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                                                                    EXHIBIT 10.6



                         RIO VISTA ENERGY PARTNERS L.P.
                              UNIT OPTION AGREEMENT

                            Void after July 10, 2006

Option No. ___                                      Date of Grant: July 10, 2003


         FOR VALUE RECEIVED and pursuant to the terms of a letter agreement
dated November 29, 2002, between Shore Capital LLC and Penn Octane Corporation,
a Delaware corporation ("Penn Octane"), and a resulting employment agreement
dated May 13, 2003, between Penn Octane and Richard Shore, Jr., the undersigned,
RIO VISTA ENERGY PARTNERS L.P., a limited partnership organized and existing
under the laws of the State of Delaware, hereby certifies that Shore Capital LLC
is entitled, subject to the terms set forth below, to purchase from the
Partnership after the completion of the distribution by Penn Octane to its
stockholders of all of the outstanding Common Units of the Partnership and
before 5:00 P.M. New York time, on July 10, 2006 (the "Expiration Date"), 97,415
Common Units of the Partnership. The purchase price per Common Unit shall be
$8.47 (the "Exercise Price").


         The details of this Option Agreement (this "Agreement") are as follows:

         1. DEFINITIONS.

                  (a) "AFFILIATE" means, with respect to any specified person,
any person that directly or through one or more intermediaries controls or is
controlled by or is under common control with the specified person. As used in
this definition, the term "control" means the possession, directly or
indirectly, of the power to direct or cause the direction of the management and
policies of a person, whether through ownership of voting securities, by
contract or otherwise. For purposes of clarification, Penn Octane Corporation is
an Affiliate of the Partnership and the General Partner.

                  (b) "CODE" means the Internal Revenue Code of 1986, as
amended.

                  (c) "GENERAL PARTNER" means Rio Vista GP LLC, a Delaware
limited liability company, and its successors or assigns.

                  (d) "MANAGER" means a manager of the General Partner under the
Delaware Limited Liability Company Act.

                  (e) "OFFICER" means any person designated by the General
Partner as an officer.

                  (f) "OPTION" means an option to acquire Common Units granted
pursuant to this Agreement.




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                  (g) "OPTIONHOLDER" means the person to whom an Option to
acquire Common Units is granted pursuant to this Agreement.

                  (h) "PARTNERSHIP" means Rio Vista Energy Partners L.P., a
Delaware limited partnership, and its successors or assigns.

                  (i) "PARTNERSHIP AGREEMENT" means the limited partnership
agreement of the Partnership, as amended.

                  (j) "PURCHASE PRICE" means the amount equal to the product of
the Exercise Price and the number of Common Units to be purchased upon exercise
of this Option.

                  (k) "COMMON UNIT" has the meaning set forth in the Partnership
Agreement or any equity interest into which a Common Unit is exchanged or
converted.

         2. ADMINISTRATION.

                  (a) ADMINISTRATION BY GENERAL PARTNER. The General Partner
shall administer this Agreement. Any interpretation of this Agreement by the
General Partner and any decision by the General Partner under this Agreement
shall be final and binding on all persons.

                  (b) POWERS OF GENERAL PARTNER. The General Partner shall have
the power, subject to, and within the limitations of, the express provisions of
this Agreement:

                           (i) to construe and interpret this Agreement, and to
establish, amend and revoke rules and regulations for its administration; the
General Partner, in the exercise of this power, may correct any defect, omission
or inconsistency in this Agreement, in a manner and to the extent it shall deem
necessary or expedient to make this Agreement fully effective; and

                           (ii) generally, to exercise such powers and to
perform such acts as the General Partner deems necessary or expedient to promote
the best interests of the Partnership which are not in conflict with the
provisions of this Agreement.

