<PAGE>

                                                                    EXHIBIT 10.5

                           AMENDMENT NO. 3 AND CONSENT

      This Amendment No. 3 and Consent ("Agreement") dated as of March 3, 2005
("Effective Date") is among Mariner Energy, Inc., a Delaware corporation (the
"Borrower"), Mariner LP LLC, Mariner Energy Texas LP, and the Lenders (as
defined below), and Union Bank of California, N.A., as administrative agent for
such Lenders (in such capacity, the "Administrative Agent") and as issuing
lender for such Lenders (in such capacity, the "Issuing Lender").

                                    RECITALS

      A. The Borrower, the Lenders, Issuing Lender and the Administrative Agent
are parties to the Credit Agreement dated as of March 2, 2004, as amended by
that certain Amendment No. 1 and Assignment Agreement dated as of July 14, 2004
and that certain Amendment No. 2 and Consent dated as of February 7, 2005 (as so
amended, the "Credit Agreement").

      B. The Borrower proposes to issue its Equity Interests (as defined in the
Credit Agreement) in a private offering and sale (the "Offering") and to use a
portion of the proceeds of such Offering to purchase and retire most of the
Equity Interests of the Borrower owned by MEI Acquisitions Holdings, LLC ("MEI
Holdings").

      C. Pursuant the Credit Agreement, the payments to be made by the Borrower
to MEI Holdings to retire such Equity Interests in the Borrower constitute
"Restricted Payments" which are not permitted to be made under Section 6.05 of
the Credit Agreement without the prior consent of the Required Lenders (as
defined in the Credit Agreement).

      D. In connection with such Offering, the Borrower also wishes to (i) amend
and restate in its entirety its certificate of incorporation in the form
attached hereto as Exhibit "A" (the "Amended and Restated Certificate") and
amend and restated in its entirety its bylaws in the form attached hereto as
Exhibit "B" (the "Amended and Restated Bylaws"), and (ii) amend the definition
of Change in Control to accommodate the different parties that will own Equity
Interests in the Borrower as a result of such Offering.

      E. At the request of the Borrower, the Administrative Agent and the
Required Lenders wish to, subject to the terms and conditions of this Agreement,
(i) consent to the making of such Restricted Payments to MEI Holdings, (ii)
consent amendments and restatements of the Borrower's certificate of
incorporation and bylaws, and (iii) amend certain provisions of the Credit
Agreement as set forth herein.

      THEREFORE, the Borrower, the subsidiaries of the Borrower signatory hereto
(the "Guarantors"), the Required Lenders, the Issuing Lender and the
Administrative Agent hereby agree as follows:

<PAGE>

                                   ARTICLE I.
                                   DEFINITIONS

      SECTION 1.01 TERMS DEFINED ABOVE. As used in this Agreement, each of the
terms defined in the opening paragraph and the Recitals above shall have the
meanings assigned to such terms therein.

      SECTION 1.02 TERMS DEFINED IN THE CREDIT AGREEMENT. Each term defined in
the Credit Agreement and used herein without definition shall have the meaning
assigned to such term in the Credit Agreement, unless expressly provided to the
contrary.

      SECTION 1.03 OTHER DEFINITIONAL PROVISIONS. The words "hereby", "herein",
"hereinafter", "hereof", "hereto" and "hereunder" when used in this Agreement
shall refer to this Agreement as a whole and not to any particular Article,
Section, subsection or provision of this Agreement. Article, Section, subsection
and Exhibit references herein are to such Articles, Sections, subsections and
Exhibits of this Agreement unless otherwise specified. All titles or headings to
Articles, Sections, subsections or other divisions of this Agreement or the
exhibits hereto, if any, are only for the convenience of the parties and shall
not be construed to have any effect or meaning with respect to the other content
of such Articles, Sections, subsections, other divisions or exhibits, such other
content being controlling as the agreement among the parties hereto. Whenever
the context requires, reference herein made to the single number shall be
understood to include the plural; and likewise, the plural shall be understood
to include the singular. Words denoting sex shall be construed to include the
masculine, feminine and neuter, when such construction is appropriate; and
specific enumeration shall not exclude the general but shall be construed as
cumulative. Definitions of terms defined in the singular or plural shall be
equally applicable to the plural or singular, as the case may be, unless
otherwise indicated.

