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                                                                    EXHIBIT 10.7

                                                                    EXHIBIT C

                             SYNTROLEUM CORPORATION
                             SECURED PROMISSORY NOTE

$19,000,000                                                       May 8, 2002

     Syntroleum Corporation, a Delaware corporation (the "Borrower"), the
principal office of which is located at Suite 1100, 1350 South Boulder, Tulsa,
Oklahoma, for value received hereby promises to pay to Marathon Oil Company, an
Ohio corporation ("Lender"), at P. O. Box 3128, Houston, Texas 77253-3128
("Registered Address"), or its registered assigns, the lesser of (i) the sum of
Nineteen Million and no/100ths Dollars ($19,000,000), or (ii) the then
outstanding principal amount of advances made to Borrower under this Note, in
each case together with interest thereon, on the terms and conditions set forth
hereinafter. The principal amount of all advances made by Lender to Borrower
under this Note together with accrued interest thereon, as set forth below,
shall be due and payable in accordance with the payment terms set forth in
Section 4 below, or when declared due and payable by the Lender upon the
occurrence of an Event of Default (as defined below). Payment for all amounts
due hereunder shall be made by mail to the Registered Address of the Lender.
This Note is issued pursuant to the provisions of that certain Participation
Agreement between the Borrower and the Lender dated May 8, 2002 (the
"Participation Agreement").

     The following is a statement of the rights of the Lender of this Note and
the obligations and conditions to which this Note are subject, and to which the
Lender and Borrower hereby agree.

     1.   Definitions. As used in this Note, the following terms, unless the
          context otherwise requires, have the following meanings:

          (a)  Borrower means Syntroleum Corporation, a Delaware corporation,
               and includes any corporation or other entity which shall succeed
               to or assume the obligations of the Borrower under this Note.

          (b)  Change of Control means if (i) any "person" or "group of related
               persons" shall have acquired "beneficial ownership" (within the
               meaning of Section 13(d) or 14(d) of the Securities Exchange Act
               of 1934, as amended, and the applicable rules and regulations
               thereunder), of shares of Voting Stock representing 35% or more
               of the Voting Power of Borrower, (ii) during any period of
               twenty-five consecutive months, commencing on the Original Issue
               Date, individuals who on the Original Issue Date were members of
               the board of directors of the Borrower (together with any
               replacement or additional directors whose election was
               recommended by

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               incumbent management of the Borrower or who were elected by a
               majority of directors then in office) cease to constitute a
               majority of the board of directors of the Borrower, (iii) any
               person or group of related persons shall merge or consolidate
               with the Borrower and, as a result of such merger or
               consolidation, the holders of a majority of the Voting Stock
               immediately before such merger or consolidation cease to hold a
               majority of the Voting Stock immediately after such merger or
               consolidation, or (iv) any person or group of related persons
               shall acquire all or substantially all of the assets of the
               Borrower. A Change of Control shall not include an increase in
               the percentage of shares of Voting Stock representing 35% or more
               of the Voting Power of the Borrower beneficially owned by any
               person solely as a result of a reduction in the number of shares
               of Voting Stock then outstanding solely due to the repurchase of
               Voting Stock by Borrower.

          (c)  Common Stock means the Borrower's common stock, $.01 par value
               per share.

          (d)  Event of Default means any of the events set forth in Article 8
               of this Note which constitute Borrower's default under this Note.

          (e)  GAAP means generally accepted accounting principles as in effect
               from time to time in the United States of America.

          (f)  Governmental Authority means (i) the government of (a) the United
               States of America or any State or other political subdivision
               thereof, or (b) any jurisdiction in which Borrower or any
               Subsidiary conducts all or any part of its business, or which
               asserts jurisdiction over any properties of Borrower or any
               Subsidiary, or (ii) any entity exercising executive, legislative,
               judicial, regulatory or administrative functions of, or
               pertaining to, any such government.

          (g)  Lender means Marathon Oil Company, an Ohio corporation, and
               includes any corporation or other entity which shall succeed to
               or assume the rights and obligations of the Lender under this
               Note.

          (h)  Market Value means the average closing price of the Common Stock
               on the five (5) trading days preceding the date in question as
               quoted on the NASDAQ Market System or, if the Common Stock is no
               longer traded on the NASDAQ Market System, on the largest U.S.
               stock exchange or market on which the Common Stock is traded or,
               if the Common Stock is not listed and traded on a stock exchange
               or market system, then as determined by an independent third
               party with expertise in the valuation of businesses. Such
               independent third party shall be selected by mutual agreement of
               Lender and Borrower.

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          (i)  Material means material in relation to the business, operations,
               affairs, financial condition, assets, properties, or prospects of
               Borrower and its Subsidiaries taken as a whole.

          (j)  Material Adverse Effect means any change to or effect on the
               business, operations, affairs, financial condition, assets or
               properties of Borrower and its Subsidiaries taken as a whole,
               other than as a result of the sole acts or omissions of Lender,
               including a default by Lender of any of its obligations under the
               Participation Agreement or this Note, that is Materially adverse
               to the ability of the Borrower to substantially perform its
               obligations under the Participation Agreement or this Note and
               will likely result in an Event of Default in the near term;
               provided, however, in no event shall cost overruns from the
               Project constitute a Material Adverse Effect.

