<PAGE>   1


                                   EXHIBIT 4.15


                               GUARANTEE AGREEMENT

                                     Between

                               SEMCO ENERGY, INC.
                                 (as Guarantor)
                                       and
                           BANK ONE TRUST COMPANY, NA
                                  (as Trustee)

                       Dated as of ________________, 1999


<PAGE>   2



                               TABLE OF CONTENTS*

                                                                          Page

ARTICLE I  DEFINITIONS....................................................1

   SECTION 1.01.    Definitions...........................................1

ARTICLE II  TRUST INDENTURE ACT...........................................3

   SECTION 2.01.    Trust Indenture Act; Application......................3
   SECTION 2.02.    Lists of Holders of Securities........................3
   SECTION 2.03.    Reports by the Trustee................................4
   SECTION 2.04.    Periodic Reports to Trustee...........................4
   SECTION 2.05.    Evidence of Compliance with Conditions Precedent......4
   SECTION 2.06.    Events of Default; Waiver.............................4
   SECTION 2.07.    Event of Default; Notice..............................4
   SECTION 2.08.    Conflicting Interests.................................5

ARTICLE III  POWERS, DUTIES AND RIGHTS OF TRUSTEE.........................5

   SECTION 3.01.    Powers and Duties of the Trustee......................5
   SECTION 3.02.    Certain Rights of Trustee.............................6
   SECTION 3.03.    Compensation; Fees; Indemnity.........................8

ARTICLE IV  TRUSTEE.......................................................8

   SECTION 4.01.    Trustee; Eligibility..................................8
   SECTION 4.02.    Appointment, Removal and Resignation of Trustee.......9

ARTICLE V  GUARANTEE......................................................9

   SECTION 5.01.    Guarantee.............................................9
   SECTION 5.02.    Waiver of Notice and Demand...........................9
   SECTION 5.03.    Obligations Not Affected..............................10
   SECTION 5.04.    Rights of Holders.....................................10
   SECTION 5.05.    Guarantee of Payment..................................11
   SECTION 5.06.    Subrogation...........................................11
   SECTION 5.07.    Independent Obligations...............................11

ARTICLE VI   SUBORDINATION................................................11

   SECTION 6.01.    Subordination.........................................11



----------------------------
* This Table of Contents does not constitute part of the Indenture or have any
bearing upon the interpretation of any of its terms and provisions.


                                        i
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ARTICLE VII  TERMINATION..................................................12

   SECTION 7.01.    Termination...........................................12

ARTICLE VIII  MISCELLANEOUS...............................................12

   SECTION 8.01.    Successors and Assigns................................12
   SECTION 8.02.    Amendments............................................12
   SECTION 8.03.    Notices...............................................12
   SECTION 8.04.    Benefit...............................................13
   SECTION 8.05.    Interpretation........................................13
   SECTION 8.06.    Governing Law.........................................14



















                                       ii
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                             CROSS-REFERENCE TABLE*



Section of                                                            Section of
Trust Indenture Act                                                    Guarantee
of 1939, as amended                                                    Agreement

310(a)...................................................................4.01(a)
310(b).............................................................4.01(c), 2.08
310(c)..............................................................Inapplicable
311(a)...................................................................2.02(b)
311(b)...................................................................2.02(b)
311(c)..............................................................Inapplicable
312(a)...................................................................2.02(a)
312(b)...................................................................2.02(b)
313.........................................................................2.03
314(a)......................................................................2.04
314(b)..............................................................Inapplicable
314(c)......................................................................2.05
314(d)..............................................................Inapplicable
314(e)..........................................................1.01, 2.05, 3.02
314(f)................................................................2.01, 3.02
315(a)...................................................................3.01(d)
315(b)......................................................................2.07
315(c)......................................................................3.01
315(d)...................................................................3.01(d)
315(e)..............................................................Inapplicable
316(a).............................................................5.04(i), 2.06
316(b)......................................................................5.03
316(c)......................................................................2.02
317(a)..............................................................Inapplicable
317(b)..............................................................Inapplicable
318(a)...................................................................2.01(b)
318(b)......................................................................2.01
318(c)...................................................................2.01(a)







------------------------
* This Cross-Reference Table does not constitute part of the Guarantee Agreement
and shall not affect the interpretation of any of its terms or provisions.

<PAGE>   5

                               GUARANTEE AGREEMENT

                  This GUARANTEE AGREEMENT ("Guarantee Agreement"), dated as of
_________, 1999 is executed and delivered by SEMCO ENERGY, INC., a Michigan
corporation (the "Guarantor"), and Bank One Trust Company, NA, a national
banking association, as trustee (the "Trustee"), for the benefit of the Holders
(as defined herein) from time to time of the Trust Preferred Securities (as
defined herein) of SEMCO CAPITAL TRUST I, a Delaware statutory business trust
(the "Trust ").

