<PAGE>
 
EXHIBIT 10.1.7

                            SUPPLEMENTAL INDENTURE


                          DATED AS OF MARCH 31, 1997

                                     AMONG

                         GERRITY OIL & GAS CORPORATION

                         PATINA OIL & GAS CORPORATION

                                      AND

                           THE CHASE MANHATTAN BANK
                      (FORMERLY KNOWN AS CHEMICAL BANK),
                                  AS TRUSTEE



                   11 3/4% SENIOR SUBORDINATED NOTES DUE 2004
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     This Supplemental Indenture dated as of March 31, 1997 is among Gerrity Oil
& Gas Corporation, a Delaware corporation (the "Company"), Patina Oil & Gas
Corporation, a Delaware corporation ("Patina"), and The Chase Manhattan Bank
(formerly known as Chemical Bank), as Trustee, and supplements, amends and
modifies that certain Indenture dated as of  June 30, 1994 between the Company
and the Trustee (the "Indenture"):

                                   RECITALS:

     WHEREAS, The Company is a wholly owned subsidiary of Patina;

     WHEREAS, the Company will merge with and into Patina (the "Merger")
pursuant to a Certificate of Ownership and Merger to be filed with the Secretary
of State of the State of Delaware;

     WHEREAS, As a result of the Merger, Patina will be the corporation
surviving the Merger, and the outstanding capital stock of the Company will be
converted into capital stock of Patina;

     WHEREAS, the Merger will be effected pursuant to Section 253 of the General
Corporation Law of the State of Delaware ("DGCL"), which permits effectuation of
such a merger without a vote of stockholders of either constituent corporation
and without any action by the Board of Directors of the Company;

     WHEREAS, Patina and the Company intend that Patina shall assume the
obligations of the Company with respect to the Indenture;

     WHEREAS, the Merger complies with Section 5.01 of the Indenture because
Patina (the Person into which the Company is to be merged) is a corporation
organized and validly existing under the laws of the State of Delaware, and
because Patina, by its execution and delivery hereof, expressly assumes all of
the obligations of the Company on all of the Securities and under the Indenture;

     WHEREAS, the Company has delivered to the Trustee the Officer's Certificate
required by Clause (v) of Section 5.01 of the Indenture, including the
arithmetic computations demonstrating compliance with clauses (iii) and (iv) of
Section 5.01;

     WHEREAS, the Company has delivered to the Trustee the Opinion of Counsel
required by Clause (v) of Section 5.01 of the Indenture;

     WHEREAS, this Supplemental Indenture is executed and delivered pursuant to
Section 9.01(2) of the Indenture;

     NOW, THEREFORE, in consideration of the premises, the covenants herein
contained  and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged by the parties hereto, the parties
hereto covenant and agree as follows:
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                                   ARTICLE I

     Section 1.1    Definitions.  Capitalized terms used but not defined herein
are defined in the Indenture and are used herein with the meanings ascribed to
them therein.
 
     Section 1.2    Assumption of the Company's Obligations under the Indenture.
Patina shall, effective as of the effective time of the Merger under the DGCL
(the "Effective Time"), assume, and shall thereafter timely pay, perform and
discharge, each and every obligation of the Company under and with respect to
the Securities and the Indenture.

     Section 1.3    No Default.  Immediately after giving effect to the Merger,
no Event of Default, and no event that, after notice or lapse of time, or both,
would become an Event of Default, has happened or is continuing.

                                   ARTICLE II

     Section 2.1    Effectiveness.  Although this Supplemental Indenture may be
executed and delivered by the parties hereto prior thereto, the provisions
hereof shall not become effective unless and until the Merger becomes effective
under the DGCL and, under such circumstances, shall become effective
concurrently with the Effective Time of such Merger.  From and after the
Effective Time, the Indenture, as hereby supplemented, amended and modified,
shall remain in full force and effect.

     Section 2.2    References.  Each reference in the Indenture or this
Supplemental Indenture to any article, section, term or provision of the
Indenture shall mean and be deemed to refer to such article, section, term or
provision of the Indenture, as modified by this Supplemental Indenture, except
where the context otherwise indicates.

     Section 2.3    Benefit.  All the covenants, provisions, stipulations and
agreements contained in this Supplemental Indenture are and shall be for the
sole and exclusive benefit of the parties hereto, their successors and assigns,
and of the holders and registered owners from time to time of the Securities, as
hereby amended and supplemented.

     Section 2.4    Counterparts. This Supplemental Indenture may be executed in
any number of counterparts, each of which shall be deemed an original and all of
which taken together shall be deemed to be a single instrument.

     Section 2.5    Governing Law.  This Supplemental Indenture shall be deemed
to be a contract under the laws of the State of New York, and for all purposes
shall be construed in accordance with the laws of such state without regard to
principles of conflicts of laws, except as may otherwise required by mandatory
provisions of law.

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     Section 2.6    Headings.  The Article and Section headings herein are for
convenience only and shall not affect the construction hereof.

     Section 2.7    Recitals.  The recitals contained herein shall be taken as
statements of Patina, and the Trustee assumes no responsibility for their
correctness. The Trustee makes no representations as to the validity or
sufficiency of this Supplemental Indenture.

     IN WITNESS WHEREOF, the said Gerrity Oil & Gas Corporation, Patina Oil &
Gas Corporation and The Chase Manhattan Bank have each caused this Supplemental
Indenture to be executed in its corporate name by the officer whose name is
subscribed below, all as of the day and year first above written.

                                    GERRITY OIL & GAS CORPORATION


                                    By:   /s/ Brian J. Cree
                                          ----------------------------------
                                    Name: Brian J. Cree,
                                    Title:  Executive Vice President and
                                             Chief Operating Officer

                                    PATINA OIL & GAS CORPORATION


                                    By:   /s/ David J. Kornder
                                          ----------------------------------
                                    Name: David J. Kornder,
                                    Title:  Vice President

 

                                    THE CHASE MANHATTAN BANK,
                                           as Trustee


                                    By:   /s/ Wanda Eiland
                                          ----------------------------------
                                    Name: Wanda Eiland,
                                    Title:  Trust Officer

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