<PAGE>   1

                                                                   EXHIBIT 10.28

                    [NATIONAL ENERGY GROUP, INC. LETTERHEAD]




April 15, 1997



Mr. W. David Willig
Atocha Exploration, Inc.
1415 Louisiana, Suite 2250
Houston, Texas 77002

Mr. Robert A. Potosky
Potosky Oil and Gas Company
1415 Louisiana, Suite 2250
Houston, Texas 77002

Sandefer Oil & Gas
1415 Louisiana, Suite 2250
Houston, Texas 77002

Re:      First Amendment to (i) Agreement dated January 1, 1996
         and (ii) Consulting Agreement dated January 1, 1996

Gentlemen:

     This First Amendment to (i) that certain Agreement dated January 1, 1996 by
and between National Energy Group, Inc. ("NEG") and Sandefer Oil & Gas Inc.
("SOG"), hereinafter (the "Agreement"), incorporated by reference herein; and
(ii) that certain Consulting Agreement dated January 1, 1996 by and between SOG
and Atocha Exploration Inc. ("Atocha") and Potosky Oil and Gas Company ("POG"),
hereinafter (the "Consulting Agreement"), incorporated by reference herein;
which provides for the services of Mr. Robert A. Potosky ("Mr. Potosky") and Mr.
W. David Willig ("Mr. Willig"), shall express the mutual understanding and
agreement of NEG and SOG and Atocha and POG as expressed in that certain
memorandum among the parties dated March 27, 1997, hereinafter (the
"Memorandum") incorporated by reference herein.  The term "First Amendment"
shall be used herein to describe corresponding amendments to both the Agreement
and the Consulting Agreement as contemplated in the Memorandum.

     In consideration of the mutual covenants and agreements contained herein
and other good  and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the parties hereto agree as follows:

1.       Paragraph 3 of the Agreement and Paragraph 4 of the Consulting
         Agreement, Initial Term, shall be amended to provide that the period
         of January 1, 1998 through December 31, 1998 shall be defined as the
         "Secondary Term," and all references in the Agreement and the
         Consulting Agreement to the Initial Term shall include the Secondary
         Term, unless otherwise expressly stated to the contrary in this First
         Amendment.
<PAGE>   2
Atocha Exploration, Inc.
Potosky Oil and Gas Company
Sandefer Oil & Gas, Inc.
April 15, 1997
Page Two


2.       Paragraph 4 of the Agreement and Paragraph 6 of the Consulting
         Agreement, Prospect Generation By SOG and Obligations of Potosky and
         Atocha, respectively, shall be amended to provide that Mr. Potosky and
         Mr. Willig shall generate Prospects exclusively for NEG; provided that
         in the event any such Prospect is declined by NEG as provided in
         Paragraph 11 of the Agreement, Rejection of Proposals; Release of
         Prospects; Atocha and POG shall be free to offer the Prospect to any
         other party as described in Paragraph 11 of the Agreement and
         Paragraph 10 of the Consulting Agreement, Sale of Prospects.

3.       Paragraph 4 of the Agreement, shall be further amended to provide that
         the following Prospects, together with any additional Prospects
         generated prior to December 31, 1997, shall be those Prospects which
         shall determine cost reimbursement to NEG as applied to the Initial
         Term Program with respect to the provisions of Paragraph 12 of the
         Agreement, Disbursement of Proceeds:

                 (a)  South Lake Arthur    (e)  West Grand Bayou
                 (b)  Southeast Gueydon    (f)  Northwest Bayou Sorrel
                 (c)  Tiger Bayou          (g)  Berry Bayou
                 (d)  South Tiger Bayou    (h)  Mushroom

4.       Paragraph 6 of the Agreement and Paragraph 7 of the Consulting
         Agreement, Compensation, shall be amended to provide that effective
         April 1, 1997 through the end of the Secondary Term and any renewals
         and/or extensions thereof, the monthly fee paid to Atocha and POG
         shall (i) be paid on or before the fifteenth day of each month, (ii)
         be increased to the amount of $29,000.00 per month and (iii) include
         all expenses incurred by Atocha and POG, including lease costs as
         described in Paragraph 9 below.

