Document:

<PAGE>
                                                                   EXHIBIT 10.17

                              [SUNOCO LETTERHEAD]

                                                             SUNOCO, INC.
                                                             PO Box 2039
                                                             Tulsa OK 74102-2039

September 19, 2001

Coho Oil & Gas, Inc.
Attn: Gary Pittman
14785 Preston Road, Ste 860
Dallas, TX 75240

                                                Re: Crude Oil Purchase Agreement
                                                    Sunoco Reference No. 502685

Dear Mr. Pittman,

Reference is made to the above subject Crude Oil Purchase Agreement whereby
Sunoco, Inc. (R&M) will purchase all of the crude oil and condensate, produced
from the lease(s) listed on Exhibit "A".

This confirms our understanding that by mutual consent said agreement shall be
amended as follows:

EFFECTIVE OCTOBER 1, 2001, THE PRICE SHALL BE AMENDED TO READ AS FOLLOWS:

         Price. For those leases listed on the attached Exhibit "A" and any
additions thereto, the U.S. dollar price per barrel for each delivery month
shall be:

         the simple average of daily settling price of the near month NYMEX
         light sweet crude oil contract during the calendar month of delivery
         MINUS (-) sixty cents ($0.60) per barrel.

         e.g. for Jan, 2001 deliveries, the average of NYMEX settling prices for
         February Light Sweet crude contract from Jan 1, 2001 through Jan 22,
         2001, and March Light Sweet crude contracts from Jan 23, 2001, through
         Jan 31, 2001 shall apply.

         This simple average shall be calculated to allow the averaging of
         settling prices only, as determined by the NYMEX. Saturdays, Sundays,
         and holidays are not included in the calculation therefore only NYMEX
         trading days will be averaged in the above calculation.

For pricing purposes, the oil delivered during any given calendar month
hereunder shall be deemed to have been delivered in equal daily quantities for
each day of the given month.

Buyer and Seller agree that for the term of this agreement and any extensions
thereof, Seller shall not incur gravity penalties.

EFFECTIVE OCTOBER 1, 2001, THE TERM SHALL BE AMENDED TO READ AS FOLLOWS:

         Term. This agreement shall remain in effect for an extended term of one
(1) year commencing on October 1, 2001, and from month to month thereafter,
unless and until terminated by either party upon written notice thereof given
thirty (30) days in advance of the end of the primary term of this agreement or
any extensions thereof.

<PAGE>
Coho Oil & Gas, Inc.-- Copa#502685
September 19, 2001
Page 2

Except as specifically modified, all terms and conditions of said Crude Oil
Purchase Agreement shall remain in full force and effect.

Please acknowledge your agreement to this amendment by signing below and mailing
or faxing a copy to:

                                   Sunoco, Inc. (R&M)
                                   Attn: Elaine Smith
                                   P.O. Box 2039
                                   Tulsa, OK 74102
                                   Fax: (918) 586-6879

If we do not receive a signed copy by mail or fax within ten (10) business days
from the date of receipt of this amendment, we will take that as evidence of
your acceptance of this amendment to the above referenced agreement.

Agreed and accepted this 26th day of September, 2001.

COHO OIL & GAS, INC.                       SUNOCO, INC. (R&M)

By /s/ GARY L. PITTMAN                     By /s/ KAREN S. COOK
   ---------------------------------          ----------------------------------
                                                       Karen S. Cook

Title  CFO                                 Title Sr. Crude Oil Representative
      ------------------------------             -------------------------------<PAGE>
                                                                   EXHIBIT 10.18

                              [TEPPCO LETTERHEAD]

                                 FIFTH AMENDMENT
                                       TO
                           CRUDE OIL PURCHASE CONTRACT

THIS FIFTH AMENDMENT TO CRUDE OIL PURCHASE CONTRACT the ("Amendment") is
entered into on November 5, 2001 by and between Coho Oil & Gas, Inc. ("Coho")
and TEPPCO Crude Oil, L.P. ("TCO").

