<PAGE>   1


                                                                 EXHIBIT 10.30





                       NATURAL GAS ADVANCE SALE CONTRACT

                                     DATED

                               DECEMBER 1, 1999


                                    BETWEEN

                       COLUMBIA NATURAL RESOURCES, INC.,
                                   AS SELLER

                                      AND

                              MAHONIA II LIMITED,
                                 AS PURCHASER
<PAGE>   2
                               TABLE OF CONTENTS


ARTICLE I - INTERPRETATION
   Section 1.01  Definitions ..........................................  1
   Section 1.02  Headings .............................................  4
   Section 1.03  Number ...............................................  5
   Section 1.04  Non-Business Days ....................................  5

ARTICLE II - SALE AND PURCHASE OF NATURAL GAS
   Section 2.01  Sale and Purchase of Natural Gas .....................  5
   Section 2.02  Measurement and Quality ..............................  6
   Section 2.03  Delivery and Receipt of Natural Gas; Right to
                 Request Alternate Delivery Points ....................  6
   Section 2.04  Payment of Delivery Fees .............................  7
   Section 2.05  Force Majeure Default Delivery Points ................  7
   Section 2.06  Failure of  Delivery  or  Receipt;  Transportation
                 and Balancing ........................................  7
   Section 2.07  Rights and Remedies; Waiver of Certain Damage Claims..  8
   Section 2.08  Possession, Title and Risk ...........................  9
   Section 2.09  Royalties ............................................  9
   Section 2.10  Taxes ................................................  9
   Section 2.11  No Warranty ..........................................  9
   Section 2.12  Preparation of Certificates; Adjustments for
                 Actual Delivery ......................................  9

ARTICLE III - REPRESENTATIONS AND WARRANTIES
   Section 3.01  Representations and Warranties of Seller .............  10
   Section 3.02  Representations and Warranties of Purchaser ..........  11

ARTICLE IV - COVENANTS
   Section 4.01  Affirmative Covenants of Seller ......................  12

ARTICLE V - EVENTS OF DEFAULT AND EARLY TERMINATION
   Section 5.01  Events of Default ....................................  13
   Section 5.02  Early Termination by Purchaser .......................  14
   Section 5.03  Calculation of Termination Payment ...................  14
   Section 5.04  Surety Bond. .........................................  15
   Section 5.05  Rights and Remedies of Purchaser .....................  17
   Section 5.06  Purchaser's Failure to Accept Delivery ...............  17

ARTICLE VI - EVENTS OF CHANGE AND ACCELERATED TERMINATION
   Section 6.01  Events of Change .....................................  18
   Section 6.02  Accelerated Termination ..............................  18
   Section 6.03  Payments on Accelerated Termination ..................  18

ARTICLE VII - MISCELLANEOUS


                                       i
<PAGE>   3
   Section 7.01  Notice ...............................................  19
   Section 7.02  Interest on Overdue Amounts ..........................  20
   Section 7.03  Governing Law; Waiver of Jury Trial ..................  20
   Section 7.04  Severability .........................................  21
   Section 7.05  Place of Payment; Currency ...........................  21
   Section 7.06  Purchaser Not an Agent ...............................  21
   Section 7.07  Benefit of the Agreement .............................  21
   Section 7.08  Assignment and Transfer ..............................  21
   Section 7.09  Entire Agreement .....................................  22
   Section 7.10  Amendments ...........................................  22
   Section 7.11  No Waivers, Remedies .................................  22
   Section 7.12  Time of the Essence ..................................  22
   Section 7.13  Counterparts .........................................  22
   Section 7.14  Guaranty Agreement ...................................  22
   Section 7.15  Intent ...............................................  22
   Section 7.16  Disclosure of Information ............................  22
   Section 7.17  Stamp and Documentary Taxes ..........................  23
   Section 7.18  Further Assurances ...................................  23


Annex 1     Seller's Conditions Precedent
Exhibit A-1 Form of Surety Bond
Exhibit A-2 Form of Demand Notice
Exhibit B   Form of Opinion of Seller and Guarantor


                                       ii
<PAGE>   4
                       NATURAL GAS ADVANCE SALE CONTRACT


      This NATURAL GAS ADVANCE SALE CONTRACT (this "Agreement") is entered into
as of December 1, 1999, between COLUMBIA NATURAL RESOURCES, INC., a Texas
corporation (the "Seller"), and MAHONIA II LIMITED, a Jersey Channel Islands
company (the "Purchaser").

      WHEREAS, Seller desires to sell and Purchaser desires to purchase certain
quantities of Natural Gas (as hereafter defined) on the terms and conditions set
forth herein;

      NOW THEREFORE, in consideration of the respective covenants and agreements
of the parties hereinafter set forth and for good and valuable consideration,
the receipt and sufficiency of which are hereby acknowledged by each of the
parties, the parties hereby agree as follows:

                          ARTICLE I - INTERPRETATION

Section 1.01 Definitions. For purposes of this Agreement, the following terms
shall have the meanings indicated:

      "Accelerated Termination Date" has the meaning given such term in Section
6.02(b).

      "Acceptable Surety Companies" means surety companies (i) having a
long-term unsecured credit rating no less than the equivalent of an A- rating by
Standard & Poor's Ratings Group and (ii) acceptable to Purchaser in its sole
discretion.

      "Applicable Instruments" of any Person means the Certificate or Articles
of Incorporation, by-laws and other organizational documents of such Person and
all contracts, indentures, agreements, instruments and documents to which such
Person is a party or by which such Person or any assets of such Person may be
bound or affected.

      "Btu" shall be defined pursuant to the transporting pipeline's FERC
approved tariff.

      "Business Day" means a day, other than a Saturday or a Sunday, on which
commercial banks are not authorized or required to be closed in New York, New
York.

      "Confirmation Letter" has the meaning given such term in Section 2.01.

      "Deficiency Quantity" means in respect of a particular Delivery Month the
amount by which the Required Delivery Quantity for that Delivery Month exceeds
the quantity of Natural Gas actually delivered and received hereunder in respect
of such Delivery Month.

      "Delivery Month" means each calendar month commencing with the month of
February, 2000, through and including the month of October, 2004.

      "Delivery Point" means a delivery point at any of the following locations
as set forth in the Confirmation Letter or as otherwise determined in accordance
with the terms and conditions of this Agreement: (i) Columbia Gas Transmission
Corporation's pool AS39145, or (ii) such other locations as


                                       1
<PAGE>   5
Seller may elect pursuant to Section 2.03(d) or as may be agreed by the parties
hereto pursuant to Section 2.05.

      "Early Termination Date" shall have the meaning ascribed thereto in
Section 5.02.

      "Event of Change" shall have the meaning given such term in Section 6.01.

      "Event of Default" shall have the meaning given such term in Section 5.01.

      "Federal Funds Rate" means, for any day, a fluctuating interest rate per
annum equal for such day to the weighted average of the rates on overnight
Federal funds transactions with members of the Federal Reserve System arranged
by Federal funds brokers, as published for such day (or, if such day is not a
Business Day, for the immediately preceding Business Day) by the Federal Reserve
Bank of New York, or, if such rate is not so published for any day which is a
Business Day, the average of the quotations for such day on such transactions
received by Purchaser from three Federal funds brokers of nationally recognized
standing selected by Purchaser.

      "FERC" shall have the meaning given such term in Section 2.02.

      "Force Majeure" shall mean a failure by either party to perform
obligations hereunder, except for the obligation to make payment due hereunder,
to the extent that such failure is caused by war, riots, insurrections, fires,
explosions, sabotage, strikes and other labor or industrial disturbances, acts
of God or the elements, government laws, regulations or requests, disruption or
breakdown of production or transportation facilities, line or compressor freeze
outs, failures of transporters in receiving and delivering Natural Gas tendered
(other than compliance with OFOs or other similar notices from a transporter) or
by any other cause reasonably beyond the control of such party, but does not
include the failure to perform obligations solely as a result of the fact that
to do so will result in economic loss or hardship to such party.

      "Governmental Requirement" shall mean all judgments, orders, writs,
injunctions, decrees, awards, laws, ordinances, statutes, regulations, rules,
franchises, permits, certificates, licenses, authorizations and the like of any
government or any commission, board, court, agency, instrumentality or political
subdivision thereof.

      "Guarantor" shall mean Columbia Energy Group, a Delaware corporation.

      "Guaranty Agreement" shall mean that certain Guaranty dated of even date
herewith given by Guarantor in favor of Purchaser to secure the payment and
performance obligations of Seller under this Agreement and the Confirmation
Letter, as set forth therein, as the same may be amended, modified or replaced
from time to time.

      "Hub Fees" means any charges, fees or costs, by either Seller or
Purchaser, relating to hubs, pooling points or title transfers charged by either
the pipeline immediately upstream of and delivering into the Delivery Point or
the pipeline immediately downstream of and taking Natural Gas from the Delivery
Points and actually incurred in connection with the transfer of Natural Gas
between Seller and Purchaser at the Delivery Point.

      "Initial Surety Bond" means the surety bond issued by the Initial Surety
Companies, as sureties, dated December 1, 1999, in favor of Purchaser, initially
in the maximum penal sum of $150,000,000


                                       2
<PAGE>   6
which maximum penal sum shall be reduced over time in accordance with Exhibit A
attached thereto, expiring December 30, 2004, as the same may be amended,
modified, or replaced from time to time.

