<PAGE>
 
Prepared by and upon                                                 EXHIBIT 4.3
recordation please return to:
Gray H. Muzzy
Bracewell & Patterson, L.L.P.
711 Louisiana, Suite 2900
Houston, Texas 77002

                                     ACT OF
                          FIRST AMENDMENT OF MORTGAGE,
                SECURITY AGREEMENT, ASSIGNMENT OF PRODUCTION AND
                              FINANCING STATEMENT



THE STATE OF TEXAS       (S)
                         (S)
COUNTY  OF  HARRIS       (S)


     Be it known that on this 23rd day of December, 1996, before me, the
     undersigned Notary Public, duly commissioned and qualified in and for the
     County and State aforesaid; and therein residing, and in the presence of
     the undersigned competent witnesses, whose names are subscribed hereto,
     personally came and appeared:

     FORMAN PETROLEUM CORPORATION, a Louisiana corporation, whose Federal Tax
     Identification Number is 72-0954774 with a mailing address of 650 Poydras
     Street, Suite 2200, New Orleans, Louisiana, 70130-6101, appearing herein
     through Marvin J. Gay, its duly authorized Treasurer, acting pursuant to
     resolutions of the Board of Directors of such corporation, a certified
     extract of which are attached hereto ("Mortgagor"), and

     JOINT ENERGY DEVELOPMENT INVESTMENTS LIMITED PARTNERSHIP, a Delaware
     limited partnership, whose Federal Tax Identification Number is 76-0407964
     with a mailing address of 1400 Smith Street, Houston, Texas 77002,
     appearing herein through its general partner Enron Capital Management
     Limited, a Delaware limited partnership, appearing herein through its
     general partner Enron Capital Corp., a Delaware corporation, appearing
     herein through Wynne M. Snoots, Jr., its duly authorized Agent and
     Attorney-in-Fact ("Mortgagee"),
<PAGE>
 
which Mortgagor and Mortgagee through their respective representatives declared
unto me, Notary, as follows:

     That, Mortgagor did execute that certain Act of Mortgage, Security
Agreement, Assignment of Production and Financing Statement dated November 21,
1996, before Diane Bailey, Notary Public, which was duly recorded in the Public
Records of Jefferson, LaFourche, St. Charles and Terrebonne Parishes, Louisiana,
as set forth on Exhibit "A" attached hereto and incorporated herein for all
purposes (the "Original Mortgage").

     That, the Original Mortgage was given as security for certain indebtedness
described therein owed by Mortgagor to Mortgagee, including any and all future
indebtedness owed by Mortgagor to Mortgagee.

     For and consideration of the benefits accruing to each of Mortgagor and
Mortgagee, Mortgagor and Mortgagee do hereby agree to amend and modify the
Original Mortgage effective for all purposes as of the date hereof, as follows:

     1.   Section 1.10.  The words "Wynne N. Snoots, Jr." in Section 1.10 are
hereby amended to read "Wynne M. Snoots, Jr." and the words "the General Partner
of the Lender" in Section 1.10 are hereby amended to read "the general partner
of the general partner of the Lender".

     2.   Section 1.12.  Section 1.12 is hereby amended in its entirety to read
as follows:

          "1.12   "MORTGAGE" means this Act of Mortgage, Security Agreement,
     Assignment of Production and Financing Statement, as amended by an Act of
     First Amendment of Mortgage, Security Agreement, Assignment of Production
     and Financing Statement dated effective as of  December 16, 1996, as it may
     be further amended, modified or supplemented from time to time."

     3.   Section 1.13.  Section 1.13 is hereby amended in its entirety to read
as follows:

          "1.13  "MORTGAGEE" means the Lender, its successors and assigns, and
     any legal owner, holder, assignee or pledgee of the Note."

     4.   Section 1.14.  Section 1.14 is hereby amended in its entirety to read
as follows:

          "1.14  "NOTE" means that certain Promissory Note of Mortgagor payable
     to the order of the Lender in the original principal amount of
     $10,000,000.00, dated as of December 16, 1996, and all modifications,
     amendments, renewals and extensions thereof."

