Document:

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                                                                     EXHIBIT 4.5

                          DEBENTURE PURCHASE AGREEMENT

                                     BETWEEN

                    SOUTH TEXAS DRILLING & EXPLORATION, INC.

                                       AND

                          WEDGE ENERGY SERVICES, L.L.C.

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                                TABLE OF CONTENTS

<TABLE>
<CAPTION>

                                                                                             Page
<S>           <C>                                                                            <C>
ARTICLE I     PURCHASE OF CONVERTIBLE DEBENTURE................................................1
   1.1    Purchase by WEDGE....................................................................1
   1.2    Letter of Credit.....................................................................1
   1.3    Definitions..........................................................................1

ARTICLE II    ADDITIONAL AGREEMENTS............................................................2
   2.1    Grant of Preemptive Rights...........................................................2
   2.2    Material Adverse Effect..............................................................3
   2.3    Company Right of First Offer.........................................................3
   2.4    The Closing..........................................................................4

ARTICLE III   REPRESENTATIONS AND WARRANTIES BY THE COMPANY AND WEDGE..........................4
   3.1    The Company hereby acknowledges and expressly reconfirms all warranties and
          representations contained in Article IV of the Debenture.............................4
   3.2    WEDGE represents and warrants that:..................................................4

ARTICLE IV    REGISTRATION RIGHTS..............................................................5
   4.1    Registration Rights..................................................................5

ARTICLE V     AFFIRMATIVE COVENANTS............................................................5
   5.1    Conditions to Obligations of WEDGE...................................................5
   5.2    Deliveries...........................................................................6
   5.3    Acquisition and Financing Transactions...............................................6

ARTICLE VI    NATURE AND SURVIVAL OF REPRESENTATIONS AND WARRANTIES............................6
   6.1    Nature of Statements.................................................................6
   6.2    Survival of Representations and Warranties...........................................6
   6.3    Indemnity by the Company.............................................................6
   6.4    Indemnity by WEDGE...................................................................6
   6.5    Limitation of Liability..............................................................7

ARTICLE VII   AFFIRMATIVE COVENANTS............................................................7
   7.1    Approval of Shareholders.............................................................7
   7.2    Board Seats..........................................................................7
   7.3    Inspection of Property, Books and Records............................................7
</TABLE>

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<TABLE>

<S>        <C>                                                                               <C>
ARTICLE VIII  NEGATIVE COVENANTS...............................................................8
   8.1    Reorganization, Stock Dividends, Reclassification,
          Subdivision or Stock Issuances.......................................................8

ARTICLE IX    DEFAULT..........................................................................8
   9.1    Event of Default.....................................................................8
   9.2    Default Remedies.....................................................................8

ARTICLE X     MISCELLANEOUS....................................................................8
   10.1   Conflict of Agreement................................................................8
   10.2   Counterparts.........................................................................8
   10.3   Amendments...........................................................................8
   10.4   Governing Law........................................................................8
   10.5   Severability.........................................................................8
   10.6   Injunctive Relief....................................................................9
   10.7   Term.................................................................................9
   10.8   Notices..............................................................................9
   10.9   Headings............................................................................10
   10.10  Arbitration.........................................................................10
</TABLE>

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<PAGE>   4

                          DEBENTURE PURCHASE AGREEMENT

         THIS DEBENTURE PURCHASE AGREEMENT (the "Agreement"), is entered into
this 30th day of March, 2001, among SOUTH TEXAS DRILLING & EXPLORATION, INC.
(the "Company"), a corporation incorporated under the laws of the State of
Texas, whose principal place of business is at 9310 Broadway, Building I, San
Antonio, Texas 78217 and WEDGE ENERGY SERVICES, L.L.C., a limited liability
company organized under the laws of the State of Delaware, whose principal place
of business is at 1415 Louisiana, Suite 3000, Houston, Texas 77002 ("WEDGE").

                                    RECITALS

         WHEREAS, on the date hereof, the Company will enter into the Debenture
Agreement of even date herewith in the form attached hereto as Exhibit A ( the
"Debenture") pursuant to the payment of $9,000,000 cash by WEDGE to the Company;

         WHEREAS, WEDGE has previously acquired 4,841,007 shares of common stock
of the Company; and

         WHEREAS, in connection with the issuance of the Debenture, the parties
have agreed, among other things, that WEDGE will have the right to convert the
Debenture as provided therein and certain preemptive rights to participate in
all future debt or equity offerings in the Company;

         NOW, THEREFORE, the parties agree as follows:

                                    ARTICLE I
                        PURCHASE OF CONVERTIBLE DEBENTURE

         1.1 Purchase by WEDGE. The Company hereby agrees to sell, and WEDGE
agrees to purchase, the 4.86% Convertible Subordinated Debenture, Series A, due
March 29, 2002, for the aggregate amount of $9,000,000 payable upon WEDGE
tendering to the Company, by wire transfer, the cash sum of $9,000,000. Attached
hereto as Exhibit "A" is the form of Debenture which sets forth all rights of
WEDGE, as a holder of such Debenture, and all duties and obligations of the
Company, as the issuer of same. The Company and WEDGE hereby expressly
incorporate all terms and conditions of the Debenture as if it were set out in
their entirety in this Section 1. 1.

