<PAGE>   1
                                                                    EXHIBIT 10.1

                      SECOND AMENDMENT TO CREDIT AGREEMENT


         THIS SECOND AMENDMENT TO CREDIT AGREEMENT (this "Second Amendment") is
made and entered into as of the 11th day of August, 1998, by and among MARINER
ENERGY, INC., a Delaware corporation ("Borrower"), NATIONSBANK, N.A., as Agent
("Agent"), TORONTO DOMINION (TEXAS), INC., as Co-Agent ("Co-Agent") and the
financial institutions listed on Schedule 1 hereto (individually a "Bank" and
collectively "Banks").

         WHEREAS, Borrower, Agent and NationsBank of Texas, N.A. (in its
individual capacity, "NationsBank of Texas") entered into that certain Credit
Agreement dated June 28, 1996 (the "Original Credit Agreement");

         WHEREAS, by various Assignment and Assumption Agreements dated August
12, 1996, NationsBank of Texas assigned thirty percent (30%) of the Total
Commitment to Toronto Dominion (Texas), Inc., twenty percent (20%) of the Total
Commitment to The Bank of Nova Scotia and twenty percent (20%) of the Total
Commitment to ABN AMRO Bank, N.V., Houston Agency, and pursuant to said
Assignment and Assumption Agreements, such financial institutions assumed the
obligations of NationsBank of Texas with respect to such assigned portions of
the Total Commitment;

         WHEREAS, NationsBank of Texas, as Agent, Toronto Dominion (Texas),
Inc., as Co-Agent, and the Banks entered into that certain First Amendment to
Credit Agreement dated August 12, 1996 (the "First Amendment") (the Original
Credit Agreement, as amended by the First Amendment, is referred to herein as
the "Credit Agreement");

         WHEREAS, NationsBank of Texas merged with and into NationsBank, N.A.,
so that Agent is the successor in interest by merger in such capacity to
NationsBank of Texas; and

         WHEREAS, Borrower, Agent, Co-Agent and Banks desire to extend the
maturity date of the facility described in the Credit Agreement and to amend
certain other terms and provisions of the Credit Agreement, as set forth herein.

         NOW, THEREFORE, FOR AND IN CONSIDERATION of the mutual covenants and
agreements contained herein, the parties hereto agree as follows:

         1. The definition of "Agent" set forth in Section 1.1 of the Credit
Agreement is deleted in its entirety, and the following is substituted in its
place:

            "Agent" means NationsBank, N.A. in its capacity as agent for Banks
         hereunder or any successor thereto.




<PAGE>   2

         2. The definition of "Maturity Date" set forth in Section 1.1 of the
Credit Agreement is deleted in its entirety, and the following is substituted in
its place:

            "Maturity Date" means October 1, 1999.

         3. Schedule 1 attached to the Credit Agreement is deleted in its
entirety, and Schedule 1 attached to this Second Amendment is substituted
therefor.

         4. Borrower hereby represents and warrants to Agent, Co-Agent and each
Bank as follows:

            (a) Borrower has (i) begun analyzing the operations of Borrower and
         its subsidiaries and affiliates that could be adversely affected by
         failure to become Year 2000 compliant (that is, that computer
         applications, embedded microchips and other systems will be able to
         perform date-sensitive functions prior to and after December 31, 1999),
         and (ii) developed a plan for becoming Year 2000 compliant in a timely
         manner, the implementation of which is on schedule in all material
         respects. Borrower reasonably believes that it will become Year 2000
         compliant for its operations and those of its subsidiaries and
         affiliates on a timely basis, except to the extent that a failure to do
         so could not reasonably expected to have a material adverse affect upon
         the financial condition of Borrower;

            (b) Borrower reasonably believes any suppliers and vendors that are
         material to the operations of Borrower or its subsidiaries and
         affiliates will be Year 2000 compliant for their own computer
         applications except to the extent that a failure to do so could not
         reasonably be expected to have a material adverse affect upon the
         financial condition of Borrower; and

            (c) Borrower will promptly notify Agent in the event Borrower
         determines that any computer application which is material to the
         operations of Borrower, its subsidiaries or any of its material vendors
         or suppliers will not be fully Year 2000 compliant on a timely basis,
         except to the extent that such failure could not reasonably be expected
         to have a material adverse affect upon the financial condition of
         Borrower.

         5. The closing of the transactions contemplated by this Second
Amendment is subject to the satisfaction of the following conditions:

            (a) All legal matters incident to the transactions herein
         contemplated shall be satisfactory to counsel to Agent; and

            (b) Agent shall have received a fully executed copy of this Second
         Amendment.




                                       -2-
<PAGE>   3

         6. Except as amended hereby, the Credit Agreement shall remain
unchanged, and the terms, conditions and covenants of the Credit Agreement shall
continue and be binding upon the parties hereto.

         7. Within thirty (30) days of the next action, whether by consent or at
a meeting, of the Board of Directors of Borrower and Guarantor, Borrower will
deliver an executed copy of resolutions of the Board of Directors of Borrower
and Guarantor in form and substance reasonably satisfactory to Agent ratifying
the execution, delivery and performance of this Second Amendment and all
documents, instruments and certificates referred to herein.

