Document:

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                                                                  EXHIBIT  10.13

                            NINTH AMENDMENT TO SECOND
                       AMENDED AND RESTATED LOAN AGREEMENT

         This NINTH AMENDMENT TO SECOND AMENDED AND RESTATED LOAN AGREEMENT (the
"Ninth Amendment") dated effective December 31, 1999, is by and between VENUS
EXPLORATION, INC., a Delaware corporation, formerly known as XPLOR CORPORATION,
a Delaware corporation (the "Borrower"), and WELLS FARGO BANK (TEXAS), N.A., a
national banking association (the "Bank").

                              W I T N E S S E T H:

         WHEREAS, Bank and Borrower entered into that certain Second Amended and
Restated Loan Agreement dated December 22, 1997 (as the same has been previously
amended through the date hereof is herein called the "Loan Agreement"), pursuant
to which Borrower obtained a credit facility in the amount of up to the lesser
of the Borrowing Base (as defined in the Loan Agreement) or the Commitment (as
defined in the Loan Agreement); and

         WHEREAS, Bank and Borrower now desire to further amend that Loan
Agreement as herein set forth.

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

         1. Amendments to the Loan Agreement. The Loan Agreement is, effective
the date hereof, and subject to the satisfaction of the conditions precedent set
forth in Section 6 hereof, hereby amended as follows:

                  (a) Except as provided below, unless the context hereof
                  indicates otherwise, all capitalized terms used herein shall
                  have the same meaning as set forth in the Loan Agreement.

                  (b) Section 2.2, Borrowing Base, of the Loan Agreement, is
                  hereby amended by adding the following subparagraph (d):

                           "(d)     Notwithstanding the above Section 2(b), for
                                    purposes of determining the Borrowing Base,
                                    the amount of the Borrowing Base as of March
                                    31, 2000 shall be zero ($0), and from March
                                    31, 2000 through the Commitment Termination
                                    Date, Borrower will maintain a zero balance
                                    on the Note."

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         2. Ratifications. The terms and provisions as set forth in this Ninth
Amendment shall modify and supersede all inconsistent terms and provisions set
forth in the Loan Agreement, and except as expressly modified and superseded by
this Ninth Amendment, the terms of the Note and any and all other Loan Documents
executed in connection therewith or hereunto are hereby ratified and confirmed
and shall continue in full force and effect. Borrower and Bank agree that the
Loan Agreement, as amended hereby, the Note and the other Loan Documents shall
continue to be the legal, valid and binding obligations of Borrower, enforceable
against Borrower in accordance with their respective terms.

         3. Representations and Warranties. Borrower hereby represents and
warrants to Bank that (i) the execution, delivery and performance of this Ninth
Amendment, and the other documents to be executed and delivered as required
hereby have been duly authorized by all requisite action on the part of
Borrower; (ii) after giving effect to this Ninth Amendment, the representations
and warranties contained in the Loan Agreement, as amended hereby, and any other
Loan Document executed in connection herewith or therewith are true, correct and
complete on and as of the date hereof as though made on and as of the date
hereof; and (iii) after giving effect to this Ninth Amendment, no Event of
Default or Potential Default has occurred and is continuing.

         4. Covenant Deviation and Waiver. Without giving effect to this Ninth
Amendment, Borrower would have failed to observe or maintain compliance with the
Current Ratio covenant set forth in Section 6.16 of the Loan Agreement and the
Tangible Net Worth covenant set forth in Section 6.17 of the Loan Agreement.
Borrower has requested, and Bank has approved, a deviation from such compliance
with respect to the aforementioned covenants for a period from the date hereof
through March 31, 2000, at which time Borrower must be in compliance therewith.
It is understood and agreed that Bank's consent to such deviation shall in no
way act as a waiver of any covenants, restrictions, rights or remedies with
respect to the Loan Agreement, but that such deviation shall apply only to the
specific matter and instance set forth hereinabove.

