<PAGE>

                                                                   EXHIBIT 10.13

                             Dated 1 February 2001


                   BURLINGTON RESOURCES (IRISH SEA) LIMITED

                                    - and -

                                 DIFCO LIMITED




                          SALE AND PURCHASE AGREEMENT
                                EAST IRISH SEA
<PAGE>

<TABLE>
                                   CONTENTS

CLAUSE                                             PAGE
<S>                                                <C>
1.       DEFINITIONS                                 2

2.       CONDITIONS                                  5

3.       SALE AND PURCHASE                           6

4.       CONSIDERATION                               8

5.       PERIOD UNTIL COMPLETION DATE               10

6.       COMPLETION                                 12

7.       INDEMNITY                                  13

8.       WARRANTIES                                 14

9.       ANNOUNCEMENTS                              16

10.      NOTICES                                    17

11.      COSTS, EXPENSES AND DELAYED PAYMENT        17

12.      TAXATION                                   18

13.      VARIATION                                  19

14.      ASSIGNMENT                                 19

15.      FURTHER ASSURANCE                          19

16.      GENERAL                                    19

17.      GOVERNING LAW                              20

SCHEDULES

1.       WARRANTIES                                 22

2.       LICENCES INTEREST DOCUMENTS                25

3.       DRAFT LICENCE ASSIGNMENT                   43

4.       DRAFT OPERATING AGREEMENT NOVATION         46

5.       DRAFT ASSIGNMENT OF LICENCE INTEREST       50

6.       WORKING CAPITAL ADJUSTMENT                 52

7.       ALLOCATION OF PAST COSTS                   53

8.       EAST IRISH SEA ASSETS                      54
</TABLE>
<PAGE>

THIS AGREEMENT is made on 1 February 2001

BETWEEN:-

(1)        BURLINGTON RESOURCES (IRISH SEA) LIMITED whose registered office is
           at One Canada Square, Canary Wharf, London E14 5AA (hereinafter
           referred to as "Buyer"); and

(2)        DIFCO LIMITED whose registered office is at 12 St. James's Square,
           London SW1Y 4RB (hereinafter referred to as "Difco").

RECITALS

(A)        The P706 Licensee is the present licensee under UK Petroleum
           Production Licence P706 dated 31 August 1989 currently comprising
           United Kingdom Continental Shelf Block 110/2b and 2c.

(B)        The P543 Licensees are the present licensees under UK Petroleum
           Production Licence P543 dated 31 October 1985 currently comprising
           United Kingdom Continental Shelf Block 110/3c.

(C)        The P099 Licensees are the present licensees under UK Petroleum
           Production Licence P099 dated 29 July 1970 currently comprising
           United Kingdom Continental Shelf Block 110/7a, 110/12a and 110/14.

(D)        The P865 Licensees are the present licensees under UK Petroleum
           Production Licence P865 dated 4 March 1994 currently comprising
           United Kingdom Continental Shelf Block 110/7c.

(E)        The P251 Licensees are the present licensees under UK Petroleum
           Production Licence P251 dated 6 July 1976 currently comprising United
           Kingdom Continental Shelf Block 110/8a (except that portion
           previously relinquished).

(F)        The P261 Licensee is the present licensee under UK Petroleum
           Production Licence P261 dated 3 February 1978 currently comprising
           United Kingdom Continental Shelf Block 110/9a.

(G)        The P287 Licensee is the present licensee under UK Petroleum
           Production Licence P287 dated 7 June 1978 currently comprising United
           Kingdom Continental Shelf Block 113/26a.


(H)        The P547 Licensees are the present licensees under UK Petroleum
           Production Licence P547 dated 16 October 1985 currently comprising
           United Kingdom Continental Shelf Block 113/27a.

(I)        The P706, P287 and P547 Licensees are the present parties to the
           Pre-Unitisation Agreement dated 13 February 1996 governing operations
           under P706, P287 and P547 on Blocks 110/2b, 113/26a and 113/27a.

(J)        The P543 Licensees are the present parties to the P543 Joint
           Operating Agreement dated 14 July 1987 governing operations under
           P543 on Block 110/3c.

(K)        Difco has a 10 per cent. beneficial interest in the Blocks 110/2b,
           110/2c, 110/7a, 110/7c, 110/8a (except that portion previously
           relinquished), 110/9a, 110/12a, 110/14 and 113/26a and 113/27a.

                                      -1-
<PAGE>

(L)      Difco has a 4.3 per cent. beneficial interest in Block 110/3c.

(M)      Difco has agreed to sell and Buyer has agreed to purchase all of
         Difco's interest under United Kingdom Production Licences P287, P706,
         P543, P865, P251, P261, P099, and P547.

(N)      Difco has agreed to sell and Buyer has agreed to purchase all of
         Difco's interest under the P543 Joint Operating Agreement and the Pre-
         Unitisation Agreement referred to in Recital I which relates to Blocks
         110/2b and 113/26a.

(O)      Difco has agreed to sell and Buyer has agreed to purchase all of
         Difco's interest in the Blocks referred to in Recitals K and L.

(P)      The Parties attach the utmost importance to the integrity of the
         package of assets comprising the Transferred Interests.

THE PARTIES AGREE AS FOLLOWS:-

1        DEFINITIONS

1.1      In this agreement (including the recitals and schedules hereto) the
         following words and expressions shall, except where the context
         otherwise requires, have the following respective meanings:-

         "Affiliate" means, in relation to any Party, any company which is a
         subsidiary of such Party or a company of which such Party is a
         subsidiary or a company which is another subsidiary of a company of
         which such Party is a subsidiary and in this definition of the word
         "subsidiary" has the meaning given to it in section 736 of the
         Companies Act 1985 as amended by section 144 of the Companies Act 1989;

         "Assignment Documents" means documents substantially in the form set
         out in schedules 3, 4 and 5 and such other documents as are necessary
         to effect the transfer of the Transferred Interests to Buyer and, where
         the context so admits, any one or more such documents;

         "Base Rate" means the mid-market London Interbank Offer Rate ("LIBOR")
         for three month deposits of Pounds Sterling or US dollars (as
         applicable) as quoted in the "Financial Times" on the Due Date plus 1%;

         "Benefit" means all income, rights, receipts and other benefits
         (including any interest which may be due on such income, rights,
         receipts and other benefits) net of operating costs and licence rentals
         in respect of the Transferred Interests;

         "Block" means a block as shown on the reference map deposited at the
         principal office of the Department of Trade and Industry;

         "Business Day" means a day (other than a Saturday or Sunday or a legal
         or public bank holiday in England) on which banks are or, as the
         context may require, were generally open for business in England and in
         Houston, Texas, USA;

         "Burlington" means Burlington Resources (Irish Sea) Limited and any of
         its relevant Affiliates;

         "Cash Call" means any request and/or demand by the Licence Operator
         (which shall include invoices generally) for payment pursuant to the
         JOA and any other agreement or arrangement related to the Transferred
         Interests;

