Document:

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                                                                   EXHIBIT 10.13

                                  AMENDMENT TO
                           PURCHASE AND SALE AGREEMENT

         This Amendment to Purchase and Sale Agreement ("Amendment") is made and
entered into this 26th day of May, 1999, by and between Shell Western E & P Inc.
and Shell Onshore Ventures Inc. ("SELLER") and Encore Operating, L.P.
("PURCHASER"). SELLER and PURCHASER may be referred to herein collectively as
the "Parties".

                                   WITNESSETH:

         WHEREAS, SELLER and PURCHASER are parties to that certain Purchase and
Sale Agreement dated March 12, 1999, providing for the sale to PURCHASER of
SELLER's interests in the Williston Basin (Cedar Creek Anticline) in Montana and
North Dakota (the "PSA"), such interests being more fully described in Exhibit A
to the PSA; and

         WHEREAS, pursuant to Section 9 of the PSA, PURCHASER has conducted a
due diligence review of the Property and has provided notice to SELLER of
certain conditions identified with respect to the PROPERTY in its Preliminary
Due Diligence Report (Items not in Data Room) dated and presented to SELLER on
April 28, 1999; and

         WHEREAS, the Parties desire to fully and finally settle and resolve the
issues raised by PURCHASER with respect to the PROPERTY and to amend the PSA as
provided herein;

         NOW THEREFORE, in consideration of the mutual promises set forth herein
and other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the Parties hereto agree as follows:

         A. Definitions.

         Capitalized terms used in this Amendment, which are not defined herein,
shall have the meaning ascribed thereto in the PSA.

         B. Settlement, Release and Amendment.

         In full and complete settlement, satisfaction and release of all
claims, liability and damages of any nature, whether contract, tort or
statutory, known or unknown, present or future (AND EVEN THOUGH SUCH CLAIM,
LIABILITY OR DAMAGE MAY HAVE BEEN CONTRIBUTED TO OR CAUSED BY THE NEGLIGENCE (OF
ANY CHARACTER) OF FAULT, OR THE STRICT OR STATUTORY LIABILITY OF SELLER OR ANY
SELLER INDEMNIFIED PARTY) which PURCHASER, its successors or assigns, or any
PURCHASER Indemnified Party may have against SELLER or any SELLER Indemnified
Party arising out of or related to the PROPERTY, or the sale thereof, save and
except for SELLER's obligations under sections 2, 5, 6, 7, 15, 18, 19, 20 and 23
and subsections 11(c), 21(c), 22(c) and (e), and 29(f), (h), (i), (j), (o) and
(p) of the PSA, the Parties agree as follows:

         1. Section 3 of the PSA is amended to change the Purchase Price to ONE
HUNDRED SEVENTY-TWO MILLION DOLLARS ($172,000,000.00).

         2. Section 4(b) of the PSA is amended to delete the parenthetical
phrase "(10% of the Purchase Price)" from line 10 on page 5.

         3. Subsection 9(c) of the PSA is deleted in its entirety and replaced
with the following:

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             (c) Review Results.

             As of the date of this Amendment, PURCHASER has completed its Pre-
     Acquisition Review and has examined the PROPERTY as fully as desired.
     EXCEPT FOR AND WITHOUT LIMITING SELLER'S INDEMNIFICATION OBLIGATIONS
     SPECIFIED IN SUBSECTIONS 22(C) AND (E) BELOW, PURCHASER HEREBY AGREES TO
     ACQUIRE THE PROPERTY "WHERE IS" AND "AS IS" WITH NO RIGHT TO RECOVER

     FROM SELLER FOR ANY LIABILITIES, COSTS OR EXPENSES RELATED TO THE PROPERTY
     (INCLUDING, WITHOUT LIMITATION, ENVIRONMENTAL CONDITIONS AND DAMAGES TO
     NATURAL RESOURCES) AND PURCHASER HEREBY FINALLY AND IRREVOCABLY WAIVES AND
     RELEASES SELLER FROM, AND AGREES TO DEFEND, INDEMNIFY AND HOLD SELLER, ITS
     AFFILIATES, DIRECTORS, OFFICERS, EMPLOYEES, AGENTS AND REPRESENTATIVES
     HARMLESS AGAINST ALL LIABILITIES,