      "Initial Surety Companies" means, collectively, American Home Assurance
Company and Travelers Casualty and Surety Company of America.

      "MMBtu" means one million Btus.

      "Natural Gas" means a mixture of gaseous hydrocarbons consisting primarily
of methane and meeting the quality standards and specifications required
pursuant to Section 2.02.

      "NYMEX" means the New York Mercantile Exchange, Inc. and any successor
thereto.

      "OFO" shall have the meaning given such term in Section 2.06(d).

      "Payment Date" shall mean the 25th day of any month following a Delivery
Month, or if such day is not a Business Day, the next succeeding Business Day.

      "Person" means any individual, corporation, company, partnership, joint
venture, trust, unincorporated association, government or any commission, board,
court, agency, instrumentality or political subdivision thereof, any other
entity or any trustee, receiver, custodian or similar official.

      "Prepaid Price" means the purchase and sales price set forth as such in
the Confirmation Letter.

      "Purchaser" has the meaning ascribed thereto in the introductory paragraph
hereto.

      "Reference Dealer" means a leading company in the natural gas marketing
business selected by Purchaser in good faith from among companies engaging in
natural gas commodity derivative transactions whose senior unsecured debt rating
is no less than BBB- as publicly announced by Standard & Poor's Ratings Group
(or its successor) and no less than Baa3 as publicly announced by Moody's
Investors Service, Inc. (or its successor).

      "Replacement Value" means (i) if Seller is the party which has failed to
perform, the price which Purchaser, acting in good faith and at arm's length,
actually pays, or has contracted to pay, for Natural Gas to replace the
Deficiency Quantity at the agreed Delivery Points plus any additional
transportation and other costs and expenses incurred by Purchaser in connection
with the purchase of such Deficiency Quantity; and (ii) if Purchaser is the
party which has failed to perform, the price which Seller, acting in good faith
and at arm's length, actually receives, or has contracted to receive, for the
sale of the Deficiency Quantity less any additional transportation and other
costs and expenses incurred by Seller in connection with the sale of such
Deficiency Quantity.

      "Required Delivery Quantity" means the amount of MMBtus of Natural Gas to
be delivered and received during a given Delivery Month pursuant to this
Agreement as agreed upon in the Confirmation Letter.

      "Seller" has the meaning ascribed thereto in the introductory paragraph
hereto.

      "Substitute Surety Bond" has the meaning given to such term in Section
5.04(b)(i).


                                       3
<PAGE>   7
      "Surety Bond" means the Initial Surety Bond or the Substitute Surety Bond,
as the case may be.

      "Surety Companies" means, collectively, the Initial Surety Companies and
all Acceptable Surety Companies selected pursuant to Section 5.04, if any.

      "Taxes" means all federal, state, municipal or local, ad valorem,
property, stamp, occupation, severance, production, gathering, pipeline,
utility, withholding, gross production, gross turnover, sales, value added, use,
excise, environmental, transaction, customs, export and any other present or
future governmental taxes, charges, duties and assessment of any kind whatsoever
(including, without limitation, deficiencies, penalties, additions to tax and
interest attributable thereto), other than taxes based on net income or net
worth.

      "Termination Date" means either the Accelerated Termination Date or the
Early Termination Date, as the case may be.

      "Termination Payment" has the meaning given such term in Section 5.03(a)
or Section 5.03(b), as applicable.

      "Transaction Documents" means this Agreement, the Confirmation Letter, the
Guaranty Agreement and the Surety Bond.

      "United States Dollars", "U.S. Dollars", or "U.S. $" means the lawful
currency of the United States of America in immediately available funds.

      "Unpaid Amounts" means, with respect to any Termination Date, the
aggregate of the amounts that became payable (whether or not due) to Purchaser
hereunder prior to the occurrence of such Termination Date and that remain
payable (whether or not due) as at such Termination Date, together with interest
thereon from (and including) the date such amounts became due and payable to
(but excluding) such Termination Date at the U.S. Base Rate less all amounts, if
any, owed by Purchaser to Seller pursuant to Section 2.03(d) and Section 2.06(c)
plus interest thereon from (and including) the date such amounts became due and
payable to (but excluding) such Termination Date at the U.S. Base Rate.

      "U.S. Base Rate" means, at any time, a fluctuating interest rate per annum
as shall be in effect from time to time, which rate per annum shall at all times
be equal to the greater of: (a) the rate of interest announced publicly by The
Chase Manhattan Bank in New York, New York, from time to time, as its prime
commercial lending rate; or (b) one-half of one percent per annum above the
Federal Funds Rate in effect from time to time. In the event there is any Unpaid
Amount, Purchaser will use reasonable efforts to inform Seller of changes in the
U.S. Base Rate promptly upon the occurrence of such changes.

Section 1.02 Headings. The division of this Agreement into Articles and Sections
and the insertion of an index and headings are for convenience of reference only
and shall not affect the construction or interpretation of this Agreement. The
terms "this Agreement", "hereof", "hereunder" and similar expressions refer to
this Agreement and not to any particular Article, Section, clause, paragraph,
annex, exhibit or other portion hereof and include any agreement supplemental
hereto. Unless something in the subject matter or context is inconsistent
therewith, references herein to Articles, Sections, clauses and paragraphs are
to Articles, Sections, clauses and paragraphs of this Agreement.


                                       4
<PAGE>   8
Section 1.03 Number. Words importing the singular number shall include the
plural and vice versa, and words importing the masculine gender shall include
the feminine and neuter genders and vice versa.

Section 1.04 Non-Business Days. Whenever any action to be taken hereunder shall
be stated to be required to be taken or any payment to be made hereunder shall
be stated to be due on a day other than a Business Day, unless otherwise
specifically provided for herein, such payment shall be made or such action
shall be taken on the next succeeding Business Day with the same force and
effect as if made on the due date, and no additional amounts shall be payable as
a result of such delay.

                ARTICLE II -  SALE AND PURCHASE OF NATURAL GAS

Section 2.01      Sale and Purchase of Natural Gas.

      (a) On December 1, 1999, Purchaser and Seller shall agree upon and execute
a letter as described in this Section 2.01 (such letter, as the same may be
amended from time to time pursuant to Section 2.03(d), being the "Confirmation
Letter") and agree upon the schedule to the Initial Surety Bond providing for
the periodic reductions to the maximum penal sum thereunder. If on December 7,
1999, a Confirmation Letter and such schedule to the Initial Surety Bond have
not been executed and delivered by the parties thereto or if the Prepaid Price
has not been paid by Purchaser to Seller, then this Agreement shall terminate
and be null and void and without any further force and effect, and without any
further actions by the parties.

            The Confirmation Letter shall specify (i) a mutually acceptable
Prepaid Price (which shall be approximately U.S. $150,000,000), (ii) the date on
which the Prepaid Price shall be paid (which shall be the same day such
Confirmation Letter is executed), and (iii) for each Delivery Month, the
Delivery Points, the Required Delivery Quantity, and the amount thereof to be
delivered and received at each such Delivery Point, each as mutually agreed by
Purchaser and Seller.

      (b) In consideration of the payment to Seller of the Prepaid Price, (i)
Seller hereby agrees to sell and deliver, or cause to be delivered, to Purchaser
or to its account in each Delivery Month, at the Delivery Points, the Required
Delivery Quantity (in the amounts set forth in the Confirmation Letter) on the
terms and conditions set forth in this Agreement; and (ii) Purchaser hereby
agrees to accept delivery of such Natural Gas. Subject to the terms of Section
2.03(d) and Section 2.06(c), payment of the Prepaid Price shall constitute
payment in full of the purchase price of the Natural Gas to be delivered
hereunder and under the Confirmation Letter.

      (c) All payments under this Agreement by Seller or Purchaser shall be made
by wire transfer in immediately available funds to an account designated by the
party entitled to receive such payment, with such designation to be made at
least two Business Days prior to the date on which this payment is to be made.

      Section 2.02 Measurement and Quality. Natural Gas delivered to a specific
Delivery Point hereunder shall be measured by the operator of such Delivery
Point in accordance with its then current Federal Energy Regulatory Commission
("FERC") approved tariff procedures. All such Natural Gas shall meet or exceed
the then current FERC approved tariff requirements of Purchaser's transporter
receiving Natural Gas at such Delivery Point, including, without limitation,
requirements of quality, composition, temperature and pressure.


                                       5
<PAGE>   9
      Section 2.03 Delivery and Receipt of Natural Gas; Right to Request
Alternate Delivery Points.

      (a) Seller agrees to deliver or caused to be delivered to Purchaser and
Purchaser agrees to accept delivery from Seller in each Delivery Month at the
Delivery Points determined pursuant to this Agreement, the Required Delivery
Quantity required to be delivered hereunder and under the Confirmation Letter in
such Delivery Month. Seller shall have no right, without the prior written
consent of Purchaser, to deliver quantities of Natural Gas hereunder in excess
of the Required Delivery Quantity in respect of any Delivery Month during such
Delivery Month, and Purchaser shall have no right to schedule and take hereunder
in excess of the Required Delivery Quantity.

      (b) Seller and Purchaser shall take such action as shall be necessary to
properly schedule the delivery and receipt of such Natural Gas at the Delivery
Points in each Delivery Month in compliance with all applicable rules,
regulations and procedures at such Delivery Points.