5.       Paragraph 8 of the Consulting Agreement, Reimbursement or Payment of
         Costs; Furnishing of Office Space and Personnel, shall be amended to
         provide that the $29,000.00 monthly fee paid to Atocha and POG shall
         be inclusive of all expenses incurred by Atocha and POG; provided that
         direct expenses related to Prospect generation as described in
         Paragraph 4 of the Agreement and Paragraph 6 of the Consulting
         Agreement, which have been approved in writing by the Company prior to
         expenditure, shall be reimbursed to Atocha and POG.

6.       Paragraph 12 of the Agreement, shall be amended to provide that cost
         reimbursement to NEG with respect to any Prospects generated during
         the Secondary Term shall be separate and distinct from any Prospects
         generated during the Initial Term and shall start a new accounting
         cycle with respect thereto.

7.       In the event there is an election to accept a Deferred Leasehold
         Interest as described in Paragraph 17 of the Agreement, Deferred
         Leasehold Interest, NEG agrees that it shall timely execute and
         deliver to Mr. Potosky and Mr. Willig et al. an Assignment
         substantially in the form of Exhibit "A", attached hereto.
<PAGE>   3
Atocha Exploration, Inc.
Potosky Oil and Gas Company
Sandefer Oil & Gas, Inc.
April 15, 1997
Page Three


8.       Atocha, POG, Mr. Potosky and Mr. Willig agree to execute, and shall
         cause SOG to execute, that certain Letter Agreement dated December 20,
         1996, attached hereto as Exhibit "B", with respect to the definition
         of the respective ORRI's and Back-In Interests as such apply to the
         PANACO, Inc. transaction described in such Letter Agreement.

9.       NEG agrees that it shall provide office space for Atocha and POG in
         Houston, Texas at the Wedge International Tower pursuant to a Lease
         Agreement, in the form of Exhibit "C", attached hereto, and Paragraph
         5 of the Agreement, Reimbursement of Expenses; Sublease of Office
         Space; Personnel, and Paragraph 9 of the Consulting Agreement, Office
         Space, shall be amended accordingly and shall provide that the costs
         associated with such lease shall be borne by Atocha and POG.

10.      NEG shall make available to each of Mr. Potosky and Mr. Willig through
         the end of the Secondary Term and any renewals and/or extensions
         thereof, the health and dental benefits available to employees of NEG;
         provided that the amount described in Paragraph 3 hereof shall be
         reduced by an amount equal to $615.00 per month for each of Mr. Willig
         and/or Mr. Potosky in any month either of them are participants in
         NEG's medical and/or dental plan.

11.      In the event NEG requests Atocha and POG to perform services with
         respect to the interpretation of 3-D data incidental to Prospect
         development of Bayou Sorrel or any other Prospect, NEG shall provide
         them with the use of a 3-D station for such time that their
         interpretive services are requested by NEG; provided that such use
         shall be exclusively for the benefit of NEG and shall not exceed the
         expiration of the Secondary Term or any renewals and/or extensions
         thereof.

12.      Miscellaneous:

                 (a)      CHOICE OF LAW.  THIS FIRST AMENDMENT AND ALL OF THE
                          RIGHTS AND OBLIGATIONS OF THE PARTIES ARISING FROM OR
                          RELATING TO THE SUBJECT MATTER HEREIN OR THE
                          TRANSACTIONS CONTEMPLATED HEREBY SHALL BE GOVERNED
                          BY, CONSTRUED AND ENFORCED IN ACCORDANCE WITH THE
                          LAWS OF THE STATE OF TEXAS, EXCLUDING THE CONFLICT OF
                          LAWS AND RULES OF SUCH STATE.

                 (b)      Other Agreements.  This First Amendment supersedes
                          any prior agreements between the parties with respect
                          to the subject matter of hereof.

                 (c)      Confidentiality.  SOG, Atocha, Mr. Potosky, Mr.
                          Willig, and/or Mr.  Potosky shall not disclose the
                          terms of this to any third party, except to the
                          extent required by law, rule, regulation or court
                          order.
<PAGE>   4

Atocha Exploration, Inc.
Potosky Oil and Gas Company
Sandefer Oil & Gas, Inc.
April 15, 1997
Page Four


                 (d)      Mediation.  In the event any dispute regarding The
                          Agreement, the Consulting Agreement and/or this First
                          Amendment cannot be reconciled by the parties, then
                          they shall attempt to resolve any such dispute
                          through (i) mediation, using a mutually acceptable
                          mediator, and, if necessary, through (ii) binding
                          arbitration, using a mutually acceptable arbitrator.
                          No dispute related to this Agreement, the Consulting
                          Agreement and/or this First Amendment shall be
                          brought before any court of law or equity.  Any
                          arbitration will be conducted in Houston, Harris
                          County, Texas using the commercial rules of the
                          American Arbitration Association.