                                      -2-
<PAGE>

         "Cash Calls Adjustment" means the sum calculated pursuant to clause
         4.4;

         "Completion" means the completion of the sale and purchase in respect
         of the Transferred Interests as provided for in clause 6;

         "Completion Date" means the date of Completion as determined in
         accordance with clause 6;

         "Consideration" means the consideration for the disposal of the
         Transferred Interests referred to in clause 4.1;

         "CT" means the tax applied pursuant to section 6 of the Income and
         Corporation Taxes Act 1988;

         "Data" means all data and information held by or on behalf of Difco and
         relating to the Transferred Interests (or any of them) including,
         without prejudice to the generality of the foregoing, accounts, tax
         information (specifically relating to the Transferred Interests),
         books, contracts, correspondence, information, data and reports
         (including without limitation petroleum engineering, reservoir
         engineering, drilling, geological, geophysical and all other kinds of
         technical data and reports, maps, samples, well-logs and analyses
         (including feasibility studies and market analyses) in whatever form
         the same are maintained) but excluding Traded Data;

         "Disclosure Letter" means a letter of even date herewith addressed by
         Difco to Buyer disclosing exceptions to the representations and
         warranties in clause 8 and schedule 1;

         "Dollars" or "$" means the lawful currency of the United States of
         America;

         "Due Date" means the date on which a payment is due to be paid (or from
         which interest is to be calculated under clause 4.1(c)) in accordance
         with the terms of this agreement;

         "Economic Date" means 1 January 2000 in respect of the Transferred
         Interests;

         "Economic Adjustment Period" means the period from the Economic Date to
         the Completion Date;

         "Force Majeure" means any event which is beyond the control of Difco
         or Buyer as the case may be, which would entitle any party to an
         agreement relating to the Transferred Interests to invoke force majeure
         under such agreement;

         "Income Adjustment" means the sum calculated pursuant to clause 4.5;

         "JOA" means the Joint Operating Agreement governing operations for
         Licence P543 (Blocks 110/3b and 3c) made on 14 July 1987 between (1)
         Canada North West Exploration Ltd, (2) Canada North West (U.K.) Ltd,
         (3) Fynegold Petroleum Limited, (4) Teredo Oils Ltd, (5) Toredo
         Exploration Ltd, and (6) Taylor Woodrow Energy Ltd, as subsequently
         amended and novated; and the Joint Operating Agreements governing
         operations for the following Licences and Blocks made on 30 October
         1998 between (1) Difco and (2) Burlington Resources (Irish Sea)
         Limited, as subsequently amended and novated:

                                      -3-
<PAGE>

                 --------------------------- ------------------------
                 Licence                     Block
                 --------------------------- ------------------------
                 P099                        110/7a
                 --------------------------- ------------------------
                 P099                        110/12a
                 --------------------------- ------------------------
                 P099                        110/14
                 --------------------------- ------------------------
                 P251                        110/8a
                 --------------------------- ------------------------
                 P261                        110/9a
                 --------------------------- ------------------------
                 P287                        113/26a
                 --------------------------- ------------------------
                 P547                        113/27a
                 --------------------------- ------------------------
                 P706                        110/2b
                 --------------------------- ------------------------
                 P706                        110/2c
                 --------------------------- ------------------------
                 P865                        110/7c F1
                 --------------------------- ------------------------
         "Licence" means one or all of the UK Petroleum Production Licences
         referred to in the recitals above;

         "Licence Interest Documents" means the documents specified in schedule
         2 and, where the context so admits, any one or more of such documents;

         "Licence Operator" means Burlington or Kerr-McGee Resources (U.K.)
         Limited in its capacity as operator (under and as defined in the
         relevant JOA or PUA);

         "Mineral Extraction Allowances" means the allowances which are
         available under Parts IV and VII of the Capital Allowances Act 1990;

         "Party" or "Parties" means a party or parties, respectively, to this
         agreement;

         "Petroleum" has the meaning ascribed to it in the Licence;

         "Plant and Machinery Allowances" means the allowances available under
         Part II of the Capital Allowances Act 1990;

         "PUA" means the Pre-Unitisation Agreement which applies to Blocks
         110/2b, 113/26a, 113/27a made on the 13 February 1996 between (1) BG
         Exploration and Production Ltd and (2) Purbeck Exploration Limited;

         "Referenced Interest Rate" means interest at the Base Rate from (save
         where expressed to the contrary) the Economic Date until payment of the
         sum due is made, including the former date but excluding the latter;

         "Secretary" means the Secretary of State for Trade and Industry;

         "Secretary's Consents" means the consents and approvals of the
         Secretary referred to in clause 2.1(a);

         "Sterling", "Pounds Sterling" or "(pound)" means the lawful currency of
         the United Kingdom;

         "Traded Data" means data which relates to the Transferred Interests and
         which has been acquired by trade, purchase or otherwise by or on behalf
         of Difco (either alone or in conjunction with other parties) from a
         third party or parties, where (after a good faith endeavour on behalf
         of Difco to obtain permission to disclose or transfer such data) such
         data cannot be provided to Buyer because such disclosure or transfer is
         prohibited by the agreement under which it was acquired or any other
         contract applicable thereto;

                                      -4-
<PAGE>

         "Transferred Interests" means all of Difco's legal and beneficial
         interests in and under the Licence Interest Documents together with all
         rights, obligations and liabilities attaching thereto or deriving
         therefrom, including but not limited to:-

         (a)      the right to take and receive the consequent share of all
                  Petroleum produced under the Licence on and (but for this
                  agreement) after the Economic Date and to receive the gross
                  proceeds from the sale or other disposition thereof; and

         (b)      the right, title and interest in and to jointly owned funds,
                  jointly owned property (including without limitation physical
                  equipment, material and other real, personal and intangible
                  property) and all other assets which are or may be owned
                  pursuant to or under any of the Licence Interest Documents;

         "VAT" means the tax applied pursuant to the Value Added Tax Act 1994;

         "Working Capital Adjustment" means the sum determined pursuant to
         clause 4.2.

1.2      All references to clauses, recitals and schedules are, unless otherwise
         expressly stated, references to clauses of and recitals and schedules
         to this agreement.

1.3      The headings in this agreement are inserted for convenience only and
         shall be ignored in construing this agreement.

1.4      Any reference to any statute or statutory instrument in this agreement
         shall be a reference to the same as amended, consolidated or extended,
         supplemented or re-enacted from time to time on or before the date of
         this agreement and shall include any orders, regulations, instruments
         or other subordinate legislation made under the relevant or the date of
         this Agreement.

1.5      Except where the context otherwise requires, words denoting the
         singular include the plural and vice versa; words denoting any gender
         include all genders; words denoting persons include firms and
         corporations and vice versa.