         1.2 Letter of Credit. As further consideration for tendering the
Debenture, WEDGE agrees to cause to be issued that certain letter of credit in
the amount of $6,000,000 on behalf of the Company as a credit enhancement on
behalf of the Frost National Bank.

         1.3 Definitions. All capitalized terms, not otherwise defined herein,
shall have the meaning ascribed to them in the Debenture.

<PAGE>   5

                                   ARTICLE II
                              ADDITIONAL AGREEMENTS

         2.1 Grant of Preemptive Rights. The Company hereby grants to WEDGE the
preemptive right to acquire a percentage of any additional capital stock of any
class or series, or debt convertible into capital stock, ("Preemptive Right
Securities") the Company may issue equal to the percentage of the Company's
outstanding common stock (assuming the conversion of all outstanding convertible
preferred stock or debt) held by WEDGE immediately preceding any such issuance
of common stock. This preemptive right shall terminate in the event WEDGE holds
less than 10% of the outstanding common stock of the Company. This preemptive
right shall also terminate four (4) years from the date hereof.

                  (a) Issuances for Cash. WEDGE'S preemptive rights occasioned
         by the issuance by the Company of Preemptive Right Securities in
         exchange for cash shall include the right to purchase such Preemptive
         Right Securities at the same purchase price and other terms as the
         other purchasers of such Preemptive Right Securities, except in the
         case of an underwritten public offering of securities registered under
         the Securities Act of 1933, as amended, in which case WEDGE's purchase
         price shall be the price at which the Preemptive Right Securities are
         offered to the public. Upon receipt of written notice from the Company
         of its preemptive right to purchase Preemptive Right Securities offered
         for cash WEDGE shall provide notice of its intent to exercise or not to
         exercise its rights to the Company in writing, within 10 days of its
         receipt of such notice from the Company and failure to provide notice
         within such 10 days shall be deemed to be a waiver of such rights. Any
         issuance of Preemptive Right Securities for cash not completed within
         60 days of the date notice is provided by the Company to WEDGE as
         provided in the previous sentence hereof shall be deemed to be a new
         issuance of Preemptive Rights Securities to which this subparagraph
         applies. This preemptive right shall not apply to the issuance of
         capital stock issued pursuant to warrants, options, or other rights to
         acquire capital stock currently outstanding or which may be granted by
         the Company to any employee, consultant or director as incentive for
         any such employee, consultant or director to become or remain
         associated with the Company or to provide services to the Company which
         has been or will be issued under the Company's 1995 Stock Option Plan
         or its 1999 Stock Option Plan for shares of common stock which may be
         issued under such 1999 Option Plan up to 1,500,000, or any other option
         plan, option, warrant or other rights to acquire capital stock which is
         or has been approved by the shareholders of the Company.

                  (b) Issuances for Other than Cash. WEDGE's preemptive rights
         occasioned by the issuance by the Company of Preemptive Right
         Securities in exchange for assets other than cash shall include the
         right to purchase such Preemptive Right Securities at a cash price per
         share (or other security unit) equal to the value per share (or other
         security unit) received by the Company as consideration for the
         issuance of the Preemptive Right Securities giving rise to WEDGE's
         rights as reflected on the regularly prepared financial statements of
         the Company. Provided, however, that if such Preemptive Right
         Securities are regularly traded, or are convertible into securities
         which are regularly traded, then the purchase price per share (or other
         security unit) shall be no less than the average of the average daily
         trading price of actual trades of such Preemptive Right Securities (or

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<PAGE>   6

         regularly trading securities into which such Preemptive Right
         Securities may be converted) for the 30 trading days preceding the date
         on which written notice of WEDGE's preemptive rights is delivered to
         WEDGE according to the following sentence. Upon receipt of written
         notice from the Company of its preemptive right to purchase Preemptive
         Right Securities offered for other than cash WEDGE shall provide notice
         of its intent to exercise or not to exercise its rights to the Company
         in writing within 10 days of its receipt of such notice from the
         Company and failure to provide notice within such 10 days shall be
         deemed to be a waiver of such rights. Any issuance of Preemptive Right
         Securities for other than cash not completed within 120 days of the
         date notice is provided by the Company to WEDGE as provided in the
         previous sentence hereof shall be deemed to be a new issuance of
         Preemptive Rights Securities to which this subparagraph applies. All
         else notwithstanding, WEDGE shall have no preemptive rights incident to
         a merger, plan of exchange or other combination involving the Company
         requiring the approval of the shareholders of the Company and regarding
         which such approval is obtained.

