<PAGE>

                                                                   EXHIBIT 10.11

                             PURE RESOURCES, INC.

                      7 1/8% SERIES A NOTE DUE 2011

                               CUSIP 74622E AA 0

                      No. 1

     Pure Resources, Inc., a Delaware corporation (the "Company"), for value
received promises to pay to Cede & Co. or registered assigns, the principal sum
of $350,000,000 on June 15, 2011 or such greater or lesser amount as is
indicated on the Schedule of Exchanges of Securities on the other side of this
Security.

     Interest Payment Dates:               June 15 and December 15

     Record Dates:                         May 31 and November 30

     Reference is hereby made to the further provisions of this Security set
forth on the reverse hereof, which further provisions shall for all purposes
have the same effect as if set forth at this place.  Capitalized terms used
herein and not otherwise defined shall have the meanings ascribed to them in the
Indenture.

     IN WITNESS WHEREOF, the Company has caused this Security to be signed
manually or by facsimile by its duly authorized officer.

Dated:         June 8, 2001                PURE RESOURCES, INC.

                                           By: /s/ Jack Hightower
                                              ------------------------
                                              Jack D. Hightower
                                              Chairman, President and
                                              Chief Executive Officer
Certificate of Authentication:

FIRST UNION NATIONAL BANK,
as Trustee, certifies that this is one of the Securities
referred to in the within-mentioned Indenture.

By: /s/ R. Douglas Milner
    ---------------------------
    Authorized Signature
<PAGE>

     UNLESS AND UNTIL IT IS EXCHANGED IN WHOLE OR IN PART FOR SECURITIES IN
DEFINITIVE FORM, THIS SECURITY MAY NOT BE TRANSFERRED EXCEPT AS A WHOLE BY THE
DEPOSITARY TO A NOMINEE OF THE DEPOSITARY OR BY A NOMINEE OF THE DEPOSITARY TO
THE DEPOSITARY OR ANOTHER NOMINEE OF THE DEPOSITARY OR BY THE DEPOSITARY OR ANY
SUCH NOMINEE TO A SUCCESSOR DEPOSITARY OR A NOMINEE OF SUCH SUCCESSOR
DEPOSITARY.  THE DEPOSITORY TRUST COMPANY SHALL ACT AS THE DEPOSITARY UNTIL A
SUCCESSOR SHALL BE APPOINTED BY THE COMPANY AND THE REGISTRAR.  UNLESS THIS
CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST
COMPANY (55 WATER STREET, NEW YORK, NEW YORK) ("DTC"), TO THE ISSUER OR ITS
AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND ANY CERTIFICATE
ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR SUCH OTHER NAME AS MAY BE
REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE TO
CEDE & CO. OR SUCH OTHER ENTITY AS MAY BE REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR
OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

     THIS SECURITY (OR ITS PREDECESSOR) WAS ORIGINALLY ISSUED IN A TRANSACTION
EXEMPT FROM REGISTRATION UNDER THE UNITED STATES SECURITIES ACT OF 1933 (The
"SECURITIES ACT"), AND THIS SECURITY MAY NOT BE OFFERED, SOLD OR OTHERWISE
TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR AN APPLICABLE EXEMPTION
THEREFROM.  EACH PURCHASER OF THIS SECURITY IS HEREBY NOTIFIED THAT THE SELLER
OF THIS SECURITY MAY BE RELYING ON THE EXEMPTION FROM THE PROVISIONS OF SECTION
5 OF THE SECURITIES ACT PROVIDED BY RULE 144A THEREUNDER.

