<PAGE>

                                                                EXHIBIT 10.13(a)

THE SECURITIES REPRESENTED BY THIS AGREEMENT HAVE NOT BEEN REGISTERED OR
QUALIFIED UNDER FEDERAL OR STATE SECURITIES LAWS. THESE SECURITIES MAY NOT BE
OFFERED FOR SALE, SOLD, PLEDGED, OR OTHERWISE DISPOSED OF UNLESS SO REGISTERED
OR QUALIFIED OR UNLESS AN EXEMPTION EXISTS, THE AVAILABILITY OF WHICH IS TO BE
ESTABLISHED BY AN OPINION OF COUNSEL TO THE REGISTERED HOLDER (WHICH OPINION AND
COUNSEL SHALL BOTH BE SATISFACTORY TO THE COMPANY).

                            BILL BARRETT CORPORATION

                             STOCK OPTION AGREEMENT
    (2002 Stock Option Plan -- Incentive Stock Options -- Tranche A Options)

      THIS STOCK OPTION AGREEMENT (the "Agreement") is made and entered into as
of the ____ day of __________ (the "Date of Grant") by and between Bill Barrett
Corporation, a Delaware corporation (the "Company"), and __________ (the
"Optionee").

                                   WITNESSETH:

      WHEREAS, effective as of __________, the Optionee received a stock option
to purchase shares of the Company's Common Stock pursuant to the Company's 2002
Stock Option Plan (as amended, restated or otherwise modified from time to time,
the "Plan") in order to provide the Optionee with an opportunity for investment
in the Company and additional incentive to pursue the success of the Company,
and this option is to be for the number of shares, at the price per share and on
the terms set forth in this Agreement;

      WHEREAS, pursuant to the Plan the Company may issue options to purchase up
to 5,500,000 shares of Common Stock at an exercise price of not less than $6.50
per share (the "Tranche A Options") and options to purchase up to 2,150,000
shares of Common Stock at an exercise price of not less than $.04412 per share
(the "Tranche B Options"). The stock option granted pursuant to this Agreement
is a Tranche A Option;

      WHEREAS, the Company intends that the stock option granted pursuant to
this Agreement qualify as an incentive stock option (an "Incentive Option") to
the full extent permitted pursuant to Section 422 of the Internal Revenue Code
of 1986, as amended (the "Code"), and that to the extent the stock option does
not qualify as an Incentive Stock Option, it shall be considered a non-qualified
stock option (a "Non-Qualified Option"). In determining whether a portion of the
stock option granted pursuant to this Agreement is a Non-Qualified Option,
Tranche B Options held by the Optionee shall be considered Incentive Options to
the fullest extent permitted by Section 422 of the Code prior to considering
whether the option granted pursuant to this Agreement or other Tranche A Options
held by the Optionee are Incentive Options; and

      WHEREAS, the Optionee desires to receive an option on the terms and
conditions set forth in this Agreement.

      NOW, THEREFORE, the parties agree as follows:

<PAGE>

      1.    Grant Of Option. The Company hereby grants to the Optionee, as a
matter of separate agreement and not in lieu of salary or any other compensation
for services, the right and option (the "Option") to purchase all or any part of
an aggregate of __________ shares of the authorized and unissued $.001 par value
common stock of the Company (the "Option Shares") pursuant to the terms and
conditions set forth in this Agreement. The Option is a Tranche A Option under
the Plan.

      2.    Option Price. At any time when shares are to be purchased pursuant
to the Option, the purchase price for each Option Share shall be $_____ (the
"Option Price").

      3.    Exercise Period.

            (a)   The exercise period shall commence on the Date of Grant and
terminate at 5:00 p.m., Denver, Colorado time, on __________, unless earlier
terminated as provided in this Agreement. The number of Option Shares that may
be purchased upon the exercise of a portion of the Option at any time during the
exercise period shall be equal to the number of Option Shares for which both
Time Vesting (as described in Section 3(b) below) and Equity Vesting (as
described in Section 3(c) below) have occurred, reduced by the number of Option
Shares previously purchased upon the exercise of a portion of the Option. No
portion of the Option shall be exercisable except to the extent that both Time
Vesting and Equity Vesting described in Section 3(c) have occurred.