         2.2 Material Adverse Effect. For purposes of this Agreement, the term
"Material Adverse Effect" shall mean an event, circumstance, loss, development
or effect (individually or in the aggregate) when considered in light of the
total operations of the Company, would prohibit the Company from engaging in any
material aspect of its business or result in a material adverse change in the
business, operations, properties, prospects or assets of the Company, or if
measured monetarily, would exceed $100,000.

         2.3 Company Right of First Offer. WEDGE agrees that it will not sell,
transfer or otherwise make a disposition of any common stock of the Company
other than into the public trading market under Rule 144 or incident to any
registration right granted by the Company to WEDGE without first offering the
stock WEDGE desires to transfer (the "Disposition Stock") to the Company in
writing (the "Disposition Notice") at the price and other terms (the
"Disposition Terms") under which WEDGE desires to transfer the Disposition
Stock. Upon receipt of any Disposition Notice the Company shall have the
assignable right to acquire the Disposition Stock from WEDGE under the
Disposition Terms at any time within 45 days following the Company's receipt of
the Disposition Notice (the "Company Disposition Period") so long as the Company
shall provide WEDGE with an affirmative written acknowledgment of its intent to
acquire the Disposition Stock within 10 days from the Disposition Notice. If the
Company or its assignee does not tender to WEDGE everything required to purchase
the Disposition Stock under the Disposition Terms within the Company Disposition
Period, WEDGE may complete a disposition of the Disposition Stock to any third
party in a matter conforming to applicable securities laws during the 45 day
period following the end of the Company Disposition Period, but not thereafter
unless the procedures of this paragraph are again complied with. The
requirements of this paragraph shall not apply to the pledge or gift of the
Company's common stock by WEDGE or a disposition to an affiliate of WEDGE or to
a disposition approved by the Board of Directors of the Company. Provided,
however, any affiliate transferee or donee of the common stock shall first be
required to agree in writing to be bound by the terms of this paragraph. WEDGE
agrees that certificates representing the common stock of the Company subject to
this paragraph may be legended in order to provide notice of the application of
this paragraph to third parties.

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         2.4 The Closing. The closing of the transactions provided for in
Section 1 of this Agreement (herein called the "Closing") shall take place at
the offices of Matthews & Branscomb, P.C., 112 East Pecan St., Suite 1100, San
Antonio, Texas, on or before March 30,2001 (the "Closing Date").

                                   ARTICLE III
             REPRESENTATIONS AND WARRANTIES BY THE COMPANY AND WEDGE

         3.1 The Company hereby acknowledges and expressly reconfirms all
warranties and representations contained in Article IV of the Debenture.

         3.2 WEDGE represents and warrants that:

                  (a) Experience. WEDGE is an "accredited investor" within the
         meaning of Regulation D promulgated by the Securities and Exchange
         Commission under the Act, and (by virtue of its experience in
         evaluating and investing in private placement transactions of
         securities in companies similar to the Company) it is capable of
         evaluating the merits and risks of its investment in the Company. WEDGE
         acknowledges that it had the opportunity to ask questions of the
         officers of the Company. In reaching the conclusion that it desires to
         acquire the Debenture and the Stock upon conversion, WEDGE has
         evaluated its financial resources and investment position and the risks
         associated with this investment and acknowledges that it is able to
         bear the economic risks of this investment.

                  (b) Restricted Securities. As of the date hereof, WEDGE
         represents, warrants and agrees that it is acquiring the Debenture, and
         upon conversion the Stock, solely for its own account, for investment,
         and not with a view to the distribution or resale thereof. WEDGE
         further represents that its present financial condition is such that it
         is not under any present necessity or constraint to dispose of such
         Debenture or the Stock into which it is convertible to satisfy any
         existing or contemplated debt or undertaking and that the investment is
         suitable for WEDGE upon the basis of WEDGE's other security holdings,
         financial situation and needs. WEDGE acknowledges and understands that
         it must bear the economic risk of this investment for an indefinite
         period of time because the Stock must be held indefinitely unless
         subsequently registered under the Act and applicable state and other
         securities laws or unless an exemption from such registration is
         available.

                  (c) Unregistered Stock. WEDGE is aware that the Debenture and
         the Stock have not been registered under the Act, and that,
         accordingly, the Stock must be held unless it is subsequently
         registered under said Act or unless, in the opinion of counsel
         reasonably satisfactory to the Company, a sale or transfer may be made
         without registration thereunder. WEDGE agrees that any certificates
         evidencing the Stock must bear a standard legend restricting the
         transfer thereof consistent with the foregoing and that a notice may be
         made in the records of the Company or its transfer agent restricting
         the transfer of the Stock in a manner consistent with the foregoing.