     THE HOLDER OF THIS SECURITY AGREES FOR THE BENEFIT OF THE COMPANY THAT (A)
THIS SECURITY MAY BE OFFERED, RESOLD, PLEDGED OR OTHERWISE TRANSFERRED, ONLY (I)
INSIDE THE UNITED STATES TO A PERSON WHOM THE SELLER REASONABLY BELIEVES IS A
QUALIFIED INSTITUTIONAL BUYER (AS DEFINED IN RULE 144A UNDER THE SECURITIES ACT)
IN A TRANSACTION MEETING THE REQUIREMENTS OF RULE 144A, (II) OUTSIDE THE UNITED
STATES IN AN OFFSHORE TRANSACTION IN ACCORDANCE WITH RULE 904 UNDER THE
SECURITIES ACT, (III) PURSUANT TO AN EXEMPTION FROM REGISTRATION UNDER THE
SECURITIES ACT PROVIDED BY RULE 144 THEREUNDER (IF AVAILABLE) OR (IV) PURSUANT
TO AN EFFECTIVE REGISTRATION STATEMENT UNDER THE SECURITIES ACT, IN EACH OF
CASES (I) THROUGH (IV) IN ACCORDANCE WITH ANY APPLICABLE SECURITIES LAWS OF ANY
STATE OF THE UNITED STATES, AND (B) THE HOLDER WILL, AND EACH SUBSEQUENT HOLDER
IS REQUIRED TO, NOTIFY ANY PURCHASER OF THIS SECURITY FROM IT OF THE RESALE
RESTRICTIONS REFERRED TO IN (A) ABOVE.

                                       2
<PAGE>

                             [REVERSE OF SECURITY]

                             PURE RESOURCES, INC.

                      7 1/8% SERIES A NOTE DUE 2011

     This Security is one of a duly authorized issue of 7 1/8% Series A
Notes due June 15, 2011 (the "Securities") of Pure Resources, Inc., a Delaware
corporation (the "Company").

     1.  Interest.  The Company promises to pay interest on the principal amount
of this Security at 7 1/8% per annum from June 8, 2001 until maturity. The
Company will pay interest semiannually on June 15 and December 15 of each year
(each an "Interest Payment Date"), or if any such day is not a Business Day, on
the next succeeding Business Day. Interest on the Securities will accrue from
the most recent Interest Payment Date on which interest has been paid or, if no
interest has been paid, from June 8, 2001; provided that if there is no existing
Default in the payment of interest, and if this Security is authenticated
between a record date referred to on the face hereof and the next succeeding
Interest Payment Date, interest shall accrue from such next succeeding Interest
Payment Date; provided, further, that the first Interest Payment Date shall be
December 15, 2001. The Company also promises to pay any additional interest
required by Section 6 of the Registration Rights Agreement (as defined in
paragraph 17 below), upon the conditions, at the rates and for the periods
specified therein. Further, the Company shall pay interest on overdue principal
and premium, if any, from time to time on demand at a rate equal to the interest
rate then in effect; it shall pay interest on overdue installments of interest
(without regard to any applicable grace periods) from time to time on demand at
the same rate to the extent lawful. Interest will be computed on the basis of a
360-day year of twelve 30-day months.

     2.  Method of Payment.  The Company will pay interest on the Securities
(except defaulted interest) to the Persons who are registered Holders of
Securities at the close of business on the record date next preceding the
Interest Payment Date, even if such Securities are canceled after such record
date and on or before such Interest Payment Date.  The Holder must surrender
this Security to a Paying Agent to collect principal and premium, if any,
payments.  The Company will pay the principal of, and premium, if any, and
interest on the Securities in money of the United States of America that at the
time of payment is legal tender for payment of public and private debts.
Payments to Holders of principal, premium, if any, and interest shall be paid by
wire transfer of immediately available funds to the accounts specified by the
Holders thereof or, if no such account is specified (or if a registered Holder
holds Securities in an aggregate principal amount of less than $1,000,000), by
mailing a check to each such Holder's registered address.