      (c) Each Delivery Month, Seller shall arrange for delivery, and Purchaser
shall arrange for receipt, of Natural Gas to begin at the Delivery Points no
later than the first day of the Delivery Month and to be completed no later than
the last calendar day of the Delivery Month. All deliveries and receipts shall
be at hourly and daily rates that are as uniform as possible over the course of
the Delivery Month in accordance with generally accepted pipeline scheduling
practices.

      (d) Seller may elect to deliver Natural Gas at alternate delivery points
other than the Delivery Points set forth in the Confirmation Letter with respect
to a Delivery Quantity for a Delivery Month provided (i) that Seller has given
Purchaser written notice (which notice shall constitute an amendment to the
Confirmation Letter) of its election no later than five (5) Business Days before
the end of the calendar month preceding the Delivery Month in which it proposes
to make delivery at such alternate delivery points, (ii) Purchaser consents in
writing to delivery at such alternate delivery points and (iii) such alternate
delivery points allow multiple title transfers at a single point or at different
points in a pool and are mutually agreeable to Seller and Purchaser. If there is
any transportation cost differential associated with delivery of such Natural
Gas at such alternate delivery points (as measured against delivery at the
Delivery Points set forth in the Confirmation Letter with respect to such
Delivery Quantity for such Delivery Month), Purchaser shall prepare and deliver
to Seller, within five (5) Business Days after the end of the applicable
Delivery Month a certificate setting out the calculation of the transportation
cost differential accompanied by reasonably available back-up documentation
therefor. Any such certificate shall, absent manifest error, be conclusive
evidence of the amount of such transportation cost differential. If Seller owes
Purchaser compensation pursuant to this Section 2.03(d), Seller shall pay
Purchaser such amount within five (5) Business Days following its receipt of
Purchaser's certificate. If Purchaser owes Seller compensation pursuant to this
Section 2.03(d), Purchaser shall pay Seller such amount on the next succeeding
Payment Date.

Section 2.04 Payment of Delivery Fees. Seller shall pay all costs in connection
with transportation of the Natural Gas to the Delivery Points and Purchaser
shall pay all costs in connection with transportation of the Natural Gas from
the Delivery Points. Seller shall be responsible for the payment of all Hub Fees
(whether charged to Seller or Purchaser) payable in connection with delivery of
Natural Gas hereunder at any Delivery Point. Seller shall not be responsible for
any insurance, storage, transportation or other costs in respect of the period
after title to any Natural Gas delivered hereunder has passed to Purchaser in
accordance with Section 2.08 of this Agreement. Purchaser shall not be
responsible for any insurance, storage, transportation or other costs in respect
of the period prior to the time title to any Natural Gas delivered hereunder has
passed to Purchaser in accordance with Section 2.08 of this Agreement.


                                       6
<PAGE>   10
Section 2.05 Force Majeure Default Delivery Points. Seller is obligated to
deliver to Purchaser and Purchaser is obligated to receive from Seller, at the
Delivery Points, the Required Delivery Quantity in accordance with the terms and
conditions of this Agreement and the Confirmation Letter. If as a result of an
event of Force Majeure, either (i) Seller is unable, after using all reasonable
business efforts, to deliver the Required Delivery Quantity to Purchaser at the
Delivery Points or (ii) Purchaser is unable, after using all reasonable business
efforts, to receive the Required Delivery Quantity from Seller at the Delivery
Points in the amounts agreed upon pursuant to this Agreement, then Seller shall
be obligated to deliver and Purchaser shall be obligated to receive, the
relevant Required Delivery Quantity at a mutually acceptable comparable delivery
point or points, which delivery point or points shall allow multiple title
transfers at a single point or at different points in a pool, with mutually
acceptable adjustments for quality and location. If the parties are unable to
agree on a comparable delivery point or points and adjustments for quality and
location, then the parties shall comply with the terms of Section 2.06(a),
Section 2.06(b) and Section 2.06(c).

Section 2.06 Failure of Delivery or Receipt; Transportation and Balancing.

      (a) (i) Without prejudice to Articles V and VI, if as a result of an event
of Force Majeure, Seller is unable to meet its delivery obligation in respect of
a Delivery Month at the Delivery Points (or at a mutually satisfactory
comparable delivery point or points under Section 2.05), then Seller shall pay
to Purchaser, as liquidated damages, the Replacement Value of the Deficiency
Quantity in respect of that Delivery Month. The Replacement Value shall be paid
to Purchaser no later than the Payment Date next following such Delivery Month,
and Purchaser will accept such payment in lieu of the Deficiency Quantity for
such Delivery Month. Where Seller is unable to meet its delivery obligation as
aforesaid, Purchaser shall use all reasonable efforts to minimize the
Replacement Value of any Deficiency Quantity and Purchaser shall prepare and
deliver to Seller, within five (5) Business Days after the end of the applicable
Delivery Month, a certificate setting out the calculation of the Replacement
Value accompanied by reasonably available back-up documentation therefor. Any
such certificate shall, absent manifest error, be conclusive evidence of the
amount due in respect of the Replacement Value. Seller shall notify Purchaser as
soon as possible of any anticipated inability to perform all or any portion of
its obligations hereunder.

            (ii) Without prejudice to Article VI, if as a result of an event of
Force Majeure, Purchaser is unable to meet its obligation to accept delivery of
all or any part of the Required Delivery Quantity in respect of a Delivery Month
at the Delivery Points (or at a mutually satisfactory comparable delivery point
or points under Section 2.05), then Seller shall pay to Purchaser, in lieu of
delivery of the Deficiency Quantity in respect of such Delivery Month, the
Replacement Value of such Deficiency Quantity in respect of that Delivery Month.
The Replacement Value shall be paid to Purchaser no later than the Payment Date
next following such Delivery Month, and Purchaser will accept such payment in
lieu of the Deficiency Quantity for such Delivery Month. Where Purchaser is
unable to meet its acceptance obligation as aforesaid, Seller shall use all
reasonable efforts to maximize the Replacement Value of any Deficiency Quantity
and Seller shall prepare and deliver to Purchaser, within five (5) Business Days
after the end of the applicable Delivery Month, a certificate setting out the
calculation of the Replacement Value accompanied by reasonably available back-up
documentation therefor. Any such certificate shall, absent manifest error, be
conclusive evidence of the amount due in respect of the Replacement Value.
Purchaser shall notify Seller as soon as possible of any anticipated inability
to perform all or any portion of its obligations hereunder as a result of an
event of Force Majeure.


                                       7
<PAGE>   11
      (b) If a party is unable to perform any of its obligations to deliver or
receive Natural Gas hereunder as a result of an event of Force Majeure, such
party shall give notice and full particulars of such event of Force Majeure to
the other party as soon as reasonably possible and shall take all reasonable
actions necessary to remedy the event of Force Majeure.

      (c) In the event that (i) Seller or Seller's agent or transporter
inadvertently delivers more or less than the Required Delivery Quantity for any
Delivery Month, or (ii) Purchaser or Purchaser's agent or transporter
inadvertently receives more or less than the Required Delivery Quantity for any
Delivery Month, and unless the parties otherwise agree or except as otherwise
required by applicable tariff provisions, if any, such overages or underages
shall be corrected or adjusted in (A) Natural Gas, if such corrections or
adjustments are made in the same Delivery Month, and (B) cash, if such
corrections or adjustments are made in the next succeeding Delivery Month; and
the party delivering or receiving excess quantities will be liable to the other
party for any associated pipeline penalties or cashouts. Each party shall notify
the other as promptly as possible of any changes in its rate of delivery or
receipt of Natural Gas at any Delivery Point and take all reasonable actions
necessary to minimize or avoid the incurrence of pipeline penalties and
imbalances.

      (d) Should either party receive an operational flow order ("OFO") or other
order or notice from a transporter requiring action to be taken in connection
with this Agreement or Natural Gas flowing under this Agreement, such party
shall immediately notify the other party of the OFO and promptly provide the
other party a copy of same by facsimile. The parties shall take all actions
required by the OFO within the time prescribed therein. Each party shall
indemnify, defend and hold harmless the other party from any damages or
liability (including, without limitation, all non-compliance penalties and
attorneys' fees) associated with an OFO (i) of which the indemnifying party
failed to give the indemnified party the notice required hereunder or (ii) under
which the indemnifying party failed to take the action required by the OFO
within the time prescribed therein.


Section 2.07 Rights and Remedies; Waiver of Certain Damage Claims. Except as set
forth in this Section 2.07, the rights and remedies of the parties set forth in
this Agreement and the other Transaction Documents are non-exclusive of the
other rights and remedies of the parties existing at law or equity. It is
expressly agreed that, notwithstanding any other rights or remedies which a
party may have, payments made in accordance with Section 2.06, Section 5.02,
Section 5.06 and Section 6.03 constitute the exclusive damages available to
Seller and Purchaser for nondelivery or nonacceptance of Natural Gas. To the
fullest extent permitted by applicable law, neither party shall be liable for
any punitive, exemplary, incidental, consequential, indirect or direct (other
than as set forth in this Section 2.07) or other damages, in tort, contract or
otherwise in respect thereof. The parties acknowledge and agree that the Natural
Gas subject of this Agreement is not unique and that no claim for specific
performance is appropriate.