1.6      Reference herein to time shall be to such time as is statutorily in
         force in the United Kingdom.

2        CONDITIONS

2.1      The respective obligations of Difco to sell and Buyer to purchase the
         Transferred Interests and thereby to effect Completion shall be subject
         to the satisfaction of the following conditions precedent:-

         (a)      the written consents and approvals of the Secretary to the
                  implementation of all matters provided for in this agreement
                  and in any other documents referred to herein, in a form and
                  substance reasonably satisfactory to Difco and Buyer;

         (b)      any necessary consents and approvals of regulatory or
                  governmental authorities in the United Kingdom and the EU
                  having been given in a form and substance reasonably
                  satisfactory to Difco and Buyer to the transactions
                  contemplated by this agreement;

         (c)      execution of the Assignment Documents by all parties thereto
                  other than Difco and Buyer;

         (d)      the receipt of the written approval or consent of every party
                  to each Licence Interest Document whose approval or consent is
                  required to the assignment or transfer of any

                                      -5-
<PAGE>

                  Licence Interest Document pursuant to this agreement, having
                  been given in form and substance reasonably satisfactory to
                  Difco and Buyer;

         (e)      there having been no material breach of the representations
                  and warranties of the Parties contained in this agreement and
                  each Party having performed in all material respects all of
                  its covenants and agreements contained in this agreement
                  (provided that neither Party shall be entitled to rely on its
                  own breach or non-performance in order to delay or avoid
                  effecting Completion);

         (f)      no co-venturer in relation to the Transferred Interests having
                  successfully exercised any binding right contained in a JOA to
                  pre-empt the sale and purchase hereby contemplated; and

         (g)      the effective discharge of all mortgages, charges (fixed or
                  floating), pledges, liens, security interests or other
                  encumbrances to the extent that the same affect or relate to
                  the Transferred Interests including, without limitation, the
                  Credit Agreement, the 1998 Debenture, the Debenture and the
                  Guaranty each as defined in the Government Agreement made
                  between The Secretary of State for Trade and Industry, Difco
                  and Toronto Dominion (Texas), Inc. on 28 June 2000.

2.2      If the conditions specified in clause 2.1 have not all been satisfied
         or waived by Buyer before 5.30 p.m. on 31 March 2001 or such other time
         or date as Difco and Buyer agree in writing either Party shall have the
         right to terminate this agreement and no Party shall have any rights or
         liabilities hereunder except in respect of any breach of this agreement
         committed before such date.

2.3      Difco and Buyer shall each use their respective reasonable endeavours
         to procure that each of the above conditions is satisfied on or before
         the time or date specified for its satisfaction.

3        SALE AND PURCHASE OF THE TRANSFERRED INTERESTS

3.1      Subject to the terms and conditions of this agreement Difco shall sell
         to Buyer for the Consideration and Buyer shall purchase the Transferred
         Interests (subject to clause 3.2) as at the Completion Date free from
         all liens, charges, mortgages, pledges, encumbrances or security
         interests or other title defects whatsoever relating thereto (other
         than in relation to any obligation to pay rent or royalty under the
         Licence or arising under applicable legislation or the Licence Interest
         Documents, including the obligations to pay rent and royalty on the
         terms and conditions of this agreement) to the extent created by,
         through or under Difco, but not otherwise.

3.2      The sale and purchase referred to in clause 3.1 above shall, as between
         the Parties, be deemed for all purposes to be made with effect on and
         from the Completion Date but so that in the period between the Economic
         Date and the Completion Date the adjustments specified in Clause 4 and
         the indemnities specified in Clause 7 shall have effect accordingly.

3.3      Difco shall, as soon as is reasonably practicable after the date hereof
         and in any event within five Business Days from the execution hereof,
         give notice to the parties entitled thereto in accordance with the
         assignment clause of each JOA.

3.4      Termination

         (a)      Notwithstanding any other provision of this Agreement, if
                  prior to this Agreement or prior to Completion one or more
                  events occur giving rise to physical damage to any Property:

                                      -6-
<PAGE>

                  (i)     which is accepted by the insurers as a total loss or a
                          constructive total loss under Difco's insurance in
                          respect of any material part of the Property; or

                  (ii)    where the aggregate of the costs arising from such
                          event or events (in terms of aggregate repair or
                          replacement costs which will or, assuming (if not a
                          fact) repair or replacement of the relevant physical
                          damage, would be borne or suffered by the owners of
                          the relevant Property as estimated in good faith by
                          the Licence Operator(s)) will or would exceed the sum
                          of (pound)5 million; then the Buyer shall have the
                          right to terminate this Agreement by notice in writing
                          to Difco given in accordance with Clause 3.4(e); or

                  (iii)   which causes a total shutdown in or delivery of
                          twenty-five percent (25%) or more of the production
                          from the Transferred Interests which total shutdown is
                          estimated in good faith by the Licence Operator as
                          likely to continue for not less than one year from the
                          date of such event.

                  As soon as possible after the occurrence of any event referred
                  to in Clause 3.4(a), Difco shall request the Licence Operator
                  to give a bona fide estimate of the estimated repair and
                  replacement costs as referred to in Clause 3.4(a)(ii) and in
                  the case of a total shutdown under clause 3.4(a)(iii) the
                  estimated duration of such total shutdown.

          (b)     If for any reason the Licence Operator fails to provide the
                  estimate(s) referred to in Clause 3.4(a) within ten (10) days
                  of a request from Difco, they shall be made by an independent
                  expert appointed by Difco and the Buyer, or if Difco or the
                  Buyer are unable to agree such appointment within twenty-four
                  (24) hours, appointed by the President of the Institute of
                  Petroleum upon request of either Difco or the Buyer.

          (c)     The estimate given by the Licence Operator or by the expert
                  pursuant to Clauses 3.4(a) or (b) shall be final and binding
                  on the Parties. In giving estimates, neither the Licence
                  Operator nor the expert shall be deemed to act as an
                  arbitrator. In any case in which an estimate is given by an
                  expert pursuant to this Clause 3.4(b), the costs of the expert
                  will be borne equally by Difco and the Buyer.

          (d)     Notice of termination pursuant to this Clause 3.4 shall be
                  given by the Buyer in accordance with the provisions of Clause
                  11 within ten (10) Business Days of:

                  (i)     the Buyer receiving written confirmation from the
                          insurers (or a copy thereof) that they accept that the
                          relevant Property is a total loss or a constructive
                          total loss (in the case of termination under Clause
                          3.4(a)(i)); or

                  (ii)    receipt by the Buyer of the Licence Operator's bona
                          fide estimate or the expert's estimate, whichever is
                          applicable (in the case of termination under Clauses
                          3.4(a)(ii) or (iii)).

                  In the event of termination of this Agreement as aforesaid,
                  there shall be no liability on the part of the Parties or any
                  of their Affiliates with respect to or in connection with this
                  Agreement and for avoidance of doubt the obligations of any
                  guarantor shall be released, save for any liabilities arising
                  prior to the date of termination.