         The parties specifically acknowledge and agree that this First
Amendment to (i) the Agreement and (ii) the Consulting Agreement shall be
effective to amend, change and otherwise modify the Agreement and the
Consulting Agreement; and wherever a conflict should arise in the
interpretation or performance contemplated in the Agreement and/or the
Consulting Agreement, the terms and conditions of this First Amendment shall
supersede the terms of the Agreement and/or the Consulting Agreement.

         If the foregoing reflects our mutual understanding and agreement of
the subject matter contained herein, please so indicate by executing in the
appropriate space below.  This First Amendment shall be effective as of April
1, 1997.

Sincerely,
National Energy Group, Inc.


By:
   ---------------------------------------------
Name:   Miles D. Bender
Title:      President and CEO

ATOCHA EXPLORATION, INC.

By: /s/ W. DAVID WILLIG
   ---------------------------------------------
Name: W. DAVID WILLIG
     -------------------------------------------
Title: PRESIDENT
      ------------------------------------------

POTOSKY OIL AND GAS COMPANY

By: /s/ ROBERT A. POTOSKY
   ---------------------------------------------
Name: ROBERT A. POTOSKY
     -------------------------------------------
Title: PRESIDENT
      ------------------------------------------

SANDEFER OIL & GAS, INC.

By: /s/ STEPHEN F. SMITH
   ---------------------------------------------
Name: STEPHEN F. SMITH
     -------------------------------------------
Title: EXECUTIVE VICE PRESIDENT
      ------------------------------------------

<PAGE>   5

                                  EXHIBIT "A"

                   ASSIGNMENT OF OVERRIDING ROYALTY INTERESTS


THE STATE OF                  }

                              }         KNOW ALL MEN BY THESE PRESENTS

PARISH OF                     }

         THAT, the undersigned, NATIONAL ENERGY GROUP, INC. whose address is
1400 One Energy Square, 4925 Greenville Avenue, Dallas, Taxes 75206 hereinafter
called "Assignor", In consideration of Ten Dollars cash and other consideration
to Assignor paid by the following named parties hereinafter called "Assignees",
the receipt and sufficiency of all consideration  being hereby acknowledged,
and subject to the terms and provisions of the Participation Agreement and as
hereinafter set forth, has TRANSFERRED, ASSIGNED AND CONVEYED, and does hereby
TRANSFER, ASSIGN AND CONVEY unto said Assignees, the following described
overriding royalty Interests with respect to oil, gas, and other minerals
produced and saved pursuant to the leases, hereinafter referred to as "Said
Leases", described in Exhibit "A" attached hereto and incorporated herein,
to-wit:

Assignees and Addresses            Overriding Royalty Interests Hereby Conveyed








         The above overriding royalty Interests hereby conveyed apply to and
are in connection with the following:

1.       The overriding royalty herein provided In all instances shall bear its
         proportionate part of production, severance and other direct tax or
         taxes applicable thereto. The overriding royalty on oil and gas shall
         be computed after deducting any lost, used or consumed on the lease
         for operating same or for treating and handling the products
         therefrom.

2.       On Assignee's proportionate share of oil, condensate or gas, including
         casinghead gas or other gaseous substances produced from said lease,
         the overriding royalty shall be paid on the amount realized from such
         sale.

3.       In the event that the lease described herein is effective as to less
         than the entire fee simple estate In the oil and gas in and

                                       1

<PAGE>   6
         under and to be produced from any tract or parcel of land purported to
         be covered thereby, then and in that event, the overriding royalty
         interest herein assigned shall be reduced in the proportion that the
         mineral interest covering said lease bears to the entire fee simple
         mineral estate therein.

4.       The overriding royalty interest herein granted shall never be deemed
         to impose any obligation upon Assignor to conduct any drilling
         operations whatsoever upon any of the property covered by the
         above-mentioned lease or to maintain any such operations if once begun
         or to maintain production of oil or gas if once established or to
         protect the lease from drainage or to maintain such lease in effect by
         payment of rentals, drilling operations or otherwise and all
         operations on the property covered by said lease and the extent and
         duration thereof as well as the preservation of such lease shall be
         solely at the will of Assignor.