     3.  Ranking and Guarantees.  The Securities are senior unsecured
obligations of the Company.  The Company's obligations to pay principal,
premium, if any, and interest with respect to the Securities are unconditionally
guaranteed on a joint and several basis (the "Guarantees") by the guarantors
(the "Guarantors"), that are from time to time parties to the Indenture.  Each
of the Guarantees is an unsecured obligation of the Guarantor providing such
Guarantee.  Certain limitations to the obligations of the Guarantors, and
certain provisions for the release of the Guarantors, are set forth in further
detail in the Indenture.  References herein to the

                                       3
<PAGE>

Indenture or the Securities shall be deemed also to refer to the Guarantees set
forth in the Indenture except where the context otherwise requires.

     4.  Optional Redemption.  The Company may redeem all or part of the
Securities at any time at its option at a redemption price equal to the greater
of (1) the principal amount of the Securities being redeemed or (2) the Make-
Whole Amount for the Securities being redeemed plus, in each case, accrued and
unpaid interest to the redemption date (the "Redemption Price").

     As used in this Security:

     "Make Whole Amount" means the sum, as determined by a Quotation Agent, of
the present values of the principal amount of the Securities to be redeemed,
together with scheduled payments of interest (exclusive of interest to the
redemption date) from the Redemption Date to the maturity date of the Securities
being redeemed, in each case discounted to the Redemption Date on a semi-annual
basis, assuming a 360-day year consisting of twelve 30-day months, at the
Adjusted Treasury Rate.

     "Adjusted Treasury Rate" means, with respect to any Redemption Date, (i)
the yield, under the heading which represents the average for the immediately
preceding week, appearing in the most recently published statistical release
designated "H.15 (519)" or any successor publication which is published weekly
by the Board of Governors of the Federal Reserve System and which establishes
yields on actively traded United States Treasury securities adjusted to constant
maturity under the caption "Treasury Constant Maturities," for the maturity
corresponding to the Comparable Treasury Issue (if no maturity is within three
months before or after the remaining term of the Securities being redeemed,
yields for the two published maturities most closely corresponding to the
Comparable Treasury Issue shall be determined and the Adjusted Treasury Rate
shall be interpolated or extrapolated from such yields on a straight line basis,
rounding to the nearest month) or (ii) if such release (or any successor
release) is not published during the week preceding the calculation date or does
not contain such yields, the rate per year equal to the semi-annual equivalent
yield to maturity of the Comparable Treasury Issue (expressed as a percentage of
its principal amount) equal to the Comparable Treasury Price for such Redemption
Date, in each case calculated on the third business day preceding the redemption
date, plus 0.25%.

     "Comparable Treasury Issue" means the United States Treasury security
selected by the Quotation Agent as having a maturity comparable to the remaining
term from the Redemption Date to the maturity date of the Securities that would
be utilized, at the time of selection and in accordance with customary financial
practice, in pricing new issues of corporate debt securities of comparable
maturity to the remaining term of the notes.

     "Comparable Treasury Price" means, with respect to any Redemption Date, if
clause (ii) of the Adjusted Treasury Rate is applicable, the average of three,
or such lesser number as is obtained by the Trustee, Reference Treasury Dealer
Quotations for such Redemption Date.

     "Quotation Agent" means the Reference Treasury Dealer selected by the
Trustee after consultation with the Company.

                                       4
<PAGE>

     "Reference Treasury Dealer" means any of Credit Suisse First Boston
Corporation and its successors and assigns and two other nationally recognized
investment banking firms selected by the Company that are primary U.S.
Government securities dealers.

     "Reference Treasury Dealer Quotations" means, with respect to each
Reference Treasury Dealer and any Redemption Date, the average, as determined by
the Trustee, of the bid and asked prices for the Comparable Treasury Issue,
expressed in each case as a percentage of its principal amount, quoted in
writing to the Trustee by such Reference Treasury Dealer at 5:00 p.m., New York
City time, on the third Business Day preceding such redemption date.

     5.  Paying Agent and Registrar.  Initially, First Union National Bank (the
"Trustee"), the Trustee under the Indenture, will act as Paying Agent and
Registrar.  The Company may change any Paying Agent, Registrar, co-registrar or
additional paying agent without notice to any Holder.  The Company may act in
any such capacity.