            (b)   "Time Vesting" shall occur based on the following schedule so
that on each date set forth under the column "Date", Time Vesting shall have
occurred with respect to the percentage of the total number of Option Shares
subject to the Option set forth under the column "Time Vested Portion Of
Option":

<TABLE>
<CAPTION>
                Date                            Time Vested Portion Of Option
-----------------------------------             -----------------------------
<S>                                             <C>
First Anniversary of Date of Grant                            40%
Second Anniversary of Date of Grant                           60%
Third Anniversary of Date of Grant                            80%
Fourth Anniversary of Date of Grant                          100%
</TABLE>

            Notwithstanding the foregoing, the Option shall become fully Time
Vested upon a Change In Control (as defined in the Plan) or immediately prior
thereto to the extent set forth in the Plan if but only if the Optionee is
employed by the Company or any of its subsidiaries immediately prior to such
Change In Control.

            (c)   The number of Option Shares for which "Equity Vesting" shall
have occurred at any particular time shall be equal to (i) the total number of
Option Shares subject to the Option divided by 5,500,000 (as appropriately
adjusted for stock splits, stock dividends and the like), (ii) multiplied by (A)
the then outstanding number of shares of Common Stock of the Company (1)
excluding Management Stock (as defined in the Stockholders' Agreement dated
March 28, 2002 among the Company and its stockholders (the "Stockholders'
Agreement")) that has not vested as a result of Dollar Vesting (as defined in
the Stockholders' Agreement), (2) excluding all Common Stock previously issued
pursuant to all Tranche A Options and Tranche B Options and (3) including all
shares of Common Stock issuable upon the conversion of outstanding convertible
securities, including the conversion of the Series B Preferred calculated on the
basis that all shares of Series B Preferred have been converted at the
"Conversion Ratio" as defined in the Certificate of Designations for the Series
B Preferred Stock and with the conversion of the Series A Preferred calculated
on the basis that all shares of Series A Preferred have been converted at the
"Conversion Ratio" as defined in the Certificate of Designations for the Series
A Preferred Stock, and multiplied by (B) 8.34066%; provided however, that upon
the earlier to occur of January 31, 2007 or the occurrence of a Liquidation
Event (as defined in the Stockholders'

                                        2
<PAGE>
Agreement), a Qualified Public Offering (as defined in the Stockholders'
Agreement) or a transaction pursuant to Section 3.10 of the Stockholders'
Agreement (each, a "Vesting Termination Event"), there shall be substituted for
"5,500,000" in Subsection 3(c)(i) above the sum of the number of shares of
Common Stock that may be purchased pursuant to all Tranche A Options granted
pursuant to the Plan that remain outstanding immediately prior to the Vesting
Termination Event plus the number of shares of Common Stock previously issued
upon the exercise of Tranche A Options; and further provided, in no event shall
the number of Option Shares for which Equity Vesting has occurred ever exceed
the total number of Option Shares set forth in Section 1. The portion of the
Option that is not vested as a result of Equity Vesting upon the occurrence of a
Vesting Termination Event shall not become exercisable and shall be forfeited
and cancelled on the option transfer records of the Company without payment
therefor to the Optionee.

            (d)   The Option shall be considered an Incentive Option to the full
extent permitted pursuant to Section 422 of the Code. To the extent that the
Option does not qualify as an Incentive Option, it shall be considered a
Non-Qualified Option. In determining whether a portion of the Option shall be
considered a Non-Qualified Option, Tranche B Options held by the Optionee shall
be considered Incentive Options to the fullest extent permitted by Section 422
of the Code prior to considering whether Tranche A Options held by the Optionee
are Incentive Options. Optionee acknowledges and agrees that the Company is
making no representation or warranty that this Option qualifies as an Incentive
Option and that the Company is not obligated to take any action or refrain from
taking any action in order to cause this Option to qualify or continue to
qualify as an Incentive Option.

      4.    Exercise Of Option.

            (a)   To the extent exercisable pursuant to Section 3(a), the Option
may be exercised in whole or in part by delivering to the Treasurer of the
Company (i) a Notice And Agreement Of Exercise Of Option, substantially in the
form attached hereto as Exhibit A (which may be modified at any time by the
Company in its discretion as necessary to comply with applicable securities
laws), specifying the number of Option Shares with respect to which the Option
is exercised, and (ii) full payment, in the manner described in Section 5 of
this Agreement, of the Option Price for such shares. The Option may not be
exercised in part unless the purchase price for the Option Shares purchased is
at least $1,000 or unless the entire remaining portion of the Option is being
exercised. In addition, if the Stockholders' Agreement is in effect at the time
of the Optionee's exercise of the Option, the Optionee shall sign and deliver to
the Company a counterpart of the Stockholders' Agreement as then in effect.