     DAMAGES, COSTS OR EXPENSES OF ANY NATURE ARISING OUT OF RELATED TO OR IN
     CONNECTION WITH THE PROPERTY (INCLUDING, WITHOUT LIMITATION, ENVIRONMENTAL
     CONDITIONS, CERCLA LIABILITY AND DAMAGES TO NATURAL RESOURCES), WHETHER
     CONTRACT, TORT OR STATUTORY, REGARDLESS OF THE NEGLIGENCE (OF ANY
     CHARACTER, WHETHER SOLE, GROSS, JOINT, CONCURRENT, CONTRIBUTORY OR
     OTHERWISE), FAULT OR STRICT (STATUTORY) LIABILITY OF SELLER, WHETHER KNOWN
     OR UNKNOWN, PRESENT OR FUTURE.

             4. Section 10 of the PSA is amended by deleting the section in its
     entirety and replacing it with "Removed".

             5. Subsection 11(b)(iv) of the PSA is amended by (1) changing
     "SUB-SECTIONS 22(c), (d) AND (e) BELOW "in lines 19 and 20 of page 17 to
     "SUB-SECTIONS 22 (c) AND (e) BELOW"; and (2) deleting "BUT EXCLUDING
     MATTERS INCLUDED IN PURCHASER's NOTICE AS PERMITTED UNDER SUBSECTION
     9(c)(1) WHICH ARE RESOLVED IN ACCORDANCE WITH THE PROVISIONS OF SUBSECTION
     9(c)" from lines 13, 14 and 15 of page 18.

             6. Subsection 13(c) of the PSA is amended by deleting ", (d)" from
     line 6 of page 23.

             7. Subsection 14 (a) of the PSA is amended by deleting ", (d)" from
     line 11 of page 24.

             8. A new subsection 14(d) is added to the PSA to read as follows:

                (d) With respect to Article 14, and except for and without
             limiting SELLER's indemnification obligations in subsection 22(e)
             below, PURCHASER HEREBY WAIVES ANY TITLE DEFECT, KNOWN OR UNKNOWN,
             RELATING TO THE PROPERTY; provided, however, (a) SELLER will, prior
             to Closing: (i) obtain and file in the records of Fallon County,
             Montana, an assignment from Mobil Oil Corporation to Seller
             covering a 66.67% interest in the properties covered by an
             operating agreement dated January 1, 1967, between Mobil and
             Northern Pacific Railway Company, recorded at Book

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             66M, page 457, and (ii) in regard to Pennel Unit Tract Nos. 48 and
             49, file with, and request the approval of, the Montana State Board
             of Land Commissioners for (A) that certain Operating Agreement
             dated April 10, 1951, recorded in Book 32M, page 143, among
             Montana-Dakota Utilities Co., Fidelity Gas Company and Shell Oil
             Company, and (B) the subsequent assignment of such operating rights
             from Shell Oil Company to Shell Western E&P Inc.

             9. Subsection 21 (b) of the PSA is amended by deleting ", (d)" from
     line 19 of page 38.

             10. Subsection 22(b) of the PSA is amended by deleting "except
     those ENVIRONMENTAL CLAIMS OR ENVIRONMENTAL LIABILITY indemnified by SELLER
     under subsection 22(d) hereof from lines 12, 13, and 14 of page 41.

             11. Subsection 22(c) of the PSA is amended by changing "subsections
     22(d) and (e)" to "subsection 22 (e)" on line 10 of page 42; and (2) by
     adding the words "insofar and only insofar as such Third Party Claim is for
     personal injury, occupational disease or death" after the phrase "Third
     Party Claim" on line 15 of page 42.