     6.  Indenture.  The Company issued the Securities under an Indenture dated
as of June 8, 2001 (the "Indenture") among the Company, the Guarantors and the
Trustee.  The terms of the Securities include those stated in the Indenture and
those made part of the Indenture by reference to the Trust Indenture Act of
1939, as amended (15 U.S. Code Sections 77aaa-77bbbb), as in effect on the date
of execution of the Indenture.  The Securities are subject to all such terms,
and Holders are referred to the Indenture and such Act for a statement of such
terms.  The Securities are unsecured general obligations of the Company limited
to $350,000,000 in aggregate principal amount in the case of the Notes issued on
the Issue Date (as defined in the Indenture).

     7.  Denominations, Transfer, Exchange.  The Securities are in registered
form without coupons in denominations of $1,000 and integral multiples of
$1,000.  The transfer of Securities may be registered and Securities may be
exchanged as provided in the Indenture.  The Registrar and the Trustee may
require a Holder, among other things, to furnish appropriate endorsements and
transfer documents and to pay any taxes and fees required by law or permitted by
the Indenture.  The Registrar need not exchange or register the transfer of any
Securities during the period between a record date and the corresponding
Interest Payment Date.

     8.  Persons Deemed Owners.  The registered Holder of a Security shall be
treated as its owner for all purposes.

     9.  Amendments and Waivers. Except as otherwise provided in Section 8.02 of
the Indenture, the Company, the Guarantors and the Trustee may amend or
supplement the Indenture and the Securities with the written consent (including
consents obtained in connection with a tender offer or exchange offer for
Securities or a solicitation of consents in respect of Securities, provided that
in each case such offer or solicitation is made to all Holders of then
outstanding Securities on equal terms) of the Holders of at least a majority in
principal amount of the then outstanding Securities. Subject to Sections 5.07
and 8.02 of the Indenture, the Holders of a majority in principal amount of the
then outstanding Securities by notice to the Trustee may waive an existing
Default or Event of Default and its consequences (including waivers obtained in
connection with a tender offer or exchange offer for Securities or a
solicitation of consents in respect of Securities, provided that in each case
such offer or solicitation is made to all Holders of then outstanding Securities
on equal terms), except (1) a continuing Default or Event of

                                       5
<PAGE>

Default in the payment of the principal of, or premium, if any, or interest on
any Security or (2) a continuing Default in respect of a provision that under
Section 8.02 of the Indenture cannot be amended with the consent of each Holder
affected. Without the consent of any Holder, the Company, the Guarantors and the
Trustee may amend or supplement the Indenture or the Securities or waive any
provision of the Indenture or the Securities: to cure any ambiguity, omission,
defect or inconsistency; to provide for uncertificated Securities in addition to
or in place of certificated Securities; to comply with Sections 4.01 and 4.02 of
the Indenture; to reflect the release of any Guarantor from its Guarantee, or
the addition of any Subsidiary of the Company as a Guarantor, in the manner
provided by Section 9.06 of the Indenture; to comply with any requirement in
order to effect or maintain the qualification of this Indenture under the Trust
Indenture Act of 1939, as amended (the "TIA"); to add additional guarantees of
the Securities; to comply with any requirements of the SEC in connection with
qualifying the Indenture under the TIA; to add to the covenants of the Company
or any Guarantor for the benefit of the Holders or to surrender any right
conferred upon the Company or any Guarantor in the Indenture; or to make any
change that does not adversely affect the rights of any Holder in any material
respect.