          (e)     If, in respect of an event referred to in Clause 3.4(a), the
                  Licence Operator's or expert's estimate referred to in Clauses
                  3.4(a) or (b) or, if applicable, written notices from insurers
                  stating that they do not accept that the relevant Property as
                  a total loss or constructive total loss have not been received
                  by the Buyer prior to the date which is ten (10) Business Days
                  prior to Completion, then the Completion Date shall be delayed
                  until ten (10) Business Days after receipt by the Buyer of the
                  Licence Operator's or expert's estimate or insurer's notice
                  (as applicable).

                                      -7-
<PAGE>

          (f)     Where a right to terminate this Agreement provided by this
                  Clause 3.4 is not exercised within the period of ten (10)
                  Business Days referred to in Clause 3.4(e), this Agreement
                  shall remain in full force and effect in accordance with its
                  terms.

4        CONSIDERATION

4.1      The Consideration shall be the sum of:-

          (a)     Sixteen  Million  Six Hundred  and  Eighty-one  Thousand  Five
                  Hundred and Four Pounds  Sterling
                  ((pound)16,681,504.00) (the "Cash Consideration"); and

          (b)     the Working Capital Adjustment (which may be a positive or
                  negative amount); and

          (c)     an amount equal to the aggregate of the sums produced by
                  applying the Referenced Interest Rate for the period between
                  the Economic Date and the actual Completion Date, to the Cash
                  Consideration; and

          (d)     the Cash Calls Adjustment (which shall be a positive amount);
                  and

          (e)     an amount equal to the sum produced by applying the Referenced
                  Interest Rate to each Cash Call after applying the adjustment
                  in Clause 4.1(h) included in the Cash Call Adjustment from the
                  date on which such Cash Call was paid until the Completion
                  Date (which shall be positive amount); and

          (f)     the Income Adjustment (which shall be a negative amount, or
                  zero); and

          (g)     an amount equal to the sum produced by applying the Referenced
                  Interest Rate to items of income within the Income Adjustment
                  after applying the adjustment at Clause 4.1(h) from the date
                  when such income is paid until the Completion Date (which
                  shall be a negative amount); and

          (h)     to the extent that any adjustments are made pursuant to this
                  clause 4 as a result of which Benefits are received by, or are
                  credited to a Party (the "Receiving Party") which have
                  accrued, or will accrue to the other Party for Corporation Tax
                  purposes, the Receiving Party shall reimburse the other Party
                  thirty percent (30%) of such Benefits.

4.2       The Working Capital Adjustment shall be the sum calculated to reflect
          the monetary value of the items set out in schedule 6 as at the
          Economic Date by:-

          (a)     adding together the amounts of material stocks, cash balances,
                  debtors and pre-payments (the term "pre-payments" shall not
                  include Licence rentals paid prior to the Economic Date) (the
                  "Positive Balance");

          (b)     adding together the amounts of accruals and creditors (the
                  "Negative Balance"); and

          (c)     deducting the Negative Balance from the Positive Balance.

           The Working Capital Adjustment shall be a negative or positive
           amount, which:-

          (d)     if negative, shall represent an amount due from Difco to
                  Buyer; or

                                      -8-
<PAGE>

         (e)      if positive, shall represent an amount due from Buyer to
                  Difco.

4.3      The Working Capital Adjustment shall be in the form set out in schedule
         6 and shall be delivered by Difco within 30 days of the date of this
         Agreement. Difco shall provide Buyer with copies of the Licence
         Operator's reports and correspondence in the possession of Difco from
         which the statement has been derived. Notwithstanding the provisions of
         clauses 4.7 and 4.8, settlement of the sum detailed in schedule 6 (as
         varied by any adjustment agreed between the parties) shall be made on
         Completion.

4.4      The Cash Calls Adjustment shall be the sum of all Cash Calls invoices
         and other payment requests related to the expenditures incurred after
         the Economic Date, made by the Licence Operators and paid by Difco
         pursuant to the Licence Interest Documents during the Economic
         Adjustment Period. The Cash Call Adjustment shall be a positive amount
         for the purpose of calculating the Consideration.

4.5      The Income Adjustment shall be the sum of all income received in
         respect of the Transferred Interests during and in respect of the
         Economic Adjustment Period. The Income Adjustment shall be a negative
         amount for the purposes of calculating the Consideration.

4.6      Not Used.

4.7      Difco shall no later than five Business Days prior to Completion
         provide Buyer with a written statement giving an estimate of the
         Consideration. Notwithstanding the provisions set out below, settlement
         of the undisputed portion of the sum detailed in the statement shall be
         made on Completion. Within 60 days after Completion Difco shall provide
         Buyer with a written statement giving the final amount of the
         Consideration and the said amount, to the extent not already paid or
         taken into account on Completion, shall be paid by Difco or Buyer (as
         the case may be) within five Business Days of delivery of such
         notification. If Difco and Buyer shall fail to agree upon such
         statement, the relevant statement shall be referred for resolution in
         accordance with the provisions of clause 4.10.

4.8      If any of the amounts  required for the  determination of the
         Consideration  has not been agreed prior to
         the Completion Date then:-

         (a)      settlement of the undisputed balance of the Consideration
                  shall be made at Completion; and

         (b)      settlement of any other amount making up the Consideration
                  shall be made (by payment from Buyer to Difco if the amount is
                  positive or by payment from Difco to Buyer if the amount is
                  negative) as soon as practicable after determination, and
                  interest shall continue to be payable on the relevant amount
                  at the Referenced Interest Rate from the Completion Date to
                  the date of the actual payment.

4.9      The Consideration shall be payable in accordance with the provisions of
         clause 6. For the avoidance of doubt (but without prejudice to the
         provisions of clause 7), the Consideration shall be adjusted to the
         extent of Buyer's assumption of the liability to pay Cash Calls after
         Completion wholly or partly in respect of the expenditure taken into
         account in the Economic Adjustment Period.

4.10     If the Parties cannot agree the statement referred to in clause 4.3 or
         clause 4.7, the same shall be referred for determination by a partner
         in a leading independent City of London firm of chartered accountants,
         which individual shall have at least ten years' experience of the oil
         and gas industry, and who shall be nominated by the Parties, or failing
         agreement on such

                                      -9-
<PAGE>

         nomination within five Business Days of a Party notifying the other
         that it proposes to refer the dispute to an expert, nominated by the
         President for the time being of the Institute of Chartered Accountants
         in England and Wales. The nominated chartered accountant shall be
         afforded the same access to books, records, accounts and documents in
         the possession of the Parties as they have in respect of each other.
         Such person shall act as an expert and not as an arbitrator. The said
         accountant's determination shall, in the absence of fraud or manifest
         error, be final and binding on the Parties. The costs of such expert
         shall be borne by the Parties in the proportions such expert thinks
         fit. Payment of the finally determined sum shall be made in accordance
         with clause 6.2(b) hereof by the relevant Party on the Completion Date
         or within five Business Days of the determination, whichever is the
         later, failing which interest shall accrue on said sum at the specified
         rate in clause 11.2 until the date of payment.