            (b)   Promptly upon receipt of the Notice And Agreement Of Exercise
Of Option together with the full payment of the Option Price, and a counterpart
of the Stockholders' Agreement signed by the Optionee, if applicable, the
Company shall deliver to the Optionee a properly executed certificate or
certificates representing the Option Shares being purchased.

            (c)   During the lifetime of the Optionee, the Option shall be
exercisable only by the Optionee; provided, that in the event of the death of
the Optionee, the personal representative or estate of the Optionee may exercise
the Option; provided, further, that in the event of the legal disability of the
Optionee, the guardian or personal representative of the Optionee may exercise
the Option if such guardian or personal representative obtains a ruling from the
Internal Revenue Service or an opinion of counsel to the effect that neither the
grant nor the exercise of such power is violative of Section 422(b)(5), or its
successor provision, of the Internal Revenue Code of 1986, as amended (the
"Code"). Any opinion of counsel must be acceptable to the Option Committee both
with respect to the counsel rendering the opinion and with respect to the form
of opinion.

                                        3
<PAGE>

            (d)   (1)   If for any reason (other than the termination of
Optionee's employment because of Optionee's death or legal disability or the
termination of Optionee's employment by the Company for Cause (as defined in the
Plan)), the Optionee ceases to be employed by the Company, then the Option may
be exercised within three (3) months after such termination of the Optionee's
employment, or, if the Optionee dies during the three-month period immediately
following such termination, within one year after Optionee's death, but, in each
case, to the extent that (A) the Option was exercisable pursuant to Section 3(a)
on the date of termination of the Optionee's employment, and (B) the period for
exercise of the Option, as set forth in Section 3(b), has not terminated as of
the date of exercise. Upon termination of the respective periods set forth in
the previous sentence, any unexercised portion of an Option shall expire. (1) If
the Optionee's employment with the Company is terminated because of the
Optionee's death or legal disability, the Option may be exercised within one (1)
year after termination, but only to the extent that (A) the Option was
exercisable pursuant to Section 3(a) on the date of termination of the
Optionee's employment, and (B) the period for exercise of the Option, as set
forth in Section 3(b), has not terminated as of the date of exercise. Upon
termination of the respective periods set forth in the previous sentence, any
unexercised portion of an Option shall expire.

                  (3)   If the Optionee's employment by the Company is
terminated for Cause (as defined in the Plan), (A) the Option shall expire upon
delivery to the Optionee of notice of termination, which may be oral or in
writing, and all rights to purchase shares pursuant to the Option shall
terminate immediately, and (B) at the Company's option, all Option Shares
acquired by Optionee shall be immediately forfeit without any action on the part
of the Company or Optionee, and the Company shall promptly reimburse Optionee
the aggregate purchase price actually paid by Optionee for such Option Shares
(excluding any Common Stock or conversion of Option Shares used as payment for
such purchase price).

      5.    Payment For Option Shares. If payment for the Option Price is made
other than by a method described in Sections 5(a) or 5(b), the Option Price
shall be paid in cash, certified funds, or Optionee's check. Payment shall be
considered made when the Treasurer of the Company receives delivery of the
payment at the Company's address, provided that a payment made by check is
honored when first presented to the Optionee's bank. Beginning 180 days after
the consummation of any firm commitment underwritten offering of Common Stock to
the public pursuant to an effective registration statement under the Securities
Act of 1933, as amended, (i) for which the aggregate gross proceeds to the
Company are not less than fifty million dollars ($50,000,000), (ii) in which
each outstanding share of Series B Preferred Stock of the Company converts
pursuant to terms thereof into shares of Common Stock that have an aggregate
value, based on the price to public in such offering, of at least $7.50 per
share, and (iii) pursuant to which shares of Common Stock are authorized and
approved for listing on the New York Stock Exchange or admitted to trading and
quoted in the Nasdaq National Market system (a "Qualified Public Offering"),
payment for the exercise of an Option may be made pursuant to the following
methods:

