<PAGE>

                                                                     Exhibit 4.8

                            CARBON ENERGY CORPORATION
                     EARLY EXERCISE STOCK PURCHASE AGREEMENT

     THIS AGREEMENT is made by and between Carbon Energy Corporation, a Colorado
corporation (the "Company"), and _______________________ ("Purchaser").


                                    RECITALS:

     Purchaser holds a stock option (the "Option") to purchase ________ shares
of common stock of the Company (the "Stock " or "Common Stock") granted on
_________________________, ____ pursuant to the Company's 1999 Stock Option Plan
(the "Plan"), which Purchaser desires to exercise.

     Purchaser wishes to take advantage of the early exercise provision of the
Option and therefore to enter into this Agreement.

                                   AGREEMENT:

     Now, therefore, the parties hereto agree as follows:

     Purchaser hereby agrees to purchase from the Company, and the Company
hereby agrees to sell to Purchaser, pursuant to the exercise of the Option, an
aggregate of ____________ shares of the Common Stock, for an exercise price of
_________ ($________) per share (total exercise price: $ _________), payable as
follows:

<TABLE>
<S>                                                              <C>
          Cash at Closing                                        $

          Value of _________ shares of                           $
          Common Stock (held outright by Purchaser for at
          least 6 months prior to the date of this
          Agreement)

          Total Exercise Price                                   $
</TABLE>

     The closing hereunder shall occur at the offices of the Company on the date
of this Agreement or at such other time and place as the parties may mutually
agree upon in writing.

     At the closing, Purchaser shall deliver three (3) stock assignments in the
form of Attachment A, duly endorsed (with date and number of shares left blank),
joint escrow instructions (the "Joint Escrow Instructions") in the form of
Attachment B, duly executed by Purchaser, and the total exercise price
(including endorsed certificates representing the appropriate number of shares
of the Company's Common Stock if a portion of the total exercise price is to be
paid by Common Stock).

     At the closing or as soon thereafter as practicable, the Company shall
deliver to the Escrow Agent (as defined below) share certificates for all of the
Stock that is to be subject to the

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<PAGE>

Purchase Option (as defined below), and shall deliver share certificates to
Purchaser for all of the Stock, if any, that is not to be subject to the
Purchase Option.

     COMPANY'S PURCHASE OPTION.

     The Stock to be purchased by Purchaser pursuant to this Agreement shall be
subject to the following option (the "Purchase Option"):

     If the Purchaser ceases to be an employee of the Company or (if the
Purchaser is a non-employee director of the Company) if the Purchaser ceases to
serve as a member of the Board of Directors of the Company, for any reason
(including his death), or no reason, with or without cause, the Purchase Option
may be exercised. The Company shall have the right at any time within ninety
(90) days after such cessation of employment or service as a director, as the
case may be, to purchase from Purchaser or his personal representative, as the
case may be, at the price per share paid by Purchaser pursuant to this Agreement
(the "Option Price"), up to but not exceeding the number of shares (including
fractional shares) of Common Stock in which the Purchaser has not vested
pursuant to the terms set forth in the schedule of the Option as of the date of
such cessation of employment or service as a director, as the case may be.

     Immediately prior to the consummation of a Change in Control, as defined in
the Plan, the Purchase Option shall automatically lapse in its entirety, except
to the extent the Purchase Option is to be assigned to the successor corporation
(or its parent company) in connection with such Change in Control. To the extent
the Purchase Option remains in effect following Change in Control, the right
shall apply to the new capital stock or other property (including cash paid
other than as a regular cash dividend) received in exchange for the stock in
consummation of the Change in Control, but only to the extent the Common Stock
is at the time covered by such right. Appropriate adjustments shall be made to
the price per share payable upon exercise of the Purchase Option to reflect the
effect of the Change in Control upon the Company's capital structure; provided,
however, that the aggregate price shall remain the same.