4.11     The settlements of amounts pursuant to this clause 4 shall be subject
         to further adjustment pursuant to clause 7.

4.12     Not Used

4.13     The benefit of any audit exceptions as a result of any audit carried
         out under a JOA in respect of any period prior to the Economic Date
         shall be for the benefit of Difco and any sums received by Buyer in
         respect thereof shall promptly be paid by Buyer to Difco.

5        PERIOD UNTIL COMPLETION DATE

5.1      During the period between the date of this Agreement and the Completion
         Date:-

         (a)      Difco shall continue to meet all expenditure and receive all
                  income relating to the Transferred Interests and shall perform
                  and comply with all of its obligations under the JOAs and
                  other Licence Interest Documents;

         (b)      Difco shall (subject to any confidentiality obligations by
                  which it is bound) provide to Buyer access to all Data and
                  other technical, financial and contractual information in its
                  possession relating to the Transferred Interests as Buyer may
                  from time to time reasonably require, excluding Board papers,
                  internal memoranda and personal correspondence;

         (c)      Difco shall, (subject to any confidentiality obligations by
                  which it is bound) prior to any meeting of the operating
                  committee, the participants or with the Licence Operator under
                  any JOA, (or any other of the Licence Interest Documents) or
                  between any of the above mentioned parties and the Secretary
                  or to any material decision being taken in relation to the
                  Transferred Interests, consult with Buyer and have due and
                  proper regard to any representations which Buyer may make
                  concerning matters to be discussed at such meeting and cast
                  Difco's vote or abstain from doing so with due and proper
                  regard to the reasonable wishes of the Buyer;

         (d)      Difco shall not without the prior written consent of Buyer
                  (not to be unreasonably withheld) agree to amend the Licence
                  Interest Documents or to execute any new agreement in respect
                  of the Transferred Interests to the extent that any such
                  amendments or new agreement would have a material adverse
                  impact on the Transferred Interests; and

         (e)      Difco shall continue to carry on its activities in relation to
                  the Transferred Interests in the ordinary and usual course so
                  as to protect and maintain the same.

                                      -10-
<PAGE>

5.2      Without prejudice to the foregoing, Difco shall (subject to any
         confidentiality obligations by which it is bound) ensure from the date
         of execution hereof that pending Completion Buyer is kept fully
         informed of developments in relation to the Transferred Interests
         including but not limited to:-

         (a)      the making of any Cash Calls;

         (b)      the approval of any AFE;

         (c)      the adoption or proposal of any work programme and budget;

         (d)      subject to any restrictions on disclosure by Difco, the
                  transfer or the receipt of any significant geological or other
                  data which Difco shall make available to Buyer in accordance
                  with the provisions of this agreement; and

         (e)      any material matters relating to insurance.

5.3      The Parties will hold in confidence all information furnished or
         disclosed to each other in connection with the transactions
         contemplated by this agreement as well as all information concerning
         the Transferred Interests contained in any analyses, compilations,
         studies or other documents prepared during such period and any business
         and/or commercial information in respect of the Parties (collectively,
         the "Transferred Interests Information"). However, Transferred
         Interests Information shall not include any information which is:-

         (a)      generally available to the public other than as a result of a
                  wrongful disclosure by a Party; or

         (b)      available to a Party on a non-confidential basis from a source
                  other than the other Party if such source is, to the
                  disclosing Party's knowledge, entitled to disclose such
                  information.

         Neither Party will, without the prior written consent of the other
         Party, release or disclose any Transferred Interests Information to any
         other person, except to its Affiliates' officers, directors, employees,
         accountants, representatives, agents, consultants and financial
         advisers who need to know the Transferred Interests Information in
         connection with the Completion or financing of the transactions
         contemplated by this agreement, who are informed of the confidential
         nature of the Transferred Interests Information and who agree to be
         bound by the terms and conditions of this clause 5.3 and except further
         to the extent required by any applicable statute, the Licence or the
         requirements of any recognised stock exchange or other regulatory
         authority in compliance with its rules and regulations or any
         Government agency (including without limitation London Stock Exchange
         and the Securities Exchange Commission of the United States of America)
         lawfully requesting such information or any court of competent
         jurisdiction acting in pursuance of its powers.

5.4      Notwithstanding the termination of this Agreement, the provisions of
         clause 5.3 above shall continue in force for a period of five years
         from the date of execution of this Agreement.

5.5      With regard to insurance matters, between the date of this agreement
         and the Completion Date, Difco shall procure that all insurance
         policies which are in force in relation to the Transferred Interests
         and to which Difco is a party or a beneficiary ("Insurance Policies")
         are maintained or caused to be maintained and shall do nothing which
         might prejudice or affect recovery thereunder.

                                      -11-
<PAGE>

5.6      If Completion does not take place for any reason provided for in this
         agreement:-

         (a)      the Transferred Interests Information, except for that portion
                  thereof contained in analyses, compilations, studies or other
                  documents prepared by or on behalf of either Party, will be
                  returned to the disclosing Party within a reasonable time of
                  its request therefor and copies thereof shall not be retained;
                  and

         (b)      that portion of the Transferred Interests Information which is
                  contained in analyses, compilations, studies or other
                  documents shall, at the direction of the disclosing Party, be
                  expunged (where it is reasonably practicable to do so) and
                  either destroyed or returned to the disclosing Party.

6        COMPLETION

6.1      Subject to the other provisions hereof completion shall take place at
         the offices of Buyer in Houston, Texas USA as soon as possible
         following the day on which the conditions specified in clause 2.1 have
         been satisfied.

6.2      On the Completion Date:-

         (a)      Difco shall deliver to Buyer (to the extent not already
                  delivered prior to Completion):-

                  (i)     the Assignment Documents duly executed by all the
                          parties thereto other than Buyer (and, in the case of
                          each Deed of Licence Assignment, the Secretary);

                  (ii)    copies of each Secretary's Consent;

                  (iii)   copies of the consents or approvals, if any, referred
                          to in clauses 2.1(b) and 2.1(d) and obtained by or on
                          behalf of Difco;

                  (iv)    a certificate executed by Difco certifying as to the
                          matters set forth in clause 2.1(e) to the extent that
                          they relate to Difco's obligations;

                  (v)     parent company guarantee; and

                  (vi)    evidence of discharge in accordance with Clause 2.1(g)
                          in a form reasonably acceptable to the Buyer.


         (b)      Buyer shall:-

                  (i)     deliver to Difco copies of the consents or approvals,
                          if any, referred to in clauses 2.1(b);

                  (ii)    subject to clause 6.2(a), execute the relevant
                          Assignment Documents;

                                      -12-
<PAGE>

                  (iii)    pay to Difco the Consideration (in accordance with
                           the provisions of clause 4) by means of telegraphic
                           transfer to Difco's bank account at:-


                            Bank of Scotland
                            St. James's Gate
                            14-16 Cockspur Street
                            London SW1Y 5BL
                            Account Number: 00951416
                            Sort Code: 12-11-03

                  (v)       deliver to Difco a certificate executed by Buyer
                            certifying as to the matters set forth in clause
                            2.1(f) to the extent that they relate to Buyer's
                            obligations.