5.       Assignor, its successors assigns and legal representatives shall have
         the right and power to pool the overriding royalty interest herein
         granted to the same extent and in the same manner as any lessor's
         interest may be pooled, whether by operation or action of law or other
         governmental authority, or under the above-mentioned lease or as may
         thereafter be provided by amendment or supplemental agreement between
         Assignor and Assignee and their respective heirs, successors, assigns,
         and legal representatives into such units as it may be deemed
         advisable for the production and development of oil or gas;

         TO HAVE AND TO HOLD the interest herein conveyed, subject to the terms
and conditions, covenants and reservations aforesaid unto Assignee and unto
Assignee's successors and assigns.

         This Assignment is made without warranty of title, either express or
implied.

         IN WITNESS WHEREOF, this instrument is executed and delivered this
_______ day of        , 1997.


WITNESSES:                        NATIONAL ENERGY GROUP, INC.
                                  
                                  
                                  By:
-----------------------              -------------------------
                                  Name:
-----------------------                -----------------------



                                       2
<PAGE>   7
      PARTIAL ASSIGNMENT OF DEFERRED LEASEHOLD INTEREST IN OIL, GAS AND
                                MINERAL LEASE


STATE OF                  }

                                           KNOW ALL MEN BY THESE PRESENTS, THAT:

PARISH OF                 }


         WHEREAS, National Energy Group, Inc. a Delaware corporation, duly
qualified to transact business in the State of Texas, hereinafter referred to
as "Assignor", is the present owner and holder of those certain Oil, Gas and
Mineral Leases situated in                          ,described on Exhibit "A",
comprising the                   ("Prospect") which is attached hereto and made
a part hereof, and being hereinafter referred to as "Subject Leases".

         NOW, THEREFORE, in consideration of the sum of TEN DOLLARS ($10.00)
and other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, National Energy Group, Inc. sometimes herein referred
to as "Deferred Leasehold Interest Assignor", has GRANTED, SOLD, TRANSFERRED,
ASSIGNED and CONVEYED, and by these presents do hereby GRANT, SELL, TRANSFER,
ASSIGN and CONVEY unto Potosky Oil and Gas, Inc. and Atocha Oil & Gas, Inc.,
collectively hereinafter sometimes referred to as "Deferred Leasehold Interest
Assignees", an     % leasehold interest in and to Subject Leases, in the
proportions set out below, this assignment to be effective and operative after
the occurrence of "Payout", as hereinafter defined.

PROSPECT               POTOSKY OIL AND GAS, INC.         ATOCHA OIL & GAS, INC.


         "Payout", as such term is herein used shall mean that point in time
when National Energy Group, Inc. has recovered from production from wells
drilled on the Subject Lease (less the production taxes, the Lessee's royalty
and overriding royalties payable out of production) the following out of pocket
costs paid solely by National Energy Group, Inc.: the geological/geophysical
services fee for the Prospect, the costs incurred in purchasing or shooting the
additional seismic data or in the reprocessing of any seismic data by SOG are
hereinafter referred to as the "Additional Seismic Data Costs" for the Prospect,
the costs incurred in such leasing or acquisition, including without limitation
brokerage costs, lease bonuses, rentals, costs of title examination and title
curative matters, are hereinafter referred to as the "Leasing Costs" for the
Prospect, the Prospect generation fee for the Prospect, the costs incurred in
drilling, testing, completing and equipping wells drilled on the Prospect, the
costs incurred in plugging and abandoning wells drilled on the Prospect and the
costs incurred in operating wells located on the Prospect during the time of
recovery of said costs less any fees or expenses paid to National Energy Group,
Inc. by others whether considered a reimbursement, Prospect Promotion, service
fee or otherwise. Such costs shall be determined in accordance with the
accounting procedure which is attached to the applicable operating agreement.

         This assignment and conveyance is made without warranty of title,
either expressed or implied, except as to conveyances or encumbrances by,
through or under Assignor.

         The provisions hereof shall inure to the benefit of and be binding
upon the parties hereto, their respective successors and assigns.

         IN WITNESS WHEREOF, this assignment is executed on the date of the
acknowledgments, but made effective for all purposes as of the _______ day of
_________, 1997, at _________ , Central Daylight Time, in the presence of the
competent witness subscribing their names hereto.
<PAGE>   8

WITNESSES                      NATIONAL ENERGY GROUP, INC.
                               

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

---------------------------
Name:                          
     ----------------------                          
                               


WITNESSES                      POTOSKY OIL AND GAS, INC.