                                    RECITALS

A.    Coho and TCO are the parties to that certain Crude Oil Purchase Contract
      COH-P00102117-M dated September 20, 2000, which has been previously
      amended (as amended, the "Original Agreement").

B.    Coho and TCO desire to amend the Original Agreement to reflect that
      effective October 1, 2001, the provisions entitled "Price" and Term" on
      the front page of the Original Agreement have changed.

FOR GOOD AND VALUABLE CONSIDERATION, the parties hereto agree as follows:

1.    The provision entitled "Price" on the front page of the Original Agreement
      is hereby amended as follows:

      The simple monthly average of the daily settlement price of the NYMEX
      crude oil contract, during the month of delivery (Nymex trading days only)
      deemed 40 degrees and deemed delivered in equal daily quantities minus the
      amounts shown on Exhibit A, Amendment 5 and Exhibit B, Amendment 5.

2.    The provision entitled "Term" on the front page of the Original Agreement
      is hereby amended as follows:
      Commencing October 1, 2001 through September 30, 2002, thereafter month to
      month until either party provides a thirty (30) days advance written
      notice of cancellation.

3.    Except as modified and amended herein, the terms and provisions of the
      Original Agreement shall remain in full force and effect and are ratified
      hereby.

THE PARTIES HERETO have executed this Amendment as of the day and year first
above written, but to be effective October 1, 2001.

COHO OIL & GAS, INC.

By: /s/ GARY L. PITTMAN
    ---------------------------
Name: Gary L. Pittman
     --------------------------
Title: CFO
      -------------------------

TEPPCO CRUDE OIL, L.P.

By TEPPCO Crude GP, LLC,
Its general partner

By: /s/ D. GREGORY SCOTT
    ------------------------------
    D. Gregory Scott
    Vice President Crude Supply<PAGE>
                                                                   EXHIBIT 10.19

                      SECOND AMENDMENT TO CREDIT AGREEMENT

         THIS SECOND AMENDMENT TO CREDIT AGREEMENT (this "Amendment"), is made
as of April 20, 2001, among COHO ENERGY, INC., a Texas corporation (the
"Borrower"), the various financial institutions as are, or may from time to time
become, parties to the Credit Agreement (the "Lenders"), THE CHASE MANHATTAN
BANK, as administrative agent for the Lenders (the "Administrative Agent"), THE
CHASE MANHATTAN BANK, as Issuing Bank, MEESPIERSON CAPITAL CORP., as
Documentation Agent, and FLEET NATIONAL BANK, as Syndication Agent.

                              WITNESSETH:

         WHEREAS, the Borrower, the Lenders, the Agents and the Issuing Bank
have heretofore entered into that certain Credit Agreement, dated as of March
31, 2000 (together with all amendments, supplements, restatements and other
modifications, if any, from time to time thereafter made thereto, the "Credit
Agreement"), and

         WHEREAS, the Borrower, the Lenders, the Agents and the Issuing Bank now
intend to amend the Credit Agreement as set forth herein;

         NOW, THEREFORE, in consideration of the premises and the mutual
agreements herein contained, each of the Borrower, the Lenders, the Agents, and
the Issuing Bank hereby agree as follows:

         SECTION 1. Defined Terms. All capitalized terms used but not otherwise
defined herein shall have the meanings given in the Credit Agreement, as amended
by this Amendment.

         SECTION 2.  Amendments to Credit Agreement.

         a. Effective January 1, 2001, the definition of "Current Ratio"
contained in Section 1.1 of the Credit Agreement hereby is amended in its
entirety to the following:

         "        "Current Ratio" means, as of any date a determination thereof
is to be made, the ratio of

                  (a) the sum of (i) the amount of the consolidated current
         assets of the Borrower and its Subsidiaries plus (ii) the amount of the
         Borrowing Base Excess

                  to

                  (b) the amount of the consolidated current liabilities of the
         Borrower and its Subsidiaries,

<PAGE>

         provided that the requirements of Statement of Financial Accounting
         Standards 133, as modified by Statement of Financial Accounting
         Standards 137, of the Financial Accounting Standards Board, with
         respect to the non-cash balance sheet and income statement treatment of
         derivative instruments, including, without limitation, the valuation of
         such instruments and accounting for fluctuations in such value, will be
         disregarded for purposes of calculating the foregoing ratio."