                  WHEREAS, pursuant to an Amended and Restated Trust Agreement
(the "Trust Agreement"), dated as of _______________, 1999, among the Trustee,
the other Trustees named therein, and SEMCO Energy, Inc., as Depositor, and the
holders of undivided beneficial interests in the assets of the Trust, the Trust
is issuing as of the date hereof $___________ aggregate liquidation amount of
its _____% Trust Originated Trust Preferred Securities (the "Trust Preferred
Securities") representing preferred undivided beneficial interests in the assets
of the Trust and having the terms set forth in the Trust Agreement;

                  WHEREAS, the Trust Preferred Securities will be issued by the
Trust and the proceeds thereof will be used to purchase Subordinated Debentures
(as defined in the Trust Agreement) of the Guarantor, which will be held by the
Trust as trust assets; and

                  WHEREAS, as incentive for the Holders to purchase the Trust
Preferred Securities, the Guarantor desires to irrevocably and unconditionally
agree, to the extent set forth herein, to pay to the Holders the Guarantee
Payments (as defined herein) and to make certain other payments on the terms and
conditions set forth herein.

                  NOW, THEREFORE, in consideration of the payment for Trust
Preferred Securities by each Holder thereof, which payment the Guarantor hereby
agrees shall benefit the Guarantor, the Guarantor executes and delivers this
Guarantee Agreement for the benefit of the Holders from time to time of the
Trust Preferred Securities.

                                    ARTICLE I

                                   DEFINITIONS

                  SECTION 1.01.     Definitions

                  As used in this Guarantee Agreement, the terms set forth below
shall, unless the context otherwise requires, have the following meanings.
Capitalized or otherwise defined terms used but not otherwise defined herein
shall have the meanings assigned to such terms in the Trust Agreement as in
effect on the date hereof.

                  "Affiliate" of any specified Person means any other Person
directly or indirectly controlling or controlled by or under direct or indirect
common control with such specified Person. For the purposes of this definition,
"control" when used with respect to any specified Person means the power to
direct the management and policies of such Person, directly or indirectly,
whether through the ownership of voting securities, by contract or otherwise;
and the terms "controlling" and "controlled" have meanings correlative to the
foregoing.



<PAGE>   6

                  "Event of Default" means a failure by the Guarantor to perform
any of its payment obligations under this Guarantee Agreement.

                  "Guarantee Payments" shall mean the following payments or
distributions, without duplication, with respect to the Trust Preferred
Securities, to the extent not paid or made by or on behalf of the Trust: (i) any
accrued and unpaid distributions that are required to be paid on such Trust
Preferred Securities but if and only if and to the extent the Trust has funds
legally and immediately available therefor to make such payment; (ii) the
redemption price, including all accrued and unpaid distributions to the date of
redemption (the "Redemption Price"), with respect to the Trust Preferred
Securities called for redemption by the Trust but if and only if and to the
extent that the Trust has funds legally and immediately available therefor
sufficient to make such payment; and (iii) upon a voluntary or involuntary
dissolution, winding-up or termination of the Trust (other than in connection
with the distribution of Subordinated Debentures to the holders of Trust
Securities or the redemption of all of the Trust Preferred Securities), the
lesser of (a) the aggregate of the liquidation amount and all accrued and unpaid
distributions on the Trust Preferred Securities to the date of payment, to the
extent the Trust has funds legally and immediately available therefor, and (b)
the amount of assets of the Trust remaining available for distribution to
Holders in liquidation of the Trust (in either case, the "Liquidation
Distribution").

                  "Holder" shall mean any holder, as registered on the books and
records of the Trust, of any Trust Preferred Securities; provided, however, that
in determining whether the holders of the requisite percentage of Trust
Preferred Securities have given any request, notice, consent or waiver
hereunder, "Holder" shall not include the Guarantor or any Affiliate of the
Guarantor.

                  "Indenture" means the Subordinated Indenture, dated as of
________ __, 199__, among the Guarantor, as issuer, and Bank One Trust Company,
NA, as trustee, as supplemented by the First Supplemental Indenture dated as of
_______________, 1999, by and between the Guarantor and Bank One Trust Company,
NA, as trustee.

                  "Majority in liquidation amount of Trust Preferred Securities"
means a vote by Holder(s) of Trust Preferred Securities, voting separately as a
class, of more than 50% of the liquidation amount of all Trust Preferred
Securities outstanding at the time of determination.

                  "Officers' Certificate" means a certificate signed by the
Chairman of the Board, the President or a Vice President, and by the Treasurer,
an Assistant Treasurer, the Secretary or an Assistant Secretary, of the
Guarantor, and delivered to the Trustee. Any Officers' Certificate delivered
with respect to compliance with a condition or covenant provided for in this
Guarantee Agreement shall include:

                  (a) a statement that each officer signing the Officers'
Certificate has read such covenant or condition and the definitions herein
relating thereto;

                  (b) a brief statement as to the nature and scope of the
examination or investigation upon which the statements or opinions contained in
the Officers' Certificate are based;

                  (c) a statement that, in the opinion of each such officer, he
has made such examination or investigation as is necessary to enable him to
express an informed opinion as to whether or not such covenant or condition has
been complied with; and


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<PAGE>   7

                  (d) a statement as to whether, in the opinion of each such
officer, such condition or covenant has been complied with.