            (a)   If the Option Price of the Option Shares purchased by Optionee
at any one time exceeds $1,000, the Company, in its sole discretion, upon
request by Optionee, may permit all or part of the Option Price to be paid by
delivery to the Company for cancellation shares of Common Stock previously owned
by Optionee ("Previously Owned Shares") with a Fair Market Value (as defined in
the Plan) as of the date of the payment equal to the portion of the Option Price
for the Option Shares that Optionee does not pay in cash. Notwithstanding the
above, Optionee may be permitted to exercise the Option by delivering Previously
Owned Shares only if (i) Optionee has held, and provides appropriate evidence of
such, the Previously Owned Shares for more than six months prior to the date of
exercise, or (ii) the Previously Owned Shares were acquired by Optionee in an
arm's length, open market transaction, or (iii) the Previously Owned Shares
consist of a combination of shares meeting the criteria described in either of
the immediately preceding clauses (i) and (ii). This period described in clause
(i) of the

                                        4
<PAGE>

preceding sentence (the "Holding Period") may be extended by the Company acting
in its sole discretion as is necessary, in the opinion of the Company, so that,
under generally accepted accounting principles, no compensation shall be
considered to have been or to be paid to Optionee as a result of the exercise of
the Option in this manner. At the time the Option is exercised, Optionee shall
provide an affidavit, and such other evidence and documents as the Company shall
request, to establish that the requirements of clauses (i), (ii) or (iii) above
have been satisfied. As indicated above, Optionee may deliver shares of Common
Stock as part of the purchase price only if the Company, in its sole discretion,
agrees, on a case by case basis, to permit this form of payment.

            (b)   Optionee also may pay the Option Price by delivering to the
Company and to a broker-dealer, which broker-dealer shall be subject to approval
by the Option Committee at the Option Committee's sole discretion, a written
notice of exercise, in the form prescribed by the Option Committee, together
with the Optionee's irrevocable instructions to the broker-dealer to promptly
deliver to the Company certified funds representing the Option Price, which
certified funds may be the result of the broker-dealer's sale of some or all of
the Option Shares received upon exercise or the result of a loan from the
broker-dealer to the Optionee.

      6.    Withholding Taxes. The Company may take such steps as it deems
necessary or appropriate for the withholding of any taxes which the Company is
required by any law or regulation or any governmental authority, whether
federal, state or local, domestic or foreign, to withhold in connection with the
Option including, but not limited to, the withholding of all or any portion of
any payment owed by the Company to the Optionee or the withholding of issuance
of Option Shares to be issued upon the exercise of the Option.

      7.    Securities Laws Requirements. The issuance of the Option has not
been registered under the Securities Act of 1933, as amended (the "1933 Act"),
in reliance upon an exemption from registration. In addition, no Option Shares
shall be issued unless and until, in the opinion of the Company, there has been
full compliance with any applicable registration requirements of the 1933 Act,
any applicable listing requirements of any securities exchange on which stock of
the same class has been listed, if any, and any other requirements of law or any
regulatory bodies having jurisdiction over such issuance and delivery. Optionee
hereby acknowledges, represents, warrants and agrees as follows, and, pursuant
to the terms of the Notice And Agreement Of Exercise Of Option (Exhibit A) that
shall be delivered to the Company upon each exercise of the Option, Optionee
shall acknowledge, represent, warrant and agree as follows:

            (a)   Optionee is acquiring the Option and the Option Shares for
investment purposes only and the Option and the Option Shares that Optionee is
acquiring will be held by Optionee without sale, transfer or other disposition
for an indefinite period unless the transfer of those securities is subsequently
registered under the federal securities laws or unless exemptions from
registration are available;

            (b)   Optionee's overall commitment to investments that are not
readily marketable is not disproportionate to Optionee's net worth and
Optionee's investment in the Option and the Option Shares will not cause such
overall commitments to become excessive;

            (c)   Optionee's financial condition is such that Optionee is under
no present or contemplated future need to dispose of any portion of the Option
or the Option Shares to satisfy any existing or contemplated undertaking, need
or indebtedness;

            (d)   Optionee has sufficient knowledge and experience in business
and financial matters to evaluate, and Optionee has evaluated, the merits and
risks of an investment in the Option and the Option Shares;

                                        5
<PAGE>

            (e)   The address set forth in this Agreement is Optionee's true and
correct residence, and Optionee has no present intention of becoming a resident
of any other state or jurisdiction;

            (f)   Optionee confirms that all documents, records and books
pertaining to an investment in the Option and the Option Shares have been made
available or delivered to Optionee, and Optionee has had the opportunity to
discuss the acquisition of the Option and the Option Shares with the Company.
Optionee also confirms that Optionee has obtained or been given access to all
information concerning the Company that Optionee has reasonably requested;