         5. Status of Claims. Borrower hereby represents and warrants to Bank
that no facts, events, status or conditions presently exist which, either now or
with the passage of time or the giving of notice or both, presently constitute
or will constitute a basis for any claim or cause of action against Bank, or any
defense to the payment of any of the Obligations. Borrower hereby releases,
relinquishes and forever discharges Bank, its successors, assigns, agents,
officers, directors, employees and representatives, of and from any and all
claims, demands, actions and causes of action of any and every kind or
character, whether known or unknown, present or future, which Borrower may have
against Bank, its successors, assigns, agents, officers, directors, employees
and representatives, arising out of or with respect to any and all transactions
relating to the Loan Agreement, this Ninth Amendment, or any Loan Document,
including any loss, cost or damage, of any kind or character, arising out of or
in any way connected with or in any way resulting from the acts, actions or
omissions of Bank, its successors, assigns, agents, officers, directors,
employees or representatives.

         6. Conditions Precedent to Effectiveness of Ninth Amendment. This Ninth
Amendment shall become effective and be deemed effective upon receipt by Bank of
the following:

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                  (i) counterparts of this Ninth Amendment duly executed by
         Borrower and Bank;

                  (ii) there shall not have been, in the sole judgment of Bank,
         any material adverse change in the financial condition, business or
         operations of Borrower;

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                  (iii) payment by Borrower of the fees and expenses of counsel
         to Bank in connection with the preparation and negotiation of this
         Ninth Amendment and all documents and instruments contemplated hereby;
         and

                  (iv) the execution and delivery by Borrower of such additional
         documents and instruments that Bank and its counsel may deem necessary
         to effectuate this Ninth Amendment or any document executed and
         delivered to Bank in connection herewith or therewith.

         7. Execution Counterparts. This Ninth Amendment may be executed in any
number of counterparts and by different parties hereto in separate counterparts,
each of which when so executed and delivered shall be deemed to be an original
and all of which when taken together shall constitute but one and the same
instrument.

         8. Governing Law. This Ninth Amendment shall be governed by and
construed in accordance with the internal laws of the State of Texas.

         9. Successors and Assigns. This Ninth Amendment is binding upon and
shall inure to the benefit of Borrower and Bank and its respective successors
and assigns; provided, however, Borrower may not assign or transfer any of their
rights or obligations hereunder without the prior written consent of Bank.

         10. Headings. The headings, captions and arrangements used in this
Ninth Amendment are for convenience only and shall not effect the interpretation
of this Ninth Amendment.

         11. NO ORAL AGREEMENTS. THIS NINTH AMENDMENT, TAKEN TOGETHER WITH THE
OTHER LOAN DOCUMENTS AND ALL SCHEDULES AND EXHIBITS THERETO, REPRESENTS THE
FINAL AGREEMENT OF 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.

         12. AGREEMENT FOR BINDING ARBITRATION. THE PARTIES AGREE TO BE BOUND BY
THE TERMS AND PROVISIONS OF THE CURRENT ARBITRATION PROGRAM OF WELLS FARGO BANK
(TEXAS), N.A., WHICH IS INCORPORATED BY REFERENCE HEREIN AND IS ACKNOWLEDGED AS
RECEIVED BY THE PARTIES, PURSUANT TO WHICH ANY AND ALL DISPUTES SHALL BE
RESOLVED BY MANDATORY BINDING ARBITRATION UPON THE REQUEST OF EITHER PARTY.

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                                                  "BORROWER"

                                                  VENUS EXPLORATION, INC.

                                                  By:   /s/ John Y. Ames
                                                     ---------------------------
                                                  Name:     John Y. Ames
                                                       -------------------------
                                                  Title:    President
                                                        ------------------------

                                                  "BANK"
                                                  WELLS FARGO BANK (TEXAS) N.A.

                                                  By:   /s/ Danny Oliver
                                                     ---------------------------
                                                  Name:     Danny Oliver
                                                       -------------------------
                                                  Title:    Relationship Manager
                                                        ------------------------

                                       5<PAGE>   1

                                                                   EXHIBIT 10.31

                                December 31, 1999

Anadarko Petroleum Corporation
17001 Northchase Dr.
Houston, Texas 77060

Attn: Mr. James J. Ward, Jr.