         8. Each of the terms defined in the Credit Agreement is used in this
Second Amendment with the same meaning, except as otherwise indicated in this
Second Amendment. Each of the terms defined in this Second Amendment is used in
the Credit Agreement with the same meaning, except as otherwise indicated in the
Credit Agreement.

         9. This Second Amendment may be executed in any number of counterparts,
each of which shall constitute an original, but all of which, when taken
together, shall constitute but one agreement.

         10. THIS SECOND AMENDMENT SHALL BE DEEMED TO BE A CONTRACT UNDER,
SUBJECT TO, AND SHALL BE CONSTRUED FOR ALL PURPOSES IN ACCORDANCE WITH THE LAWS
OF THE STATE OF TEXAS.

         11. THE CREDIT AGREEMENT, AS AMENDED, REPRESENTS THE FINAL AGREEMENT
BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR,
CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES.

         THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

         IN WITNESS WHEREOF, the parties have caused this Second Amendment to be
executed by their duly authorized officers as of the day and year first above
written.

                                  MARINER ENERGY, INC.


                                  By: /s/ Frank A. Pici
                                      ---------------------------------------
                                      Frank A. Pici
                                      Title:Vice President of Finance and CFO
                                            ---------------------------------




                                       -3-
<PAGE>   4

                                  NATIONSBANK, N.A., as Agent


                                  By: /s/ James R. Allred
                                      ---------------------------------------
                                      James R. Allred
                                      Title:Senior Vice President
                                            ---------------------------------


                                  TORONTO DOMINION (TEXAS), INC.,
                                  as Co-Agent


                                  By: /s/ Jimmy Simien
                                      ---------------------------------------
                                      Jimmy Simien
                                      Title:Vice President
                                            ---------------------------------


                                  NATIONSBANK, N.A.


                                  By: /s/ James R. Allred
                                      ---------------------------------------
                                      James R. Allred
                                      Title:Senior Vice President
                                            ---------------------------------


                                  TORONTO DOMINION (TEXAS), INC.


                                  By: /s/ Jimmy Simien
                                      ---------------------------------------
                                      Jimmy Simien
                                      Title:Vice President
                                            ---------------------------------

                                  THE BANK OF NOVA SCOTIA


                                  By: /s/ F.C.H. Ashby
                                      ---------------------------------------
                                      F.C.H. Ashby
                                      Title:Senior Manager - Loan Operations
                                            ---------------------------------




                                       -4-
<PAGE>   5

                                  ABN AMRO BANK, N.V., Houston Agency

                                  By: ABN AMRO North America, Inc., as agent


                                      By: /s/ Charles W. Randall
                                          -----------------------------------
                                          Charles W. Randall
                                          Title:Senior Vice President
                                                -----------------------------

                                      By: /s/ Stephanie Balette
                                          -----------------------------------
                                          Stephanie Balette
                                          Title:Assistant Vice President
                                                -----------------------------

         Guarantor joins in the execution of this Second Amendment to evidence
that it hereby agrees and consents to all of the matters contained in this
Second Amendment and further agrees that (i) its liability under those three
certain Guarantys, each dated August 12, 1996, each being executed by Guarantor
for the benefit of a Bank (the "Guarantys") shall not be reduced, altered,
limited, lessened or in any way affected by the execution and delivery of this
Second Amendment or any of the instruments or documents referred to herein by
the parties hereto, except as specifically set forth herein or therein, (ii) the
Guarantys are hereby renewed, extended, ratified, confirmed and carried forward
in all respects, (iii) the Guarantys are and shall remain in full force and
effect and are and shall remain enforceable against Guarantor in accordance with
their terms and (iv) the Guarantys shall cover all indebtedness of Borrower to
the Banks described in the Credit Agreement as amended hereby.

                                  MARINER HOLDINGS, INC.


                                  By: /s/ Frank A. Pici
                                      ---------------------------------------
                                      Frank A. Pici
                                      Title:Vice President of Finance and CFO
                                            ---------------------------------

                                                           "GUARANTOR"


                                       -5-
<PAGE>   6

                                   SCHEDULE 1

                             Financial Institutions

<TABLE>
<CAPTION>
   Financial                   Domestic               Eurodollar Lending                   Commitment
  Institution               Lending Office                  Office                          Percentage                 Commitment
  -----------               --------------                  ------                          ----------                 ----------
<S>                         <C>                       <C>                                  <C>                         <C>
NationsBank, N.A.           700 Louisiana,               700 Louisiana,                         30%                    $45,000,000
                            8th Floor                    8th Floor
                            Houston, Texas               Houston, Texas
                            77002                        77002
Toronto Dominion            909 Fannin,                  909 Fannin,                            30%                    $45,000,000
(Texas), Inc.               Suite 1700                   Suite 1700
                            Houston, Texas               Houston, Texas
                            77010                        77010
The Bank of Nova            600 Peachtree St.,           600 Peachtree St.,                     20%                    $30,000,000
Scotia                      N.E., Suite 2700             N.E., Suite 2700
                            Atlanta, Georgia             Atlanta, Georgia
                            30308                        30308
ABN AMRO                    Three Riverway,              Three Riverway,                        20%                    $30,000,000
Bank, N.V.,                 Suite 1700                   Suite 1700
Houston Agency              Houston, Texas               Houston, Texas
                            77056                        77056
</TABLE>