            (g)   Optionee has had the opportunity to ask questions of, and
receive the answers from, the Company concerning the terms of the investment in
the Option and the Option Shares and to receive additional information necessary
to verify the accuracy of the information delivered to Optionee, to the extent
that the Company possesses such information or can acquire it without
unreasonable effort or expense;

            (h)   Optionee understands that the Option has not been, and the
Option Shares issuable upon exercise of the Option will not be, registered under
the 1933 Act or any state securities laws in reliance on an exemption for
private offerings, and no federal or state agency has made any finding or
determination as to the fairness of this investment or any recommendation or
endorsement of the issuance of the Option or the Option Shares;

            (i)   The Option and the Option Shares that Optionee is acquiring
will be solely for Optionee's own account, for investment, and are not being
purchased with a view to or for the resale, distribution, subdivision or
fractionalization thereof. Optionee has no agreement or arrangement for any such
resale, distribution, subdivision or fractionalization thereof;

            (j)   Optionee acknowledges and is aware of the following:

                  (i)   The Company has a history of losses. The Option and the
Option Shares constitute a speculative investment and involve a high degree of
risk of loss by Optionee of Optionee's total investment in the Option and the
Option Shares.

                  (ii)  There are substantial restrictions on the
transferability of the Option and the Option Shares. The Option is not
transferable except by will or the laws of descent and distribution, and any
attempt to do so shall void the Option. The Option Shares cannot be transferred,
pledged, hypothecated, sold or otherwise disposed of unless they are registered
under the 1933 Act or an exemption from such registration is available and
established to the satisfaction of the Company; Optionee has no rights to
require that the Option Shares be registered; there is no right of presentment
of the Option Shares and there is no obligation by the Company to repurchase any
of the Option Shares; and, accordingly, Optionee may have to hold the Option
Shares indefinitely and it may not be possible for Optionee to liquidate
Optionee's investment in the Company;

                  (iii) Each certificate issued representing the Option Shares
shall be imprinted with a legend that sets forth a description of the
restrictions on transferability of those securities, which legend will read
substantially as follows:

            "THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
      REGISTERED OR QUALIFIED UNDER FEDERAL OR STATE SECURITIES LAWS. THESE
      SECURITIES MAY NOT BE OFFERED FOR SALE, SOLD, PLEDGED, OR OTHERWISE
      DISPOSED OF UNLESS SO REGISTERED OR QUALIFIED OR UNLESS AN EXEMPTION
      EXISTS, THE AVAILABILITY OF WHICH IS TO BE ESTABLISHED BY AN OPINION OF
      COUNSEL TO THE REGISTERED HOLDER (WHICH OPINION AND COUNSEL SHALL BOTH BE
      SATISFACTORY TO THE COMPANY)."

                                        6
<PAGE>

            (k)   Optionee shall report all sales of Option Shares to the
Company in writing on a form prescribed by the Company.

            (l)   If and so long as Optionee is subject to reporting
requirements under Section 16(a) of the 1934 Act, Optionee shall (i) be aware
that any sale by Optionee or Optionee's immediate family of shares of the
Company's common stock or any of the Option Shares within six months before or
after any transaction deemed to be a "purchase" of an equity security of the
Company may create liability for Optionee under Section 16(b) of the 1934 Act,
(ii) consult with Optionee's counsel regarding the application of Section 16(b)
of the 1934 Act prior to any exercise of the Option, and prior to any sale of
shares of the Company's common stock or the Option Shares, (iii) furnish the
Company with a copy of each Form 4 filed by Optionee, and (iv) timely file all
reports required under the federal securities laws.

            (m)   Optionee shall immediately notify the Company in writing of
any sale, transfer, assignment or other disposition (or action constituting a
disqualifying disposition within the meaning of Section 421 of the Code) of any
Option Shares, within two (2) years after the Date of Grant or within one (1)
year after the acquisition of such Option Shares, setting forth the date and
manner of disposition, the number of Option Shares disposed of and the price at
which such shares were disposed. The Company shall be entitled to withhold from
any compensation or other payments then or thereafter due to the Optionee such
amounts as may be necessary to satisfy any withholding requirements of Federal
or state law or regulation and, further, to collect from the Optionee any
additional amounts which may be required for such purpose. The Company may, in
its discretion, require Option Shares acquired by a Optionee upon exercise of
the Option to be held in an escrow arrangement for the purpose of enabling
compliance with the provisions of this section.

            The restrictions described above, or notice thereof, may be placed
on the certificates representing the Option Shares purchased pursuant to the
Option, and the Company may refuse to issue the certificates or to transfer the
shares on its books unless it is satisfied that no violation of such
restrictions will occur.