Section 2.08 Possession, Title and Risk. Possession of and title to Natural Gas
delivered pursuant hereto and the Confirmation Letter shall pass from Seller to
Purchaser at the Delivery Points when the Natural Gas is nominated and confirmed
for delivery from the pool pursuant to the transporting pipeline's FERC tariff.
Until such time, Seller shall be deemed to be in control and possession of, have
title to, risk of loss of and be responsible for such Natural Gas and, after
such time, Purchaser shall be deemed to be in control and possession of, have
title to, risk of loss of and be responsible for such Natural Gas.

Section 2.09 Royalties. Seller shall at all times have the obligation to make
settlements for all royalties and payments to mineral and royalty owners,
governmental entities or agencies (whether federal, state, municipal or local)
and all other Persons having an ownership interest in the Natural Gas delivered
by


                                       8
<PAGE>   12
Seller to Purchaser hereunder. Seller hereby agrees to indemnify Purchaser and
save it harmless from all suits, actions, debts, accounts, damages, costs,
losses and expenses arising from or out of adverse claims of any and all Persons
in respect of royalties, taxes, license fees or charges thereon which are
applicable before the title passes to Purchaser or which may be levied and
assessed upon Seller in respect of a sale of the Natural Gas to Purchaser.

Section 2.10 Taxes. Seller is liable for and shall pay, cause to be paid or
reimburse Purchaser if Purchaser shall have paid, all Taxes applicable to the
Natural Gas sold hereunder prior to the time title to the Natural Gas has passed
to Purchaser, unless allocated to Purchaser as hereinafter provided. Purchaser
is liable for and shall pay, cause to be paid or reimburse Seller if Seller
shall have paid, all Taxes applicable to the Natural Gas sold hereunder at or
after the time title to the Natural Gas has passed to Purchaser. Both parties
shall use reasonable efforts to administer this Agreement and implement the
provisions in accordance with their intent to minimize Taxes. Purchaser
represents that it is engaged in the business of reselling the Natural Gas
delivered under this Agreement and the Confirmation Letter and Purchaser is
purchasing the Natural Gas for resale to third parties, and accordingly
Purchaser is entitled to purchase the Natural Gas hereunder free of any Taxes
and Purchaser agrees to pay any Taxes incurred as a result of the absence of
such resale exemption. Each party agrees to cooperate with obtaining any
exemption from or reduction of Tax upon request by the other party.

Section 2.11 No Warranty. PURCHASER ACKNOWLEDGES THAT IT HAS ENTERED INTO THIS
AGREEMENT AND IS CONTRACTING FOR THE NATURAL GAS TO BE SUPPLIED BY SELLER BASED
SOLELY UPON THE EXPRESS COVENANTS, REPRESENTATIONS AND WARRANTIES HEREIN SET
FORTH (including Section 2.02) AND, SUBJECT TO SUCH COVENANTS, REPRESENTATIONS
AND WARRANTIES (including Section 2.02), ACCEPTS SUCH NATURAL GAS "AS IS, WHERE
IS" AND "WITH ALL FAULTS."

Section 2.12 Preparation of Certificates; Adjustments for Actual Delivery. With
respect to the preparation of any certificate pursuant to Section 2.03(d) or
Section 2.06(a), to the extent that the actual quantity is not available to
Purchaser or Seller, as the case may be, by the fifth Business Day after the end
of the applicable Delivery Month, Purchaser or Seller, as the case may be, may
prepare any certificate required to be prepared hereunder based on nominated
quantities, subject to reduction for any known periods when nominated quantities
were not delivered and subject to later correction based on actual data. If such
certificate is rendered based on nominated quantities rather than actual
quantities, Purchaser or Seller, as the case may be, shall render a corrected
certificate as soon as possible after actual quantities are known.

                 ARTICLE III - REPRESENTATIONS AND WARRANTIES

Section 3.01 Representations and Warranties of Seller. Seller represents and
warrants to Purchaser as of the date hereof and as of the date of execution of
the Confirmation Letter and payment of the Prepaid Price as follows:

      (a) Status and Authority. Seller is a corporation incorporated under the
laws of the jurisdiction of its formation and has all necessary corporate power
and authority to carry on its business as now being conducted by it. Seller has
full corporate power and authority to enter into this Agreement and the
Transaction Documents to which it is a party and to do all acts and things and
execute and deliver all other documents as are required hereunder or thereunder
to be done, observed or performed by it in accordance with the terms hereof or
thereof.


                                       9
<PAGE>   13
      (b) Power and Authority. The execution, delivery and performance by Seller
of this Agreement and the Transaction Documents to which it is a Party and the
consummation of the transactions contemplated hereby and thereby are within
Seller's corporate power and authority and have been duly authorized by all
necessary corporate action.

      (c) Consents, Approvals, Etc. No authorization, consent or approval of, or
other action by, or notice to or filing with, any governmental authority,
regulatory body or any other Person is required for the due authorization,
execution, delivery or performance by Seller of this Agreement, or the
consummation of the transactions contemplated by this Agreement, except those
approvals which have been obtained, and those notices and filings which have
been made, copies of all of which have been delivered to Purchaser.

      (d) Validity of Documents and Enforceability. This Agreement and the
Transaction Documents to which Seller is a party are the legal, valid and
binding obligations of Seller enforceable against Seller in accordance with
their terms, except as the enforceability thereof may be limited by the effect
of any applicable bankruptcy, insolvency, reorganization, moratorium or similar
laws affecting creditors' rights generally and by general principles of equity.
Neither the execution and delivery of this Agreement or any Transaction Document
to which Seller is a party nor compliance with the terms and conditions hereof
or thereof by Seller (i) will result in a violation of the terms of any
Applicable Instrument of Seller or (ii) requires any approval or consent of any
governmental authority or agency, including the Securities and Exchange
Commission, having jurisdiction except such as has already been obtained.

      (e) Compliance with Laws. Neither the execution, delivery and performance
by Seller of this Agreement or the Transaction Documents to which it is a party
nor the consummation of the transactions contemplated hereby or thereby (i)
violates any provision of any Applicable Instrument of Seller or any
Governmental Requirement applicable to Seller or (ii) does or will result in or
require the creation or imposition of any lien on any properties, assets or
revenues of Seller. Seller is in compliance with the Applicable Instruments of
Seller and all Governmental Requirements applicable to Seller except as
non-compliance would not have a material adverse effect on Seller's performance
of the Transaction Documents.

      (f)   Investment Company.  Seller is not an "investment company" subject
to regulation under the Investment Company Act of 1940, as amended.

      (g) Public Utility Holding Company. Seller is subject to regulation as a
"subsidiary" of a "holding company" in each case as such term is defined in the
Public Utility Holding Company Act of 1935, as amended.

      (h) Ownership of Natural Gas. The Natural Gas to be delivered by Seller to
Purchaser hereunder shall be delivered to Purchaser with good and marketable
title thereto, free and clear of all liens, encumbrances or any other adverse
claims whatsoever, including royalties and Taxes for which Seller is responsible
under Sections 2.09 and 2.10, except for those that may be imposed by statute
and which are released upon payment by Seller of the purchase price under its
agreements to acquire the Natural Gas.

      (i) Commercial Purpose. Seller has entered into this transaction for
commercial purposes related to its business as a producer, processor or
merchandiser of Natural Gas or natural gas liquids and


                                       10
<PAGE>   14
not for speculative purposes. Seller has the capability (either directly or
indirectly), and intends, to make delivery of the Natural Gas to be delivered
hereunder. Seller is selling the Natural Gas in the ordinary course of its
business. Seller is also acting as a principal and not as an agent, understands
and acknowledges that Purchaser has been and will be acting only on an arm's
length basis and not as its agent, broker, advisor or fiduciary in any respect;
Seller is relying solely upon its own evaluation of this Agreement and the
transactions contemplated hereby (including the present and future results,
consequences, risks and benefits thereof, whether financial, accounting, tax,
legal or otherwise) and upon advice from its own professional advisors,
understands this Agreement and the transactions contemplated hereby and the
risks associated therewith, has determined that those risks are appropriate for
it, and is willing to assume those risks, and has not relied and will not be
relying upon any evaluation or advice (including any recommendation, opinion or
representation) from Purchaser or its affiliates or the representatives or
advisors of Purchaser or its affiliates.

      (j) Margin Stock. Seller is not engaged principally, or as one of its
important activities, in the business of extending credit for the purpose,
whether immediate, incidental or ultimate, of buying or carrying margin stock
(within the meaning of Regulation U or X of the Board of Governors of the
Federal Reserve System) and no part of the proceeds of the Prepaid Price
hereunder will be used to buy or carry any margin stock.

Section 3.02 Representations and Warranties of Purchaser. Purchaser represents
and warrants to Seller as follows:

      (a) Status and Authority. Purchaser is a company incorporated under the
laws of the jurisdiction of its formation and has all necessary corporate power
and authority to carry on its business as now being conducted by it. Purchaser
has full corporate power and authority to enter into this Agreement and the
Transaction Documents to which it is a party and to do all acts and things and
execute and deliver all other documents as are required hereunder or thereunder
to be done, observed or performed by it in accordance with the terms hereof or
thereof.

      (b) Power and Authority. The execution, delivery and performance by
Purchaser of this Agreement and the Transaction Documents to which it is a Party
and the consummation of the transactions contemplated hereby and thereby are
within Purchaser's corporate power and authority and have been duly authorized
by all necessary corporate action.