          (k)  Maturity Date means the June 30, 2004.

          (l)  Original Issue Date means May 8, 2002.

          (m)  Participation Agreement means the Participation Agreement dated
               May 8, 2002 between the Borrower and Lender.

          (n)  Subsidiary means, as to any person, any corporation, association
               or other entity in which such person or one or more of its
               Subsidiaries or such person and one or more of its Subsidiaries
               owns sufficient equity or voting interests to enable it or them
               (as a group) to elect a majority of the directors (or persons
               performing similar functions) of such entity.

          (o)  Voting Power means the total voting power represented by all
               outstanding Voting Stock.

          (p)  Voting Stock means stock of any class or classes (however
               designated) of Borrower having ordinary voting power for the
               election of the directors of Borrower, other than stock having
               such power only by reason of the happening of a contingency.

          (q)  Other Defined Terms. Unless otherwise defined herein, any
               capitalized term used herein shall have its respective meaning as
               defined in the Participation Agreement. Terms defined herein
               shall include the plural, as well as the singular use of such
               words.

     2.   Advances. The Lender shall make advances under this Note as and when
          requested by Borrower from time to time up to and not to exceed
          $19,000,000 in principal amount, the proceeds of which will be used by
          Borrower to carry out the Project. Borrower requests for advances
          shall be in minimum amounts of $1,000,000 or more. Such requests for
          advances shall be supported by actual and

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          forecasted Project expenditures in accordance with the unanimously
          approved Project budget and approved and submitted by the Executive
          Committee to Lender on or before the first (1st) day of the month and
          funded by Lender on or before the fifteenth (15th) day of such month.
          Lender and Borrower shall reconcile such expenditures and advances on
          a monthly basis. Lender reserves the right to temporarily or
          permanently suspend any such advances upon (i) the occurrence of an
          Event of Default, or (ii) a termination under Article VI of the
          Participation Agreement or suspension of performance under Article XV
          of the Participation Agreement. The Lender is hereby authorized by the
          Borrower to endorse on the schedule forming a part of this Note
          appropriate notations evidencing the date and the amount of each
          advance made by the Lender to Borrower.

     3.   Interest.

          (a)  Subject to Section 17 hereof, simple interest on the unpaid
               balance of the Note shall accrue from the Original Issue Date
               until this Note is paid at the rate of interest equal to eight
               percent (8%) per annum (the "Interest Rate").

          (b)  Subject to Section 17 hereof, upon the occurrence of an Event of
               Default (as defined below) the interest rate on this Note will
               increase immediately to a rate equal to the Interest Rate plus 2%
               until such time as no Event of Default exists (whether through
               cure, repayment or otherwise).

     4.   Repayment. This Note shall be repaid by Borrower and Lender shall have
          no recourse against Borrower or its affiliates for the repayment of
          this Note (or interest accrued thereon), except in the manner and only
          in the manner set forth below.

          (a)  Subject to Lender's conversion right and notice provisions as set
               forth in Section 4(d), Borrower may prepay, from the date hereof
               until and including May 31, 2003, in cash, in whole or in part
               and without penalty, the outstanding principal balance, together
               with accrued interest to date of payment, of this Note upon five
               (5) days written notice to Lender. Other than as provided above,
               Borrower shall not prepay the Note.

          (b)  In the event a third party participant (excluding the DOE)
               provides capital contributions to the Project on or before May
               31, 2003, Borrower shall apply such capital contributions to the
               outstanding principal advanced to Borrower under this Note, plus
               accrued interest thereon with payment due within five (5) days of
               receipt of such capital contributions.

          (c)  Notwithstanding Sections 4(a) and 4(b) hereof and within five (5)
               business days of the date of Lender's written notice to Borrower,
               Lender shall have the option until this Note has been fully
               satisfied by Borrower and from time-to-time to convert all or any
               part of the then outstanding principal

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               balance and interest accrued thereon owed to Lender under this
               Note in any combination of the following methods:

                    (i)   in the form of validly issued, fully paid and
                          nonassessable shares of Common Stock at a per share
                          conversion price of (y) $7.50 on or prior to June 30,
                          2003 or (z) $8.50 between July 1, 2003 and June 30,
                          2004 (adjusted in each such case for any stock splits,
                          reverse stock splits, stock dividends or any other
                          recapitalizations or reorganizations subsequent to the
                          date hereof);

                    (ii)  in the form of current or future credits on a
                          $1-for-$1 basis against Net License Fees payable by
                          Lender to Borrower in accordance with Section 4.05(a)
                          of the Participation Agreement; or

                    (iii) in the form of credit on a $1-for-$1 basis against any
                          amounts due Borrower resulting from Lender's equity
                          participation in a gas-to-liquids specialty chemicals
                          plant located in Equatorial Guinea using Lender's
                          natural gas as plant feedstock and in which Lender and
                          Borrower each own an equity interest.

               Upon satisfaction by Borrower of its obligations under this
               Section 4(c), this Note shall be deemed repaid in the amount of
               principal and interest to the extent applied by Lender to the
               conversion to Common Stock or credits pursuant to this Section
               4(c).

