1, 1997, as previously
amended (the "Credit Agreement"); and
              ----------------

     WHEREAS the Borrower desires to amend certain provisions of the Credit
Agreement;

     NOW, THEREFORE, the parties hereto hereby agree as follows:

     1.   DEFINITIONS.  Unless otherwise defined herein or the context otherwise
requires, or except as the definition may be amended by this Amendment, terms
used in this Amendment, including its preamble and recitals, shall have the
meanings provided in the Credit Agreement, as hereby amended.

     2.   AMENDMENT TO CREDIT AGREEMENT.  Subsection 8.4(a) of the Credit
                                          -----------------
Agreement is amended hereby by replacing "525,000,000" with "$565,000,000".

     3.   REPRESENTATIONS AND WARRANTIES.

     In order to induce the Lenders and the Agents to enter into this Amendment,
the Borrower hereby reaffirms, as of the date hereof, its representations and
warranties contained in Article VI of the Credit Agreement (except to the extent
                        ----------
any such representation and warranty relates solely to an earlier date) and
additionally represents and warrants as follows:

     3.1  Organization.  The Borrower and each of its corporate Subsidiaries is
          ------------
a corporation validly organized and existing and in good standing under the laws
of the state, or country, of its incorporation, and is duly qualified to do
business and is in good standing as a foreign corporation in each jurisdiction
where the nature of its business requires such qualification, except where
failure to qualify would not have a material adverse effect on the business or
financial condition of the Borrower and its Subsidiaries taken as a whole or the
Borrower's ability to perform the Loan Documents, as such may be amended hereby,
or this Amendment.  Each of the Borrower's Subsidiaries which is organized as a
partnership is validly organized and existing and in good standing under the
laws of the state of its formation, and is duly qualified to do business and is
in good
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standing as a foreign partnership where the nature of its business requires such
qualification, except where failure to qualify would not have a material adverse
effect on the business or financial condition of the Borrower, or the Borrower
and its Subsidiaries taken as a whole or the Borrower's ability to perform under
the Loan Documents, as such may be amended hereby, or this Amendment. The
Borrower and each of its Subsidiaries has full power and authority and holds all
requisite governmental licenses, permits and other approvals to enter into and
perform its Obligations under the Credit Agreement, as amended hereby, each
other Loan Document and this Amendment and to own and hold under lease its
property and to conduct its business substantially as currently conducted by it.

     3.2  Due Authorization, Non-Contravention.  The execution, delivery and
          ------------------------------------
performance by the Borrower of this Amendment and the consummation of the
transactions contemplated hereby and by the Credit Agreement as so amended, are
within the Borrower's corporate powers, have been duly authorized by all
necessary corporate action, and do not

               (a) contravene the Borrower's Organic Documents;

               (b) contravene any contractual restriction, law or governmental
     regulation or court decree or order binding on or affecting the Borrower or
     any Subsidiary; or

               (c) result in, or require the creation or imposition of, any Lien
     on any properties of the Borrower or its Subsidiaries except as Liens will
     be imposed, created, or required upon execution and delivery of the
     Security Documents pursuant to Section 7.11 of the Credit Agreement.
                                    ------------

     3.3  Governmental Approval.  No authorization or approval or other action
          ---------------------
by, and no notice to or filing with, any governmental authority or regulatory
body is required for the due execution, delivery or performance by the Borrower
of this Amendment.

     3.4  Validity, etc.  This Amendment and the Credit Agreement as amended
          --------------
hereby constitute the legal, valid and binding obligations of the Borrower,
enforceable in accordance with their respective terms except as such
enforceability is subject to the effect of (i) any applicable bankruptcy,
insolvency, reorganization or similar law relating to or affecting creditors'
rights generally and (ii) general principles of equity (regardless of whether
such enforceability is considered in a proceeding in equity or at law),
including concepts of materiality, reasonableness, good faith and fair dealing.

     4.   EFFECT OF AMENDMENT.

     This Amendment shall be deemed to be an amendment to the Credit Agreement,
and the Credit Agreement, as amended hereby, is hereby ratified, approved and
confirmed in each and every respect.  All references to the Credit Agreement in
any other document, instrument, agreement or writing shall hereafter be deemed
to refer to the Credit Agreement as amended hereby.