                                      -2-
<PAGE>
 
     5.   Section 1.15.  Clauses (d), (e) and (f) of Section 1.15 are hereby
amended in their entirety to read as follows:

     "(d) all sums advanced or costs or expenses incurred by Mortgagee, which
     are made or incurred pursuant to, or allowed by, the terms of this
     instrument or any other instrument executed in connection with the Note,
     including but not limited, that certain Loan Agreement by and between
     Mortgagor and Mortgagee dated effective as of the effective date of the
     Note, including but not limited to, all legal fees and all engineering and
     other costs incurred in connection with Mortgagee's due diligence, plus
     interest thereon from the date of the advance until reimbursement of
     Mortgagee charged at the Default Rate; (e) all renewals, extensions,
     amendments and substitutions of the above whether or not Mortgagor executes
     any renewal or extension agreement, any amounts due and payable by
     Mortgagor to ECT Securities Corp. and/or its successors and assigns,
     including Mortgagee; and (f) any amounts due and payable by Mortgagor under
     any Price Protection Agreement approved by Mortgagee, including any costs
     and expenses incurred by any counter-party in connection therewith."

     6.   Section 1.21.  A new Section 1.21 is hereby added to the Mortgage as
follows:

          "1.21  "PRICE PROTECTION AGREEMENTS" means each price swap agreement,
     option agreement or other agreement entered into by and between Mortgagor
     and Mortgagee or a counter-party acceptable to Mortgagee pursuant to which
     the price of Hydrocarbons is hedged by the Mortgagor."

     7.   Section 3.2(c).  Section 3.2(c) is amended in its entirety to read as
follows:

          "(c)  Third, to the payment of any then due and owing principal
     constituting part of the Obligations and any other Obligations then due and
     payable; and"

     8.   Section 4.4(m).  Section 4.4(m) is amended by deleting the words ",
including reasonable expenditures necessary for completion of the current
workover operations in the LaFourche Parish School Board No.  1 Well", and
adding the following new sentence to the end thereof:

          "Notwithstanding the foregoing, in the event that the Mortgagor's
     EBITDA for any calendar month following the date hereof, as evidenced by
     the monthly Pro Forma Cash Flow Statements to be provided by the Mortgagor
     to the Mortgagee in accordance with the Loan Agreement, should ever exceed
     $1,000,000, the Mortgagor may use up to $120,000 of such EBITDA for such
     month in excess of said $1,000,000, as partial payment of any indebtedness
     of the Mortgagor to McLain J. Forman, so long as McLain J. Forman has any
     outstanding indebtedness to Whitney National Bank; provided that the
     Mortgagor may not 

                                      -3-
<PAGE>
 
     use such cash for repayment of indebtedness to McLain J. Forman if, and so
     long as, a Default (as defined in that certain Loan Agreement by and
     between Mortgagor and Mortgagee dated of even date with the Note (the "Loan
     Agreement")) is outstanding and not cured. For purposes of the foregoing,
     "EBITDA" shall be calculated in a manner consistent with the methodologies
     reflected in the Pro Forma Cash Flow Report attached to the Loan Agreement,
     including sources of income and all cost and expense items included in the
     Pro Forma Cash Flow Report attached to the Loan Agreement".

     9.   Section 5.1(a).  Subsection 5.1(a) is hereby amended in its entirety
to read as follows:

          "(a)  Mortgagor or any endorser, guarantor, surety, accommodation
     party, or other person liable upon or for payment of any of the Obligations
     secured hereby fails to pay when due any of the Obligations secured hereby
     or to perform punctually any other obligation, covenant, term or provision
     contained in or referred to in this instrument or any other instrument
     executed in connection herewith, including without limitation, the
     occurrence of an "Event of Default" as defined in that certain Loan
     Agreement by and between Mortgagor and Mortgagee dated of even date with
     the Note."