          (d)  Without limiting Lender's other rights and remedies contained
               herein, if any unpaid balance of this Note (including accrued
               interest) remains outstanding on the Maturity Date or has not
               been converted pursuant to any of the provisions contained in
               this Section 4 prior to the Maturity Date, the unpaid balance
               outstanding under this Note (including accrued interest) shall be
               payable to Lender as follows:

                    (i)   if the Market Value is at or above $8.50 per share on
                          the Maturity Date, Lender shall convert in the form of
                          validly issued, fully paid and nonassessable shares of
                          Common Stock at a per share conversion price of $8.50
                          (adjusted in each such case for any stock splits,
                          reverse stock splits, stock dividends or any other
                          recapitalizations or reorganizations subsequent to the
                          date hereof); or

                    (ii)  if the then Market Value is below $8.50 per share on
                          the Maturity Date, Borrower shall have the option to
                          pay Lender in cash; provided however, that if Borrower
                          does not pay Lender all amounts due and owing under
                          this Note within ten (10)

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                          business days after the Maturity Date, Lender shall
                          have the option to receive, at Lender's option, any
                          combination of the following upon notification to
                          Borrower:

                              (x)  payment in the form of validly issued, fully
                                   paid and nonassessable shares of Common Stock
                                   at a per share conversion price of the higher
                                   of (1) the Market Value on the Maturity Date,
                                   or (2) $6.00 (adjusted in each such case for
                                   any stock splits, reverse stock splits, stock
                                   dividends or any other recapitalizations or
                                   reorganizations subsequent to the date
                                   hereof);

                              (y)  payment in the form of current or future
                                   credits, on a $1-for-$1 basis, against Net
                                   License Fees payable by Lender to Borrower in
                                   accordance with Section 4.05(a) of the
                                   Participation Agreement; or

                              (z)  payment in the form of credits on a $1-for-$1
                                   basis against any amounts due Borrower
                                   resulting from Lender's equity participation
                                   in a gas-to-liquids specialty chemicals plant
                                   located in Equatorial Guinea using Lender's
                                   natural gas as plant feedstock and in which
                                   Lender and Borrower each own an equity
                                   interest.

               Borrower agrees to provide written notice to Lender on or about
               the Maturity Date reminding Lender of its election rights under
               this Section 4. In the event that Lender has not notified
               Borrower of its election under this Section 4(d)(ii) within
               twenty (20) business days after the Maturity Date, Lender shall
               be deemed to have elected payment in the form specified in
               Section 4(d)(ii)(y) (license fee credits). Upon satisfaction by
               Borrower of all of its obligations under this Section 4(d), this
               Note shall be deemed repaid in full.

     5.   Common Stock Payment Procedure.

          (a)  Upon Lender's election to receive payment of this Note in the
               form of Common Stock, the payment of this Note will be deemed to
               have been made as of the close of business on the date on which
               Lender receives the number of validly issued, fully paid and
               nonassessable shares of Common Stock equal to the principal and
               interest due and owing on the Note divided by the applicable
               conversion price per share as provided herein (adjusted for any
               stock splits, reverse stock splits, stock dividends or any other
               recapitalizations or reorganizations subsequent to the date
               hereof). Upon such event, Lender shall surrender this Note at the
               principal office

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               of the Borrower. At such time as such payment has been effected,
               the rights of the Lender of this Note as such a Lender will cease
               and the person or persons in whose name or names any certificate
               or certificates for shares of Common Stock are to be issued upon
               such conversion will be deemed to have been the holder or holders
               of record of the shares of Common Stock represented thereby.

          (b)  Upon any conversion to Common Stock by Lender as provided herein,
               Borrower will deliver to the Lender within ten (10) business days
               of Lender's election to convert to such Common Stock:

               (i)  a certificate or certificates representing the number of
                    shares of Common Stock issuable by reason of such payment in
                    such name or names and such denomination or denominations as
                    the Lender has specified; and

               (ii) payment in an amount equal to the principle and interest
                    payable to the Lender in lieu of a fractional share of
                    Common Stock, if any.

          (c)  The issuance of certificates for shares of Common Stock upon
               payment of this Note upon Lender's election to receive payment in
               the form of Common Stock will be made without charge to the
               Lender for any issuance tax in respect thereof or other cost
               incurred by the Borrower in connection with such payment and the
               related issuance of shares of Common Stock. Upon payment of this
               Note in the form of Common Stock, the Borrower will take all such
               actions as are necessary in order to insure that the Common Stock
               issuable with respect to such payment will be validly issued,
               fully paid and nonassessable.

          (d)  The Borrower will not close its books against the transfer of
               this Note or of Common Stock issued or issuable upon payment of
               this Note, in any manner which interferes with the timely payment
               of this Note in such manner.

          (e)  The Borrower shall at all times reserve and keep available out of
               its authorized but unissued shares of Common Stock, solely for
               the purpose of effecting the payment of this Note, such number of
               its shares of Common Stock as shall from time to time be
               sufficient to effect a payment of this Note and if at any time
               the number of authorized but unissued shares of Common Stock
               shall not be sufficient to effect the payment of the Note, the
               Borrower shall promptly seek such corporate action as may, in the
               opinion of its counsel, be necessary to increase its authorized
               but unissued shares of Common Stock to such number of shares as
               shall be sufficient for such purpose.