         3. NUMBER OF COMMON UNITS, EXERCISE PRICE AND CAPITALIZATION
ADJUSTMENTS. The number of Common Units subject to this Option and the exercise
price per Common Unit may be adjusted from time to time for capitalization
adjustments described in this Section. If any change is made in the Common Units
subject to this Agreement, without the receipt of consideration by the
Partnership (through conversion, merger, consolidation, reorganization,
recapitalization, unit distribution, distribution in property other than cash,
Common Units split, liquidating distribution, combination of Common Units,
exchange of Common Units, change in structure or other transaction not involving
the receipt of consideration by the Partnership), the Common Units subject to
this Agreement will be appropriately adjusted in the class(es) and number of
securities and price per Common Unit. The General Partner, the determination of
which shall be final, binding and conclusive, shall make such adjustments. (The
conversion of any convertible securities of the Partnership shall not be treated
as a transaction "without receipt of consideration" by the Partnership.)

         4. METHOD OF PAYMENT. Payment of the Purchase Price is due in full upon
exercise of all or any part of this Option. The Optionholder may elect to make
payment of the Purchase Price in cash or by check.




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         5. WHOLE COMMON UNITS. This Option may only be exercised for whole
Common Units.

         6. SECURITIES LAW COMPLIANCE. Notwithstanding anything to the contrary
contained herein, this Option may not be exercised unless the Common Units
issuable upon exercise of this Option are then registered under the Securities
Act or, if such Common Units are not then so registered, the Partnership has
determined that such exercise and issuance would be exempt from the registration
requirements of the Securities Act. The Partnership shall use its commercially
reasonable efforts to register such Common Units under the Securities Act.

         7. TERM. The term of this Option commences on the date Penn Octane
completes the distribution to its stockholders of all the outstanding Common
Units of the Partnership and expires on the Expiration Date.

         8. EXERCISE.

                  (a) Subject to Section 8(c), the Optionholder may exercise
this Option during its term by delivering a Notice of Exercise (in a form
designated by the General Partner) together with the Purchase Price to the
Secretary of the General Partner, or to such other person as the General Partner
may designate, during regular business hours, together with such additional
documents as the General Partner may then require.

                  (b) By exercising this Option, the Optionholder agrees that,
as a condition to any exercise of this Option, the Partnership or the General
Partner may require the Optionholder to enter an arrangement providing for the
payment by the Optionholder to the Partnership of any tax withholding obligation
of the Partnership, as reasonably determined by the Partnership, arising by
reason of (1) the exercise of this Option, or (2) the disposition of Common
Units acquired upon such exercise.

                  (c) Notwithstanding anything herein to the contrary, during
the term of this Option, this Option may only be exercised during the first ten
(10) days of the first month of each fiscal quarter of the Partnership and
during such other periods as the General Partner may designate in its sole
discretion for purposes of minimizing the accounting costs to the Partnership
resulting from such exercise.

         9. TRANSFERABILITY. This Option is not transferable, except to any
Affiliate of the Optionholder, and is exercisable only by the Optionholder or
such Affiliate. The transferability of the Common Units issued upon exercise of
this Option shall be subject to the conditions, restrictions and limitations set
forth in this Agreement, the Partnership Agreement and any other agreements the
Optionholder may have with the Partnership.

                  (a) The provisions of this Section 9 may be waived with
respect to any transfer by the Partnership, upon duly authorized action of the
General Partner.

                  (b) Any sale or transfer, or purported sale or transfer, of
the Common Units by the Optionholder shall be null and void unless the terms,
conditions, and provisions of this Section 9 are observed and followed.





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         10. WITHHOLDING OBLIGATIONS

                  (a) At the time this Option is exercised, in whole or in part,
or at any time thereafter as requested by the Partnership, you hereby authorize
withholding from payroll and any other amounts payable to you, and otherwise
agree to make adequate provision for, any sums required to satisfy the federal,
state, local and foreign tax withholding obligations of the Partnership or an
Affiliate, if any, which arise in connection with this Option.

                  (b) This Option is not exercisable unless the tax withholding
obligations of the Partnership and/or any Affiliate are satisfied. Accordingly,
you may not be able to exercise this Option when desired even though this Option
is vested, and the Partnership shall have no obligation to issue a certificate
for such Common Units or release such Common Units from any escrow provided for
herein.