     10.  Defaults and Remedies. If an Event of Default (other than an Event of
Default specified in clause (6) or (7) of Section 5.01 of the Indenture with
respect to the Company or any Material Subsidiary) occurs and is continuing, the
Trustee by notice to the Company, or the Holders of at least 25% in principal
amount of the then outstanding Securities by notice to the Company and the
Trustee, may declare the principal of and premium, if any, and accrued and
unpaid interest on all then outstanding Securities to be due and payable
immediately. If an Event of Default specified in clause (6) or (7) of Section
5.01 of the Indenture with respect to the Company or any Material Subsidiary
occurs, such amounts shall ipso facto become and be immediately due and payable
without any declaration, notice or other act on the part of the Trustee or any
Holder.  The Holders of a majority in principal amount of the then outstanding
Securities by written notice to the Trustee may rescind an acceleration and its
consequences (other than nonpayment of principal of, or premium, if any, or
interest on the Securities) if the rescission would not conflict with any
judgment or decree and if all existing Events of Default have been cured or
waived, except nonpayment of principal, or premium, if any, or interest that has
become due solely because of the acceleration.  Holders may not enforce the
Indenture or the Securities except as provided in the Indenture. The Trustee may
refuse to perform any duty or exercise any right or power unless it receives
indemnity reasonably satisfactory to it against any loss, liability or expense.
Except in the case of a Default or Event of Default in payment of principal of,
or premium, if any, or interest on any Security, the Trustee may withhold notice
of the Default or Event of Default if and so long as a committee of its Trust
Officers in good faith determines that withholding the notice is in the
interests of Holders.  The Company must furnish an annual compliance certificate
to the Trustee, as more fully described in Section 3.04 of the Indenture.

     11.  Discharge Prior to Maturity. As more fully described in, and subject
to, Article VII of the Indenture, the Indenture shall be discharged and canceled
upon the payment of all of the Securities and shall be discharged except for
certain obligations upon the irrevocable deposit with the Trustee of funds or
U.S. Government Obligations sufficient for such payment.

                                       6
<PAGE>

     12.  Trustee Dealings with Company and Guarantors.  The Trustee in its
individual or any other capacity may become the owner or pledgee of Securities
and may otherwise deal with the Company, the Guarantors or any of their
Affiliates with the same rights it would have if it were not Trustee.

     13.  No Recourse Against Others. A director, officer, employee or
stockholder, as such, of the Company or any Guarantor shall not have any
liability for any obligations of the Company or such Guarantor under the
Securities or the Indenture or for any claim based on, in respect of or by
reason of such obligations or their creation. Each Holder by accepting a
Security waives and releases all such liability. The waiver and release are part
of the consideration for the issuance of the Securities.

     14.  Authentication.  This Security shall not be valid until authenticated
by the manual signature of the Trustee or an authenticating agent.

     15.  CUSIP Numbers.  Pursuant to a recommendation promulgated by the
Committee on Uniform Security Identification Procedures, the Company has caused
CUSIP numbers to be printed on the Securities as a convenience to the Holders of
the Securities.  No representation is made as to  the accuracy of such numbers
as printed on the Securities and reliance may be placed only on the other
identification numbers printed thereon.

     16.  Abbreviations.  Customary abbreviations may be used in the name of a
Holder or an assignee, such as:  TEN COM (= tenants in common), TEN ENT (=
tenants by the entireties), JT TEN (= joint tenants with right of survivorship
and not as tenants in common), CUST (= Custodian), and U/G/M/A (= Uniform Gifts
to Minors Act).

     17.  Additional Rights of Holders of Transfer Restricted Securities.  In
addition to the rights provided to Holders of Securities under the Indenture,
Holders of Transfer Restricted Securities shall have all the rights set forth in
the Registration Rights Agreement relating to such Securities (the "Registration
Rights Agreement"), among the Company, the Guarantors named therein and the
Initial Purchasers.