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          (f)  The Borrower shall pay all issue taxes and other governmental
               charges (other than income or other taxes imposed upon profits
               realized by the recipient) that may be imposed in respect of the
               issue or delivery of shares of Common Stock or other securities
               or property upon payment of this Note.

          (g)  No fractional share shall be issued upon payment of this Note in
               the form of Common Stock. All shares of Common Stock (including
               fractions thereof) issuable upon payment of this Note in the form
               of Common Stock by the Borrower shall be aggregated for purposes
               of determining whether the payment would result in the issuance
               of any fractional share. If, after such aggregation, the payment
               would result in the issuance of a fraction of a share of Common
               Stock, the Borrower shall, in lieu of issuing any fractional
               share, pay the Lender a sum in cash equal to the remainder of
               principal and interest otherwise not payable pursuant to such
               payment.

          (h)  Except as specified in this Section 5(h), Borrower is under no
               obligation to file a registration under the Securities Act of
               1933 for shares of Common Stock issued to Lender under this Note.

               (i)    For all Common Stock collectively issued to Lender under
                      this Note, Lender may on one occasion demand the Borrower
                      to file a registration under the Securities Act of 1933 in
                      respect of all or a portion of such Common Stock shares by
                      delivering to Borrower written notice stating that such
                      right is being exercised, specifying the number of shares
                      to be included in such registration and describing the
                      intended method of distribution thereof. In response to
                      such request, Borrower shall as promptly as practicable,
                      file with the SEC a registration statement providing for
                      the registration of such number of shares as the Lender
                      shall have requested be registered for distribution and
                      perform all other acts reasonably necessary to insure that
                      such registration statement becomes effective in a timely
                      manner.

               (ii)   If, at any time during the term of this Note, Borrower on
                      its own initiative or on behalf of any other holder of
                      registration rights proposes to register any Common Stock
                      on a registration statement on Form S-1, Form S-2 or Form
                      S-3 for purposes of a primary or secondary offering,
                      Borrower shall give prompt written notice to Lender of its
                      intention to do so and Lender shall, upon prompt written
                      request specifying the number of shares to be disposed, be
                      entitled to "piggy-back" upon such registration subject to
                      the limitations contained herein. In the event that
                      Borrower and its investment advisors reasonably and in
                      good faith determine that the inclusion of such
                      "piggy-back" shares creates a substantial risk that the
                      price per share of Common Stock will be materially and

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                      adversely affected or that the number of shares of Common
                      Stock sought to be registered is a greater number than can
                      reasonably be sold or the success of the offering would
                      otherwise be materially and adversely affected, Borrower
                      shall include in such registration statement such number
                      of shares of Common Stock as Borrower, any other holder of
                      registration rights, and Lender are advised can be sold in
                      such offering without such an effect (the "Maximum
                      Number") as follows and in the following order of
                      priority: (A) first, such number of shares of Common Stock
                      as Borrower intended to be registered and sold by
                      Borrower; and (B) second, such additional shares, up to
                      the Maximum Number, requested by Lender on a pro rata
                      basis with all other holders of registration rights
                      desiring to participate in the registration.

          (i)  Lender agrees that, with respect to any Common Stock issued
               pursuant to this Note, if Lender sells such Common Stock, the
               amount of Common Stock sold, together with all sales of Common
               Stock sold by or for the account of Lender within the preceding
               three months, excluding any Common Stock held by Lender prior to
               the Original Issue Date, shall not exceed the greater of (i) one
               percent of the shares of Common Stock outstanding as shown by the
               most recent Form 10-K, 10-Q, or other report or statement
               published by Borrower, or (ii) the average weekly reported volume
               of trading in such securities on all national securities
               exchanges and/or reported through the automated quotation system
               of a registered securities association during the four calendar
               weeks preceding the date of sale. Notwithstanding the sales
               limitations contained in this Section 5(i), Lender shall be
               entitled to dispose of its shares of Common Stock through
               privately negotiated block sales in excess of 50,000 shares.

     6.   Representations, Warranties and Covenants.

          (a)  Borrower's Representations, Warranties and Covenants. In addition
               to other representations, warranties and covenants of Borrower
               set forth herein, Borrower further represents, warrants and
               covenants to Lender that as of the Original Issue Date:

               (i)    The Borrower is a corporation duly organized, validly
                      existing and in good standing under the laws of its
                      jurisdiction or incorporation and is duly qualified as a
                      foreign corporation and is in good standing in each
                      jurisdiction in which such qualification is required by
                      law, other than those jurisdictions as to which the
                      failure to be so qualified or in good standing could not,
                      individually or in the aggregrate, reasonably be expected
                      to have a Material Adverse Effect.

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               (ii)   This Note and the transactions contemplated herein have
                      been duly authorized by all necessary corporate action on
                      the part of the Borrower, and upon execution and delivery,
                      this Note will constitute, a legal, valid and binding
                      obligation of the Borrower enforceable against the
                      Borrower in accordance with its terms, except as such
                      enforceability may be limited by (i) applicable
                      bankruptcy, insolvency, reorganization, moratorium or
                      other similar laws affecting the enforcement of creditors'
                      rights generally and (ii) general principles of equity
                      (regardless of whether such enforceability is considered
                      in a proceeding in equity or at law).