                  "Person" means any individual, corporation, partnership,
limited liability company, joint venture, trust, unincorporated organization or
government or any agency or political subdivision thereof.

                  "Responsible Officer" means, with respect to the Trustee, any
Vice President, any Assistant Vice President, the Secretary, any Assistant
Secretary, the Treasurer, any Assistant Treasurer, any Senior Trust Officer,
Trust Officer or Assistant Trust Officer or any other officer of the Corporate
Trust Department of the Trustee customarily performing functions similar to
those performed by any of the above designated officers and also means, with
respect to a particular corporate trust matter, any other officer to whom such
matter is referred because of that officer's knowledge of and familiarity with
the particular subject.

                  "Successor Trustee" means a successor Trustee possessing the
qualifications to act as Trustee under Section 4.01.

                  "Trust Common Securities" means the securities representing
common undivided beneficial interests in the assets of the Trust.

                  "Trust Indenture Act" means the Trust Indenture Act of 1939,
as amended.

                  "Trust Securities" means the Trust Preferred Securities and
the Trust Common Securities.

                  "Trustee" means Bank One Trust Company, NA until a Successor
Trustee has been appointed and has accepted such appointment pursuant to the
terms of this Guarantee Agreement and thereafter means each such Successor
Trustee.

                                   ARTICLE II

                               TRUST INDENTURE ACT

                  SECTION 2.01.     Trust Indenture Act; Application.

                  (a) This Guarantee Agreement is subject to the provisions of
the Trust Indenture Act that are required to be part of this Guarantee Agreement
and shall, to the extent applicable, be governed by such provisions; and

                  (b) If and to the extent that any provision of this Guarantee
Agreement limits, qualifies or conflicts with the duties imposed by Sections 310
to 317, inclusive, of the Trust Indenture Act, such imposed duties shall
control.

                  SECTION 2.02. Lists of Holders of Securities.

                  (a) The Guarantor shall furnish or cause to be furnished to
the Trustee (a) semiannually, not later than June 1 and December 1 in each year,
a list, in such form as the Trustee may reasonably require, of the names and
addresses of the Holders ("List of Holders") as of a date not more than 15 days
prior to the time such list is furnished, and (b) at such other times as the


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<PAGE>   8

Trustee may request in writing, within 30 days after the receipt by the
Guarantor of any such request, a List of Holders as of a date not more than 15
days prior to the time such list is furnished; provided that, the Guarantor
shall not be obligated to provide such List of Holders at any time the List of
Holders does not differ from the most recent List of Holders given to the
Trustee by the Guarantor or at any time the Trustee is the Securities Registrar
under the Trust Agreement. The Trustee may destroy any List of Holders
previously given to it on receipt of a new List of Holders.

                  (b) The Trustee shall comply with its obligations under
Sections 311(a), 311(b) and 312(b) of the Trust Indenture Act.

                  SECTION 2.03.     Reports by the Trustee

                  Within 60 days after May 15 of each year commencing May 15,
2000, the Trustee shall provide to the Holders of the Trust Preferred Securities
such reports as are required by Section 313(a) of the Trust Indenture Act, if
any, in the form and in the manner provided by Section 313 of the Trust
Indenture Act. The Trustee shall also comply with the other requirements of
Section 313 of the Trust Indenture Act.

                  SECTION 2.04.     Periodic Reports to Trustee

                  The Guarantor shall provide to the Trustee such documents,
reports and information as required by Section 314 of the Trust Indenture Act
(if any) in the form, in the manner and at the times required by Section 314 of
the Trust Indenture Act, and shall provide, within 120 days after the end of
each fiscal year of the Guarantor, the compliance certificate required by
Section 314(a)(4) of the Trust Indenture Act in the form and in the manner
required by such Section.

                  SECTION 2.05.     Evidence of Compliance with Conditions
                                    Precedent

                  The Guarantor shall provide to the Trustee such evidence of
compliance with any conditions precedent, if any, provided for in this Guarantee
Agreement that relate to any of the matters set forth in Section 314(c) of the
Trust Indenture Act. Any certificate or opinion required to be given by an
officer pursuant to Section 314(c)(1) may be given in the form of an Officers'
Certificate.

                  SECTION 2.06.     Events of Default; Waiver

                  The Holders of a Majority in liquidation amount of Trust
Preferred Securities may, by vote, on behalf of all of the Holders, waive any
past Event of Default and its consequences. Upon such waiver, any such Event of
Default shall cease to exist, and any Event of Default arising therefrom shall
be deemed to have been cured and not to have existed, for every purpose of this
Guarantee Agreement, but no such waiver shall extend to any subsequent or other
default or Event of Default or impair any right consequent thereon.

                  SECTION 2.07. Event of Default; Notice.

                  (a) The Trustee shall, within 90 days after the occurrence of
an Event of Default actually known to the Trustee, transmit by mail, first class
postage prepaid, to the Holders, notices of all such Events of Default, unless
such defaults have been cured before the giving of such notice,

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<PAGE>   9

provided that the Trustee shall be protected in withholding such notice if and
so long as the board of directors, the executive committee, or a trust committee
of directors and/or Responsible Officers of the Trustee in good faith determines
that the withholding of such notice is in the interests of the Holders.