                                  ARTICLE II.
                                    CONSENTS

      SECTION 2.01 CONSENT; ACKNOWLEDGMENT; AGREEMENT. Subject to the terms of
this Agreement, the Lenders hereby consent to the making of Restricted Payments
by the Borrower to MEI Holdings; provided that (a) such Restricted Payments are
made solely from the proceeds of the Offering, (b) such Restricted Payments are
made solely as consideration for the retirement of Equity Interests held by MEI
Holdings, and (c) after giving effect to the Offering, no "Change in Control" as
defined in the Credit Agreement, as amended, hereby shall have occurred. The
Administrative Agent hereby consents to (i) the amendment of the Borrower's
certificate of incorporation as set forth in the Amended and Restated
Certificate and (ii) the amendment to the Borrower's bylaws as set forth in the
Amended and Restated Bylaws. Each of the Lenders hereby acknowledges and agrees
to such consent by the Administrative Agent. The consents by the Lenders and by
the Administrative described in this Section 2.01 are collectively referred to
herein as the "Consents." The Consents by the Lenders and by the Administrative
Agent described in this Section 2.01 are contingent upon the satisfaction of the
condition described in the proviso set forth in the first sentence of this
Section 2.01 and the conditions precedent set forth below. Such Consents are
strictly limited to the extent described herein. Nothing contained herein shall
be construed to be a consent to or a permanent waiver of the Sections covered by
the Consents provided for herein or any other terms, provisions, covenants,
warranties or agreements contained in the Credit Agreement or in any of the
other Loan

                                       2-

<PAGE>

Documents. The Lenders reserve the right to exercise any rights and remedies
available to them in connection with any other present or future defaults with
respect to the Credit Agreement or any other provision of any Loan Document. The
descriptions herein of the Consents are based upon the information provided to
the Lenders on or prior to the date hereof and shall not be deemed to exclude
the existence of any Defaults or Events of Default. The failure of the Lenders
to give notice to the Borrower or the Guarantors of any such Defaults or Events
of Default is not intended to be nor shall be a waiver thereof. The Borrower
hereby agrees and acknowledges that the Lenders require and will require strict
performance by the Borrower of all of its obligations, agreements and covenants
contained in the Credit Agreement and the other Loan Documents, and no inaction
or action regarding any Default or Event of Default is intended to be or shall
be a waiver thereof.

                                  ARTICLE III.
                          AMENDMENT TO CREDIT AGREEMENT

      SECTION 3.01 AMENDMENTS TO SECTION 1.01.

            (a) New Defined Terms: Section 1.01 of the Credit Agreement is
hereby amended by adding the following new defined terms in alphabetical order:

            "Offering" shall mean that certain private offering and sale of
      Equity Interests in the Borrower the proceeds of such are used to, in
      part, purchase and retire a portion of the Equity Interests of the
      Borrower held by MEI Acquisitions Holdings, LLC .

            "Permitted Holders" shall mean, collectively, MEI Holdings, ACON
      Investments LLC, Riverstone Holdings, LLC and their respective Affiliates
      (other than portfolio companies of such institutions), Affiliates of the
      Administrative Agent and employees and directors of the Borrower.

            "Wholly-Owned Subsidiary" shall mean a Subsidiary which is (a) owned
      100%, either directly or indirectly, by the Borrower, (b) a consolidated
      Subsidiary of the Borrower, and (c) a Guarantor.