     THE COMPANY WILL FURNISH TO ANY HOLDER UPON WRITTEN REQUEST AND WITHOUT
CHARGE A COPY OF THE INDENTURE.  REQUEST MAY BE MADE TO:

     PURE RESOURCES, INC.
     500 WEST ILLINOIS
     MIDLAND, TEXAS 79701
     ATTENTION:  CORPORATE SECRETARY

                                       7
<PAGE>

                             NOTATION ON SECURITY

                            RELATING TO GUARANTEES

     Each Guarantor (which term includes any successor Person under the
Indenture), has fully, unconditionally and absolutely guaranteed, to the extent
set forth in the Indenture and subject to the provisions in the Indenture, the
due and punctual payment of the principal of, and premium, if any, and interest
on the Securities and all other amounts due and payable under the Indenture and
the Securities by the Company.

     The obligations of the Guarantors to the Holders of Securities and to the
Trustee pursuant to the Guarantees and the Indenture are expressly set forth in
Article 9 of the Indenture and reference is hereby made to the Indenture for the
precise terms of the Guarantees.

                                        TITAN EXPLORATION, INC.
                                        PURE RESOURCES I, INC.
                                        HALLWOOD PETROLEUM, INC.
                                        PURE GP, LLC
                                        HEP PURE ACQUISITION, LLC


                                        By:   /s/ Jack Hightower
                                            ------------------------------
                                                Jack D. Hightower
                                                President

                                        PURE RESOURCES HOLDINGS, INC.
                                        PURE RESOURCES HOLDINGS, LLC


                                        By:   /s/ Richard Klumpp
                                            ------------------------------
                                        Name:   Richard Klumpp
                                        Title:  Vice President

                                       8
<PAGE>

                                        PURE RESOURCES, L.P.
                                        PK I, L.P.
                                        PK II, L.P.
                                        PK III, L.P.
                                        PK IV, L.P.


                                        By:  PURE RESOURCES I, INC.,
                                             its general partner

                                        By:  /s/ Jack Hightower
                                           -------------------------------
                                             Jack D. Hightower
                                             President

                                        HEP PURE, LP


                                        By:  PURE GP, LLC, its general partner


                                        By:  /s/ Jack Hightower
                                           --------------------------
                                             Jack D. Hightower
                                             President

                                        HALLWOOD ENERGY PARTNERS, L.P.


                                        By:  HEP PURE ACQUISITION, LLC,
                                             its general partner


                                        By:  /s/ Jack Hightower
                                           ------------------------------
                                             Jack D. Hightower
                                             President

                                       9
<PAGE>

                                        LA PLATA ASSOCIATES, LLC
                                        HALLWOOD LA PLATA, LLC


                                        By:  HALLWOOD PETROLEUM, INC.,
                                             its manager

                                        By: /s/ Jack Hightower
                                            ------------------------------
                                            Jack D. Hightower
                                            President

                                       10
<PAGE>

                                ASSIGNMENT FORM

     To assign this Security, fill in the form below: (I) or (we) assign and
transfer this Security to _________________________________________

______________________________________________________________________________
(Insert assignee's social security or tax I.D. number)

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________
(Print or type assignee's name, address and zip code)

and irrevocably appoint _______________________________________________________
as agent to transfer this Security on the books of the Company.  The agent may
substitute another to act for him.

______________________________________________________________________________


Date: ___________________________


Your Signature: ________________________________________________________________
                (Sign exactly as your name appears on the face of this Security)

Signature Guarantee:____________________________________________________________
                    (Participant in a Recognized Signature Guaranty Medallion
                     Program)

                                       11
<PAGE>

                     SCHEDULE OF EXCHANGES OF SECURITIES*

     The following exchanges, redemptions or repurchases of a part of this
Global Security have been made:

<TABLE>
<CAPTION>
<S>                     <C>                       <C>                        <C>                      <C>
Date of Transaction     Amount of decrease in     Amount of increase in      Principal Amount of      Signature of authorized
                         Principal Amount of       Principal Amount of         Global Security          Officer, Trustee or
                           Global Security           Global Security        following decrease (or      Security Custodian
                                                                            increase)



</TABLE>
     ______________________

     *  This should be included only if the Security is issued in global form.

                                       12