             12. Subsection 22(d) of the PSA is amended by deleting the
     subsection in its entirety and replacing it with "Removed".

             13. Subsection 22 (f) of the PSA is amended by (1) deleting "(d)"
     from line 12 of page 45, and (2) by changing "subsections 22 (c) and (d)"
     on lines 13 and 14 of page 45 to read "subsection 22(c)".

             14. Subsection 22(f)(4) is amended to read as follows: "The
     combined financial obligations of both SELLERS under subsection 22(c) shall
     be limited to and shall never exceed, in the aggregate, $44,194,444.00;".

             C. PSA

                Except as expressly set forth herein, the terms and conditions
of the PSA are unaffected and shall remain in full force and effect as written.

                IN WITNESS WHEREOF, this Amendment is executed as of the date
first above written.

SHELL WESTERN E&P INC.

By:  /s/
   ------------------------------
          Attorney-in-Fact

SHELL ONSHORE VENTURES INC.

By:  /s/
   ------------------------------
          Attorney-in-Fact

ENCORE OPERATING, L. P.
By: EAP Operating Inc.
General Partner

By:  /s/ I. JON BRUMLEY
   ------------------------------
         I. Jon Brumley
         Chief Executive Officer

                                       3<PAGE>   1
                                                                   EXHIBIT 10.14

                               SECOND AMENDMENT TO

                           PURCHASE AND SALE AGREEMENT

         This Second Amendment to Purchase and Sale Agreement ("Amendment") is
made and entered into this 28th day of May, 1999, by and between Shell Western
E&P Inc. ("SWEPI") and Shell Onshore Ventures Inc. ("SOVI") (collectively
"SELLER") and Encore Operating, L.P. ("PURCHASER"). SELLER and PURCHASER may be
referred to herein collectively as the "Parties" and individually as a "Party."

                                   WITNESSETH

         WHEREAS, SELLER and PURCHASER are parties to that certain Purchase and
Sale Agreement dated March 12, 1999, as amended by agreement dated May 26, 1999,
providing for the sale to PURCHASER of SELLER's interests in the Williston Basin
(Cedar Creek Anticline) in Montana and North Dakota (the "PSA"); and

         WHEREAS, the Parties desire to amend the PSA to provide for additional
transitional services from SWEPI to PURCHASER;

         NOW THEREFORE, in consideration of the mutual promises set forth herein
and other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the Parties hereto agree as follows:

A.        Definitions

         Capitalized terms used in this Amendment, which are not defined herein,
shall have the meaning ascribed thereto in the PSA.

B.        Amendments

         1. Subsection 19(d) of the PSA is amended by deleting the last two
sentences and inserting the following:

         "At PURCHASER's request, after Closing, SELLER or its affiliates shall
         provide transitional services (including without limitation accounting,
         payment and collection, purchasing, communications and information
         system services) for PURCHASER pursuant to the terms of the Transition
         Services Agreement attached as EXHIBIT "I" hereto."

         2. EXHIBIT "I" to the PSA shall be replaced in its entirety with the
EXHIBIT "I" attached to this Amendment and incorporated by reference herein.

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C.       PSA

         Except as expressly set forth herein, the terms and conditions of the
PSA are unaffected and shall remain in full force and effect as written.

         IN WITNESS WHEREOF, this Amendment is executed as of the date first
above written.

SHELL WESTERN E&P INC.

By: /s/
    ----------------------------
         Attorney-in-Fact

SHELL ONSHORE VENTURES INC.

By: /s/
    ----------------------------
         Attorney-in-Fact

ENCORE OPERATING, L. P.
By: EAP Operating Inc.
General Partner

By: /s/ I. JON BRUMLEY
    ----------------------------
        I. Jon Brumley,
        Chief Executive Officer

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                                                                       EXHIBIT I

                            ATTACHED TO THAT CERTAIN
                               SECOND AMENDMENT TO
                           PURCHASE AND SALE AGREEMENT
                      BY AND BETWEEN SHELL WESTERN E&P INC.
                   AND SHELL ONSHORE VENTURES INC., AS SELLER,
                    AND ENCORE OPERATING, L.P., AS PURCHASER

                          TRANSITION SERVICES AGREEMENT

         This Transition Services Agreement ("Agreement") is made and entered
into as of this 1st day of June, 1999, by and between Shell Western E&P Inc.
("SWEPI") and Encore Operating, L.P. ("Encore"). "SWEPI" and "Encore" are
referred to collectively herein as "Parties".