      (c) Validity of Documents and Enforceability. This Agreement and the
Transaction Documents to which Purchaser is a party are the legal, valid and
binding obligations of Purchaser enforceable against Purchaser in accordance
with their terms, except as the enforceability thereof may be limited by the
effect of any applicable bankruptcy, insolvency, reorganization, moratorium or
similar laws affecting creditors' rights generally and by general principles of
equity. Neither the execution and delivery of this Agreement or any Transaction
Document to which Purchaser is a party nor compliance with the terms and
conditions hereof or thereof by Purchaser (i) will result in a violation of the
terms of any Applicable Instrument of Purchaser or (ii) requires any approval or
consent of any governmental authority or agency having jurisdiction except such
as has already been obtained.

      (d) Consents, Approvals, Etc. No authorization, consent or approval of, or
other action by, or notice to or filing with, any governmental authority,
regulatory body or any other Person is required for the due authorization,
execution, delivery or performance by Purchaser of this Agreement, or the
consummation of the transactions contemplated by this Agreement, except those
approvals which have


                                       11
<PAGE>   15
been obtained, and those notices and filings which have been made, copies of all
of which have been delivered to Seller.

      (e) Compliance with Laws. Neither the execution, delivery and performance
by Purchaser of this Agreement or the Transaction Documents to which it is a
party nor the consummation of the transactions contemplated hereby or thereby
violates any provision of any Applicable Instrument of Purchaser or any
Governmental Requirement applicable to Purchaser. Purchaser is in compliance
with the Applicable Instruments of Purchaser and all Governmental Requirements
applicable to Purchaser except as non-compliance would not have a material
adverse effect on Purchaser's performance of the Transaction Documents.

      (f) Commercial Purpose. Purchaser has entered into this transaction for
commercial purposes related to its business in conjunction with its line of
business and not for speculative purposes. Purchaser has the capability (either
directly or indirectly), and intends, to take delivery of the Natural Gas to be
delivered hereunder. Purchaser is acquiring the Natural Gas in the ordinary
course of business.

                            ARTICLE IV - COVENANTS

Section 4.01 Affirmative Covenants of Seller. Seller covenants and agrees with
Purchaser that so long as any obligation of Seller to deliver Natural Gas or to
make any payment is outstanding hereunder:

      (a) Compliance with Laws, etc. Seller will comply with all Governmental
Requirements applicable to the performance of Seller's obligations hereunder,
except where noncompliance therewith would not have a material adverse effect on
Seller or on the performance of the Transaction Documents. Seller will comply
with the Applicable Instruments of Seller except as non-compliance would not
have a material adverse effect on Seller's performance of the Transaction
Documents.

      (b) Maintenance of Corporate Existence. Seller will maintain its corporate
existence, except as otherwise permitted pursuant to Section 7.08.

      (c) Notice of Event of Default or Event of Change. Seller shall notify
Purchaser of the occurrence of any event which with the passage of time or the
giving of notice, or both, would be an Event of Default or an Event of Change
promptly after becoming aware of the same.

      (d) Qualification. Seller will be duly qualified to do business as a
foreign corporation and will be in good standing under the laws of all
jurisdictions in which the failure to be so qualified could have a material
adverse effect on Seller's performance of the Transaction Documents by or before
the first Delivery Month.


                                       12
<PAGE>   16
              ARTICLE V - EVENTS OF DEFAULT AND EARLY TERMINATION

Section 5.01 Events of Default. Each of the following events shall constitute an
"Event of Default" by Seller under this Agreement:

      (a) Seller shall (i) fail to deliver the Required Delivery Quantity either
(A) to the Delivery Points in accordance with the terms of this Agreement and
the Confirmation Letter or (B) to one or more mutually acceptable alternate
delivery points in accordance with Section 2.05 if an event of Force Majeure has
occurred in a Delivery Month; and (ii) if an event of Force Majeure has occurred
and delivery to or receipt by Purchaser at one or more alternate Delivery Points
is not possible, fail to make a payment of the Replacement Value in respect
thereof in accordance with the provisions of Section 2.06 and any such failure
is not remedied within three (3) Business Days; or

      (b) Seller shall fail to pay any amounts due under any other Transaction
Document and such failure is not remedied within the grace period set forth
therein or, if no grace period is specified, within three (3) Business Days
after Purchaser's written notice to Seller thereof (designated a "Notice of
Default"); or

      (c) Seller shall fail to perform or observe any material term, covenant or
agreement contained herein or in any Transaction Document to which it is a party
on its part to be performed or observed (other than any term, covenant or
agreement whose breach or default in performance is specifically dealt with
elsewhere in this Section 5.01) and such failure shall remain unremedied for
twenty-five (25) days after written notice (designated a "Notice of Default")
thereof to Seller by Purchaser; or

      (d) Seller shall admit in writing its inability to pay its debts
generally, or shall make a general assignment for the benefit of creditors; or
any proceeding shall be instituted by or against Seller seeking to adjudicate it
as bankrupt or insolvent, or seeking liquidation, winding up, reorganization,
arrangement, adjustment, protection, relief, or composition of it or its debts
under any law relating to bankruptcy, insolvency or reorganization or relief of
debtors, or seeking the entry of an order for relief or the appointment of a
receiver, trustee, or other similar official for it or for any substantial part
of its property, and in the case of any such proceeding instituted against it
(but not yet instituted by it), shall remain undismissed or unstayed for a
period of sixty (60) days or Seller shall take any corporate action to authorize
any of the actions set forth above in this Section 5.01(d); or

      (e) any representation or warranty made by Seller in this Agreement or any
Transaction Document shall prove to have been incorrect in any material respect
when made; or

      (f) an Event of Default under the Guaranty Agreement shall occur and be
continuing or the Guaranty Agreement shall cease to be the legal, valid and
binding obligation of Guarantor or Guarantor shall so state in writing.

Section 5.02 Early Termination by Purchaser. At any time while an Event of
Default is continuing, Purchaser may, by notice to Seller specifying the
relevant Event of Default, designate a Business Day not earlier than the fifth
(5th) Business Day nor later than the tenth (10th) Business Day following
Seller's receipt of such notice (as provided in Section 7.01) as an early
termination date ("Early Termination Date"); provided, however, that if an Event
of Default pursuant to Section 5.01(d) shall have occurred, the Early
Termination Date shall occur immediately on the occurrence of such Event of
Default without the need for Purchaser to give any prior notice. Upon the
designation or occurrence of an Early Termination


                                       13
<PAGE>   17
Date, the obligation of Seller to make any further deliveries of Natural Gas to
Purchaser under this Agreement shall terminate and Seller shall pay to Purchaser
the Termination Payment together with any Unpaid Amounts. All amounts payable
under this Section 5.02 shall become due on the Early Termination Date and shall
be payable on the third Business Day immediately following delivery by Purchaser
of the statement required pursuant to Section 5.03(c). Such amount will be paid
together with (to the extent permitted by applicable law) interest thereon
(before as well as after judgment or the filing of any petition for bankruptcy)
from (and including) the Early Termination Date to (but excluding) the date such
amount is paid, at the rate specified in Section 7.02. The parties agree that
the Termination Payment is a reasonable pre-estimate of the damages which would
be incurred by Purchaser as a result of an Event of Default and not a penalty.

Section 5.03 Calculation of Termination Payment.

      (a) "Termination Payment" shall be the mean amount determined on the basis
of quotations from at least four Reference Dealers as the amount that would have
been payable on the Termination Date by Purchaser in consideration of an
agreement between Purchaser and the quoting Reference Dealer, and subject to
such documentation evidencing agreement on price as they may in good faith
agree, with the relevant Termination Date as the date of commencement of such
agreement that would have the effect of preserving for Purchaser the equivalent
of the remaining Required Delivery Quantities as set forth in the Confirmation
Letter that, but for the occurrence of the relevant Termination Date, Seller
would have been obligated to perform hereunder after such Termination Date.
Purchaser will request each Reference Dealer to provide its quotation to the
extent practicable as of the same time (without regard to different time zones)
on the relevant Termination Date (or, if a Termination Date is deemed to occur,
as of a time as soon thereafter as practicable). The time as of which such
quotations are to be provided will be selected in good faith by Purchaser. If
exactly three such quotations are provided, the Termination Payment will be the
quotation remaining after disregarding the highest and lowest quotations. For
this purpose, if more than one quotation has the same highest value or lowest
value, then one of such quotations shall be disregarded. If fewer than three
quotations are provided, then the Termination Payment shall be calculated in
accordance with Section 5.03(b).

      (b) Subject to Section 2.07, if the Termination Payment cannot be
determined in accordance with Section 5.03(a) because fewer than three
quotations are provided, "Termination Payment" shall be an amount (discounted at
a U.S. Dollar interest rate reflecting the interpolated discount rate for the
forward London Interbank Offered Rate curve for the period commencing on such
Termination Date and ending on the last date of the last scheduled Delivery
Month) equal to the total amount required, as determined as of the relevant
Early Termination Date by Purchaser in good faith, to compensate it for its
direct actual losses and costs (including loss of bargain and reasonable legal
fees and other out-of-pocket expenses) that it incurs as a result of the early
termination of Seller's delivery obligations hereunder, including, without
limitation, any damages, losses, or expenses incurred in obtaining, terminating,
liquidating, reestablishing or employing hedges or related trading positions
against Purchaser's position hereunder.