               (iii)  The execution, delivery and performance of the Borrower of
                      this Note will not (i) contravene, result in any breach
                      of, or constitute a default under, or result in the
                      creation of any Lien (except as provided herein) in
                      respect of any property of the Borrower or any Subsidiary
                      under, any indenture, mortgage, deed of trust, loan,
                      purchase or credit agreement, lease, corporate charter or
                      by-laws, or any other agreement or instrument to which the
                      Borrower or any Subsidiary is bound or by which the
                      Borrower or any Subsidiary or any of their respective
                      properties may be bound or affected, (ii) conflict with or
                      result in a breach of any of the terms, conditions or
                      provisions of any order, judgment, decree, or ruling of
                      any court, arbitrator or Governmental Authority applicable
                      to the Borrower or any Subsidiary or (iii) violate any
                      provision of any statute or other rule or regulation of
                      any Governmental Authority applicable to the Borrower or
                      any Subsidiary.

               (iv)   No consent, approval or authorization of, or registration,
                      filing or declaration with, any Governmental Authority is
                      required in connection with the execution, delivery or
                      performance by the Borrower of this Note.

               (v)    There are no actions, suits or proceedings pending against
                      the Borrower or any Subsidiary or any property of the
                      Borrower or any Subsidiary in any court or before any
                      arbitrator of any kind or before or by any Governmental
                      Authority that, individually or in the aggregrate, could
                      reasonably be expected to have a Material Adverse Effect.

               (vi)   To the knowledge of Borrower, neither the Borrower nor any
                      Subsidiary is in default under any term of any agreement
                      or instrument to which it is a party or by which it is
                      bound, or any order, judgment, decree or ruling of any
                      court, arbitrator or Governmental Authority or is in
                      violation of any applicable law, ordinance, rule or
                      regulation (including without limitation

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                      environmental laws) of any Governmental Authority, which
                      default or violation, individually or in the aggregrate,
                      could reasonably be expected to have a Material Adverse
                      Effect.

               (vii)  The Borrower and its Subsidiaries have filed all tax
                      returns that are required to have been filed in any
                      jurisdiction, and have paid all taxes shown to be due and
                      payable on such returns and all other taxes and
                      assessments levied upon them or their properties, assets,
                      income or franchises, to the extent such taxes and
                      assessments have become due and payable and before they
                      have become delinquent, except for any taxes and
                      assessments (i) the amount of which is not individually or
                      in the aggregrate Material or (ii) the amount,
                      applicability or validity of which is currently being
                      contested in good faith by appropriate proceedings and
                      with respect to which the Borrower or a Subsidiary, as the
                      case may be, has established adequate reserves in
                      accordance with GAAP. The Borrower knows of no basis for
                      any other tax or assessment that could reasonably be
                      expected to have a Material Adverse Effect.

               (viii) Except for the rights of the DOE in the Project and the
                      existing real property lease upon which the Project will
                      be constructed, Borrower is not aware of any mortgage,
                      lien, pledge, charge, security interest or other
                      encumbrance, or any interest or title of any vendor,
                      lessor, lender or other secured party to or of such person
                      under any agreement including, without limitation, any
                      conditional sale or other title retention agreement or
                      capital lease, upon or with respect to such lease or the
                      Project assets.

          (b)  Lender's Representations, Warranties, and Covenants. With respect
               to Lender's right to convert outstanding balances due under this
               Note into Common Stock pursuant to Section 4 of this Note, Lender
               represents, warrants and covenants to Borrower the following:

               (i)    Lender is a sophisticated investor and has experience in
                      evaluating and investing in private placement transactions
                      of securities in companies similar to Borrower so that
                      Lender is capable of evaluating the merits and risks of
                      its investment in Borrower and has the capacity to protect
                      Lender's own interests. Further, Lender recognizes that an
                      investment in Borrower is highly speculative and involves
                      significant risks (including those identified in
                      Borrower's Annual Report on Form 10-K for its fiscal year
                      ended December 31, 2001 and the Proxy Statement dated
                      March 25, 2002 (the "SEC Reports")) including a complete
                      loss of such investment. In addition, Lender is an
                      "accredited investor" as such term is defined in Rule
                      501(a) of Regulation D under the Securities Act. Lender is
                      able to bear the substantial economic risk of an

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                      investment in the Common Stock for an indefinite period
                      and could afford the complete loss of Lender's investment
                      in the Common Stock.

               (ii)   Lender has read carefully and understands this Note and
                      has consulted with Lender's own attorney, accountant or
                      investment advisor with respect to the investment
                      contemplated hereby and its suitability for Lender. Lender
                      has received a copy of the SEC Reports. Lender has had an
                      opportunity to discuss Borrower's business, management and
                      financial affairs with its management and has had the
                      opportunity to review Borrower's facilities. Lender also
                      has had opportunity to ask questions of officers of
                      Borrower. Lender's taking advantage of any such
                      opportunity however, does not limit or modify the
                      representations and warranties of Borrower in Section 6(a)
                      hereof or the right of Lender to rely thereon. Lender has
                      relied solely upon the information provided by Borrower in
                      the SEC Reports and this Note in making the decision to
                      invest in the Common Stock.