                                   WITNESSETH:

         WHEREAS, SWEPI and Shell Onshore Ventures Inc. (collectively "Seller")
and Encore have entered into that certain Purchase & Sale Agreement dated March
12, 1999, as amended, pursuant to which Encore will acquire certain oil and gas
properties in the States of Montana and North Dakota (the "Property") from
Seller, effective June 1, 1999; and

         WHEREAS, in order to provide for an efficient transfer of operations
from Seller to Encore following Closing, Encore desires to retain SWEPI's
services to assist Encore in the operation of the Property during the term of
this Agreement; and

         WHEREAS, SWEPI is agreeable to providing such services pursuant to the
terms hereof;

         NOW, THEREFORE, in consideration of the premises, mutual covenants and
agreements herein contained, the Parties hereto agree as follows:

 1.       SERVICES

          During the term of this Agreement, SWEPI will provide or otherwise
 make available employees or employees of its affiliates to perform (i) such
 land, operations, purchasing, communications, information system, and other
 general services relating to the Property as Encore may from time to time
 request and SWEPI may from time to time agree to perform; and (ii) such of the
 accounting and payment services described on Schedule 1 hereof as Encore may
 from time to time request, provided that any request for accounting and payment
 services for June 1999, must be made on or before 9:00 a.m. Central Time on
 June 14, 1999. This Agreement shall not extend to services that would require
 the service provider (a) to be authorized to do business in any jurisdiction
 other than Montana, Texas or North Dakota or (b) to make any decisions
 concerning the business activities or rights or obligations or regulatory
 responsibilities of Encore.

2.       PERFORMANCE OBLIGATIONS

         During such time as SWEPI's or its affiliates' employees are rendering
services hereunder on behalf of Encore, those employees who are engaged in the
performance of such services shall remain employees of SWEPI or its affiliates
and continue to be paid by and to enjoy the benefits to which they are entitled
as employees of SWEPI or its affiliates.

         The obligations of SWEPI and its affiliates and their employees to
perform any services hereunder shall be subject to the business requirements of
SWEPI and its affiliates.

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         The obligations of SWEPI and its affiliates and their employees to
perform the accounting and payment services set forth on Schedule 1 hereof are
conditioned upon and subject to Encore and SWEPI, at Encore's cost and expense,
maintaining the existing communications link between Encore's office at Baker,
Montana, and SWEPI's offices in Houston, Texas. It is acknowledged and agreed
that SWEPI and its affiliates shall be relieved of their obligations to provide
the services set forth on Schedule 1 at any time and from time to time that such
communications link is unavailable or inoperable.

3.       COMPENSATION

         To the extent the services described in clause (i) of Section 1 shall
be furnished, payment therefor shall be made by Encore to SWEPI at a rate equal
to all direct costs (including, without limitation, salaries of the employees
providing the services, labor burden, out-of-pocket expenses, expenses for
computers, materials and supplies, and for travel and living, etc.) plus all
applicable indirect costs, all of which costs, whether direct or indirect, shall
be determined in accordance with generally accepted accounting methods and
practices, consistently applied.

         The services described on Schedule 1 (if requested by Encore) shall be
provided for production occurring in the month of June 1999 for a flat fee of
$107,000.00. If Encore desires to receive any such services for any production
month other than June, 1999, the requested services shall be provided at a
mutually agreeable fee.

         SWEPI shall maintain adequate accounting records, which in reasonable
detail fairly reflect the services contemplated hereunder, and shall maintain a
system of internal controls, sufficient to provide reasonable assurances that
the services are provided in accordance with this Agreement.