                                       2
<PAGE>

     5.   GOVERNING LAW, SEVERABILITY, ETC.

     THIS AMENDMENT SHALL BE A CONTRACT MADE UNDER AND GOVERNED BY THE INTERNAL
LAWS OF THE STATE OF ILLINOIS.  Whenever possible each provision of this
Amendment shall be interpreted in such manner as to be effective and valid under
applicable law, but if any provision of this Amendment shall be prohibited by or
invalid under applicable law, such provision shall be ineffective to the extent
of such prohibition or invalidity, without invalidating the remainder of such
provision or the remaining provisions of this Amendment.

     THIS WRITTEN AMENDMENT AND THE CREDIT AGREEMENT AS AMENDED BY THIS
AMENDMENT REPRESENT THE FINAL AGREEMENT AMONG THE PARTIES AND MAY NOT BE
CONTRADICTED BY PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OF THE
PARTIES.  THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

     6.   MISCELLANEOUS.

     6.1  Successors and Assigns.  This Amendment shall be binding upon and
          ----------------------
shall inure to the benefit of the parties hereto and their respective successors
and assigns.

     6.2  Counterparts.  This Amendment may be executed in one or more
          ------------
counterparts, each of which shall be deemed an original but all of which
together shall constitute one and the same instrument.

     6.3  Effectiveness.  This Amendment shall become effective when
          -------------
counterparts hereof executed on behalf of the Borrower and each Lender (or
notice thereof satisfactory to the Agent) shall have been received by the Agent.

                                       3
<PAGE>

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

                              POGO PRODUCING COMPANY

                              By: /s/ James P. Ulm II
                                 -----------------------------------------------
                              Name:  James P. Ulm II
                              Title: Vice President and Chief Financial Officer

                                      S-1
<PAGE>

                              BANK OF MONTREAL, acting through its U.S. branches
                              and agencies, including initially its Chicago
                              Illinois branch, as Agent

                              By: /s/ Sara J. Teasdale
                                 ----------------------------------------
                              Name: Sara J. Teasdale
                              Title: Director

                                      S-2
<PAGE>

                              PARIBAS, formerly known as Banque Paribas, as
                              Documentation Agent

                              By: /s/ Barton D. Schouest
                                 -------------------------------------
                              Name:  Barton D. Schouest
                              Title: Managing Director

                              By: /s/ A. David Dodd
                                 --------------------------------------
                              Name:  A. David Dodd
                              Title: Vice President

                                      S-3
<PAGE>

                              BANK OF MONTREAL, as a Lender

                              By: /s/   Melissa Bauman
                                 -----------------------------------------
                              Name: Melissa Bauman
                              Title: Director

                                      S-4
<PAGE>

                              PARIBAS, formerly known as Banque Paribas, as a
                              Lender

                              By: /s/ Barton D. Schouest
                                 -----------------------------------------
                              Name:  Barton D. Schouest
                              Title: Managing Director

                              By: /s/ A. David Dodd
                                 -----------------------------------------
                              Name:  A. David Dodd
                              Title: Vice President

                                      S-5
<PAGE>

                              FLEET NATIONAL BANK, formerly BankBoston, N.A., as
                              a Lender

                              By: /s/ Terrence Ronan
                                 -----------------------------------------
                              Name:  Terrence Ronan
                              Title: Director

                                      S-6
<PAGE>

                              BANK OF AMERICA, N.A., formerly NationsBank, N.A.,
                              as a Lender

                              By: /s/ Mary Louise Allen
                                 -----------------------------------------
                              Name:  Mary Louise Allen
                              Title: Vice President

                                      S-7
<PAGE>

                              ABN AMRO BANK N.V., as a Lender

                              By: /s/ Jamie A. Conn
                                 -----------------------------------------
                              Name:  Jamie A. Conn
                              Title: Vice President

                              By: /s/ Matt McCain
                                 -----------------------------------------
                              Name:  Matt McCain
                              Title: Assistant Vice President

                                      S-8
<PAGE>

                              SOCIETE GENERALE, as a Lender

                              By: /s/ Paul E. Cornell
                                 -----------------------------------------
                              Name:  Paul E. Cornell
                              Title: Managing Director

                                      S-9
<PAGE>

                              TORONTO DOMINION (TEXAS), INC., as a Lender

                              By: /s/ Carol Brandt
                                 -----------------------------------------
                              Name:  Carol Brandt
                              Title: Vice President

                                      S-10
<PAGE>

                              THE SANWA BANK LIMITED, NEW YORK BRANCH, as a
                              Lender

                              By: /s/ C. Lawrence Murphy
                                 -----------------------------------------
                              Name:  C. Lawrence Murphy
                              Title: Senior Vice President

                                      S-11

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