         SECTION 3. Waiver. The Borrower has notified the Administrative Agent
and the Lenders that mergers involving the following Subsidiaries of the
Borrower have taken place as of the indicated date (the "Mergers"):

         a.       Interstate Natural Gas Company, a Delaware corporation
                  ("INGC"), merged into Coho Resources, Inc., a Nevada
                  corporation ("CRI"), effective as of December 31, 2000;

         b.       Coho Louisiana Production Company, a Delaware corporation
                  ("CLPC"), merged into CRI effective as of December 31, 2000;

         c.       Coho Marketing & Transportation, Inc., a Nevada corporation
                  ("CMTI"), merged into CRI effective as of December 31, 2000;

         d.       Coho Exploration, Inc., a Delaware corporation ("CEI"), merged
                  into CRI effective as of December 31, 2000; and

         e.       Coho Shell Company, a Delaware corporation ("CSC"), merged
                  into Borrower effective as of January 26, 2001.

In addition, the Borrower has notified the Administrative Agent and the Lenders
that the Borrower intends to merge Coho Resources Limited, an Alberta
corporation ("CRL"), into the Borrower in the near future (the "Proposed
Merger").

         Borrower has requested the waiver by the Lenders of, and the Lenders
hereby waive, any Default or Event of Default occurring under any terms or
provisions of the Credit Agreement or the other Loan Documents in connection
with the Mergers or the Proposed Merger; provided that such waiver only shall
continue to be effective if the Administrative Agent receives all modifications
to the Loan Documents reasonably requested in writing by the Administrative
Agent necessitated by such Mergers or the Proposed Merger, in form and substance
acceptable to the Administrative Agent, in its reasonable discretion. This
waiver shall not be deemed to be a waiver of the requirements of any other terms
or provisions of the Credit Agreement or the other Loan Documents with respect
to the Mergers or the Proposed Merger or of any prohibitions in the Loan
Documents relating to future mergers involving the Borrower or any Subsidiaries
of the Borrower.

                                        2

<PAGE>

         SECTION 4. Conditions to Effectiveness. Subject to Section 2 hereof
(the effective date of which shall be as therein provided), the effectiveness of
this Amendment is conditioned upon the Administrative Agent receiving the
following, in form and substance acceptable to the Administrative Agent in its
reasonable discretion:

         a.       This Amendment executed by the Borrower;

         b.       Ratification, and appropriate UCC-3 Financing Statement Change
                  forms, executed by CRI,;

         c.       Ratification, and appropriate UCC-3 Financing Statement Change
                  forms, executed by Borrower;

         d.       Evidence of the consummation of the Mergers, including, copies
                  of the certificates of merger, certified as being true and
                  correct copies by an Authorized Officer of the Borrower,
                  satisfactory to the Administrative Agent, in its reasonable
                  discretion; and

         e.       Such other documents as the Administrative Agent shall have
                  requested in writing, in its reasonable discretion.

All documents executed or submitted pursuant hereto by or on behalf of Borrower
or any of its Subsidiaries shall be satisfactory in form and substance to the
Administrative Agent and Administrative Agent shall have received all
information, approvals, opinions, documents or instruments as Lender or its
counsel may reasonably request.