     The Company shall be entitled to pay for any shares purchased pursuant to
its Purchase Option at the Company's option in cash, by offset against any
indebtedness owing to the Company by Purchaser including without limitation any
recourse note given in payment for the Stock, or a combination of both.

     The Purchase Option may be exercised by giving written notice of exercise
delivered or mailed as provided herein. Upon provision of such notice and
payment or tender of the purchase price, the Company shall become the legal and
beneficial owner of the Stock being purchased and all rights and interests
therein or related thereto.

     If from time to time during the term of the Purchase Option there is any
stock dividend or liquidating dividend or distribution of cash and/or property,
stock split or other change in the character or amount of any of the outstanding
securities of the Company, then, in such event, any and all new, substituted or
additional securities or other property to which Purchaser is entitled by reason
of his ownership of Stock will be immediately subject to the Purchase Option and
be included in the word "Stock" for all purposes of the Purchase Option with the
same force and effect as the shares of Stock then subject to the Purchase
Option. While the total Option Price

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<PAGE>

shall remain the same after each such event, the Option Price per share of Stock
upon exercise of the Purchase Option shall be appropriately adjusted.

     As used herein, employment with the Company shall include employment with
an affiliate of the Company.

     COMPANY'S DESIGNEE.

     The Company shall have the right to designate one or more persons or
entities, or a combination of both (the "Company's Designee"), to exercise all
or any part of the Company's rights, assume all or any part of the Company's
benefits, and bear all or any part of the Company's burdens pursuant to this
Agreement, and a reference to the Company shall mean the Company and/or the
Company's Designee, as the case may be. Notwithstanding any provision herein to
the contrary, a designation by the Company shall not relieve the Company of the
responsibility to pay any part of the purchase price for Stock which is not paid
by the Company's Designee.

     NOTICES.

     All notices or other communications required under this Agreement or given
in connection herewith shall be in writing and shall either be delivered
personally, in which event the effective date shall be the date of delivery, or
shall be sent by United States mail addressed as hereinafter set forth, postage
pre-paid, registered or certified, return receipt requested, in which event the
effective date shall be the delivery date as specified on the return receipt.
Unless otherwise directed by notice in writing, all notices shall be addressed
as follows:

          (a)  To the Company at:

               Carbon Energy Corporation
               1700 Broadway, Suite 1150
               Denver, Colorado 80290
               Attention: Secretary

          (b)  To the Purchaser at: The address of the Purchaser set forth on
the transfer records of the Company.

     LEGEND ON STOCK.

     All certificates representing any shares of Stock of the Company subject to
the provisions of this Agreement shall bear applicable legends which read
substantially as follows:

          (a)  THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO AN
OPTION SET FORTH IN AN AGREEMENT BETWEEN THE ISSUER AND THE REGISTERED HOLDER,
OR HIS PREDECESSOR IN INTEREST. A COPY OF THAT AGREEMENT MAY BE INSPECTED AT THE
PRINCIPAL OFFICE OF THE ISSUER. ANY TRANSFER OR ATTEMPTED TRANSFER OF ANY SHARES
SUBJECT TO SUCH OPTION IS VOID WITHOUT THE PRIOR EXPRESS WRITTEN CONSENT OF THE
ISSUER OF THESE SHARES.

                                       3
<PAGE>

          (b)  Any legend required to be placed thereon by appropriate state
blue sky officials.

     As security for his faithful performance of the terms of this Agreement and
to ensure the availability for delivery of Purchaser's Stock upon exercise of
the Purchase Option herein provided for, Purchaser agrees, at the closing
hereunder (or as soon thereafter as practicable), to deliver (or have the
Company deliver on the Purchaser's behalf) to and deposit with the Secretary of
the Company ("Escrow Agent"), as Escrow Agent in this transaction, three (3)
stock assignments duly endorsed (with date and number of shares left blank) in
the form attached hereto as Attachment A, together with a certificate or
certificates evidencing all of the Stock subject to the Purchase Option; said
documents are to be held by the Escrow Agent and delivered by said Escrow Agent
pursuant to the Joint Escrow Instructions of the Company and Purchaser set forth
in Attachment B attached hereto and incorporated herein by this reference, which
instructions shall also be delivered to the Escrow Agent at the closing
hereunder (or as soon thereafter as practicable).