      (c) On or as soon as reasonably practicable following the occurrence of a
Termination Date, Purchaser will calculate the Termination Payment and will
provide Seller with a statement (1) showing, in reasonable detail, such
calculations (including all relevant quotations) and (2) giving details of the
relevant account to which the Termination Payment is payable. Any such statement
shall, absent manifest error, be conclusive evidence of the amount due in
respect of the Termination Payment.


                                       14
<PAGE>   18
Section 5.04 Surety Bond.

      (a) Initial Surety Bond. To support its obligation to make the Termination
Payment hereunder, Seller hereby agrees to deliver the Initial Surety Bond on or
before the date set forth in the Confirmation Letter for payment of the Prepaid
Price.

      (b) Substitute Support Arrangements. If, at any time, the long-term
unsecured credit rating of any of the Surety Companies of the then-current
Surety Bond shall be downgraded below the level specified in clause (i) of the
definition of "Acceptable Surety Companies" or such rating shall be withdrawn,
Seller agrees that within 30 Business Days following delivery of a request of
Purchaser, it shall deliver or cause to be delivered one of the following:

      (i) a replacement surety bond (the "Substitute Surety Bond") in
      substantially the form of Exhibit A-1 or otherwise acceptable to Purchaser
      issued by one or more Acceptable Surety Companies having a schedule of
      payment obligations equal to, and reducing periodically in accordance
      with, the remaining schedule of payment obligations under the Surety Bond
      being replaced;

      (ii) an irrevocable performance letter of credit in form and substance
      satisfactory to Purchaser from a bank satisfactory to Purchaser in its
      sole discretion covering payment amounts equal to, and reducing
      periodically in accordance with, the remaining schedule of payment
      obligations under the Surety Bond being replaced;

      (iii) cash collateral in an amount equal to, and reducing periodically in
      accordance with, the remaining schedule of payment obligations under the
      Surety Bond being replaced, pursuant to a collateral agreement in form and
      substance satisfactory to Purchaser in its sole discretion; or

      (iv) any combination of clauses (i), (ii) or (iii) set forth above
      providing support in the amount of, and reducing periodically in
      accordance with, the remaining schedule of payment obligations under the
      Surety Bond being replaced;

provided however, Seller shall be deemed to have satisfied the foregoing
obligations of this Section 5.04(b) by the payment of a monthly premium in the
amount of 35 basis points per annum of the applicable Surety Company's
Commitment Percentage (as such term is defined in the Surety Bond) of the
Monthly Maximum Penal Sum (as such term is defined in the Surety Bond) with
respect to each month during which the long-term unsecured credit rating of any
of such Surety Companies shall remain below the level specified in clause (i) of
the definition of "Acceptable Surety Companies". Such premium shall begin to
accrue on the 31st Business Day after Seller's receipt of the request specified
above and shall be payable monthly in arrears on each Payment Date thereafter so
long as (A) the long-term unsecured credit rating of any of such Surety
Companies shall remain below the level specified in clause (i) of the definition
of "Acceptable Surety Companies" or such rating remains withdrawn, and (B)
Seller has not otherwise satisfied the obligations of Section 5.04(b)(i), (ii),
(iii) or (iv).

      Upon delivery by Seller and acceptance by the Purchaser of any of the
items described in clauses (i) through (iv) of this Section 5.04(b), the Surety
Bond being replaced shall be deemed to be immediately null and void and
Purchaser shall promptly return the same to Seller and shall provide such
acknowledgment that same is released and of no further force or effect and that
the liability of the Surety


                                       15
<PAGE>   19
Companies in respect of the Surety Bond being replaced is discharged, in each
case as such Surety Companies or Seller may reasonably request.

      (c) Payment Demands Under Surety Bonds. Upon the failure by Seller to make
the Termination Payment in full in accordance with this Agreement, which unpaid
amount of Termination Payment is not made by Guarantor in accordance with the
Guaranty Agreement within any applicable grace period provided for therein, in
addition to drawing under the irrevocable performance letter of credit or the
cash collateral provided for in Section 5.04(b), Purchaser shall have the right,
by providing written notice in the form attached hereto as Exhibit A-2, with
appropriate insertions, to the Surety Companies specifying the unpaid amount of
the Termination Payment then due, to instruct the Surety Companies to pay the
lesser of such unpaid amount of the Termination Payment or the current remaining
payment obligation under and in accordance with the Surety Bond. Notwithstanding
the foregoing, in no event shall Purchaser have the right to demand or receive,
collectively, from the Surety Companies and drawing under the irrevocable
performance letter of credit or cash collateral, any amount in excess of such
unpaid amount of the Termination Payment.

      (d) Payment Under Surety Bond. The Surety Companies shall make their pro
rata payment under the Surety Bond to Purchaser not later than 10 Business Days
after receipt of a demand for payment pursuant to Section 5.04(c). It is
expressly agreed that, as provided in Section 5 to the Initial Surety Bond (and
the corresponding section in any Substitute Surety Bond), the payment by the
Surety Companies of their respective Commitment Percentage of the amount
demanded in accordance with this Section 5.04 shall, as to such Surety Company,
constitute satisfaction in full of all of its obligations under the Surety Bond.
Such payment shall be the exclusive remedy of the Purchaser under the Surety
Bond. Upon payment by a Surety Company, the Purchaser shall assign its rights to
payment against Seller and Guarantor to such Surety Company.

      (e) No Waiver. Seller and Guarantor acknowledge that payment shall be made
by the Surety Companies in strict compliance with their respective obligations
under the Surety Bond (including Section 4 of the Initial Surety Bond or the
equivalent provision in any Substitute Surety Bond) and agree not to dispute
prior to payment any payment by the Surety Companies in compliance with the
Surety Bond. Notwithstanding the foregoing, and notwithstanding the provisions
of any Surety Bond (including but not limited to Sections 4 and 8 of the Initial
Surety Bond) no written notice as between Purchaser and the Surety Companies,
and no payment to or for the benefit of Purchaser by Seller, Guarantor or the
Surety Companies shall constitute a waiver of any other right, remedy or power
of Seller or Guarantor under this Agreement or any document executed in
connection herewith.

Section 5.05 Rights and Remedies of Purchaser.

      (a) If an Early Termination Date has been designated under Section 5.02 or
6.02 and Seller has not paid the amounts due under Section 5.02 or Section 6.03
when such amounts are due, then Purchaser: (i) shall have the right to exercise
any and all rights and remedies available to it under any Transaction Document
or available at law or equity, including calling upon the Surety Bond in
accordance with the terms of Section 5.04; and (ii) shall have the right and be
entitled, at its option, to offset balances held by it for account of Seller or
Guarantor at any of its offices or other amounts owed to it under any
Transaction Document, in U.S. Dollars or in any other currency, against any
Termination Payments or Unpaid Amounts (regardless of whether such balances are
then due to Seller or Guarantor), in which case it shall promptly notify Seller
or Guarantor thereof, provided that Purchaser's failure to give such notice
shall not affect the validity thereof. Purchaser's right to offset balances held
by it for the account of Guarantor pursuant to this Section 5.05 shall be
subject to the limits set forth in the Guaranty Agreement.


                                       16
<PAGE>   20
      (b) All proceeds received after an Early Termination Date shall be applied
first to reimbursement of expenses and indemnities provided for in this
Agreement and other Transaction Documents owed by Seller to Purchaser and then
to the Termination Payment and any Unpaid Amounts; and any excess shall be paid
to Seller or as otherwise required by any governmental requirement.

Section 5.06 Purchaser's Failure to Accept Delivery. If other than as a result
of an event of Force Majeure, Purchaser defaults in its obligation to accept
delivery of all or any part of the Required Delivery Quantity in respect of a
Delivery Month at the Delivery Points and such default shall remain unremedied
for one (1) Business Day after notice thereof to Purchaser by Seller, then
Seller, as its sole and exclusive remedy, (i) shall sell the Deficiency Quantity
for such Delivery Month and pay to Purchaser, in lieu of delivery of the
Deficiency Quantity in respect of such Delivery Month, the Replacement Value of
such Deficiency Quantity in respect of that Delivery Month, and (ii) if
Purchaser fails (other than as a result of an event or events of Force Majeure)
to accept delivery of the entire Delivery Quantity in respect of any two
Delivery Months at the Delivery Points during a 12 month period, Seller may
terminate this Agreement in the same manner as it may be terminated by Purchaser
pursuant to Sections 5.02 and 5.03, except that Seller will give the relevant
notices and calculate the amount of the Termination Payment to be made by Seller
to Purchaser. The Replacement Value shall be paid to Purchaser no later than the
Payment Date next following such Delivery Month, and Purchaser will accept such
payment in lieu of the Deficiency Quantity for such Delivery Month. Where
Purchaser defaults in its acceptance obligation as aforesaid, Seller shall use
all reasonable business efforts to maximize the Replacement Value of any
Deficiency Quantity and Seller shall prepare and deliver to Purchaser, within
five Business Days after the end of the applicable Delivery Month, a certificate
setting out the calculation of the Replacement Value accompanied by reasonably
available back-up documentation therefor. Any such certificate shall, absent
manifest error, be conclusive evidence of the amount due in respect of the
Replacement Value.