      8.    Transferability Of Option. No Option shall be transferable by the
Optionee otherwise than by will or by the laws of descent and distribution or,
in the case of a Non-Qualified Option, pursuant to a domestic relations order
(within the meaning of Rule 12a-12 promulgated under the Exchange Act), and
Options shall be exercisable during the lifetime of an Optionee only by the
Optionee or the Optionee's guardian or legal representative. Notwithstanding the
foregoing, with advance written consent of the Company, Non-Qualified Options
may be transferred to Permitted Transferees (as defined below) of the Optionee,
and for purposes of this Agreement, a Permitted Transferee of an Optionee shall
be deemed to be the Optionee. The terms of an Option shall be final, binding and
conclusive upon the beneficiaries, executors, administrators, heirs and
successors of the Optionee. A "Permitted Transferee" means (i) the spouse of the
Optionee, (ii) a trust, or family partnership, the sole beneficiary of which is
the Optionee, the spouse of or, any person related by blood or adoption to, the
Optionee; provided, that any such transfers to a Permitted Transferee do not
conflict with or constitute a violation of state or federal securities laws.

      9.    Adjustment By Stock Split, Stock Dividend, Etc. In the event that
each of the outstanding shares of Common Stock (other than shares held by
dissenting stockholders which are not changed or exchanged) of the Company
should be changed into, or exchanged for, a different number or kind of shares
of stock or other securities of the Company, or if further changes or exchanges
of any stock

                                       7
<PAGE>

or other securities into which the Common Stock shall have been changed,
or for which it shall have been exchanged, shall be made (whether by reason of
merger, consolidation, reorganization, recapitalization, stock dividends,
reclassification, split-up, combination of shares or otherwise), then the Option
Shares shall be subject to adjustment as provided in the Plan.

      10.   Change in Control; Subsequent Change in Control. In the event of a
Change in Control or a Subsequent Change in Control (each as defined in the
Plan), the Option and the Option Shares shall be governed by the provisions of
Section 20 of the Plan.

      11.   Common Stock To Be Received Upon Exercise. Optionee understands that
the Company is under no obligation to register the issuance of the Option Shares
or the resale of the Option Shares under the Securities Act of 1933, as amended
(the "Act"), and that in the absence of any such registration, the Option Shares
cannot be sold unless they are sold pursuant to an exemption from registration
under the Act. The Company is under no obligation to comply, or to assist the
Optionee in complying, with any exemption from such registration requirement,
including supplying the Optionee with any information necessary to permit
routine sales of the Option Shares under Rule 144 of the Securities and Exchange
Commission. Optionee also understands that with respect to Rule 144, routine
sales of securities made in reliance upon such Rule can be made only in limited
amounts in accordance with the terms and conditions of the Rule, and that in
cases in which the Rule is inapplicable, compliance with either Regulation A or
another disclosure exemption under the Act will be required. Thus, the Option
Shares will have to be held indefinitely in the absence of registration under
the Act or an exemption from registration.

            Furthermore, the Optionee fully understands that issuance of the
Option Shares may not be registered under the Act and that if their issuance is
not registered, they will be issued in reliance upon an exemption which is
available only if Optionee acquires such shares for investment and not with a
view to distribution. Optionee is familiar with the phrase "acquired for
investment and not with a view to distribution" as it relates to the Act and the
special meaning given to such term in various releases of the Securities And
Exchange Commission.

      12.   Privilege Of Ownership. Optionee shall not have any of the rights of
a stockholder with respect to the shares covered by the Option except to the
extent that one or more certificates for such shares shall be delivered to him
upon exercise of the Option.

      13.   Relationship To Employment Or Position. Nothing contained in this
Agreement (i) shall confer upon the Optionee any right with respect to
continuance of Optionee's employment by, or position or affiliation with, or
relationship to, the Company, or (ii) shall interfere in any way with the right
of the Company at any time to terminate the Optionee's employment by, position
or affiliation with, or relationship to, the Company.