     The Company shall not be required (i) to transfer on its books any shares
of Stock of the Company which shall have been sold or transferred in violation
of any of the provisions set forth in this Agreement or (ii) to treat as owner
of such shares or to accord the right to vote as such owner or to pay dividends
to any transferee to whom such shares shall have been so transferred.

     Subject to the provisions of this Agreement, Purchaser (but not any
unapproved transferee) shall, during the term of this Agreement, exercise all
rights and privileges of a stockholder of the Company with respect to the Stock.

     The parties agree to execute such further instruments and to take such
further action as reasonably may be necessary to carry out the intent of this
Agreement.

     This Agreement is not an employment contract and nothing in this Agreement
shall be deemed to create in any way whatsoever any obligation on the part of
Purchaser to continue in the employ of the Company, or of the Company to
continue Purchaser in the employ of the Company. In the event that the Option
was granted in connection with the performance of services as a consultant or
director, references to employment, employee and similar terms shall be deemed
to include the performance of services as a consultant or a director, as the
case may be, provided, however, that no rights as an employee shall arise by
reason of the use of such terms.

     TERMINATION.

     Notwithstanding any provision herein to the contrary, this Agreement shall
terminate, and neither party shall have any further obligation to the other,
upon the occurrence of any of the following events:

          (a)  The insolvency, receivership or dissolution of the Company.

          (b)  The voluntary written agreement of the Company, upon the approval
of its Board of Directors, and the Purchaser.

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<PAGE>

          (c)  September 1, 2014.

     SPECIFIC ENFORCEMENT.

     Because of the unique value of the Stock, in addition to any other remedies
which the Company may have upon the breach of the agreements contained herein,
the obligations of the Purchaser shall be specifically enforceable.

     MODIFICATION.

     This Agreement may only be altered or amended by a written instrument
signed by the Company and the Purchaser setting forth such changes.

     COSTS OF ENFORCEMENT.

     In any action at law or in equity to enforce any of the provisions or
rights under this Agreement, the unsuccessful party of such litigation, as
determined by any court of competent jurisdiction in a final judgment or decree,
shall pay the successful party or parties all costs, expenses and reasonable
attorneys' fees incurred therein by such party or parties (including without
limitation such costs, expenses and fees on any appeals), and if such successful
party shall recover judgment in any action or proceeding, such costs, expenses
and attorneys' fees shall be included as part of the judgment.

     SEVERABILITY.

     The invalidity or unenforceability of any provision hereof shall in no way
affect the validity or enforceability of any other provision hereof.

     ENTIRE AGREEMENT; COUNTERPARTS; HEADINGS.

     This Agreement constitutes the entire agreement between the parties
pertaining to the subject matter hereof and supersedes all prior and
contemporaneous agreements, representations and understandings. This Agreement
may be executed in counterparts, all of which shall be deemed to be one and the
same instrument, and it shall be sufficient for each party to have executed at
least one, but not necessarily the same, counterpart. The headings contained in
this Agreement are for reference purposes only and shall not affect the meaning
or interpretation of this Agreement in any way.

     ASSIGNMENT.

     This Agreement shall be binding upon the parties and their respective legal
representatives, beneficiaries, successors and assigns.

     GOVERNING LAW.

     This Agreement and the rights and obligations of the parties hereto shall
be governed by and construed and enforced in accordance with the laws of the
State of Colorado.

                                       5
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the ____ day of _______________, 20__.