         (c)      Each Party shall, and shall procure that its Affiliates shall,
                  execute all such other documents and do all such other acts
                  and things as may reasonably be required in order to effect
                  the transfer of the Transferred Interests to Buyer and
                  otherwise carry out the true intent of this agreement.

         (d)      On or within ten Business Days after the Completion Date,
                  Difco will deliver to the Buyer the originals of the Licence
                  Interest Documents and the Data in its possession. Any costs,
                  charges or expenses payable to any third party data storage
                  companies in respect of the transfer or relocation of any Data
                  or photocopying charges shall be borne by Buyer.

7        INDEMNITY

Without prejudice to the provisions of clause 8 and, except as otherwise
expressly provided in this agreement:-

7.1      To the extent that any costs, charges, expenses, liabilities and
         obligations relating to Transferred Interests (together "Obligations")
         are properly incurred by Difco in respect of any period after the
         Economic Date and the Consideration has not been adjusted pursuant to
         clause 4 in respect thereof, Buyer shall re-imburse and indemnify Difco
         against any of such Obligations which are borne by Difco.

7.2      To the extent that Buyer properly incurs any Obligations attributable
         to periods prior to the Economic Date and the Consideration has not
         been adjusted pursuant to clause 4 in respect thereof, Difco shall
         re-imburse and indemnify Buyer against any such Obligations which are
         borne by Buyer.

7.3      To the extent that any income, receipts, rebates or other benefits
         (other than any benefit accruing pursuant to the provisions of clause
         5.5 which shall be dealt with in accordance with the provisions of that
         clause) relating to the Transferred Interests (together "Benefits") are
         received by or credited to Difco in respect of any period after the
         Economic Date, and the Consideration has not been adjusted pursuant to
         clause 4 in respect thereof, Difco shall, except as otherwise provided
         herein, re-imburse Buyer for any such Benefits.

7.4      To the extent that any such Benefits are received by or credited to
         Buyer in respect of any period prior to the Economic Date and the
         Consideration has not been adjusted pursuant to clause 4 in respect
         thereof Buyer shall re-imburse Difco for any such Benefits.

                                      -13-
<PAGE>

7.5      To the extent that any adjustments are made pursuant to this clause 7
         as a result of which Benefits are received by, or are credited to a
         Party (the "Receiving Party") which have accrued, or will accrue to the
         other Party for Corporation Tax purposes, the Receiving Party shall
         reimburse the other Party thirty percent (30%) of such Benefits

7.6      To the extent any Obligations accrue or arise on or after the Economic
         Date, including but not limited to plugging and abandonment and
         environmental liabilities, and such Obligations are not otherwise
         provided for or dealt with under this agreement, Buyer shall assume and
         timely pay such Obligations and shall protect, defend and indemnify
         Difco from any such Obligations.

8        REPRESENTATIONS AND WARRANTIES

8.1      Subject to the provisions of this clause, Difco hereby represents and
         warrants to Buyer in the terms set out in part 1 of schedule 1
         ("Difco's Warranties") and Buyer hereby represents and warrants to
         Difco in the terms set out in part 2 of schedule 1 ("Buyer's
         Warranties"). Notwithstanding any other provisions of this agreement,
         Buyer shall have the right of full substitution and subrogation in and
         to any and all rights and actions of warranty which Difco has or may
         have in respect of the Transferred Interests against all preceding
         owners of the Transferred Interests

8.2      Difco's Warranties are subject to the following:-

         (a)      the matters disclosed by the Licence, the JOA and the PUA and
                  the Disclosure Letter;

         (b)      any matter hereafter done or omitted to be done by or with the
                  written consent of or at the written request of Buyer;

         (c)      there shall be no claim if in respect of any single
                  circumstances the amount of the claim is less than fifty
                  thousand Dollars ($50,000);

         (d)      there shall be no claim if the amount of the claim when
                  aggregated with all other claims in excess of fifty thousand
                  Dollars ($50,000) already made is less than two hundred fifty
                  thousand Dollars ($250,000).

8.3      Difco's and Buyer's Warranties shall be deemed to be repeated as at
         Completion with reference to circumstances then existing and, subject
         to the other provisions of this agreement, shall survive Completion so
         that the remedies for breach of any of such warranties shall continue
         to subsist notwithstanding Completion for a period of one (1) year
         following Completion whereupon such Warranties shall expire and be of
         no further force or effect except with respect to any breaches of
         Warranties as to which specific written notice has been provided within
         such one (1) year time period.

8.4      In the event of any matter or thing materially inconsistent with any of
         Difco's Warranties arising at or before Completion and Difco having
         failed to remedy such material inconsistency by Completion Buyer shall
         not be bound to complete the acquisition of the Transferred Interests
         and Buyer may rescind this agreement prior to Completion. If Buyer
         elects to rescind this agreement pursuant to this clause, such
         rescission shall be its sole remedy for such continuing inconsistency.

8.5      Difco will immediately notify Buyer in the event that it becomes aware
         of any matter or thing which would or might constitute a material
         breach of any of Difco's Warranties or otherwise entitle Buyer to
         rescind this agreement pursuant to clause 8.4 above. Buyer will
         immediately notify Difco in the event that it becomes aware of any
         matter or thing, which would or might

                                      -14-
<PAGE>

         constitute a material breach of any of Buyer's Warranties or otherwise
         entitle Difco to rescind this agreement pursuant to clause 8.4 above.

8.6      Neither Party shall be liable under this agreement for any claim for
         breach of any of Difco's Warranties (in the case of Difco) or Buyer's
         Warranties (in the case of Buyer):-

         (a)   unless notice of the claim is given in writing by the Party
               claiming breach to the other Party setting out particulars of the
               grounds on which the claim is based, on or before the first
               anniversary of the Completion Date, it being understood, however,
               that Buyer shall have the right of full substitution and
               subrogation in and to any and all rights and actions of warranty
               which Difco has or may have in respect of the Transferred
               Interests against all preceding owners of the Transferred
               Interests.

         (b)   to the extent that the liability of a Party for all claims made
               would thereby exceed the Consideration;

         (c)   to the extent that such claim has already been taken into account
               pursuant to clause 4.

8.7      Where any warranty is qualified by any reference to the knowledge of
         any Party, that Party shall be deemed to have made all reasonable
         inquiries concerning the subject matter of that warranty.

8.8      Not Used.

8.9      Nothing herein shall be deemed to relieve Buyer from any common law
         duty to mitigate any loss or damage incurred by it and in any event
         Buyer undertakes that it will procure that following Completion insofar
         as relevant to Difco's Warranties or other obligations of Difco
         hereunder it shall to the extent not detrimental to Buyer take all
         reasonable steps to perform its obligations owing to and enforce its
         rights against third parties including (without limitation) promptly to
         recover any sums relevant thereto.