      14.   Notices. All notices, requests, demands, directions and other
communications ("Notices") concerning this Agreement shall be in writing and
shall be mailed or delivered personally or sent by telecopier or facsimile to
the applicable party at the address of such party set forth below in this
Section 14. When mailed, each such Notice shall be sent by first class,
certified mail, return receipt requested, enclosed in a postage prepaid wrapper,
and shall be effective on the fifth business day after it has been deposited in
the mail. When delivered personally, each such Notice shall be effective when
delivered to the address for the respective party set forth in this Section 14,
provided that it is delivered on a business day and further provided that it is
delivered prior to 5:00 p.m., local time of the party to whom the notice is
being delivered, on that business day; otherwise, each such Notice shall be
effective on the first business day occurring after the Notice is delivered.
When sent by telecopier or facsimile, each such Notice shall be effective on the
day on which it is sent provided that it is sent on a business day and further
provided that it is sent prior to 5:00 p.m., local time of the party to whom the
Notice is being sent,

                                        8
<PAGE>

on that business day; otherwise, each such Notice shall be effective on the
first business day occurring after the Notice is sent. Each such Notice shall be
addressed to the party to be notified as shown below:

            (a)   if to the Company:          Bill Barrett Corporation
                                              1099 18th Street
                                              Suite 2300
                                              Denver, Colorado 80202
                                              Facsimile No. (303) 291-0420
                                              Attention:________________

            (b)   if to the Optionee:         __________________________
                                              __________________________
                                              __________________________
                                              Facsimile No.:____________

      Either party may change its respective address for purposes of this
Section 14 by giving the other party Notice of the new address in the manner set
forth above.

      15.   General Provisions. This instrument (a) contains the entire
agreement between the parties, (b) may not be amended nor may any rights
hereunder be waived except by an instrument in writing signed by the party
sought to be charged with such amendment or waiver, (c) shall be construed in
accordance with, and governed by the laws of the state in which the Company is
then incorporated, and (d) shall be binding upon and shall inure to the benefit
of the parties and their respective personal representatives and assigns, except
as above set forth. All pronouns contained herein and any variations thereof
shall be deemed to refer to the masculine, feminine or neuter, singular or
plural as the identity of the parties hereto may require.

      IN WITNESS WHEREOF, the parties have executed this Agreement on the dates
set forth below.

                                                BILL BARRETT CORPORATION

Date:____________________                       By:_________________________

                                                ____________________________
                                                Printed Name And Title

                                                OPTIONEE

Date:____________________                       ____________________________

                                                Address:____________________
                                                        ____________________
                                                        ____________________

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                                    EXHIBIT A
                          (To Bill Barrett Corporation
                             Stock Option Agreement)

                            BILL BARRETT CORPORATION

                   NOTICE AND AGREEMENT OF EXERCISE OF OPTION

      The undersigned, __________ (the "Optionee") hereby exercises Optionee's
stock Option pursuant to the Stock Option Agreement (the "Option Agreement")
dated as of __________ between Optionee and Bill Barrett Corporation (the
"Company") as to ________ shares of the $.001 par value common stock (the
"Option Shares") of the Company at a purchase price of $_____ per share. The
total exercise price for these Option Shares is $_______.

      Enclosed is the payment specified in Section 5 of the Option Agreement.
(Check the following box if the payment includes Previously Owned Shares: ___.
Check the following box if the payment will be made from a broker-dealer: ___.)

      Optionee understands that no Option Shares will be issued unless and
until, in the opinion of the Company, there has been full compliance with any
applicable registration requirements of the Securities Act of 1933, as amended
(the "1933 Act"), any applicable listing requirements of any securities exchange
on which stock of the same class is then listed, and any other requirements of
law or any regulatory bodies having jurisdiction over such issuance and
delivery. Optionee hereby acknowledges, represents, warrants and agrees to and
with the Company as follows:

            (a)   Optionee is acquiring the Option Shares for investment
purposes only and the Option Shares that Optionee is acquiring will be held by
Optionee without sale, transfer or other disposition for an indefinite period
unless the transfer of those securities is subsequently registered under the
federal securities laws or unless exemptions from registration are available;

            (b)   Optionee's overall commitment to investments that are not
readily marketable is not disproportionate to Optionee's net worth and
Optionee's investment in the Option Shares will not cause such overall
commitments to become excessive;

            (c)   Optionee's financial condition is such that Optionee is under
no present or contemplated future need to dispose of any portion of the Option
Shares to satisfy any existing or contemplated undertaking, need or
indebtedness;

            (d)   Optionee has sufficient knowledge and experience in business
and financial matters to evaluate, and Optionee has evaluated, the merits and
risks of an investment in the Option Shares;

            (e)   The address set forth in this Notice And Agreement Of Exercise
Of Option is Optionee's true and correct residence, and Optionee has no present
intention of becoming a resident of any other state or jurisdiction;