                  (b) The Trustee shall not be deemed to have knowledge of any
Event of Default unless the Trustee shall have received written notice, or a
Responsible Officer charged with the administration of the Trust Agreement shall
have obtained written notice, of such Event of Default.

                  SECTION 2.08.     Conflicting Interests

                  The Trust Agreement shall be deemed to be specifically
described in this Guarantee Agreement for the purposes of clause (i) of the
first proviso contained in Section 310(b) of the Trust Indenture Act.

                                   ARTICLE III

                      POWERS, DUTIES AND RIGHTS OF TRUSTEE

                  SECTION 3.01. Powers and Duties of the Trustee.

                  (a) This Guarantee Agreement shall be held by the Trustee for
the benefit of the Holders, and the Trustee shall not transfer this Guarantee
Agreement to any Person except the Trustee shall assign rights hereunder to a
Holder to the extent such assignment is necessary to exercise such Holder's
rights pursuant to Section 5.04 or to a Successor Trustee upon acceptance by
such Successor Trustee of its appointment to act as Successor Trustee. The
right, title and interest of the Trustee shall automatically vest in any
Successor Trustee, and such vesting and cessation of title shall be effective
whether or not conveyancing documents have been executed and delivered pursuant
to the appointment of such Successor Trustee.

                  (b) If an Event of Default has occurred and is continuing, the
Trustee shall enforce this Guarantee Agreement for the benefit of the Holders.

                  (c) The Trustee, before the occurrence of any Event of Default
and after the curing or waiving of all Events of Default that may have occurred,
shall undertake to perform only such duties as are specifically set forth in
this Guarantee Agreement, and no implied covenants shall be read into this
Guarantee Agreement against the Trustee. In case an Event of Default has
occurred (that has not been cured or waived pursuant to Section 2.06), the
Trustee shall exercise such of the rights and powers vested in it by this
Guarantee Agreement, and use the same degree of care and skill in its exercise
thereof, as a prudent person would exercise or use under the circumstances in
the conduct of his or her own affairs.

                  (d) No provision of this Guarantee Agreement shall be
construed to relieve the Trustee from liability for its own negligent action,
its own negligent failure to act, or its own willful misconduct, except that:

                  (i) prior to the occurrence of any Event of Default and after
         the curing or waiving of all such Events of Default that may have
         occurred:


                                      -5-
<PAGE>   10

                           (A) the duties and obligations of the Trustee shall
                  be determined solely by the express provisions of this
                  Guarantee Agreement, and the Trustee shall not be liable
                  except for the performance of such duties and obligations as
                  are specifically set forth in this Guarantee Agreement; and

                           (B) in the absence of bad faith on the part of the
                  Trustee, the Trustee may conclusively rely, as to the truth of
                  the statements and the correctness of the opinions expressed
                  therein, upon any certificates or opinions furnished to the
                  Trustee and conforming to the requirements of this Guarantee
                  Agreement; but in the case of any such certificates or
                  opinions that by any provision hereof are specifically
                  required to be furnished to the Trustee, the Trustee shall be
                  under a duty to examine the same to determine whether or not
                  they conform to the requirements of this Guarantee Agreement;

                  (ii) the Trustee shall not be liable for any error of judgment
         made in good faith by a Responsible Officer of the Trustee, unless it
         shall be proved that the Trustee was negligent in ascertaining the
         pertinent facts upon which such judgment was made;

                  (iii) the Trustee shall not be liable with respect to any
         action taken or omitted to be taken by it in good faith in accordance
         with the direction of the Holders of a Majority in liquidation amount
         of the Trust Preferred Securities relating to the time, method and
         place of conducting any proceeding for any remedy available to the
         Trustee, or exercising any trust or power conferred upon the Trustee
         under this Guarantee Agreement; and

                  (iv) no provision of this Guarantee Agreement shall require
         the Trustee to expend or risk its own funds or otherwise incur personal
         financial liability in the performance of any of its duties or in the
         exercise of any of its rights or powers, if the Trustee shall have
         reasonable grounds for believing that the repayment of such funds or
         liability is not reasonably assured to it under the terms of this
         Guarantee Agreement or adequate indemnity against such risk or
         liability is not reasonably assured to it.

                  SECTION 3.02. Certain Rights of Trustee.