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

---------------------------                               
Name:                          
     ----------------------
                               
                               

WITNESSES                      ATOCHA OIL & GAS, INC.


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


STATE OF TEXAS

COUNTY OF ___________

         On this ________ day of ________________, 1997, before me appeared
___________________,to me personally known, who being by me duly sworn did say
that he is the __________________________ of National Energy Group, Inc., a
Delaware corporation, and that said instrument was signed in behalf or the
corporation by authority of its Board of Directors, and that ___________
acknowledged the instrument to be the free act and deed of the corporation.



                                  -------------------------------------------
                                  Name (print):
                                               ------------------------------
                                  Notary Public in and for the State of Texas
                                  My Commission Expires:
                                                        ---------------------


STATE OF TEXAS

COUNTY OF HARRIS

         On this ___________ day of _____________, 1997, before me appeared
__________, to me personally known, who being by me duly sworn did say that he
is the ___________________ of Potosky Oil and Gas, Inc., a Texas corporation,
and that said instrument was signed in behalf or the corporation by authority of
its Board of Directors, and that ____________________ acknowledged the
instrument to be the free act and deed of the corporation.



                                  -------------------------------------------
                                  Name (print):
                                               ------------------------------
                                  Notary Public in and for the State of Texas
                                  My Commission Expires:
                                                        ---------------------
<PAGE>   9
STATE OF TEXAS

COUNTY OF HARRIS

         On this ____________ day of ________________, 1997, before me appeared
________________________, to me personally known, who being by me duly sworn did
say that he is the ______ of Atocha Oil & Gas, Inc. a Louisiana corporation, and
that said instrument was signed in behalf of the corporation by authority of its
Board of Directors, and that _____________________ acknowledged the instrument
to be the free act and deed of the corporation.




                                  -------------------------------------------
                                  Name (print):
                                               ------------------------------
                                  Notary Public in and for the State of Texas
                                  My Commission Expires:
                                                        ---------------------
<PAGE>   10

                                  EXHIBIT "B"

                    [NATIONAL ENERGY GROUP, INC. LETTERHEAD]


                               December 20, 1996

Mr. Robert Potosky
Potosky Oil & Gas
1201 Louisiana, Suite 1050
Houston, Texas 77002

Sandefer Oil & Gas
Two Houston Center
Suite 3250
Houston, Texas 77010

Mr. W. David Willig
Atocha Exploration, Inc.
1201 Louisiana, Suite 1050
Houston, Texas 77002

Re:      East Bayou Sorrel Prospect
         Iberville Parish, Louisiana

Gentlemen:

Pursuant to that certain Joint Operating Agreement ("JOA") dated December 15,
1995, between National Energy Group, Inc.  ("NEG") (successor in interest to
W&T Offshore, Inc.) as Operator and Supply Development Group, Inc. et al., as
Non-Operator, pertaining to the East Bayou Sorrel Prospect, Sandefer Oil & Gas
("SOG"), Robert Potosky ("Potosky") and W. David Willig ("Willig") are entitled
to a certain overriding royalty ("ORRI") and Potosky Oil & Gas ("POG") and
Atocha Exploration, Inc. ("Atocha") are entitled to a reversionary back-in
interest ("Back-In") pursuant to the terms of such JOA.

WHEREAS, it is the intention of the parties that the ORRI and Back-In shall be
paid as consideration for generation and presentation of new oil and gas
exploration prospects; and

WHEREAS, NEG has acquired certain oil and gas leases from PANACO, Inc.
("PANACO") which are partially subject to the Area of Interest ("AOI")
provisions of the JOA and which include oil and gas wells with producing
horizons down to a depth of 11,000 feet; and
<PAGE>   11
Mr. Robert Potosky
Potosky Oil & Gas
Sandefer Oil & Gas
Mr. W. David Willig
Atocha Exploration, Inc.
December 20, 1996
Page Two





WHEREAS, the parties agree SOG, Potosky, Willig, POG and Atocha are entitled to
their respective ORRI and Back-In as consideration for generation and
presentation of new oil and gas prospects related to certain portions of the
PANACO acquisition; and

WHEREAS, the parties desire to clarify their rights and obligations with
respect to the ORRI to be earned with respect to production acquired in the
PANACO acquisition.