            (f)   Optionee confirms that all documents, records and books
pertaining to an investment in the Option Shares have been made available or
delivered to Optionee, and Optionee has had the opportunity to discuss the
acquisition of the Option Shares with the Company. Optionee also confirms that
Optionee has obtained or been given access to all information concerning the
Company that Optionee has requested;

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            (g)   Optionee has had the opportunity to ask questions of, and
receive the answers from, the Company concerning the terms of the investment in
the Option Shares and to receive additional information necessary to verify the
accuracy of the information delivered to Optionee, to the extent that the
Company possesses such information or can acquire it without unreasonable effort
or expense;

            (h)   Optionee understands that the issuance of the Option Shares
has not been registered under the 1933 Act or any state securities laws in
reliance on an exemption for private offerings, and no federal or state agency
has made any finding or determination as to the fairness of this investment or
any recommendation or endorsement of the issuance of the Option Shares;

            (i)   Optionee is acquiring the Option Shares solely for Optionee's
own account, for investment, and are not being purchased with a view to or for
the resale, distribution, subdivision or fractionalization thereof. Optionee has
no agreement or arrangement for any such resale, distribution, subdivision or
fractionalization thereof;

            (j)   Optionee acknowledges and is aware of the following:

                  (i)   The Company has a history of losses. The Option Shares
constitute a speculative investment and involve a high degree of risk of loss by
Optionee of Optionee's total investment in the Option Shares.

                  (ii)  There are substantial restrictions on the
transferability of the Option Shares. The Option Shares cannot be transferred,
pledged, hypothecated, sold or otherwise disposed of unless they are registered
under the 1933 Act or an exemption from such registration is available and
established to the satisfaction of the Company; investors in the Company have no
rights to require that the Option Shares be registered; there is no right of
presentment of the Option Shares and there is no obligation by the Company to
repurchase any of the Option Shares; and, accordingly, Optionee may have to hold
the Option Shares indefinitely and it may not be possible for Optionee to
liquidate Optionee's investment in the Company;

                  (iii) Each certificate issued representing the Option Shares
shall be imprinted with a legend that sets forth a description of the
restrictions on transferability of those securities, which legend will read
substantially as follows:

            "THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
      REGISTERED OR QUALIFIED UNDER FEDERAL OR STATE SECURITIES LAWS. THESE
      SECURITIES MAY NOT BE OFFERED FOR SALE, SOLD, PLEDGED, OR OTHERWISE
      DISPOSED OF UNLESS SO REGISTERED OR QUALIFIED OR UNLESS AN EXEMPTION
      EXISTS, THE AVAILABILITY OF WHICH IS TO BE ESTABLISHED BY AN OPINION OF
      COUNSEL TO THE REGISTERED HOLDER (WHICH OPINION AND COUNSEL SHALL BOTH BE
      SATISFACTORY TO THE COMPANY)."

            (k)   Optionee will not sell or dispose of Optionee's Option Shares
in violation of the 1933 Act or any other applicable federal or state securities
laws;

            (l)   Optionee agrees that the Company may, without liability for
its good faith actions, place legend restrictions upon Optionee's Option Shares
and issue "stop transfer" instructions requiring compliance with applicable
securities laws and the terms of the Option Agreement;

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            (m)   Optionee shall report all sales of Option Shares to the
Company in writing on a form prescribed by the Company;

            (n)   If and so long as Optionee is subject to reporting
requirements under Section 16(a) of the Securities Exchange Act of 1934, as
amended (the "1934 Act"), Optionee recognizes that any sale by Optionee or
Optionee immediate family of the Company's $.001 par value common stock may
create liability for Optionee under Section 16(b) of the 1934 Act ("Section
16(b)"). Therefore, Optionee has consulted with Optionee's counsel regarding the
application of Section 16(b) to this exercise of Optionee's Option; and

            (o)   Optionee will consult with Optionee's counsel regarding the
application of Section 16(b) before Optionee can make any sale of the Company's
$.001 par value common stock, including the Option Shares, and Optionee will
furnish the Company with a copy of each Form 4 filed by Optionee and will timely
file all reports that Optionee may be required to file under the federal
securities laws.

      The number of Option Shares specified above are to be issued in the name
or names set forth below in the left-hand column.

__________________________________       _______________________________________
(Print Your Name)                        Signature

__________________________________       _______________________________________
(Optionee - Print Name of Spouse         Address
if you wish joint registration)          _______________________________________
                                         City, State and Zip Code

                                     * * * *

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