8.10     Buyer shall not be entitled to recover any sum in respect of any claim
         under Difco's Warranties or otherwise obtain reimbursement or
         restitution more than once in respect of any one circumstance or event
         in relation to which such claim is brought.

8.11     If Difco pays to Buyer an amount in discharge of a claim under Difco's
         Warranties and Buyer recovers (whether by payment, discount, credit or
         otherwise) from a third party (including any taxation authority) a sum
         which would not have been received but for the circumstances giving
         rise to the claim in respect of which the payment by Difco was made,
         Buyer shall repay to Difco an amount equal to the lesser of the amount
         Difco shall have so paid to Buyer or the sum received from the third
         party less any costs or expenses incurred in recovering the same
         (whichever shall be the lower).

8.12     No breach of any warranty, covenant or undertaking hereunder or
         misrepresentation or misstatement of fact by either Party shall give
         rise to a right on the part of the other Party to rescind or terminate
         this agreement following Completion.

8.13     Neither Party shall be liable in respect of any claim hereunder by the
         other Party to the extent (and only to the extent) that such liability
         is increased wholly or partly out of or as a result of or in connection
         with any act or omission after the date hereof of the other Party.

8.14     Save as and to the extent set forth in their respective Warranties, the
         Disclosure Letter and elsewhere in this agreement, the Parties make no
         representations or warranties in respect of

                                      -15-
<PAGE>

      any matter or thing and disclaims all liability and responsibility (except
      in the case of fraud or misfeasance) for any representation, warranty,
      statement, opinion or information made or communicated (orally or in
      writing) to each other (including without limiting the generality of the
      foregoing, any representation, warranty, statement, opinion, information
      or advice made or communicated to Buyer by any other shareholder,
      stockholder, director, employee, agent, consultant or representative of
      the other party or of any of its Affiliates) and each of the Parties
      acknowledges and affirms that it has not relied upon any representation,
      warranty, statement, opinion or information in entering into and carrying
      out the transactions contemplated by this agreement. Without limiting the
      generality of the foregoing, Difco makes no representations or warranties
      as to:-

      (a)  the quantity of Petroleum reserves attributable to the Transferred
           Interests;

      (b)  the quality or deliverability of said Petroleum reserves or the
           existence of a public market; or

      (c)  any geological, geophysical, engineering, economic or other
           interpretations, forecasts or evaluations concerning the Transferred
           Interests.

8.15  Buyer shall make its own independent investigation, analysis and
      evaluation of the geological, geophysical, engineering, marketing,
      economic or other interpretations, costs and prospects for further
      development of the Transferred Interests. Immediately following execution
      of this agreement, Difco, in Difco's offices, will make the Data available
      to Buyer and Buyer's authorised representatives. Difco shall permit Buyer
      and Buyer's authorised representatives to consult with Difco's employees
      during reasonable business hours. Buyer shall not contact any Licence
      Operator under the JOAs directly without Difco's consent.

8.16  Without restricting the rights of Buyer or the ability of Buyer to claim
      damages on any basis available to it, in the event of a breach of Difco's
      Warranties, Difco shall pay to Buyer the amount necessary to put Buyer
      into the position which would have existed in the absence of such breach.

8.17  Each of Difco's Warranties and Buyer's Warranties shall be construed as a
      separate representation, warranty, covenant or undertaking (as the case
      may be) and (except as expressly provided to the contrary) shall not be
      limited by the terms of any of the other representations or warranties or
      by the other terms of this agreement.

9     ANNOUNCEMENTS

      Subject to the terms of the JOAs, no Party nor any Affiliate of either of
      them shall, without the prior written consent of the other Party, issue or
      make any public announcement or statement regarding this agreement or any
      matter which is the subject of this agreement, unless it is necessary for
      that Party or its Affiliate to make such public announcement or statement
      in order to comply with a statutory obligation, an obligation to include
      information in published or audited accounts, or with the requirement of a
      competent government agency, the Securities and Exchange Commission or
      other regulatory body, or a recognised stock exchange on which that Party
      or such Affiliate has its shares listed or an unlisted securities market
      in which its shares are dealt, in which event, the Party proposing to make
      such an announcement or statement shall consult with the other Party and
      shall take account of any representations made by the other Party. A copy
      of such announcement or statement shall be furnished to the other Party at
      least 72 hours prior to publication and such announcement or statement
      shall be limited to the minimum so required.

                                      -16-
<PAGE>

10    NOTICES

10.1  Any notice under this agreement shall be in writing and signed by or on
      behalf of the Party giving it and may be served by leaving it at or
      sending it by fax (subject to receipt of a confirmation copy by prepaid
      first class post), prepaid recorded delivery or registered post to the
      address and for the attention of the relevant Party set out in clause
      10.2. Any notice so served by fax (subject to receipt of a confirmation
      copy by prepaid first class post), recorded delivery or registered post
      shall be deemed to have been received:-

      (a)  in the case of fax on the Business Day next following the date of
           dispatch and upon receipt of the correct answerback; and

      (b)  in the case of recorded delivery or registered post, on the Business
           Day next following the date of posting.

10.2  The addresses of the Parties for the purpose of clause 10.1 are as
      follows:-

      (a)  Difco:-

           Difco Limited

           Address: 4200 E. Skelly Drive, Suite 1000
           Tulsa, Oklahoma  USA  74135

           Fax:       918.494.4918

           Attention: John A. Keenan

      Buyer:-

           Burlington Resources (Irish Sea) Limited

           Address:  One Canada Square
           Canary Wharf, London  E14 5AA

           Fax:      (0044) [0]2073 699548

           Attention:  The Director and General Manager

      or such other address and/or person as the Parties shall from time to time
      notify to each other.

11    COSTS, EXPENSES AND DELAYED PAYMENT

11.1  Difco and Buyer shall each pay its and its Affiliates' own costs and
      expenses in relation to the preparation and execution of this agreement
      and the documents contemplated hereby or executed pursuant hereto.

11.2  Without prejudice to any other rights hereunder, if any amount payable
      hereunder is not paid when due, the defaulting Party shall pay interest on
      such amount from the due date of payment (after as well as before
      judgment) at a rate equal to two point five per cent. (2.5%) above the
      Referenced Interest Rate.

                                      -17-
<PAGE>

12    TAXATION

12.1  (a)  Difco and Buyer agree that the amount shown on schedule 7 shall be
           allocated accordingly as expenditure qualifying for Mineral
           Extraction Allowances at twenty-two decimal five percent (22.5%) and
           Plant and Machinery Allowances at twenty-two decimal five percent
           (22.5%) and the balance of such Cash Consideration shall be allocated
           to the Licences.