                  Re:      Amendments to: (i) Purchase, Sale and Exchange
                           Agreement dated as of December 17, 1999, between EXCO
                           Resources, Inc., as Seller, and Anadarko Petroleum
                           Corporation ("Anadarko"), as Buyer (the "EXCO PSA")
                           and (ii) Purchase and Sale Agreement dated December
                           17, 1999 between Venus Exploration, Inc., as Seller,
                           and Anadarko as Buyer (the "Venus PSA")

Gentlemen:

         By the EXCO PSA and Venus PSA (collectively the "Agreements"), EXCO
Resources, Inc. ("EXCO") and Venus Exploration, Inc. ("Venus") agreed to sell to
Anadarko certain properties located in Jackson Parish, Louisiana. Reference to
the Agreements is hereby made for all purposes. Unless otherwise stated, words
beginning with capital letters in this Agreement shall have the same meaning
ascribed thereto in the Agreements. The Closing Date specified in the Agreements
is December 31, 1999. Because of potential problems in arranging for the wire
transfer of money as a result of the "Y2K" phenomenon, and other matters, EXCO,
Venus and Anadarko have concluded that it is in everyone's best interest to
modify the closing as set forth below, including deferring the actual cash
transfer of the adjusted Purchase Price until a few days after the first of
January, 2000 as set forth hereinbelow. Such deferral necessitates an amendment
to the Agreements. This letter, when accepted by you in the manner hereinafter
set forth, shall constitute the agreement of EXCO, Venus and Anadarko to amend
the Agreements as follows:

         1.       The method and date of Closing as set forth in the Agreements
                  is hereby changed as follows:

                  1.1      On December 31, 1999, the following documents (the
                           "Escrow Documents") which are contemplated by the
                           Agreements, will be fully executed and delivered to
                           American Escrow Company:

                           (1)      The Assignment and Bill of Sale attached as:
                                    (a) Exhibit D to the EXCO PSA, (b) Exhibit B
                                    to the Venus PSA, and (c) Exhibit D to the
                                    Exchange Agreement between Anadarko and
                                    EXCO;

                           (2)      The escrow agreement attached hereto as
                                    Addendum "A" (the "Escrow Agreement");

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Mr. James J. Ward, Jr.
December 31, 1999
Page 2

                           (3)      The letters-in-lieu of transfer orders
                                    described in the Agreements;

                           (4)      The affidavit of non-foreign status
                                    contemplated by the Agreements;

                           (5)      The Exchange Agreement between Anadarko and
                                    EXCO; and

                           (6)      The Assignment and Bill of Sale conveying
                                    from EXUS Energy LLC ("EXUS") to EXCO and
                                    Venus the properties subject to the
                                    Agreements.

                  1.2      Provided that all of the conditions to payment set
                           forth in the Escrow Agreement and the Agreements have
                           been satisfied, on January 6, 2000 (the "Initial
                           Escrow Payment Date"), Anadarko will make the payment
                           of the Purchase Price adjusted as provided for in the
                           Agreements as follows, which payments, subject to the
                           next sentence of this section, shall not be less
                           than:

                           (a)      $16,848,307 under the EXCO PSA ("Initial
                                    EXCO PSA Payment"); and

                           (b)      $17,104,571 under the Venus PSA ("Initial
                                    Venus PSA Payment", collectively the
                                    "Initial Escrow Payment").

                           Title defects which may be cured by obtaining a
                           corrective assignment from Apache representing a
                           Title Defect Amount of $919,188 as to the EXCO Assets
                           and $887,710 as to the Venus Assets is anticipated to
                           be received by Anadarko prior to the Initial Escrow
                           Payment Date. In the event such assignment is not
                           received, then the Initial EXCO PSA Payment shall be
                           reduced by $919,188 and the Initial Venus PSA
                           Agreement shall be reduced by $887,710.

                  A.       Payments attributable to the EXCO PSA shall be made
                           to Texas Escrow Company, Inc. to the following
                           account:

                                    Bank One Texas
                                    ABA No. 111000614
                                    For Credit to Texas Escrow Company, Inc.
                                    Account No. 189 234 8820
                                    Re: GF No. 99S12314 SJ1E
                                    Notify Holly Garfield at (214) 855-8837

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Mr. James J. Ward, Jr.
December 31, 1999
Page 3

                  B.       Payments attributable to the Venus PSA shall be made
                           to American Escrow Company to the following account:

                                    Bank One Dallas, Texas
                                    ABA No. 111000614
                                    For Credit to American Escrow Company
                                    Account No. 1825159336
                                    Re: GF No. 99S13498 SJ1
                                    Notify Melissa Garfield at (214) 855-8862

Distributions of the Initial Escrow Payment and any subsequent payments from
Anadarko by Texas Escrow Company, Inc. and American Escrow Company shall be made
as set forth in the Escrow Agreement.