     10.  Section 5.2.  Section 5.2 is hereby amended by deleting the following
words from the first sentence thereof:

     "In addition to the right and privilege to demand payment of Note at any
     time, as set forth therein, and without limiting the Mortgagee's rights and
     privileges thereunder."

     11.  Section 6.12.  The words "c/o Enron Corp." and "Attn: Wynne N. Snoots"
in the addresses for Mortgagee  in Section 6.12 are hereby amended to read "c/o
Enron Capital Corp." and "Attn: Wynne M. Snoots, Jr.", respectively.

     12.  Ratification and Reaffirmation.  Mortgagor does hereby ratify the
Mortgage as being in full force and effect, does hereby covenant and agree that,
except as set forth herein the Mortgage has not been amended or modified, and
does hereby reaffirm each and every representation and warranty set forth
therein as being true and correct as of the date hereof.  Mortgagor does hereby
affirm unto Mortgagee that Mortgagor, as of the date hereof, has timely
performed each and every covenant and obligation under the Mortgage, and that no
Event of Default as defined therein, has occurred.  This amendment shall be
effective for all purposes as of December 16, 1996.

     Thus done and passed in my office in Houston, Harris County, Texas, this
23rd day of December, 1996, in the presence of the undersigned competent
witnesses who hereunto sign their names with the Mortgagor, Mortgagee and me,
Notary, after due reading of the whole.

                                      -4-
<PAGE>
 
                                  MORTGAGOR

Witnesses to all signatures:      FORMAN PETROLEUM CORPORATION


  /s/ Gray H. Muzzy
 -----------------------------
Name: Gray H. Muzzy               By:   /s/ McLain J. Forman
      ------------------------       -------------------------------
                                            McLain J. Forman
  /s/ Harold C. Block                       President
 -----------------------------
Name:  Harold C. Block
     -------------------------

                                  MORTGAGEE:


                                  JOINT ENERGY DEVELOPMENT      
                                  INVESTMENTS LIMITED PARTNERSHIP,
                                  a Delaware limited partnership 

                                  By:  ENRON CAPITAL MANAGEMENT,
                                       LIMITED PARTNERSHIP, a Delaware
                                       limited partnership, its general 
                                       partner 

                                       By:  ENRON CAPITAL CORP., a
                                            Delaware corporation, its general
                                            partner


                                         By:   /s/ Wynne M. Snoots, Jr.
                                            ---------------------------------
                                                   Wynne M. Snoots, Jr.
                                                   Agent and Attorney-in-Fact

Stamp:
Judy M. Sullivan                          /s/ Judy M. Sullivan
Notary Public - State of Texas         ---------------------------
My Commission                                 Notary Public
Expires 7/3/99                  
                                


   (Signature Page to Act of First Amendment of Mortgage, Security Agreement,
               Assignment of Production and Financing Statement)

                                      -5-
<PAGE>
 
                      CERTIFICATE OF CORPORATE RESOLUTIONS

The undersigned, Secretary of FORMAN PETROLEUM CORPORATION, a Louisiana
corporation ("Company"), does hereby certify to JOINT ENERGY DEVELOPMENT
INVESTMENTS LIMITED PARTNERSHIP ("Lender") as follows:

     1.  The following is a true and correct copy of the resolutions duly
     adopted by the Board of Directors of the Company, and such resolutions have
     not been altered, amended, rescinded or repealed and are now in full force
     and effect:

               WHEREAS, the Company did execute a Promissory Note in the
          original principal amount of $10,000,000.00, payable to the order of
          Lender (the "Note"); and

               NOW THEREFORE, the Board of Directors of the Company does hereby
          ratify and reaffirm the execution and delivery of the Note to the
          Lender and does hereby ratify and reaffirm all covenants and
          obligations of the Company thereunder; and all liens and other
          collateral heretofore given as security for the payment of the Note
          (as hereinafter defined), including specifically, without limitation,
          the Mortgage and the Financing Statements (each as hereinafter
          defined); and