                  (a)      Subject to the provisions of Section 3.01:

                  (i) the Trustee may rely and shall be fully protected in
         acting or refraining from acting upon any resolution, certificate,
         statement, instrument, opinion, report, notice, request, direction,
         consent, order, bond, debenture, note, other evidence of indebtedness
         or other paper or document believed by it to be genuine and to have
         been signed, sent or presented by the proper party or parties;

                  (ii) any direction or act of the Guarantor contemplated by
         this Guarantee Agreement shall be sufficiently evidenced by an
         Officers' Certificate;

                  (iii) whenever, in the administration of this Guarantee
         Agreement, the Trustee shall deem it desirable that a matter be proved
         or established before taking, suffering or omitting any action
         hereunder, the Trustee (unless other evidence is herein specifically
         prescribed) may, in the absence of bad faith on its part, request and
         rely upon an Officers'

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<PAGE>   11

         Certificate which, upon receipt of such request, shall be promptly
         delivered by the Guarantor;

                  (iv) the Trustee may consult with counsel of its choice, and
         the written advice or opinion of such counsel with respect to legal
         matters shall be full and complete authorization and protection in
         respect of any action taken, suffered or omitted by it hereunder in
         good faith and in accordance with such advice or opinion; such counsel
         may be counsel to the Guarantor or any of its Affiliates and may
         include any of its employees; the Trustee shall have the right at any
         time to seek instructions concerning the administration of this
         Guarantee Agreement from any court of competent jurisdiction;

                  (v) the Trustee shall be under no obligation to exercise any
         of the rights or powers vested in it by this Guarantee Agreement at the
         request or direction of any Holder, unless such Holder shall have
         provided to the Trustee such adequate security and indemnity as would
         satisfy a reasonable person in the position of the Trustee, against the
         costs, expenses (including attorneys' fees and expenses) and
         liabilities that might be incurred by it in complying with such request
         or direction, including such reasonable advances as may be requested by
         the Trustee; provided that nothing contained in this Section 3.02(a)(v)
         shall be taken to relieve the Trustee, upon the occurrence of an Event
         of Default, of its obligation to exercise the rights and powers vested
         in it by this Guarantee Agreement;

                  (vi) the Trustee shall not be bound to make any investigation
         into the facts or matters stated in any resolution, certificate,
         statement, instrument, opinion, report, notice, request, direction,
         consent, order, bond, debenture, note, other evidence of indebtedness
         or other paper or document, but the Trustee, in its discretion, may
         make such further inquiry or investigation into such facts or matters
         as it may see fit;

                  (vii) the Trustee may execute any of the trusts or powers
         hereunder or perform any duties hereunder either directly or by or
         through agents or attorneys, and the Trustee shall not be responsible
         for any misconduct or negligence on the part of any agent or attorney
         appointed with due care by it hereunder; and

                  (viii) whenever in the administration of this Guarantee
         Agreement the Trustee shall deem it desirable to receive instructions
         with respect to enforcing any remedy or right or taking any other
         action hereunder, the Trustee (i) may request instructions from the
         Holders, (ii) may refrain from enforcing such remedy or right or taking
         such other action until such instructions are received, and (iii) shall
         be protected in acting in accordance with such instructions.

                  (b) No provision of this Guarantee Agreement shall be deemed
to impose any duty or obligation on the Trustee to perform any act or acts or
exercise any right, power, duty or obligation conferred or imposed on it in any
jurisdiction in which it shall be illegal, or in which the Trustee shall be
unqualified or incompetent in accordance with applicable law, to perform any
such act or acts or to exercise any such right, power, duty or obligation. No
permissive power or authority available to the Trustee shall be construed to be
a duty.


                                      -7-
<PAGE>   12


                  SECTION 3.03.     Compensation; Fees; Indemnity.

                  The Guarantor agrees:

                  (a) to pay to the Trustee from time to time reasonable
compensation for all services rendered by the Trustee hereunder (which
compensation shall not be limited by any provision of law in regard to the
compensation of a trustee of an express trust);

                  (b) except as otherwise expressly provided herein, to
reimburse the Trustee upon request for all reasonable expenses, disbursements
and advances incurred or made by the Trustee in accordance with any provision of
this Guarantee Agreement (including the reasonable compensation and the expenses
and disbursements of its agents and counsel), except any such expense,
disbursement or advance as may be attributable to its negligence or bad faith;
and

                  (c) to indemnify the Trustee for, and to hold the Trustee
harmless against, any and all loss, damage, claims, liability or expense
incurred without negligence or bad faith on its part, arising out of or in
connection with the acceptance or administration of this Guarantee Agreement,
including the costs and expenses of defending itself against any claim or
liability in connection with the exercise or performance of any of its powers or
duties hereunder.

                  The provisions of this Section 3.03 shall survive the
termination of this Guarantee Agreement or the registration or removal of the
Trustee.

                                   ARTICLE IV

                                     TRUSTEE

                  SECTION 4.01. Trustee; Eligibility.

                  (a)      There shall at all times be a Trustee which shall:

                  (i)      not be an Affiliate of the Guarantor; and

                  (ii) be a corporation organized and doing business under the
         laws of the United States of America or any State or Territory thereof
         or of the District of Columbia, or a corporation or Person permitted by
         the Securities and Exchange Commission to act as an institutional
         trustee under the Trust Indenture Act, authorized under such laws to
         exercise corporate trust powers, having a combined capital and surplus
         of at least 50 million U.S. dollars ($50,000,000), and subject to
         supervision or examination by Federal, State, Territorial or District
         of Columbia authority. If such corporation publishes reports of
         condition at least annually, pursuant to law or to the requirements of
         the supervising or examining authority referred to above, then, for the
         purposes of this Section 4.01(a)(ii), the combined capital and surplus
         of such corporation shall be deemed to be its combined capital and
         surplus as set forth in its most recent report of condition so
         published.