                                        CARBON ENERGY CORPORATION


                                        By:
                                           -------------------------------------
                                        Title:
                                              ----------------------------------
                                        Address: 1700 Broadway, Suite 1150
                                                 Denver, CO 80290
                                                 -------------------------------


                                        PURCHASER:
                                                  ------------------------------
                                        Address:
                                                --------------------------------



ATTACHMENTS:

Attachment A  Assignment Separate from Certificate
Attachment B  Joint Escrow Instructions


                                       6
<PAGE>

                                  ATTACHMENT A

                      ASSIGNMENT SEPARATE FROM CERTIFICATE

           FOR VALUE RECEIVED and pursuant to that certain Early Exercise
Stock  Purchase Agreement (the "Agreement") dated as of ____________, ______,
__________ hereby sells, assigns and  transfers unto (____) shares of common
stock of Carbon Energy Corporation, a  Colorado corporation, standing in the
undersigned's name on the books of said corporation represented by
Certificate No. ___ herewith, and does hereby irrevocably constitute and
appoint the Secretary of Carbon Energy Corporation as attorney in fact to
transfer the said stock on the books of the said corporation with full power
of substitution in the premises. This Assignment may be used only in
accordance with and subject to the terms and conditions of the Agreement, in
connection with the repurchase of shares of common stock issued to the
undersigned pursuant to the Agreement, and only to the extent that such
shares remain subject to the Company's Purchase Option under the Agreement.

Dated:
      ---------------------             -------------------------------------
                                        [Signature]


                                        -------------------------------------
                                        [Print Name]



                                       7
<PAGE>

                                  ATTACHMENT B
                            JOINT ESCROW INSTRUCTIONS


Carbon Energy Corporation
Attention: Secretary
1700 Broadway, Suite 1150
Denver, Colorado 80290

Dear Sir or Madam:

     As Escrow Agent for both Carbon Energy Corporation, a Colorado corporation
(the "Company"), and the undersigned purchaser of stock of the Company
("Purchaser"), you are hereby authorized and directed to hold the documents
delivered to you pursuant to the terms of that certain Early Exercise Stock
Purchase Agreement (the "Agreement"), dated ________________, to which a copy of
these Joint Escrow Instructions is attached as Attachment B, in accordance with
the following instructions:

1.   In the event the Company or an assignee shall elect to exercise the
Purchase Option set forth in the Agreement, the Company or its assignee will
give to Purchaser and you a written notice specifying the number of shares of
stock to be purchased, the purchase price, and the time for a closing hereunder
at the principal office of the Company. Purchaser and the Company hereby
irrevocably authorize and direct you to close the transaction contemplated by
such notice in accordance with the terms of said notice.

2.   At the closing you are directed (a) to date any stock assignments necessary
for the transfer in question, (b) to fill in the number of shares being
transferred, and (c) to deliver same, together with the certificate evidencing
the shares of stock to be transferred, to the Company against the simultaneous
delivery to you of the purchase price (which may include suitable acknowledgment
of cancellation of indebtedness) of the number of shares of stock being
purchased pursuant to the exercise of the Purchase Option.

3.   Purchaser irrevocably authorizes the Company to deposit with you any
certificates evidencing shares of stock to be held by you hereunder and any
additions and substitutions to said shares as specified in the Agreement.
Purchaser does hereby irrevocably constitute and appoint you as his
attorney-in-fact and agent for the term of this escrow to execute with respect
to such securities and other property all documents of assignment and/or
transfer and all stock certificates necessary or appropriate to make all
securities negotiable and complete any transaction herein contemplated.

4.   This escrow shall terminate upon expiration or exercise in full of the
Purchase Option, whichever occurs first.

5.   If at the time of termination of this escrow you should have in your
possession any documents, securities, or other property belonging to Purchaser,
you shall deliver all of same to Purchaser and shall be discharged of all
further obligations hereunder; provided, however, that if at the time of
termination of this escrow you are advised by the Company that the property

                                       8
<PAGE>

subject to this escrow is the subject of a pledge or other security agreement,
you shall deliver all such property to the pledgeholder or other person
designated by the Company.

6.   Except at otherwise provided in these Joint Escrow Instructions, your
duties hereunder may be altered, amended, modified or revoked only by a writing
signed by all of the parties hereto.