            (b) Revision to Defined Term. Section 1.01 of the Credit Agreement
is hereby further amended by deleting the definitions of "Change in Control" and
"Project Company Mortgage" and replacing them in their entirety with the
following, respectively:

            "Change in Control" shall mean the occurrence of any of the
      following events:

            (a) the Borrower shall cease to, directly or indirectly, own 100% of
      the Equity Interests of its Subsidiaries;

            (b) prior to the closing of the Offering, the Permitted Holders
      cease to collectively own more than 50% of the Equity Interests of the
      Borrower;

            (c) after the closing of the Offering, any "person" (as such term is
      used in Section 13(d) and 14(d) of the Exchange Act), or related persons
      constituting a "group" (as such term is used in Rule 13d-5 under the
      Exchange Act), other than a Permitted Holder, is or becomes or has the
      absolute,

                                       3-

<PAGE>

      unconditional right to become the "beneficial owner" (as defined in Rules
      13d-3 and 13d-5 under the Exchange Act), directly or indirectly, of 30% or
      more of the total voting power of the voting stock of the Borrower;

            (d) during any period of 12 consecutive months occurring after the
      Closing Date, a majority of the members of the board of directors or other
      equivalent governing body of the Borrower ceases to be composed of
      individuals (i) who were members of that board or equivalent governing
      body on the first day of such period, (ii) whose election or nomination to
      that board or equivalent governing body was approved by individuals
      referred to in clause (i) above constituting at the time of such election
      or nomination at least a majority of that board or equivalent governing
      body or (iii) whose election or nomination to that board or other
      equivalent governing body was approved by individuals referred to in
      clauses (i) and (ii) above constituting at the time of such election or
      nomination at least a majority of that board or equivalent governing body
      (excluding, in the case of both clause (ii) and clause (iii), any
      individual whose initial nomination for, or assumption of office as, a
      member of that board or equivalent governing body occurs as a result of an
      actual or threatened solicitation of proxies or consents for the election
      or removal of one or more directors by any person or group other than a
      solicitation for the election of one or more directors by or on behalf of
      the board of directors); or

            (e) Scott Josey ceases to be the Chief Executive Officer or
      President of the Borrower or to be actively engaged in the executive
      management of the Borrower and is not replaced with an individual of
      comparable qualifications within six months after he ceases to be the
      Chief Executive Officer or President of the Borrower or to be actively
      engaged in the executive management of the Borrower (and the Borrower
      hereby agrees to consult with the Lenders regarding any such potential
      replacement).

            "Project Company Mortgage" means, collectively, the (a) the
      Mortgage, Security Agreement, Assignment of Production, Financing
      Statement (Moving and Personal Property including Hydrocarbons) and
      Fixture Filing dated as of March 2, 2004 by the Borrower to JEDI, (b) the
      Deed of Trust, Security Agreement, Assignment of Production, Financing
      Statement (Moving and Personal Property including Hydrocarbons) and
      Fixture Filing dated as of March 2, 2004 by the Borrower to JEDI, and (c)
      any other mortgage or deed of trust given to JEDI solely in replacement of
      the mortgage or deed of trust described in clause (a) or (b) above and
      given by any Wholly-Owned Subsidiary that acquired the JEDI Collateral in
      accordance with Section 6.04 of this Agreement, and, in case of clause
      (a), (b) and (c) above, copies of which have been delivered to the
      Administrative Agent and which shall only encumber the JEDI Collateral.

      SECTION 3.02 AMENDMENT TO SECTION 6.04- MERGER OR CONSOLIDATION; ASSET
SALES. Section 6.04 is hereby amended by deleting the period appearing at the
end thereof and adding the following new clause to the end thereof:

                                       4-

<PAGE>

            "and (v) the sale or transfer of Property from the Borrower to a
      Wholly-Owned Subsidiary, from a Wholly-Owned Subsidiary to the Borrower,
      or from a Wholly-Owned Subsidiary to another Wholly-Owned Subsidiary
      provided that (A) if prior to such sale or transfer such Property is
      subject to a Lien securing the Obligations, then (y) such Property shall
      remain subject to such Lien after such transfer or sale and the new owner
      of such Property shall execute and deliver all such agreements,
      instruments and other instruments requested by the Administrative Agent
      which are necessary or desirable in order for the new owner to grant an
      Acceptable Security Interest in such Property to secure the Obligations,
      and (z) the Borrower shall give to the Administrative Agent a 10 days'
      prior written notice of such sale or transfer of Property, (B) if such
      Property is, in whole or in part, encumbered under the Project Company
      Mortgage and any new mortgages, security documents, or assumptions of
      mortgages encumbering such Property are entered into, then (y) JEDI and
      the Administrative Agent shall have, prior to or concurrently with such
      transfer or sale, entered into an intercreditor agreement substantially in
      the form of the Intercreditor Agreement covering any new mortgages or
      security documents encumbering such Property and (z) at least three
      Business Days prior to such transfer or sale, the Administrative Agent
      shall have received a true, correct and complete copy of such new
      mortgages or security documents certified as such by the Borrower, and (C)
      if such Property is, in whole or in part, encumbered under the Project
      Company Mortgage and no new mortgages, security documents or assumptions
      of mortgages encumbering such Property are entered into, then the Borrower
      shall give to the Administrative Agent prompt written notice of such
      transfer or sale."

                                  ARTICLE IV.
                         REPRESENTATIONS AND WARRANTIES

      SECTION 4.01 BORROWER REPRESENTATIONS AND WARRANTIES. The Borrower
represents and warrants that: (a) the representations and warranties contained
in Article IV of the Credit Agreement and the representations and warranties
contained in the Security Instruments, the Guaranties, and each of the other
Loan Documents are true and correct in all material respects on and as of the
Effective Date, after giving effect to the terms of this Agreement, as though
made on and as of such date, except those representations and warranties that
speak of a certain date, which representations and warranties were true and
correct as of such date; (b) after giving effect to the terms of this Agreement,
no Default has occurred and is continuing; (c) the execution, delivery and
performance of this Agreement are within the corporate power and authority of
the Borrower and have been duly authorized by appropriate corporate action and
proceedings; (d) this Agreement constitutes a legal, valid, and binding
obligation of the Borrower enforceable in accordance with its terms, except as
limited by applicable bankruptcy, insolvency, reorganization, moratorium, or
similar laws affecting the rights of creditors generally and general principles
of equity; (e) there are no governmental or other third party consents, licenses
and approvals required in connection with the execution, delivery, performance,
validity and enforceability of this Agreement; (f) the Liens under the Security
Instruments are valid and subsisting and secure Borrower's obligations under the
Loan Documents; and (g) the Amended and Restated Bylaws and the Amended and
Restated Certificate attached as exhibits hereto are the true, correct and
complete copies of the bylaws and certificate of incorporation of the Borrower,
and the same has not been amended, supplemented, or otherwise modified and,
after

                                       5-

<PAGE>

such bylaws have been fully executed and such certificate has been filed with
the Secretary of the State of Delaware, they will remain in full force and
effect as of the date hereof.

      SECTION 4.02 GUARANTORS REPRESENTATIONS AND WARRANTIES. Each Guarantor
represents and warrants that: (a) the representations and warranties contained
in Article IV of the Credit Agreement and the representations and warranties
contained in the Security Instruments, the Guaranties, and each of the other
Loan Documents are true and correct in all material respects on and as of the
Effective Date, as though made on and as of such date, except those
representations and warranties that speak of a certain date, which
representations and warranties were true and correct as of such date; (b) after
giving effect to the terms of this Agreement, no Default has occurred and is
continuing; (c) the execution, delivery and performance of this Agreement are
within the corporate power and authority of such Guarantor and have been duly
authorized by appropriate corporate action and proceedings; (d) this Agreement
constitutes a legal, valid, and binding obligation of such Guarantor enforceable
in accordance with its terms, except as limited by applicable bankruptcy,
insolvency, reorganization, moratorium, or similar laws affecting the rights of
creditors generally and general principles of equity; (e) there are no
governmental or other third party consents, licenses and approvals required in
connection with the execution, delivery, performance, validity and
enforceability of this Agreement; (f) it has no defenses to the enforcement of
its Guaranty; and (g) the Liens under the Security Instruments are valid and
subsisting and secure such Guarantor's obligations under the Loan Documents.