                  (b) If at any time the Trustee shall cease to be eligible to
so act under Section 4.01(a), the Trustee shall immediately resign in the manner
and with the effect set out in Section 4.02(c).


                                      -8-
<PAGE>   13

                  (c) If the Trustee has or shall acquire any "conflicting
interest" within the meaning of Section 310(b) of the Trust Indenture Act, the
Trustee and Guarantor shall in all respects comply with the provisions of
Section 310(b) of the Trust Indenture Act, subject to the rights of the Trustee
under the penultimate paragraph thereof.

                  SECTION 4.02. Appointment, Removal and Resignation of Trustee.

                  (a) Subject to Section 4.02(b), the Trustee may be appointed
or removed without cause at any time by the Guarantor.

                  (b) The Trustee shall not be removed until a Successor Trustee
has been appointed and has accepted such appointment by written instrument
executed by such Successor Trustee and delivered to the Guarantor.

                  (c) The Trustee appointed to office shall hold office until a
Successor Trustee shall have been appointed or until its removal or resignation.
The Trustee may resign from office (without need for prior or subsequent
accounting) by an instrument in writing executed by the Trustee and delivered to
the Guarantor, which resignation shall not take effect until a Successor Trustee
has been appointed and has accepted such appointment by instrument in writing
executed by such Successor Trustee and delivered to the Guarantor and the
resigning Trustee.

                  (d) If no Successor Trustee shall have been appointed and
accepted appointment as provided in this Section 4.02 within 60 days after
delivery to the Guarantor of an instrument of resignation, the resigning Trustee
may petition any court of competent jurisdiction for appointment of a Successor
Trustee. Such court may thereupon, after prescribing such notice, if any, as it
may deem proper, appoint a Successor Trustee.

                                    ARTICLE V

                                    GUARANTEE

                  SECTION 5.01.     Guarantee

                  The Guarantor irrevocably and unconditionally agrees to pay in
full to the Holders the Guarantee Payments (without duplication of amounts
theretofore paid by or on behalf of the Trust), as and when due, regardless of
any defense, right of set-off or counterclaim which the Guarantor may have or
assert against any person. The Guarantor's obligation to make a Guarantee
Payment may be satisfied by direct payment of the required amounts by the
Guarantor to the Holders or by causing the Trust to pay such amounts to the
Holders.

                  SECTION 5.02.     Waiver of Notice and Demand

                  The Guarantor hereby waives notice of acceptance of this
Guarantee Agreement and of any liability to which it applies or may apply,
presentment, demand for payment, any right to require a proceeding first against
the Trust or any other Person before proceeding against the Guarantor, protest,
notice of nonpayment, notice of dishonor, notice of redemption and all other
notices and demands.


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<PAGE>   14


                  SECTION 5.03.     Obligations Not Affected

                  The obligation of the Guarantor to make the Guarantee Payments
under this Guarantee Agreement shall in no way be affected or impaired by reason
of the happening from time to time of any of the following:

                  (a) the release or waiver, by operation of law or otherwise,
of the performance or observance by the Trust of any express or implied
agreement, covenant, term or condition relating to the Trust Preferred
Securities to be performed or observed by the Trust;

                  (b) the extension of time for the payment by the Trust of all
or any portion of the Distributions, Redemption Price, Liquidation Distribution
or any other sums payable under the terms of the Trust Preferred Securities or
the extension of time for the performance of any other obligation under, arising
out of, or in connection with, the Trust Preferred Securities (other than an
extension of time for payment of Distributions, Redemption Price, Liquidation
Distribution or other sum payable that results from the extension of any
interest payment period on the Subordinated Debentures permitted by the
Indenture);

                  (c) any failure, omission, delay or lack of diligence on the
part of the Holders to enforce, assert or exercise any right, privilege, power
or remedy conferred on the Holders pursuant to the terms of the Trust Preferred
Securities, or any action on the part of the Trust granting indulgence or
extension of any kind;

                  (d) the voluntary or involuntary liquidation, dissolution,
sale of any collateral, receivership, insolvency, bankruptcy, assignment for the
benefit of creditors, reorganization, arrangement, composition or readjustment
of debt of, or other similar proceedings affecting, the Trust or any of the
assets of the Trust;

                  (e) any invalidity of, or defect or deficiency in, the Trust
Preferred Securities;

                  (f) the settlement or compromise of any obligation guaranteed
hereby or hereby incurred; or

                  (g) any other circumstance whatsoever that might otherwise
constitute a legal or equitable discharge or defense of a guarantor, it being
the intent of this Section 5.03 that the obligations of the Guarantor hereunder
shall be absolute and unconditional under any and all circumstances.

                  There shall be no obligation of the Holders to give notice to,
or obtain consent of, the Guarantor with respect to the happening of any of the
foregoing.