7.   You shall be obligated only for the performance of such duties as are
specifically set forth herein and may rely and shall be protected in relying or
refraining from acting on any instrument reasonably believed by you to be
genuine and to have been signed or presented by the proper party or parties or
their assignees. You shall not be personally liable for any act you may do or
omit to do hereunder as Escrow Agent or as attorney-in-fact for Purchaser while
acting in good faith and any act done or omitted by you pursuant to the advice
of your own attorneys shall be conclusive evidence of such good faith.

8.   You are hereby expressly authorized to disregard any and all warnings given
by any of the parties hereto or by any other person or corporation, excepting
only orders or process of courts of law, and are hereby expressly authorized to
comply with and obey orders, judgments or decrees of any court. In case you obey
or comply with any such order, judgment or decree of any court, you shall not be
liable to any of the parties hereto or to any other person, firm or corporation
by reason of such compliance, notwithstanding any such order, judgment or decree
being subsequently reversed, modified, annulled, set aside, vacated or found to
have been entered without jurisdiction.

9.   You shall not be liable in any respect on account of the identity,
authority or rights of the parties executing or delivering or purporting to
execute or deliver the Agreement or any documents or papers deposited or called
for hereunder.

10.  You shall not be liable for the outlawing of any rights under any statute
of limitations with respect to these Joint Escrow Instructions or any documents
deposited with you.

11.  You shall be entitled to employ such legal counsel and other experts as you
may deem necessary properly to advise you in connection with your obligations
hereunder, may rely upon the advice of such counsel, and may pay such counsel
reasonable compensation therefor.

12.  Your responsibilities as Escrow Agent hereunder shall terminate if you
shall cease to be Secretary of the Company or if you shall resign by written
notice to each party. In the event of any such termination, the Company may
appoint any officer or assistant officer of the Company as successor Escrow
Agent and Purchaser hereby confirms the appointment of such successor or
successors as attorney-in-fact and agent to the full extent of your appointment.

13.  If you reasonably require other or further instruments in connection with
these Joint Escrow Instructions or obligations in respect hereto, the necessary
parties hereto shall join in furnishing such instruments.

14.  It is understood and agreed that should any dispute arise with respect to
the delivery and/or ownership or right of possession of the securities, you may
(but are not obligated to) retain in your possession without liability to anyone
all or any part of said securities until such

                                       9
<PAGE>

dispute shall have been settled either by mutual written agreement of the
parties concerned or by a final order, decree or judgment of a court of
competent jurisdiction after the time for appeal has expired and no appeal has
been perfected, but you shall be under no duty whatsoever to institute or defend
any such proceedings.

15.  Any notice required or permitted hereunder shall be given in writing and
shall be deemed effectively given upon personal delivery or upon deposit in any
United States Post Box, by registered or certified mail with postage and fees
prepaid, addressed to each of the other parties hereunto entitled at the
following addresses specified below, or at such other addresses as a party may
designate by ten days' written notice to each of the other parties hereto:

16.  By signing these Joint Escrow Instructions you become a party hereto only
for the purpose of said Joint Escrow Instructions; you do not become a party to
the Agreement.

17.  This instrument shall be binding upon and inure to the benefit of the
parties hereto and their respective successors and permitted assigns. It is
understood and agreed that references to "you" or "your" herein refer to the
original Escrow Agent and to any and all successor Escrow Agents. It is
understood and agreed that the Company may at any time or from time to time
assign its rights under the Agreement and these Joint Escrow Instructions in
whole or in part.


                                        CARBON ENERGY CORPORATION


                                        By:
                                           -------------------------------------
                                        Title:
                                              ----------------------------------
                                        Address: 1700 Broadway, Suite 1150
                                                 Denver, CO 80290
                                                 -------------------------------


                                        PURCHASER:
                                                  ------------------------------
                                        Address:
                                                --------------------------------



                                       10