           ARTICLE VI - EVENTS OF CHANGE AND ACCELERATED TERMINATION

Section 6.01 Events of Change. Each of the following events shall constitute an
"Event of Change" for the purposes of this Agreement:

      (a) the enactment, promulgation, execution or ratification of, or any
change in or amendment to, any regulation or law (or the application or
interpretation of any regulation or law, as determined by a court or regulatory
authority of competent jurisdiction or as determined by the opinion of
independent counsel mutually acceptable to Seller and Purchaser) that occurs
after the date hereof which would result in the imposition of a Tax (other than
an increase in the rate of a general tax on overall income) or of state or
federal government royalties (in excess of those royalties currently in effect)
or of export or price controls in a material amount by any government or taxing
authority upon a party hereto with respect to delivery of, or acceptance of
delivery of, Natural Gas under this Agreement or with respect to any cash
payment made pursuant to Section 2.06 which in any case would materially
adversely affect the net cost to Seller or Purchaser of performing its
obligations hereunder; or

      (b) the enactment, promulgation, execution or ratification of, or any
change in or amendment to, any regulation or law (or the application or
interpretation of any regulation or law, as determined by a court or regulatory
authority of competent jurisdiction or as determined by the opinion of
independent counsel mutually acceptable to Seller and Purchaser) that occurs
after the date hereof which would result in the performance of any obligation of
Seller to deliver Natural Gas or Purchaser to accept delivery of Natural Gas
under this Agreement being unlawful; or


                                       17
<PAGE>   21
      (c) any one or more events of Force Majeure (other than any event
specified in Section 6.01(b)) that occur and continue for a consecutive period
of longer than ninety (90) days.

Section 6.02      Accelerated Termination.

      (a) During the continuation of an Event of Change contemplated by Section
6.01(a), both parties shall make reasonable efforts to make arrangements to
avoid the imposition of any Tax contemplated by Section 6.01(a); provided that
this Section 6.02 shall not impose on either party any obligation other than to
negotiate in good faith to make such arrangements as will not adversely affect
the parties.

      (b) During the continuation of any Event of Change, or, in the case of an
Event of Change under Section 6.01(a), if any arrangement is not made pursuant
to Section 6.02(a), then either party may designate an accelerated termination
date ("Accelerated Termination Date") upon not less than two (2) and not more
than ten (10) Business Days' notice to the other party. Upon the Accelerated
Termination Date, the parties' obligations hereunder shall terminate, except for
the obligations contained in Section 6.03 and Section 7.02.

Section 6.03 Payments on Accelerated Termination. Upon the designation of an
Accelerated Termination Date, Seller shall pay to Purchaser an amount equal to
the Termination Payment plus any Unpaid Amounts. All amounts payable under this
Section 6.03 shall become due on the Accelerated Termination Date and shall be
payable on the Business Day following delivery by Purchaser of the certificate
of calculation of the Termination Payment contemplated by Section 5.03(c). The
remedy under this Section 6.03 shall be Purchaser's sole remedy upon the
occurrence of an Accelerated Termination Date; provided that nothing herein
shall affect a party's obligation to make payments of amounts which were due and
owing (whether or not payable) on or prior to the occurrence of such Accelerated
Termination Date.


                          ARTICLE VII - MISCELLANEOUS

Section 7.01 Notice. Any demand, notice or communication to be made or given
hereunder shall be in writing and may be made or given by personal delivery or
by transmittal by telecopy addressed to the respective party as follows:

To Seller:

      Columbia Natural Resources, Inc.
      900 Pennsylvania Avenue
      Charleston, West Virginia 25362-0070
      Attention:  Chief Financial Officer
      Telecopier No.:  (304) 353-5249
      Telephone No.:  (304) 353-5067

with a copy to:

      Columbia Energy Group Service Corporation


                                       18
<PAGE>   22
      13880 Dulles Corner Lane
      Herndon, Virginia 20171
      Attention:  Benga Farina
      Telecopier No.:  (703) 561-7303
      Telephone No.:  (703) 561-6792

To Purchaser:

      Mahonia II Limited
      22 Grenville Street
      St. Helier
      Jersey, Channel Islands JE4 8PX
      Attention:  Ian James
      Telecopier No.:  44-1534-609333
      Telephone No.:  44-1534-609000

with copy to:

      The Chase Manhattan Bank
      270 Park Avenue, 6th Floor
      New York, New York 10017
      Attention:  Dinsa Mehta
      Telecopier No.:  (212) 834-6084
      Telephone No.:  (212) 834-2032

or to such other address or telecopy number as any party may from time to time
notify the others in accordance with this Section 7.01. Any demand, notice or
certification made or given by personal delivery shall be conclusively deemed to
have been given on the day of actual delivery thereof, or, if made or given by
telecopy, on the date that such transmittal is received by a responsible
employee of the recipient in legible form (it being agreed that the burden of
proving receipt will be on the sender and will not be met by a transmission
report generated by the sender's facsimile machine) or if such date is not a
Business Day, on the first Business Day following the transmittal thereof.

Section 7.02 Interest on Overdue Amounts. If any monetary amounts payable under
this Agreement are not paid when due, then such overdue amount shall bear
interest for each day until paid in full, payable on demand, both before and
after judgment or petition for bankruptcy, at the U.S. Base Rate plus one and
one-half percent per annum on the basis of the actual number of days elapsed and
on the basis of a year of 360 days, as the case may be, but in no event shall
the interest rate exceed the maximum rate allowed by law. Such interest shall be
determined daily and compounded monthly in arrears on the last day of each
calendar month.

Section 7.03 Governing Law; Waiver of Jury Trial.

      (a) THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH
THE LAWS OF THE STATE OF NEW YORK AND APPLICABLE FEDERAL LAWS OF THE UNITED
STATES OF AMERICA.

      (b) ANY PROCEEDING WITH RESPECT TO THIS AGREEMENT OR ANY OTHER TRANSACTION
DOCUMENT MAY BE BROUGHT IN THE COURTS OF THE STATE OF NEW


                                       19
<PAGE>   23
YORK IN THE COUNTY OF NEW YORK OR OF THE UNITED STATES FOR THE SOUTHERN DISTRICT
OF NEW YORK AND, BY EXECUTION AND DELIVERY OF THIS AGREEMENT, EACH OF THE
PARTIES HERETO HEREBY IRREVOCABLY ACCEPTS FOR ITSELF AND IN RESPECT OF ITS
PROPERTY, THE JURISDICTION OF THE AFORESAID COURTS. TO THE EXTENT PERMITTED BY
APPLICABLE LAW, EACH OF THE PARTIES HERETO HEREBY IRREVOCABLY CONSENTS TO THE
SERVICE OF PROCESS WITH RESPECT TO ANY PROCEEDING (WHETHER OR NOT IN NEW YORK),
BY THE MAILING OF COPIES THEREOF BY REGISTERED OR CERTIFIED MAIL, POSTAGE
PREPAID, TO SUCH PERSON, AT ITS RESPECTIVE ADDRESS, SUCH SERVICE TO BECOME
EFFECTIVE 30 DAYS AFTER SUCH MAILING.

      (c) EACH OF THE PARTIES HERETO HEREBY IRREVOCABLY WAIVES, TO THE FULLEST
EXTENT PERMITTED BY APPLICABLE LAW, ANY OBJECTION WHICH IT MAY NOW OR HEREAFTER
HAVE TO THE LAYING OF VENUE OF ANY OF THE AFORESAID PROCEEDINGS ARISING OUT OF
OR IN CONNECTION WITH THIS OR ANY OTHER TRANSACTION DOCUMENT BROUGHT IN THE
COURTS REFERRED TO IN SECTION 7.03(b) HEREOF AND HEREBY FURTHER IRREVOCABLY
WAIVES, TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW, AND AGREES NOT TO
PLEAD OR CLAIM IN ANY SUCH COURT THAT ANY SUCH PROCEEDING BROUGHT IN ANY SUCH
COURT HAS BEEN BROUGHT IN AN INCONVENIENT FORUM.

      (d) EACH OF THE PARTIES HERETO HEREBY IRREVOCABLY WAIVES TO THE FULLEST
EXTENT PERMITTED BY LAW ALL RIGHT TO TRIAL BY JURY IN ANY PROCEEDING ARISING OUT
OF OR RELATING DIRECTLY OR INDIRECTLY TO ANY OF THIS AGREEMENT OR ANY OTHER
TRANSACTION DOCUMENTS OR THE TRANSACTIONS CONTEMPLATED HEREBY OR THEREBY
(WHETHER BASED ON CONTRACT, TORT OR ANY OTHER THEORY). EACH PARTY HERETO (A)
CERTIFIES THAT NO REPRESENTATIVE, AGENT OR ATTORNEY OF ANY OTHER PARTY HAS
REPRESENTED EXPRESSLY OR OTHERWISE, THAT SUCH PARTY WOULD NOT, IN THE EVENT OF
LITIGATION, SEEK TO ENFORCE THE FOREGOING WAIVER AND (B) ACKNOWLEDGES THAT IT
AND THE OTHER PARTY HERETO HAVE BEEN INDUCED TO ENTER INTO THIS AGREEMENT BY,
AMONG OTHER THINGS, THE MUTUAL WAIVERS AND CERTIFICATIONS IN THIS PARAGRAPH.