     7.   Grant of Security Interest and Leasehold Mortgage; Other Agreements.

          (a)  Within ninety (90) days of the Original Issue Date and subject to
               the interest of the DOE in the Project, Borrower shall exercise
               its commercially reasonable efforts to execute those agreements
               (including, without limitation, a security agreement and
               leasehold mortgage) and other documents and to do all things
               necessary to grant to Lender a first security interest, leasehold
               mortgage and mortgage in the Project's assets for the purpose and
               only for the purpose of allowing Lender to complete the Project
               (as described in Article II of the Participation Agreement) in
               the event of an Event of Default by Borrower. The security
               granted by Borrower to Lender shall include without limitation:

               (i)    a first perfected security interest in all tangible and
                      intangible personal property associated with the SFP or
                      located from time to time on the property on which the SFP
                      is located;

               (ii)   a first position with respect to the leasehold mortgage in
                      the lease between Borrower and the Port of Catoosa
                      Authority relating to the real property (including without
                      limitation any buildings and fixtures) on which the SFP is
                      located;

               (iii)  a first mortgage position with respect to any buildings
                      and fixtures located on the real property on which the SFP
                      is located; and

               (iv)   a first perfected security interest in the rights-of-way
                      and easements necessary to operate the SFP.

                                       12

<PAGE>

               In the case of an Event of Default, Borrower further covenants
               and agrees to (i) grant Lender such further rights-of-way and
               easements necessary to complete and operate the SFP, and (ii)
               enter into a services agreement with Lender for providing
               utilities and services to Lender necessary to operate the SFP.
               Such agreement shall provide that such utilities and services
               shall be provided by Borrower to Lender at Borrower's cost and
               shall include, without limitation, any water, electricity, gas
               and other utilities and services necessary to operate the SFP and
               complete the Project. Borrower shall further assist Lender with
               obtaining all necessary licensing rights, catalysts and other
               facilities and services necessary to operate the SFP and complete
               the Project in the event that Lender exercises its rights
               hereunder.

          (b)  Each of the agreements referenced in Section 7(a) shall provide
               that upon completion of the Project (as described in Article II
               of the Participation Agreement), Lender shall reasonably promptly
               release its security interest in and mortgages on any Project
               assets and shall reasonably promptly return possession and
               control of the SFP to Borrower, with the SFP to be in
               substantially the same condition as when Lender took control,
               normal wear and tear excepted along with any Project additions
               completed by Lender as contemplated by the Participation
               Agreement.

     8.   Events of Default. If any of the events specified in this Section 8
          shall occur (herein individually referred to as an "Event of
          Default"), Lender may, so long as such condition exists, declare the
          entire outstanding principal and unpaid accrued interest under this
          Note immediately due and payable and exercise its remedies pursuant to
          Section 9 of this Note by notice in writing to the Borrower.

          (a)  The Borrower shall default in the payment of principal, interest
               or any other amounts due under this Note when the same shall
               become due and payable, whether at any stated due date, at
               maturity or by acceleration or otherwise and such default shall
               not have been remedied within five (5) business days after
               written notice thereof shall have been given to the Borrower; or

          (b)  The Borrower shall default in the performance of or compliance
               with any representation and warranty, covenant, agreement,
               condition or term contained in this Note or the Participation
               Agreement and such default shall not have been remedied within
               the applicable cure period herein or therein, if any; or

          (c)  The Borrower shall make an assignment for the benefit of
               creditors, or shall admit in writing its inability to pay its
               debts as they become due, or an order for relief is entered
               against the Borrower under any bankruptcy, insolvency or similar
               laws or the Borrower shall file any petition or

                                       13

<PAGE>

               answer seeking for itself any reorganization, arrangement,
               composition, readjustment, dissolution or similar relief under
               any present or future statute, law or regulation, or shall file
               an answer admitting the material allegations of a petition filed
               against the Borrower in any such proceeding, or shall seek or
               consent to the acquiesce in the appointment of any trustee,
               receiver or liquidator of the Borrower of all or any substantial
               part of the properties of the Borrower, or the Borrower or its
               board of directors or a majority of its stockholders shall take
               any action looking to the dissolution or liquidation of the
               Borrower and such default shall not have been remedied within 60
               days after written notice thereof shall have been given by Lender
               to the Borrower; or

          (d)  Within 60 days after the commencement of any proceeding against
               the Borrower seeking any reorganization, arrangement,
               composition, readjustment, liquidation, dissolution or similar
               relief under any present or future statute, law or regulation,
               such proceeding shall not have been dismissed or, within 60 days
               after the appointment without the consent or acquiescence of the
               Borrower of any trustee, receiver or liquidator of the Borrower
               or of all or any substantial part of the properties of the
               Borrower such appointment shall not have been vacated; or

          (e)  Borrower shall have a Material Adverse Effect; or

          (f)  Borrower shall experience a Change of Control; or

          (g)  Borrower's current assets minus current liabilities as set forth
               on its most recent audited or unaudited balance sheet filed with
               the Securities and Exchange Commission with Form 10-K or Form
               10-Q fall below $10 million; provided, however, that for purposes
               of calculating such amount, amounts due under this Note and
               liabilities associated with prepaid license fees shall be
               excluded from current liabilities.