         SECTION 5. Representations and Warranties. To confirm each Lender's
understanding concerning Borrower and its businesses, properties and
obligations, and to induce the Agents, the Issuing Bank and each Lender to enter
into this Amendment, the Borrower hereby reaffirms to the Agents, the Issuing
Bank and each Lender that, as of the date hereof and for this purpose giving
effect to this Amendment as if in force at all times from and after January 1,
2001, its representations and warranties contained in Article III of the Credit
Agreement and in the other Loan Documents to which it is a party (except to the
extent such representations and warranties relate solely to an earlier date) are
true and correct and additionally represents and warrants as follows:

a.       The execution, delivery and performance by the Borrower of this
         Amendment and each other Loan Document executed or to be executed by
         it, and the execution, delivery and performance by each other Obligor
         of each Loan Document executed or to be executed by it, are within the
         Borrower's and each such Obligor's corporate, limited liability company
         and/or partnership powers, and have been duly authorized by all
         necessary corporate, limited liability company and/or partnership
         action, and if required, stockholder, member and/or partner action.
         This Amendment has been duly executed and delivered by the Borrower and
         this Amendment and the Credit Agreement as amended by this Amendment
         each constitute, and each other Loan Document executed or to be
         executed by any Obligor, when executed

                                        3
<PAGE>

         and delivered by such Obligor, will constitute, a legal, valid and
         binding obligation of the Borrower or such Obligor (as the case may
         be), enforceable in accordance with their respective terms.

b.       The execution, delivery and performance by the Borrower of this
         Amendment, the Credit Agreement and each other Loan Document executed
         or to be executed by it, and the execution, delivery and performance by
         each other Obligor of each Loan Document executed or to be executed by
         it, (a) do not require any Governmental Approval or third party
         approvals, except such as have been obtained or made and are in full
         force and effect and except filings necessary to perfect Liens created
         under the Loan Documents, (b) will not violate any applicable
         Governmental Rule or the Organic Documents of the Borrower or any such
         Obligor or any order of any Governmental Authority, (c) will not
         violate or result in a default under any indenture, agreement or other
         instrument binding upon the Borrower or any such Obligor or its assets,
         or give rise to a right thereunder to require any payment to be made by
         the Borrower or any such Obligor, and (d) will not result in the
         creation or imposition of any Lien on any asset of the Borrower or any
         such Obligor except Liens created under the Loan Documents.

         SECTION 6. Reaffirmation of Credit Agreement. This Amendment
constitutes a "Loan Document" as defined in the Credit Agreement and shall be
deemed to be an amendment of the Credit Agreement, and the Credit Agreement, as
amended and restated hereby, is hereby ratified, approved and confirmed in each
and every respect. All references to the Credit Agreement in any other document,
instrument, agreement or writing shall hereafter be deemed to refer to the
Credit Agreement as amended by this Amendment.

         SECTION 7. Counterparts. This Amendment may be executed in counterparts
(and by different parties hereto on different counterparts), each of which shall
constitute an original, but all of which when taken together shall constitute
one and the same Amendment.

         SECTION 8. GOVERNING LAW. THIS AMENDMENT AND THE OTHER LOAN DOCUMENTS
SHALL BE CONSTRUED IN ACCORDANCE WITH AND GOVERNED BY THE INTERNAL LAWS OF THE
STATE OF TEXAS.

         SECTION 9. Severability. Any provision of this Amendment held to be
invalid, illegal or unenforceable in any jurisdiction shall, as to such
jurisdiction, be ineffective to the extent of such invalidity, illegality or
unenforceability without affecting the validity, legality and enforceability of
the remaining provisions hereof; and the invalidity of a particular provision in
a particular jurisdiction shall not invalidate such provision in any other
jurisdiction.

         SECTION 10. Entire Agreement. THIS WRITTEN AMENDMENT AND THE OTHER LOAN
DOCUMENTS REPRESENT THE FINAL AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE
CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL
AGREEMENTS OF THE PARTIES.

                                        4
<PAGE>

         THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                      [SIGNATURES BEGIN ON FOLLOWING PAGE]

                                        5
<PAGE>

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

                                     COHO ENERGY, INC.