Section 7.04 Severability. In the event that one or more of the provisions
contained in this Agreement shall be finally judicially determined to be
invalid, illegal or unenforceable in any respect by a court of competent
jurisdiction the validity, legality or enforceability of the remaining
provisions hereof shall not be affected or impaired thereby and the parties
agree to negotiate in good faith to agree on a provision which is enforceable
and which preserves the economic bargain of the parties to the greatest extent
possible. Each of the Sections of this Agreement is hereby declared to be
separate and distinct.

Section 7.05 Place of Payment; Currency. Unless otherwise stated, all payments
herein or in any Transaction Document to Purchaser shall be made at Purchaser's
account, account number 323874541, maintained by Purchaser with The Chase
Manhattan Bank in New York City, New York. Unless otherwise stated, all amounts
expressed herein in terms of money refer to the United States Dollar and all
payments to be made hereunder shall be made in such currency.

Section 7.06 Purchaser Not an Agent. Purchaser acknowledges and confirms that
all purchases of Natural Gas hereunder are being made by it as a principal and
that it is not acting as agent for any other


                                       20
<PAGE>   24
Person in connection with purchases of Natural Gas hereunder. Seller
acknowledges and agrees that Purchaser may employ (and perform through) agents
or servicers to perform its obligations in respect of scheduling delivery and
receipt of Natural Gas hereunder, arranging for actual delivery and performance
of Purchaser's other obligations; and Seller agrees to accept performance
through such agents or servicers.

Section 7.07 Benefit of the Agreement. This Agreement shall inure to the benefit
of and be binding upon Seller, Purchaser and their respective successors and
assigns.

Section 7.08 Assignment and Transfer. Except as expressly provided in this
Section 7.08, neither party may assign any rights or delegate any obligations
hereunder without the prior written consent of the other party. Seller may
assign any rights or delegate any obligations hereunder without the consent of
Purchaser in the event of a transaction involving the sale, transfer or other
disposition of all or substantially all of the assets of Seller, or a
recapitalization, merger or other combination involving Seller, and in either
event, the survivor entity, if not Seller, expressly assumes the obligations of
Seller hereunder. Seller may otherwise assign any rights or delegate any
obligations hereunder with the consent of Purchaser, which consent will not be
unreasonably withheld or delayed. With the consent of Seller, which consent will
not be unreasonably withheld or delayed, Purchaser may assign or grant a
security interest in Purchaser's interests in this Agreement and the other
Transaction Documents as collateral security for any financing or hedging
provided to Purchaser or to any of its affiliates, whether now existing or
hereafter created; and the terms of such assignment may permit such assignee or
secured party to foreclose or substitute itself in lieu of Purchaser hereunder
and thereunder.

Section 7.09 Entire Agreement. This Agreement and the other Transaction
Documents constitute the entire agreement between the parties hereto and with
respect to the subject matter hereof supersede any prior agreement, undertaking,
declarations, commitments or representations, written or oral, in respect
thereof. There are no unwritten oral agreements among the parties.

Section 7.10 Amendments. This Agreement may not be modified or amended except by
an instrument in writing signed by Purchaser and Seller or by their respective
successors or permitted assigns.

Section 7.11 No Waivers, Remedies. No failure to exercise and no delay in
exercising any right hereunder shall operate as a waiver thereof nor shall any
single or partial exercise of any right hereunder preclude any other or further
exercise thereof or the exercise of any other right. The remedies herein
provided are cumulative and not exclusive of any remedies provided by law except
as otherwise expressly provided herein.

Section 7.12 Time of the Essence. Time shall be of the essence in this
Agreement.

Section 7.13 Counterparts. This Agreement may be executed in counterparts, each
of which so executed shall be deemed to be an original and such counterparts
together shall constitute one and the same instrument.

Section 7.14 Guaranty Agreement. Seller's payment obligations under this
Agreement are guaranteed by the Guaranty Agreement to the extent set forth
therein.

Section 7.15 Intent. The parties intend that this Agreement shall constitute a
purchase and sale of inventoriable goods and a forward contract within the
meaning of Section 556 of the United States Bankruptcy Code of 1978, as amended
from time to time.


                                       21
<PAGE>   25
Section 7.16 Disclosure of Information. In the event that any party provides the
other with written confidential information belonging to such disclosing party
or its affiliates which has been denominated in writing as "confidential", the
non-disclosing party agrees to thereafter maintain such information in
confidence in accordance with the standards of care and diligence that each
utilizes in maintaining its own confidential information. This obligation of
confidence shall not apply to such portions of the information which (i) are in
the public domain, (ii) hereafter become part of the public domain without such
party breaching its obligation of confidence hereunder, (iii) are hereafter
developed by such party without using the disclosing party's information, (iv)
are hereafter obtained by or available to such party from a third party who owes
no obligation of confidence to the disclosing party with respect to such
information, (v) are disclosed with the disclosing party's consent, or (vi) as
may be required by Persons regulating the activities of the non-disclosing
party, or law or regulation or order of any governmental authority in any
judicial, arbitration or governmental proceeding; provided, however, a party
shall (A) give the other party timely notice of the service or subpoena or other
process so that the other party may seek a protective order or other legal
remedy to prevent disclosure or to take steps to have the confidentiality of
such information protected to the extent possible under such legal process, and
(B) disclose only such information as is required by such process. Further, any
party may disclose any such information to any independent petroleum engineers
or consultants, any independent certified public accountants, any legal counsel
employed by it in connection with this Agreement or any Transaction Document,
including without limitation, the enforcement or exercise of all rights and
remedies thereunder; provided, however, that it imposes on such Person to whom
such information is disclosed the same obligation to maintain the
confidentiality of such information as is imposed upon it hereunder.
Notwithstanding anything to the contrary provided herein, this obligation of
confidence shall cease three (3) years from the date the information was
furnished, unless the disclosing party requests in writing at least 30 days
prior to the expiration of such three year period, that the non-disclosing party
maintain the confidentiality of such information for an additional three year
period.

Section 7.17 Stamp and Documentary Taxes. To the fullest extent permitted by
applicable law, Seller agrees to pay, and shall indemnify Purchaser for any and
all liabilities incurred by it in connection with, any present or future stamp
or documentary taxes or any other excise or property taxes, duties, charges or
similar levies that arise from any payment made hereunder or from the execution,
delivery or registration of, or otherwise with respect to, this Agreement or any
other Transaction Document.

Section 7.18 Further Assurances. The parties hereto agree to take all such
further actions and to execute, acknowledge and deliver all such further
documents that are necessary or useful to carry out the purposes of this
Agreement.


                                       22
<PAGE>   26
                                                                  EXECUTION COPY



      IN WITNESS WHEREOF the parties hereto have executed this Agreement as of
the date first written above.

                                          COLUMBIA NATURAL RESOURCES, INC.



                                          By:    /s/ W.H. Harmon
                                          Name:  W.J. Harmon
                                          Title: President and CEO



                                          MAHONIA II LIMITED



                                          By:    /s/ Ian James
                                          Name:  Ian James
                                          Title: Director



              [Signature Page - Natural Gas Advance Sale Contract
       between Columbia Natural Resources, Inc. and Mahonia II Limited]
<PAGE>   27
                                                                  EXECUTION COPY



                                    ANNEX 1

                     To Natural Gas Advance Sale Contract

                             Conditions Precedent


1.    The Guaranty Agreement shall have been executed by the Guarantor.

2.    The Initial Surety Bond shall have been issued.

3.    An opinion of counsel to each Surety Company shall have been delivered to
      Purchaser and The Chase Manhattan Bank.

4.    An opinion of LeBoeuf, Lamb, Greene & MacRae, L.L.P., counsel to Seller
      and Guarantor, in substantially the form set out in Exhibit B, shall have
      been delivered to Purchaser.

5.    A certificate of the Secretary or an Assistant Secretary of Seller setting
      forth (i) resolutions of its Board of Directors with respect to the
      authorization of Seller to execute and deliver the Agreement and the
      Transaction Documents to which it is a party and to enter into the
      transactions and perform its obligations contemplated in those documents,
      (ii) the officers of such Person (y) who are authorized to sign such
      agreements and (z) who will, until replaced by another officer or officers
      duly authorized for that purpose, act as its representative for the
      purposes of signing documents in connection with such agreements and the
      transactions contemplated hereby, (iii) specimen signatures of the
      authorized officers, and (iv) certificate of incorporation, by-laws, if
      any, and other organizational documents of Seller, certified as being true
      and complete.  Purchaser may conclusively rely on such certificate until
      it receives notice in writing from Seller to the contrary.

6.    A certificate of the Secretary or an Assistant Secretary of Guarantor
      setting forth (i) resolutions of its board of directors authorizing
      officers of Guarantor to issue guarantees in the name of Guarantor on
      behalf of its subsidiaries, (ii) the officers of the Guarantor (y) who are
      authorized to sign such agreement and (z) who will, until replaced by
      another officer or officers duly authorized for that purpose, act as its
      representative for the purposes of signing documents in connection with
      such agreement and the transactions contemplated hereby, (iii) specimen
      signatures of the authorized officers, and (iv) certificate of
      incorporation, by-laws, if any, and other organizational documents of
      Guarantor, certified as being true and complete.  Purchaser may
      conclusively rely on such certificate until it receives notice in writing
      from Seller to the contrary.


                                   Annex 1-1