         2.       Section 3.5 of each of the Agreements provides that Buyer
                  (Anadarko) may elect to treat unsatisfied consent requirements
                  and preferential purchase right requirements as Title Defects
                  by giving Seller (EXCO and Venus) notice of such treatment no
                  later than one business day prior to the Closing Date. Section
                  3.5 of the Agreements is hereby amended to permit Buyer to
                  treat such unsatisfied consent and preferential purchase
                  rights as Title Defects by giving Seller notice no later than
                  January 5, 2000, at 5:00 p.m. In the event (i) Anadarko treats
                  unsatisfied consent requirements and/or preferential purchase
                  right requirements as a Title Defect; (ii) makes adjustments
                  to the Purchase Price on account thereof; (iii) such consent
                  requirements remain unsatisfied as of March 31, 2000; and (iv)
                  Anadarko does not waive all or any portion of such unsatisfied
                  consent requirements and/or preferential purchase rights by
                  making additional payments attributable to EXCO's Assets to
                  Texas Escrow Company, Inc. and additional payments
                  attributable to Venus' Assets to American Escrow Company in an
                  amount equal to the adjustments to the Purchase Price made on
                  account thereof, then and in such event, Anadarko agrees that
                  it shall reconvey unto EXCO and Venus, all of the Assets
                  attributable to such unsatisfied consent requirements,
                  effective as of October 1, 1999. In the event of such
                  reconveyance, the parties shall make adjustment of the type
                  contemplated by the Agreements as if such Property had never
                  been conveyed to Anadarko with EXCO and Venus being: (a)
                  responsible for all reasonable costs and expenses of every
                  nature incurred by Anadarko arising out of or related to the
                  reconveyed Property; and (b) entitled to net proceeds
                  attributable to production from the reconveyed Property
                  received by Anadarko. As Title Defects with respect to a
                  Property are cured to Anadarko's satisfaction, Anadarko shall
                  make additional payments to the escrow account which are equal
                  to the Title Defect Amount for which Anadarko received a
                  reduction in the Purchase Price.

         3.       Notwithstanding anything contained in the Agreements to the
                  contrary, upon receipt of the Assignment and Bill of Sale as
                  to a particular property, Anadarko, its

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Mr. James J. Ward, Jr.
December 31, 1999
Page 4

                  successors and assigns, shall not thereafter be required
                  to obtain from EXUS, EXCO, Venus or their successors or
                  assigns any consent to assign any of the Properties or
                  interests described in such Assignment and Bill of Sale.

         4.       EXCO, Venus and Anadarko have reached an agreement regarding
                  Title Defect Amount attributable to the Properties. The total
                  Title Defect Amount under both of the Agreements is
                  $5,414,020. The allocation of the Title Defect Amount amongst
                  the Properties and the Agreements is set forth on Addendum "B"
                  hereto. EXCO and Venus shall have the right to cure the Title
                  Defects which comprise the Title Defect Amount for a period up
                  to and including March 31, 2000. If the total Title Defect
                  Amount still outstanding as of March 31, 2000 is less than the
                  Title Defect Amount set forth on Addendum "B" hereto but is
                  equal to or greater than at least $300,000.00, and Anadarko
                  shall have reduced the initial payment to the escrow agent by
                  at least $300,000, then Anadarko shall, except for Title
                  Defect Amounts for Property it reconveys to EXCO or Venus, pay
                  to EXCO and Venus, in equal proportions, the difference
                  between the Title Defect Amount set forth on Addendum "B"
                  hereto and the Title Defect Amount outstanding as of March 31,
                  2000. If the total Title Defect Amount still outstanding as of
                  March 31, 2000 is less than $300,000.00, and Anadarko shall
                  have reduced the initial payment to the escrow agent by
                  $300,000, then Anadarko shall, except for Title Defect Amounts
                  for Property it reconveys to EXCO or Venus, pay to EXCO and
                  Venus the entire Title Defect Amount.