                                     By:      /s/ Gary L. Pittman
                                           -------------------------------------
                                     Name:    Gary L. Pittman
                                     Title:   Vice President and
                                              Chief Financial Officer

                                       S-1
<PAGE>

                                     THE CHASE MANHATTAN BANK, individually
                                     as a Lender and as Administrative Agent

                                     By:      /s/ Russell A. Johnson
                                           -------------------------------------
                                     Name:    Russell A. Johnson
                                     Title:   Vice President

                                       S-2
<PAGE>

                                     THE CHASE MANHATTAN BANK,
                                     as Issuing Bank

                                     By:      /s/ Russell A. Johnson
                                           -------------------------------------
                                     Name:    Russell A. Johnson
                                     Title:   Vice President

                                       S-3
<PAGE>

                                     FORTIS CAPITAL CORP.,
                                     successor-in-interest to Meespierson
                                     Capital Corp., individually as a Lender
                                     and as Documentation Agent

                                     By:      /s/ Darrell W. Holley
                                           -------------------------------------
                                     Name:    Darrell W. Holley
                                     Title:   Managing Director

                                     By:      /s/ Christopher S. Parada
                                           -------------------------------------
                                     Name:    Christopher S. Parada
                                     Title:   Vice President

                                       S-4
<PAGE>

                                     FLEET NATIONAL BANK, individually as a
                                     Lender and as Syndication Agent

                                     By:      /s/ Terrence Ronan
                                           -------------------------------------
                                     Name:    Terrence Ronan
                                     Title:   Managing Director

                                       S-5
<PAGE>

                                     CREDIT LYONNAIS, NEW YORK BRANCH,
                                     individually as Lender

                                     By:      /s/ Philippe Soustra
                                           -------------------------------------
                                     Name:    Philippe Soustra
                                     Title:   Executive Vice President

                                     By:
                                           -------------------------------------
                                     Name:
                                     Title:

                                       S-6
<PAGE>

                                     ABN AMRO BANK N.V., individually as Lender

                                     By:      /s/ Jamie A. Conn
                                           -------------------------------------
                                     Name:    Jamie A. Conn
                                     Title:   Group Vice President

                                     By:      /s/ Stuart Murray
                                           -------------------------------------
                                     Name:    Stuart Murray
                                     Title:   Group Vice President

                                       S-7
<PAGE>

                                     GENERAL ELECTRIC CAPITAL CORPORATION,
                                     individually as Lender

                                     By:      /s/ Michael W. Depriest
                                           -------------------------------------
                                     Name:    Michael W. Depriest
                                     Title:   Portfolio Manager

                                       S-8
<PAGE>

                                     CIBC INC., individually as Lender

                                     By:      /s/ Nora Q. Catiis
                                           -------------------------------------
                                     Name:    Nora Q. Catiis
                                     Title:   Authorized Signature

                                     By:
                                           -------------------------------------
                                     Name:
                                     Title:

                                       S-9
<PAGE>

                                     CREDIT AGRICOLE INDOSUEZ, individually
                                     as Lender

                                     By:      /s/ Patrick Cocquerel
                                           -------------------------------------
                                     Name:    Patrick Cocquerel
                                     Title:   First Vice President, Managing
                                              Director, Head of Houston
                                              Representative Office

                                     By:      /s/ Brian D. Kniezeak
                                           -------------------------------------
                                     Name:    Brian D. Kniezeak
                                     Title:   First Vice President

                                      S-10
<PAGE>

                                     NATEXIS BANQUES POPULAIRES, individually
                                     as Lender

                                     By:      /s/ Donovan C. Broussard
                                           -------------------------------------
                                     Name:    Donovan C. Broussard
                                     Title:   Vice President

                                     By:      /s/ Renaud J. d'Herbes
                                           -------------------------------------
                                     Name:    Renaud J. d'Herbes
                                     Title:   Senior Vice President
                                              and Regional Manager

                                      S-11

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