      (b)  The elements of the Cash Consideration in clauses 4.3 and 4.4 and the
           Economic Adjustment Period Consideration in clause 4.6 shall be
           allocated so as to reflect the nature of the expenditures for each
           element of the Consideration as shown in the appropriate Licence
           Operator's billing statement.

      (c)  The elements of the Cash Consideration in clauses 4.1(e) and 4.1(f)
           shall be allocated to the Licence.

      Both Difco and Buyer agree to file their CT computations on this basis as
      a just and reasonable allocation and shall use all reasonable endeavours
      to agree the foregoing Consideration allocation with the Inland Revenue
      and in the event of a challenge by the Inland Revenue to such allocation
      the Parties shall use reasonable endeavours and shall provide reasonable
      co-operation to each other in order to resist such a challenge, provided
      that if, nevertheless, such a challenge is successful, so much of the
      consideration as is no longer allocated to plant and machinery as a result
      shall be reduced by twenty-two decimal five percent (22.5%).

12.2  Buyer acknowledges that the element of Consideration allocated to Mineral
      Extraction Allowances represents expenditure incurred by Difco, Burlington
      or BG Exploration and Production Limited in respect of mineral exploration
      and access as defined within Parts IV and VII of the Capital Allowances
      Act 1990. Difco accepts that Buyer will claim Capital Allowances under
      Part IV of the Capital Allowances Act 1990 in respect of this element of
      the Consideration.

12.3  (a)  Each Party confirms that it is registered for Value Added Tax ("VAT")
           in the United Kingdom or has applied for registration for VAT and
           that the Transferred Interests are acquired by Buyer for its own
           trade of exploration and exploitation.

      (b)  Each Party  confirms that it has not elected to waive exemption under
           schedule 10 of the Value Added Tax Act 1994.

      (c)  Each Party confirms that the transfer of the Transferred Interests
           shall be treated as a transfer of a going concern under Article 5 of
           the Value Added Tax (Special Provisions) Order 1995 (SI 1995 No.
           1268), so that the transfer of the Transferred Interests is neither a
           supply of goods nor a supply of services for VAT purposes and prior
           to Completion Difco shall use its reasonable endeavours to obtain
           from HM Customs and Excise clearance that this transaction will be
           regarded as a transfer of going concern.

      (d)  Notwithstanding that the Parties believe that the transfer hereunder
           is a transaction which is outside the scope of VAT by virtue of
           Article 5 of the Value Added Tax (Special Provisions) order 1995 (SI
           1995 No. 1268), in the event that either Party is advised in writing
           by HM Customs & Excise that the transaction is subject to VAT and if
           so called upon to do so by Difco, Buyer undertakes to pay to Difco,
           on presentation of a proper VAT invoice therefore, any amount due in
           respect of VAT

                                      -18-
<PAGE>

           set out in such invoice within 30 days of demand. In the event that
           VAT is chargeable, Difco will assist in any way possible to enable
           Buyer to recover VAT properly chargeable on the transaction, subject
           to Difco's requirements under VAT legislation in force at the time.

      (e)  Difco shall prepare a notice and the Parties shall jointly notify the
           Board of Inland Revenue of the sale and purchase of the Transferred
           Interests in compliance with paragraph 3 of Schedule 17 of the
           Finance Act 1980 (hereinafter the "FA80") and shall not make an
           application under Paragraph 4 of Schedule 17 of FA80 for the
           provisions of Parts II and III of Schedule 17 of FA80 not to apply to
           such sale and purchase.

      (f)  Difco shall indemnify the Buyer in respect of any liabilities which
           the Buyer may incur in relation to the Transferred Interests arising
           in connection with Schedule 15 of the Finance Act 1973 to the extent
           that they arise in respect of events occurring before the Completion
           Date.

12.4  Where any amount is required to be paid under this agreement as interest,
      and tax is required by law to be deducted from such amount, the party
      making such payments shall be entitled to make such deduction, and account
      for the same to the Inland Revenue.

12.5  The Parties acknowledge that any expenditure incurred in relation to the
      Transferred Interest prior to the Economic Date will be claimed by Difco
      to the extent that it is eligible for relief under the provisions of
      schedule 7 of the Oil Taxation Act 1975.

13    VARIATION

      The terms and conditions of this agreement shall only be varied by an
      agreement in writing by each of the Parties and specifically referring to
      this agreement.

14    ASSIGNMENT

      No Party shall assign all or part of its rights, title or interest in this
      agreement without the prior consent in writing of the other Party.

15    FURTHER ASSURANCE

      The parties shall execute or cause to be executed and deliver to each
      other such further deeds and documents and do all such further acts and
      things as may reasonably be necessary or as may reasonably be requested by
      either party more fully to vest in and assign to Buyer all rights, powers,
      privileges and remedies herein intended to be vested in or assigned to
      Buyer.

16    GENERAL

16.1  Subject to the provisions of clause 8.3 hereof, this agreement including,
      without limitation, the representations, warranties and undertakings
      hereunder shall remain in full force and effect notwithstanding
      Completion.

16.2  No waiver by either Party of any breach of a provision of this agreement
      shall be binding unless made expressly in writing and any such waiver
      shall relate only to the matter to which it expressly relates and shall
      not apply to any subsequent or other matter.

                                      -19-
<PAGE>

16.3  This agreement shall enure to the benefit of and be binding upon the
      respective successors and permitted assigns of the Parties hereto.

16.4  Time shall be deemed to be of the essence of this agreement.

16.5  This agreement constitutes the whole agreement between the Parties
      relating to the subject matter hereof and supersedes and excludes all
      negotiations and preliminary agreements whether oral or written.

16.6  Notwithstanding termination of this agreement in accordance with its
      terms, the provisions of clauses 5.3, 5.4, 9, and 11 shall survive
      termination, shall be deemed to remain in full force and effect and the
      parties shall continue to be bound thereby.

17    GOVERNING LAW

17.1  This agreement (and any dispute, controversy, proceedings or claim of
      whatever nature arising out of or in any way relating to this agreement or
      its formation or performance) shall be exclusively governed by and
      construed in accordance with English Law, without reference to any
      conflicts of laws principles applied by the courts of England.

17.2  The Parties hereby submit to the non-exclusive jurisdiction and forum of
      the English Courts for the purpose of hearing and determining any dispute,
      controversy or claim of whatsoever nature arising out of this agreement
      and to the non-exclusive jurisdiction of the English Courts for the
      purpose of enforcement of any judgment against their respective assets.

17.3  Without prejudice to any other permitted mode of service the Parties agree
      that service of any writ, notice or other document for the purpose of any
      proceedings begun in England shall be duly served upon it if delivered or
      sent by first class post, in the case of:

      (a)  Buyer to the address given under clause 10.2 (marked for the
           attention of Commercial Director); and

      (b)  Difco to the address given under clause 10.2 (marked for the
           attention of Commercial Director);

      or such other person and address in England and/or Wales as either Party
      shall notify the others in writing from time to time.

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