         5.       Title Defect number 108 described on Addendum "B" hereto
                  pertains to the drilling and production unit known as the
                  "Lower Cotton Valley Reservoir A SUA", or "LCV RA SUA". Such
                  unit includes the Davis Bros. H-1 and K-1 wells. In the event
                  the Title Defect set forth on Addendum "B" hereto which
                  pertains to such unit is cured prior to March 31, 1999 by
                  obtaining a lease or a ratification of a lease from mineral
                  owners in such unit, any payments made to such mineral owners
                  by Anadarko in order to secure such lease or ratification
                  shall be added to the Title Defect Amount in determining
                  whether the Title Defect Amount is lesser or greater than
                  $300,000 pursuant to paragraph 4, above.

         6.       All representations, warranties and covenants of the Parties
                  and other conditions set forth in the Agreements as being
                  applicable on or extending through the Closing Date shall be
                  applicable as of the Initial Escrow Payment Date.

         7.       Operation of the properties shall be transferred to Buyer at
                  1:00 p.m. on December 31, 1999, and Anadarko shall assume risk
                  of loss to the Properties attributable to its operations
                  commencing with its assumption of operations. Anadarko agrees
                  to defend, indemnify and hold harmless Venus and EXCO from all
                  Liabilities arising

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Mr. James J. Ward, Jr.
December 31, 1999
Page 5

                  out of or related to operations conducted by Anadarko on the
                  Properties including, without limitation, Liabilities
                  attributable to operations on the Louisiana Minerals 15-1
                  Alternate well.

         8.       The number of days after the Closing Date in which Seller
                  shall provide a statement to Buyer, as set forth in Section
                  8.4(b) of the Agreements, setting forth the final calculation
                  of the Adjusted Purchase Price shall be 90 days, instead of 60
                  days, and the parties shall undertake to agree on the final
                  statement within 180 days after the Closing Date, instead of
                  150 days after the Closing Date.

         9.       The place of Closing as set forth in Section 8.1(a) of the
                  Agreements shall be Anadarko's office at 17001 Northchase
                  Drive, Houston, Texas.

         10.      Sections 9.1 and 9.2 of the Agreements shall be deleted in
                  their entirety and the following substituted in lieu thereof:

                  Section 9.1 Termination

                        This Agreement may be terminated at any time prior to
                  the Initial Escrow Payment Date if either party has failed to
                  perform or observe in any material respect its covenants and
                  agreements hereunder. In the event this Agreement terminates
                  because a party has failed to perform or observe in any
                  material respect any of its agreements or covenants contained
                  herein which are to be performed at or prior to the Initial
                  Escrow Payment Date, then the other party shall be entitled to
                  all remedies available at law or in equity and shall be
                  entitled to recover attorneys' fees in addition to any other
                  relief to which such party may be entitled.

         11.      Except as amended hereby, the Agreements shall remain in full
                  force and effect in accordance with their original terms.

         If the foregoing correctly sets forth your understanding of our
agreement, please indicate your acceptance hereof by executing three copies of
this agreement in the space provided below and by returning two copies to the
undersigned at the address set forth above.

                                                     EXCO Resources, Inc.

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Mr. James J. Ward, Jr.
December 31, 1999
Page 6

                                                     By: /s/ T. W. EUBANK
                                                         -----------------------
                                                         T. W. Eubank, President

Accepted and agreed to this 31st
day of December, 1999.

Venus Exploration, Inc.

By: /s/ JOHN Y. AMES
    ----------------
    John Y. Ames, President

Accepted and agreed to this 31st
day of December, 1999.

Anadarko Petroleum Corporation

By: /s/ BRUCE H. STOVER
    -------------------
    Bruce H. Stover, Vice President
    Worldwide Business Development

<PAGE>   7

Mr. James J. Ward, Jr.
December 31, 1999
Page 7

                                LIST OF ADDENDUMS

<TABLE>

         <S>                <C>
         ADDENDUM A         ESCROW  AGREEMENT FOR CLOSING FUNDS
                            AND CLOSING DOCUMENTS

         ADDENDUM B         EXCO RESOURCES, INC. AND ANADARKO
                            PETROLEUM CORPORATION'S AGREED TO
                            TITLE DEFECTS
</TABLE>

Addendums for this Exhibit 10.2 have not been filed herewith. These addendums
will be filed if requested by the Securities and Exchange Commission.

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