     9.   Remedies for an Event of Default. Without limiting any other rights or
          remedies of Lender contained herein or under the Participation
          Agreement, Lender shall have the right to exercise any and all of the
          following remedies upon an Event of Default:

          (a)  Lender may exercise any of its rights and remedies with respect
               to the security interests, leasehold mortgage, mortgage,
               rights-of-way and easements granted to Lender hereunder pursuant
               to Section 7; and

          (b)  Lender shall have the right to appoint a majority of the
               Executive Committee under the Participation Agreement, and in
               such event, shall be entitled to such other rights as provided by
               Section 3.02(b) of the Participation Agreement.

                                       14

<PAGE>

          The limitation of Lender's rights and remedies contained herein shall
          in no event limit Lender's remedies available at law or equity to
          enforce such rights and remedies.

     10.  Assignment. Subject to the restrictions on transfer and assignment
          described in Section 18 below, the rights and obligations of the
          Borrower and the Lender of this Note shall be binding upon and benefit
          the permitted successors, assigns, heirs, administrators and
          transferees of the parties.

     11.  Waiver and Amendment. Any provision of this Note may be amended,
          waived or modified only upon the written agreement of the Borrower and
          Lender.

     12.  Notices. Any notice relating to this Note shall be deemed sufficiently
          given and served for all purposes when receipt has been evidenced (i)
          if by facsimile notification, by the sending facsimile confirming
          receipt, or (ii) if by courier, certified or registered mail, by
          signature of recipient confirming receipt. Notice shall be given to
          the address of the party set forth in the Participation Agreement.

     13.  No Stockholder Rights. Nothing contained in this Note shall be
          construed as conferring upon the Lender or any other person the right
          to vote or to consent or to receive notice as a stockholder in respect
          of meetings of stockholders for the election of directors of the
          Borrower or any other matters or any rights whatsoever as a
          stockholder of the Borrower and no dividends shall be payable or
          accrued in respect of the Common Stock obtainable hereunder until, and
          only to the extent that, this Note shall have been paid in the form of
          Common Stock.

     14.  Course of Dealing. No course of dealing between the Borrower and the
          Lender or any delay on the part of the Lender in exercising any rights
          hereunder shall operate as a waiver of any rights of the Lender.

     15.  Waiver of Notice. Except as otherwise expressly specified in this
          Note, the Borrower and each surety, guarantor, endorser, or other
          party liable for payment on this Note hereby waive diligence,
          presentment, demand, protest, and notice of any kind whatsoever, and
          agree that their liability on this Note shall not be affected by any
          renewal or extension in the time of payment hereof, or any
          indulgences, or by any taking, release, or change in any security for
          payment of this Note.

     16.  Costs and Fees. If this Note is placed in the hands of an attorney for
          collection after occurrence of an Event of Default, or if it is
          collected through legal or bankruptcy proceedings, the Borrower agrees
          to pay all costs of collection, including but not limited to, court
          costs and reasonable attorneys fees.

     17.  Interest Limitation. All agreements between or among the Borrower and
          the Lender, whether now existing or hereafter arising and whether
          written or oral, are hereby limited so that in no contingency, whether
          by reason of demand or

                                       15

<PAGE>

          acceleration of the maturity hereof or otherwise, shall the interest
          contracted for, charged, received, paid or agreed to be paid to the
          Lender under this Note exceed the maximum amount permissible under
          applicable law. If, from any circumstance whatsoever, interest would
          otherwise be payable to the Lender in excess of the maximum lawful
          amount, the interest payable to the Lender shall be reduced to the
          maximum amount permitted under applicable law; and if from any
          circumstance the Lender shall ever receive anything of value deemed to
          be interest by applicable law in excess of the maximum lawful amount,
          an amount equal to any excessive interest shall be applied to the
          reduction of the principal hereof and not to the payment of interest,
          or if such excessive interest exceeds the unpaid balance of principal
          hereof, such excess shall be refunded to the Borrower. All interest
          paid or agreed to be paid to the Lender shall, to the extent permitted
          by applicable law, be amortized, prorated, allocated, and/or spread
          throughout the full period until payment in full of the principal
          (including the period of any renewal or extension hereof) so that the
          interest hereon for such full period shall not exceed the maximum
          amount permitted by applicable law. This paragraph shall control all
          agreements between or among the Lender and the Borrower.

     18.  Transfer. This Note is transferable only on the books of the Borrower
          by the Lender or the Lender's duly authorized attorney-in-fact. Unless
          notified by Lender, the Borrower shall be entitled to treat the
          registered Lender of this Note as the true and lawful owner hereof for
          all purposes, including payment. Lender may transfer or assign its
          rights and obligations to any party and shall provide reasonably
          prompt notice to Borrower. Borrower shall not transfer or assign its
          rights or obligations hereunder.

     19.  Governing Law. This Note shall be governed by and construed in
          accordance with the laws of the State of Oklahoma, excluding that body
          of law relating to conflict of laws.