NOW THEREFORE, in consideration of the mutual promises and agreements set forth
herein, the above recitals and other good and valuable consideration, the
receipt and sufficiency of which is hereby acknowledged, the parties hereto
agree as follows:

1.       It is agreed with respect to the leases acquired in the PANACO
         acquisition which are within the AOI (as defined in the JOA), that it
         was not the intent of the parties, nor contemplated, to burden such
         leases with the ORRI for established production (being defined as any
         productive or once productive sand as described in State of Louisiana
         Conservation Department Unit Orders) included in any and all existing
         well bores which fall within certain existing Louisiana Department of
         Conservation units; specifically the following as may be described on
         Exhibit "A", attached hereto:

                 C2 RA SUB; C2 RA SUA; D RA SUA; E ZONE
                 RA SUA, SUB, AND SUC; H2 RA SUA; I RB SUA
                 K RA SUA; Y RA SUA (hereinafter " Existing Units").

         However, to further clarify, the following exceptions are subject to
         the ORRI as defined in the JOA:

         (a)     For existing wells within the Existing Units, and for any well
                 drilled hereafter within the Existing Units, all rights below
                 the L4 Sand, the base of which is defined as occurring in the
                 National Energy Group, Inc. Schwing #1 well (2-T1OS-R11E)
                 at the measured log depth of 11,340' shall be subject to the
                 ORRI.

         (b)     With respect to new wells drilled in those portions of
                 sections 3, 4, 5 and 6-T1OS-R11E that fall within the
                 Existing Units, all rights, except those unitized rights
                 pertaining to the Existing Units and only with respect to the
                 sand interval pertaining to such unit, shall be subject to the
                 ORRI.
<PAGE>   12
Mr. Robert Potosky
Potosky Oil & Gas
Sandefer Oil & Gas
Mr. W. David Willig
Atocha Exploration, Inc.
December 20, 1996
Page Three





         (c)     All rights determined by the State of Louisiana Department of
                 Conservation unitization proceedings to be upthrown and fault
                 separated from the Existing Units by Fault "1" as documented
                 by established State of Louisiana Conservation Department
                 adopted unitized geology shall be subject to the ORRI,
                 irrespective of depth or formation.

         (d)     For any wells drilled outside the Existing Units but within
                 the AOI, all rights shall be subject to an ORRI on 8/8ths of
                 the production from any such well.

2.       Accordingly, each of SOG, Potosky, Willig, POG, and Atocha for
         themselves on behalf of their respective successors and assigns, waive
         all claims to any ORRI with respect to the PANACO acquisition derived
         from established production included in any and all existing well
         bores which fall within the Existing Units and which fall within the
         AOI except for the exceptions stated in Paragraphs 1 (a) through (d)
         above.

3.       Nothing contained in this correspondence shall act to supersede,
         change, amend or otherwise modify  the terms and provisions of the
         JOA, except as otherwise provided herein.

4.       This agreement may be executed in several counterparts, each of which
         will be deemed to be an  original, but all of which together will
         constitute one and the same instrument.

If the foregoing correctly states our mutual understanding and agreement,
please so indicate your acceptance by executing in the appropriate space below.

Sincerely,
National Energy Group, Inc.


/s/ WILLIAM T. JONES
William T. Jones
Title: Senior Vice President

WTJ:1jg

cc: Miles D. Bender (President and CEO)
<PAGE>   13
Mr. Robert Potosky
Potosky Oil & Gas
Sandefer Oil & Gas
Mr. W. David Willig
Atocha Exploration, Inc.
December 20, 1996
Page Four





ACCEPTED AND AGREED
THIS 20TH DAY OF DECEMBER, 1996.


Robert A. Potosky, an Individual

/s/ ROBERT A. POTOSKY                 
-------------------------------------
By: ROBERT A. POTOSKY


Potosky Oil & Gas

/s/ ROBERT A. POTOSKY                  
-------------------------------------
By: /s/ ROBERT A. POTOSKY
Title: President


Sandefer Oil & Gas

/s/ STEPHEN F. SMITH                   
-------------------------------------
By: STEPHEN F. SMITH
Title: EXECUTIVE VICE PRESIDENT


W. David Willig, an Individual

/s/ W. DAVID WILLIG                     
-------------------------------------
By: W. DAVID WILLIG


Atocha Exploration, Inc.

/s/ W. DAVID WILLIG                     
-------------------------------------
By: W. DAVID WILLIG
Title: PRESIDENT
<PAGE>   14
                          [BAYOU SORREL FIELD DIAGRAM]


                                  EXHIBIT "A"