         All related books and accounts of SWEPI applicable to the performance
of its obligations hereunder shall at all reasonable times be open to inspection
by auditors for Encore for a period of one year after the date of termination of
this Agreement.

4.       INDEPENDENT CONTRACTOR STATUS

         Nothing contained in this Agreement shall constitute either party
hereto the agent of the other party for any purpose. SWEPI and all affiliates
and employees providing services hereunder shall be engaged in a capacity as an
independent contractor with full control over the manner and method of
performance subject to the work authorizations of Encore.

5.       STANDARD OF CARE

         While performing the services hereunder, SWEPI and its affiliates and
their employees shall perform their duties in a manner they reasonably believe
to be in the best interests of Encore, BUT SHALL HAVE NO LIABILITY WHATSOEVER TO
ENCORE FOR ANY DAMAGES, LIABILITIES OR CLAIMS ARISING OUT OF OR RELATED TO THE
PERFORMANCE OF SERVICES HEREUNDER, EXCEPT SUCH AS ARE CAUSED BY THEIR GROSS
NEGLIGENCE OR WILLFUL MISCONDUCT IN THE PERFORMANCE OF THE SERVICES HEREUNDER.

6.       INDEMNITY

         TO THE MAXIMUM EXTENT PERMITTED BY APPLICABLE LAW, ENCORE SHALL
INDEMNIFY, DEFEND AND HOLD HARMLESS SWEPI, ITS AFFILIATES, AND THEIR RESPECTIVE
OFFICERS, DIRECTORS, EMPLOYEES, AGENTS, CONTRACTORS AND SUBCONTRACTORS ("SWEPI
INDEMNIFIED PARTIES") FROM AND AGAINST ANY AND ALL CLAIMS COSTS OR LIABILITIES
OF ANY KIND OR NATURE WHATSOEVER ("CLAIMS"), ARISING OUT OF ANY ACTS OR
OMISSIONS OF THE SWEPI INDEMNIFIED

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PARTIES IN THE PERFORMANCE OF THE SERVICES HEREUNDER, EXCEPT FOR THE GROSS
NEGLIGENCE OR WILLFUL MISCONDUCT OF THE SWEPI INDEMNIFIED PARTIES. THE FOREGOING
OBLIGATIONS OF INDEMNITY SHALL APPLY EVEN THOUGH SUCH CLAIMS MAY HAVE BEEN
CONTRIBUTED TO OR CAUSED BY THE NEGLIGENCE OR FAULT, OR THE STRICT OR STATUTORY
LIABILITY OF THE SWEPI INDEMNIFIED PARTIES UNDER ANY LAW (INCLUDING STATUTORY,
REGULATORY AND CASE LAW), EXCEPT FOR THE GROSS NEGLIGENCE OR WILLFUL MISCONDUCT
OF THE SWEPI INDEMNIFIED PARTIES.

         Nothing contained in this Agreement shall be deemed to relieve either
the partners or management of Encore from the performance of their respective
duties or limit the exercise of their powers and authority under the law.

         This Agreement, upon execution hereof, shall be effective as of the
date hereof and shall continue in effect for a period of six months thereafter,
subject, however, to termination at any time by either party giving thirty (30)
days written notice to the other.

         IN WITNESS WHEREOF, the Parties have executed this Agreement effective
as of the date first written above.

                                        SHELL WESTERN E&P INC.

                                        By:
                                           -------------------------------------
                                                     Attorney-in-Fact

                                        ENCORE OPERATING, L.P.
                                        BY: EAP OPERATING INC.,
                                            GENERAL PARTNER

                                        By:
                                           -------------------------------------

                                        Title:
                                              ----------------------------------

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                                   SCHEDULE 1

                               Accounting Services

Revenue calculation

Severance tax calculation

Payment to working interest owners

Payment to royalty interest owners

Net proceeds calculation/payment

State/Federal reports

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