         11. TAX CONSEQUENCES AND CAPITAL ACCOUNT UPON EXERCISE OF OPTION. It is
intended that neither the Optionholder, the Partnership nor any of the partners
(owners) of the Partnership shall have any federal income tax consequences upon
grant of this Option, or, except as described herein, upon exercise of this
Option. Additionally, it is intended that the Optionholder shall not be treated
as a partner with respect to the Common Units subject to this Agreement until
such time as this Option is exercised. It is intended that, upon exercise of
this Option, the Optionholder shall include in income ("Exercise Income") in the
tax year of exercise an amount equal to the excess, if any, of the fair market
value of Common Units on the date of exercise over the sum of the amount the
Optionholder has paid or will pay to the Partnership ("Option Consideration")
upon the grant and exercise of the Option. It is intended that the Partnership
shall be entitled to a deduction equal to the amount of Exercise Income, which,
if the Partnership continues to be a partnership for federal income tax
purposes, shall be allocated solely to the partners of the Partnership other
than the Optionholder to whom the Exercise Income is attributable. Upon exercise
of this Option, Optionholder shall receive a positive capital account in the
Partnership (assuming the Partnership continues to be classified as a
partnership for federal income purposes) equal to the sum of the Exercise Income
to the Optionholder and the amount of Option Consideration paid by the
Optionholder. Should the federal income tax consequences differ from that
described in this section, the General Partner shall have the authority to
change the capital account or allocations of income and deduction to the
Optionholder as described herein in manner which in its discretion most closely
achieves the same economic effect of the consequences described in this Section.
Notwithstanding anything herein to the contrary, the General Partner shall have
the authority to change the tax reporting described above with respect to the
issuance or exercise of this Option or alter the capital accounts or allocations
of Partnership items as necessary to preserve or achieve the uniformity of
Common Units.

         12. NOTICES. Any notices provided for in this Option or this Agreement
shall be given in writing and shall be deemed effectively given upon receipt or,
in the case of notices delivered by the Partnership to the Optionholder, five
(5) days after deposit in the United States mail, postage prepaid, addressed to
the Optionholder at the last address provided to the Partnership.





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         13. THIS AGREEMENT; PARTNERSHIP AGREEMENT. This Option is subject to
all the provisions of this Agreement, the provisions of which are hereby made a
part of this Option, and is further subject to all interpretations, amendments,
rules and regulations which may from time to time be promulgated and adopted
pursuant to this Agreement, and the Common Units to be delivered are subject to
the terms of the Partnership Agreement. In the event of any conflict among
Optionholder's rights under the Common Units, this Agreement and the Partnership
Agreement, the terms of the Partnership Agreement shall control.

         14. MISCELLANEOUS.

                  (a) AVAILABILITY OF COMMON UNITS. During the term of this
Option, the Partnership shall keep available at all times the number of Common
Units required to satisfy the Option.

                  (b) PARTNER RIGHTS. No Optionholder shall be deemed to be
partner of the Partnership, or to have any of the rights of a partner or rights
of an assignee from a partner with respect to, any Common Units subject to the
Option unless and until such Optionholder has satisfied all requirements for
exercise of the Option pursuant to its terms.

                  (c) INVESTMENT ASSURANCES. Unless the Common Units issuable
upon exercise of this Option are then registered under the Securities Act, the
General Partner may require an Optionholder, as a condition of exercising or
acquiring Common Units under the Option, to give written assurances that he is
an "accredited investor," as defined in the rules and regulations under the
Securities Act. The General Partner may, upon advice of counsel to the General
Partner, place legends on Common Units certificates issued under the Option as
such counsel deems necessary or appropriate in order to comply with applicable
securities laws, including, but not limited to, legends restricting the transfer
of the Common Units.

                  (d) PARTNERSHIP AGREEMENT. Before receiving Common Units, the
Optionholder shall take such action and execute such documents as the General
Partner may require to become a partner of the Partnership.

         IN WITNESS WHEREOF, this Option Agreement has been duly executed by the
parties hereto as of the Date of Grant.

                                  RIO VISTA ENERGY PARTNERS L.P.

                                  By:  RIO VISTA GP LLC, general partner


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


ACCEPTED AND AGREED:

SHORE CAPITAL LLC


By:
   ----------------------------------
     Richard Shore, Jr., President







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