     20.  Heading; References. All headings used herein are used for convenience
          only and shall not be used to construe or interpret this Note. Except
          where otherwise indicated, all references herein to Sections refer to
          Sections hereof.

     IN WITNESS WHEREOF, the Borrower has caused this Note to be issued this 8th
day of May 2002.

SYNTROLEUM CORPORATION                  MARATHON OIL COMPANY

By:                                  By:
   --------------------------            -------------------------------------
    Mark A. Agee, President               John T. Mills, Senior Vice President

                                       16

<PAGE>

                             SYNTROLEUM CORPORATION
                            UNSECURED PROMISSORY NOTE
                       ADVANCES AND PAYMENTS OF PRINCIPAL

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                                     AMOUNT OF
              AMOUNT OF   TYPE OF    PRINCIPAL
    DATE      ADVANCES    ADVANCES    REPAID            NOTATION MADE BY
--------------------------------------------------------------------------------

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                                       17<PAGE>

                                                                    EXHIBIT 10.8

                                 NOTE AGREEMENT

     This Note Agreement (the "Agreement") is made and entered into effective
the 7th day of June, 2002 by and between Mark A. Agee ("Maker") and Syntroleum
Corporation, a Delaware corporation ("Lender").

     WHEREAS, Maker is indebted to Lender pursuant to a Secured Promissory Note
given by Maker to Lender dated June 25, 2001 (the "2001 Note") and pursuant to a
Secured Promissory Note dated November 9, 1997 given by Maker to Lender (the
"1997 Note"); and

     WHEREAS, the foregoing obligations are secured by a pledge of shares of
Lender Common Stock ("Lender Stock") by Maker to Lender pursuant to the Security
and Stock Pledge Agreement dated June 25, 2001 as amended (the "2001 Security
Agreement"), and the Security and Stock Pledge dated November 9, 1997 as amended
by Amendment No. 1 to Security and Stock Pledge Agreement made effective August
18, 1998 (the "1997 Security Agreement") (the 2001 Note, the 1997 Note, the 2001
Security Agreement and the 1997 Security Agreement are referred to collectively
as the "Loan Documents");

     WHEREAS, pursuant to the Loan Documents, Maker is required to pledge a
number of shares of Lender Stock having a market value equal to or greater than
two times the amount of the indebtedness, including accrued interest.

     WHEREAS, beginning in April 2002, and continuing thereafter, the number of
shares which have been held as collateral has been, and is now, inadequate to
cover the amount of the indebtedness plus accrued interest; and

     WHEREAS, Maker has now advised Lender that he is uncertain whether he will
be able to deliver additional shares of Lender Stock under the Loan Documents;
and

     WHEREAS, as result, Lender has the right to declare a default and foreclose
upon its existing collateral under the Loan Documents and the parties have
agreed that it is in their best interest to have a sufficient number of shares
of Lender Stock which are held as collateral redeemed by Lender and applied
toward outstanding indebtedness.

     NOW, THEREFORE, the parties agree as follows.

     1. Maker acknowledges that the failure to provide additional collateral as
set forth in the recitals is a breach of the Loan Documents and that the Lender
may declare a default thereunder based upon such failure. Maker waives any right
to object to the declaration of such default by Lender and waives any defense to
the foreclosure by Lender under the Loan Documents upon any collateral held by
Lender. The parties stipulate that the amount of principal and interest owed
under the 2001 Note and the 1997 Note collectively, as of June 7, 2002 is
$2,141,632.87 (the "Current Indebtedness"). In lieu of foreclosure, and in order
to cause a more orderly transfer of such shares, Maker acknowledges that with
respect to the Current Indebtedness, Lender is, effective June 7, 2002,
exercising its right to take ownership of and

<PAGE>

redeem that number of shares of Lender Stock currently held as collateral as is
equal in amount to the Current Indebtedness based upon the value of such shares
as set forth herein. For purpose hereof, the shares of Lender Stock shall be
valued at $4.10 per share, the Market Value thereof (as defined below) as of
June 7, 2002. Market Value means the closing price of a share of Common Stock of
Lender, on June 7, 2002, as reported on NASDAQ. The Lender Stock to be redeemed
by Lender hereunder shall be shares of Common Stock pledged by Maker to Lender
under the 2001 Security Agreement and the 1997 Security Agreement. Based upon
the foregoing calculations, the number of shares being redeemed by the Lender
effective June 7, 2002 is 522,350 shares. Maker agrees to cooperate with Lender
with respect to carrying out the provisions hereof.

     2. Maker represents, warrants and agrees as follows: (a) Lender shall be
the owner of such shares which it redeems free of any liens, claims or
encumbrances; (b) the provisions hereof will not violate any court order or
ruling; (c) he will not contest or defend the actions of the Lender in redeeming
the shares hereunder; and (d) he will indemnify and hold Lender harmless for any
loss, damage or claim arising from any breach of these representations or his
obligations hereunder.

     Dated effective the 7th day of June 2002.

                          Remainder of Page Left Blank

<PAGE>

                                             Syntroleum Corporation

                                             By:
                                                --------------------------------

                                             Title:
                                                   -----------------------------

                                             -----------------------------------
                                             Mark A. Agee

                                             Date:
                                                  ------------------------------

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