                                   ARTICLE V.
                                   CONDITIONS

      This Agreement and the Consents contained herein shall become effective
and enforceable against the parties hereto, and the Credit Agreement shall be
amended as provided herein, upon the occurrence of the following conditions
precedent:

      SECTION 5.01 DOCUMENTS; CERTIFICATES. The Administrative Agent shall have
received multiple original counterparts, as requested by the Administrative
Agent, of:

            (a) this Agreement duly and validly executed and delivered by duly
authorized officers of the Borrower, the Guarantors, the Administrative Agent,
and the Required Lenders, and

            (b) such other instruments, documents and amendments or supplements
as the Administrative Agent may reasonably request.

      SECTION 5.02 NO DEFAULT. No Default shall have occurred and be continuing
as of the Effective Date.

      SECTION 5.03 REPRESENTATIONS. The representations and warranties in this
Agreement shall be true and correct in all material respects.

                                       6-

<PAGE>

                                  ARTICLE VI.
                                  MISCELLANEOUS

      SECTION 6.01 EFFECT ON LOAN DOCUMENTS; ACKNOWLEDGMENTS.

            (a) The Borrower acknowledges that on the date hereof all
Obligations are payable without defense, offset, counterclaim or recoupment.

            (b) The Administrative Agent, the Issuing Lender, and the Lenders
hereby expressly reserve all of their rights, remedies, and claims under the
Loan Documents. Nothing in this Agreement shall constitute a waiver or
relinquishment of (i) any Default or Event of Default under any of the Loan
Documents other than as expressly set forth above, (ii) any of the agreements,
terms or conditions contained in any of the Loan Documents other than as
expressly set forth above, (iii) any rights or remedies of the Administrative
Agent, the Issuing Lender or any Lender with respect to the Loan Documents, or
(iv) the rights of the Administrative Agent, any Issuing Lender or any Lender to
collect the full amounts owing to them under the Loan Documents.

            (c) Each of the Borrower, the Guarantors, Administrative Agent,
Issuing Lender, and Lenders does hereby adopt, ratify, and confirm the Credit
Agreement, as amended hereby, and acknowledges and agrees that the Credit
Agreement, as amended hereby, and all other Loan Documents are and remain in
full force and effect, and each of the Borrower and the Guarantors acknowledges
and agrees that its liabilities under the Credit Agreement and the other Loan
Documents are not impaired in any respect by this Agreement or the consents
granted hereunder.

            (d) From and after the Effective Date, all references to the Credit
Agreement and the Loan Documents shall mean such Credit Agreement and such Loan
Documents as amended by this Agreement.

            (e) This Agreement is a Loan Document for the purposes of the
provisions of the other Loan Documents. Without limiting the foregoing, any
breach of representations, warranties, and covenants under this Agreement shall
be a Default or Event of Default, as applicable, under the Credit Agreement.

      SECTION 6.02 REAFFIRMATION OF THE GUARANTY. Each Guarantor hereby
ratifies, confirms, acknowledges and agrees that its obligations under its
Guaranty are in full force and effect and that such Guarantor continues to
unconditionally and irrevocably guarantee the full and punctual payment, when
due, whether at stated maturity or earlier by acceleration or otherwise, all of
the Guaranteed Obligations (as defined in its Guaranty), as such Guaranteed
Obligations may have been amended by this Agreement, and its execution and
deliver of this Agreement does not indicate or establish an approval or consent
requirement by such Guarantor under its Guaranty in connection with the
execution and delivery of amendments to the Credit Agreement, the Notes or any
of the other Loan Documents.

      SECTION 6.03 COUNTERPARTS. This Agreement may be signed in any number of
counterparts, each of which shall be an original and all of which, taken
together, constitute a single instrument. This Agreement may be executed by
facsimile signature and all such signatures shall be effective as originals.

                                       7-

<PAGE>

      SECTION 6.04 SUCCESSORS AND ASSIGNS. This Agreement shall be binding upon
and inure to the benefit of the Lenders, the Borrower and the Administrative
Agent hereto and their respective successors and assigns permitted pursuant to
the Credit Agreement.