                  SECTION 5.04.     Rights of Holders

                  The Guarantor expressly acknowledges that: (i) this Guarantee
Agreement will be deposited with the Trustee to be held for the benefit of the
Holders; (ii) the Trustee has the right to enforce this Guarantee Agreement on
behalf of the Holders; (iii) the Holders of a Majority in liquidation amount of
the Trust Preferred Securities have the right to direct the time, method and
place of conducting any proceeding for any remedy available to the Trustee in
respect of this Guarantee Agreement or to direct the exercise of any trust or
power conferred upon the Trustee

                                      -10-
<PAGE>   15

under this Guarantee Agreement, provided that such direction shall not be in
conflict with any rule of law or with this Guarantee Agreement, and could not
involve the Trustee in personal liability in circumstances where reasonable
indemnity would not be adequate; and (iv) any Holder may institute a legal
proceeding directly against the Guarantor to enforce its rights under this
Guarantee Agreement, without first instituting a legal proceeding against or
requesting or directing that action be taken by the Trustee or any other person;
it being understood and intended that no one or more of such Holders shall have
any right in any manner whatsoever by virtue of, or by availing of, any
provision of this Guarantee Agreement to affect, disturb or prejudice the rights
of any other of such Holders or to obtain or to seek to obtain priority or
preference over any other of such Holders or to enforce any right under this
Guarantee Agreement, except in the manner herein provided and for the equal and
ratable benefit of all of such Holders.

                  SECTION 5.05.     Guarantee of Payment

                  This Guarantee Agreement creates a guarantee of payment and
not of collection. This Guarantee Agreement will not be discharged except by
payment of the Guarantee Payments in full (without duplication) or upon the
distribution of Subordinated Debentures to the Holders in exchange for all of
the Trust Preferred Securities.

                  SECTION 5.06.     Subrogation

                  The Guarantor shall be subrogated to all (if any) rights of
the Holders against the Trust in respect of any amounts paid to the Holders by
the Guarantor under this Guarantee Agreement; provided, however, that the
Guarantor shall not (except to the extent required by mandatory provisions of
law) be entitled to enforce or exercise any rights which it may acquire by way
of subrogation or any indemnity, reimbursement or other agreement, in all cases
as a result of payment under this Guarantee Agreement, if, at the time of any
such payment, any amounts of Guarantee Payments are due and unpaid under this
Guarantee Agreement. If any amount shall be paid to the Guarantor in violation
of the preceding sentence, the Guarantor agrees to hold such amount in trust for
the Holders and to pay over such amount to the Holders.

                  SECTION 5.07.     Independent Obligations

                  The Guarantor acknowledges that its obligations hereunder are
independent of the obligations of the Trust with respect to the Trust Preferred
Securities and that the Guarantor shall be liable as principal and as debtor
hereunder to make Guarantee Payments pursuant to the terms of this Guarantee
Agreement notwithstanding the occurrence of any event referred to in subsections
(a) through (g), inclusive, of Section 5.03 hereof.

                                   ARTICLE VI

                                  SUBORDINATION

                  SECTION 6.01.     Subordination

                  This Guarantee Agreement will constitute an unsecured
obligation of the Guarantor and will rank (i) subordinate and junior in right of
payment to all other liabilities of

                                      -11-
<PAGE>   16

the Guarantor, including the Subordinated Debentures, except those obligations
or liabilities made pari passu or subordinate by their terms, (ii) pari passu
with the most senior preferred stock now or hereafter issued or guaranteed by
the Guarantor, and (iii) senior to all common stock of the Guarantor.

                                   ARTICLE VII

                                   TERMINATION

                  SECTION 7.01.     Termination

                  This Guarantee Agreement shall terminate and be of no further
force and effect upon: (i) full payment of the Redemption Price of all Trust
Preferred Securities, (ii) the distribution of Subordinated Debentures to the
Holders in exchange for all of the Trust Preferred Securities, or (iii) full
payment of the amounts payable in accordance with the Trust Agreement upon
liquidation of the Trust. Notwithstanding the foregoing, this Guarantee
Agreement will continue to be effective or will be reinstated, as the case may
be, if at any time any Holder must restore payment of any sums paid with respect
to Trust Preferred Securities or under this Guarantee Agreement.

                                  ARTICLE VIII

                                  MISCELLANEOUS

                  SECTION 8.01.     Successors and Assigns

                  All guarantees and agreements contained in this Guarantee
Agreement shall bind the successors, assigns, receivers, trustees and
representatives of the Guarantor and shall inure to the benefit of the Holders
then outstanding. Except in connection with a consolidation, merger, conveyance,
transfer, or lease involving the Guarantor that is permitted under Article Eight
of the Indenture, the Guarantor shall not assign its obligations hereunder.

                  SECTION 8.02.     Amendments

                  Except with respect to any changes which do not materially and
adversely affect the rights of Holders (in which case no consent of Holders will
be required), this Guarantee Agreement may only be amended with the prior
approval of the Holders of not less than 66-2/3% in liquidation amount of all
the outstanding Trust Preferred Securities. The provisions of Article Six of the
Trust Agreement concerning meetings of Holders shall apply to the giving of such
approval.