               RESOLVED, that the Board of Directors of the Company does hereby
          authorize the Company to execute and deliver to the Lender (i) an Act
          of First Amendment of Mortgage, Security Agreement, Assignment of
          Production and Financing Statement covering certain properties owned
          by the Company as described therein (the "Mortgage"), (ii) Financing
          Statements perfecting the security interests created by the Mortgage
          (the "Financing Statements"), and (iii) the Note, all in substantially
          the forms presented to this meeting, with such changes therein as the
          person executing the same shall approve, such approval to be
          conclusively evidenced by his execution thereof; and further

               RESOLVED, that each officer of the Company is hereby authorized
          to execute and deliver on behalf of the Company, in such forms as the
          Lender may require any and all other agreements, instruments and
          documents which may be requested or required by the Lender to take any
          and all other action relating to or in connection with the Note, the
          Mortgage, or the Financing Statements; and further

               RESOLVED, that any and all documents, instruments and agreements
          executed by an officer of the Company pursuant to these resolutions
          may contain such terms and conditions as the person executing the same
          shall approve, including confessions of judgment, pacts de non
          alienando, waivers of appraisement, and 

                                      -6-
<PAGE>
 
          waivers of notice and delay, such approval to be conclusively
          evidenced by his execution thereof; and further

               RESOLVED, that the signature of any officer of the Company on any
          agreement, instrument or document is sufficient to bind the Company,
          and no other signature shall be required; and further

               RESOLVED, that the Lender may rely on these resolutions and these
          resolutions shall remain in full force and effect until such time as
          notice to the contrary is duly delivered to the Lender and receipted
          for in writing by a Vice President of the Lender.

     2.  The person who, as an officer of the Company, executed the Note, the
     Mortgage, and the Financing Statements that are referred to in such
     resolutions was at the time of such signing and delivery, and is now duly
     elected, qualified and acting as such officer and the signature appearing
     on such Note, the Mortgage, and the Financing Statements, is the genuine
     signature of such officer.

     3.  The Note, the Mortgage, and the Financing Statement actually executed
     by the Company and delivered to the Lender are in substantially the forms
     of the documents submitted to and approved by the Board of Directors of the
     Company pursuant to such resolutions.

     IN WITNESS HEREOF, I have hereunto signed my name and set the seal of the
Company this 23rd of December, 1996.


                                    By:  /s/ Susan N. Richards
                                       ---------------------------------
                                       Susan N. Richards, Secretary

                                      -7-
<PAGE>
 
                             NOTARIAL CERTIFICATION


     I certify that a copy of these resolutions, certified by the Secretary of
Forman Petroleum Corporation is attached to an Act of First Amendment of
Mortgage, Security Agreement, Assignment of Production and Financing Statement
executed by said corporation before me, Notary this 23rd day of December 1996.



                                     /s/ Thomas J. Say
                                    ----------------------------------
                                    Notary Public
                                    For the State of Louisiana
                                    My commission expires at death

                                      -8-
<PAGE>
 
                           SCHEDULE OF RECORDING DATA

     Act of First Amendment of Mortgage, Security Agreement, Assignment of
     Production and Financing Statement dated December 23, 1996, executed by
     Forman Petroleum Corporation, passed before Judy M. Sullivan, Notary
     Public, recorded in the following parishes in the State of Louisiana:
<TABLE>
<CAPTION>
 
          Parish             Book       Page/Folio  Entry No.
          ------         -------------  ----------  ---------
<S>                      <C>            <C>         <C>
 
          Jefferson      Mineral Lease
                         Book 136           166      96-68975
                         MOB 3778           580      96-68975
                                                   
          Lafourche      COB 1293           358        807596
                         MOB 731            136        807596
                                                   
          St. Charles    COB 516            659        207615
                         MOB 624            583        207615
 
          Terrebonne     COB 1538                      989317
                         MOB 1085                      989317
 
</TABLE>

                                      -9-