      SECTION 6.05 INVALIDITY. In the event that any one or more of the
provisions contained in this Agreement shall for any reason be held invalid,
illegal or unenforceable in any respect, such invalidity, illegality or
unenforceability shall not affect any other provision of this Agreement.

      SECTION 6.06 GOVERNING LAW. This Agreement shall be deemed to be a
contract made under and shall be governed by and construed in accordance with
the laws of the State of Texas.

      SECTION 6.07 ENTIRE AGREEMENT. THIS AGREEMENT, THE CREDIT AGREEMENT AS
AMENDED BY THIS AGREEMENT, THE NOTES, AND THE OTHER LOAN DOCUMENTS CONSTITUTE
THE ENTIRE UNDERSTANDING AMONG THE PARTIES HERETO WITH RESPECT TO THE SUBJECT
MATTER HEREOF AND SUPERSEDE ANY PRIOR AGREEMENTS, WRITTEN OR ORAL, WITH RESPECT
THERETO.

            THERE ARE NO UNWRITTEN ORAL AGREEMENTS AMONG THE PARTIES.

                         [SIGNATURES BEGIN ON NEXT PAGE]

                                       8-

<PAGE>

                                                               EXECUTION VERSION

      EXECUTED effective as of the date first above written.

                         MARINER ENERGY, INC.

                         By: /s/ Rick G. Lester
                             ---------------------------------------------------
                                 Rick G. Lester
                                 Vice President and Chief Financial Officer

                         Mariner LP LLC

                         By: /s/ Rick G. Lester
                             ---------------------------------------------------
                                 Rick G. Lester
                                 Vice President and Chief Financial Officer

                         Mariner Energy Texas LP, a Delaware limited
                         partnership

                          By: Mariner Energy, Inc., its sole general partner

                              By: /s/ Rick G. Lester
                                  ----------------------------------------------
                                      Rick G. Lester
                                      Vice President and Chief Financial Officer

                         UNION BANK OF CALIFORNIA, N.A.,
                         as Administrative Agent, as Issuing Lender, and as a
                         Lender

                         By: /s/ Damien Meiburger
                             -----------------------------------------------
                                 Damien Meiburger, Senior Vice President

                         By: /s/ Randall Osterberg
                             -----------------------------------------------
                         Name:   Randall Osterberg
                        Title:   Senior Vice President

                  Signature Page to Amendment No. 3 and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                           BNP PARIBAS

                                           By: /s/ David Dodd
                                               ------------------------------
                                           Name:   David Dodd
                                           Title:  Director

                                           By: /s/ Betsy Jocher
                                               ------------------------------
                                           Name:   Betsy Jocher
                                           Title:  Vice President

                  Signature Page to Amendment No. 3 and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                         JPMorgan Chase Bank, N.A. (successor by
                                         merger to Bank One, NA)

                                         By: /s/ Jo Linda Papadakis
                                             -----------------------------------
                                         Name:   Jo Linda Papadakis
                                         Title:  Associate Director

                  Signature Page to Amendment No. 3 and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                    SOUTHWEST BANK OF TEXAS, N.A.

                                    By: /s/ Kenneth R. Batson, III
                                       --------------------------------------
                                    Name:   Kenneth R. Batson, III
                                    Title:  Vice President

                  Signature Page to Amendment No. 3 and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                    GUARANTY BANK

                                    By: /s/ Arthur R. Garcia Jr.
                                       -------------------------------------
                                    Name:   Arthur R. Garcia Jr.
                                    Title:  Senior Vice President

                  Signature Page to Amendment No. 3 and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                HARRIS NESBITT FINANCING, INC.

                                By: /s/ James V. Ducote
                                   ----------------------------------------
                                Name:   James V. Ducote
                                Title:  Vice President

                  Signature Page to Amendment No. 3 and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                   Exhibit "A" to Amendment No. 3 and Consent

                        Amended and Restated Certificate

                                 [See attached.]

<PAGE>

                   Exhibit "B" to Amendment No. 3 and Consent

                           Amended and Restated Bylaws

                                 [See attached.]