                  SECTION 8.03.     Notices

                  Any notice, request or other communication required or
permitted to be given hereunder shall be in writing, duly signed by the party
giving such notice, and delivered, telecopied or mailed by first class mail as
follows:

                  (a) if given to the Guarantor, to the address set forth below
or such other address as the Guarantor may give notice of to the Trustee and the
Holders:

                                      -12-
<PAGE>   17

                  SEMCO Energy, Inc.
                  405 Water Street
                  Port Huron, Michigan 48060
                  Attn: ___________________

                  (b) if given to the Trust, in care of the Trustee, or to the
Trustee at the Trust's (and the Trustee's) address set forth below or such other
address as the Trustee on behalf of the Trust may give notice of to the Holders:

                  SEMCO Capital Trust I

                  c/o [______________________]

with a copy, in the case of a notice to the Trust (other than a notice from the
Guarantor), to the Guarantor.

                  (c) if given to any Holder, at the address set forth on the
books and records of the Trust.

                  All notices hereunder shall be deemed to have been given when
received in person, telecopied with receipt confirmed, or mailed by first class
mail, postage prepaid except that if a notice or other document is refused
delivery or cannot be delivered because of a changed address of which no notice
was given, such notice or other document shall be deemed to have been delivered
on the date of such refusal or inability to deliver.

                  SECTION 8.04.     Benefit

                  This Guarantee Agreement is solely for the benefit of the
Holders and, subject to Section 3.01(a), is not separately transferable from the
Trust Preferred Securities.

                  SECTION 8.05.     Interpretation

                  In this Guarantee Agreement, unless the context otherwise
requires:

                  (a) capitalized terms used in this Guarantee Agreement but not
defined in the preamble hereto have the respective meanings assigned to them in
Section 1.01;

                  (b) a term defined anywhere in this Guarantee Agreement has
the same meaning throughout;

                  (c) all references to "the Guarantee Agreement" or "this
Guarantee Agreement" are to this Guarantee Agreement as modified, supplemented
or amended from time to time;

                  (d) all references in this Guarantee Agreement to Articles and
Sections are to Articles and Sections of this Guarantee Agreement unless
otherwise specified;

                  (e) a term defined in the Trust Indenture Act has the same
meaning when used in this Guarantee Agreement unless otherwise defined in this
Guarantee Agreement or unless the context otherwise requires;


                                      -13-
<PAGE>   18

                  (f) a reference to the singular includes the plural and vice
versa; and

                  (g) the masculine, feminine or neuter genders used herein
shall include the masculine, feminine and neuter genders.

                  SECTION 8.06. Governing Law.

                  THIS GUARANTEE AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED
AND INTERPRETED IN ACCORDANCE WITH THE INTERNAL LAWS OF THE STATE OF NEW YORK.
THE GUARANTOR HEREBY IRREVOCABLY SUBMITS TO THE JURISDICTION OF THE UNITED
STATES DISTRICT COURT FOR THE SOUTHERN DISTRICT OF NEW YORK AND ANY COURT IN THE
STATE OF NEW YORK LOCATED IN THE CITY AND COUNTY OF NEW YORK IN ANY ACTION, SUIT
OR PROCEEDING BROUGHT AGAINST IT AND RELATED TO OR IN CONNECTION WITH THIS
GUARANTEE AGREEMENT OR THE TRANSACTIONS CONTEMPLATED THEREBY, AND TO THE EXTENT
PERMITTED BY APPLICABLE LAW, THE GUARANTOR HEREBY WAIVES AND AGREES NOT TO
ASSERT BY WAY OF MOTION, AS A DEFENSE OR OTHERWISE IN ANY SUCH SUIT, ACTION OR
PROCEEDING, ANY CLAIM THAT IT IS NOT PERSONALLY SUBJECT TO THE JURISDICTION OF
SUCH COURTS, THAT THE SUIT, ACTION OR PROCEEDING IS BROUGHT IN AN INCONVENIENT
FORUM, THAT THE VENUE OF THE SUIT, ACTION OR PROCEEDING IS IMPROPER, OR THAT
THIS GUARANTEE AGREEMENT OR ANY DOCUMENT OR ANY INSTRUMENT REFERRED TO HEREIN OR
THE SUBJECT MATTER HEREOF MAY NOT BE LITIGATED IN OR BY SUCH COURTS. THE
GUARANTOR AGREES THAT SERVICE OF PROCESS MAY BE MADE UPON IT BY CERTIFIED OR
REGISTERED MAIL TO THE ADDRESS FOR NOTICES SET FORTH IN THIS GUARANTEE AGREEMENT
OR ANY METHOD AUTHORIZED BY THE LAWS OF NEW YORK.

                  This instrument may be executed in any number of counterparts,
each of which so executed shall be deemed to be an original, but all such
counterparts shall together constitute but one and the same instrument.







                      [THIS SPACE INTENTIONALLY LEFT BLANK]




                                      -14-
<PAGE>   19


                  THIS GUARANTEE AGREEMENT is executed as of the day and year
first above written.

                                      SEMCO ENERGY, INC.

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





                                      BANK ONE TRUST COMPANY, NA
                                      as Trustee


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










                                     -15-