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                                   ARTICLE IV
                               REGISTRATION RIGHTS

         4.1 Registration Rights. Upon conversion of the Debenture into
common stock as provided in Article VII of the Debenture, the Company and the
Holder shall agree to abide by and honor the terms of that certain Registration
Rights Agreement executed simultaneously herewith and attached hereto as Exhibit
"B".

                                    ARTICLE V
                              AFFIRMATIVE COVENANTS

         5.1 Conditions to Obligations of WEDGE. The obligations of WEDGE under
this Agreement shall, except as may be waived in writing by WEDGE, be subject to
the following conditions:

                  (a) Company's Representations and Warranties True at Closing.
         WEDGE shall not have discovered any material error, misstatement or
         omission in the representations and warranties made by the Company in
         Article III above which such error, misstatement or omission remains
         uncured; and the Company shall have performed and complied with in all
         material respects all agreements and conditions required by this
         Agreement to be performed or complied with by it at or prior to the
         Closing.

                  (b) Opinion of Counsel for the Company. Attached hereto as
         Exhibit "C" is a true and correct copy of the Opinion of Counsel to be
         delivered by Matthews and Branscomb, P.C., attorneys for the Company,
         as approved by counsel for WEDGE.

                  (c) Absence of Restraint. No order to restrain, enjoin or
         otherwise prevent the consummation of this Agreement or the
         transactions contemplated hereby shall have been entered; and, on the
         Closing Date, there shall not be any pending or threatened litigation
         in any court, or any proceeding by or before any governmental
         commission, board or agency, with a view to seeking to restrain or
         prohibit consummation of this Agreement or the .transactions
         contemplated hereby or in which divestiture, rescission or significant
         damages are sought in connection with this Agreement or the
         transactions contemplated hereby, and no investigation by any
         governmental agency shall be pending or threatened which might result
         in any such litigation or other proceeding.

                  (d) Company Officers' Certificate. WEDGE shall have received a
         certificate, dated the Closing Date, of the president, executive vice
         president and financial officers of the Company (the "Officers'
         Certificate") to the effect that the representations and warranties
         relating to the Company or its business, financial condition,
         properties or assets are true in all material respects at and as of the
         Closing Date or, to the extent such representations and warranties are
         made at and as of a specific date, such representations and warranties
         were true in all material respects, and as of such date.

                  (e) No Material Adverse Effect. No Material Adverse Effect in
         the results of operations, financial condition or business of the
         Company taken as a whole shall have occurred, and the Company shall not
         have suffered any material loss or damages to any

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         of its properties or assets which change, loss or damage materially
         affects or impairs the ability of the Company to conduct its respective
         businesses.

                  (f) Consents and Other Approvals. WEDGE shall have received
         all consents and other approvals which are necessary or required, if
         any, to consummate this Agreement.

         5.2 Deliveries. WEDGE shall have received (a) Debenture certificates,
(b) a fully executed Registration Rights Agreement, and (c) any other documents
which WEDGE may reasonably request to consummate this Agreement and the
transactions contemplated hereby.

         5.3 Acquisition and Financing Transactions. The Company shall have
closed the acquisition of Mustang Drilling and a debt financing package with The
Frost National Bank in an amount of not less than twelve million dollars
($12,000,000).

                                   ARTICLE VI
              NATURE AND SURVIVAL OF REPRESENTATIONS AND WARRANTIES

         6.1 Nature of Statements. All statements contained in any Exhibit or
schedule hereto, including the Debenture, or in any certificate or other
instrument delivered by or on behalf of the Company pursuant to this Agreement
shall be deemed representations and warranties by the Company.

         6.2 Survival of Representations and Warranties. All covenants,
agreements, representations and warranties made hereunder or pursuant hereto or
in connection with the transactions contemplated hereby shall survive the
Closing. Generally, all covenants, representations and warranties shall remain
effective for a period of 24 months from the date of closing. The
representations and warranties of the Company with respect to litigation, ERISA
and environmental matters shall remain effective for a period of 48 months from
the Closing Date. The representations and warranties of the Company with respect
to taxes and title to Stock shall survive for the applicable limitations period
established by law. Notwithstanding the foregoing, any bona fide claim which
shall have been asserted during any such survival period and the obligation to
indemnify for such claim shall continue in effect until such time as such claim
shall have been resolve or settled.

         6.3 Indemnity by the Company. The Company shall indemnify and hold
harmless WEDGE and the officers, directors, managers, agents, affiliates and
representatives of WEDGE or any of them (the "WEDGE Indemnitees") from and
against, and shall reimburse the WEDGE Indemnitees from any loss, liability,
damage or expense, including reasonable attorneys' fees and costs of
investigation incurred as a result thereof, that the WEDGE Indemnitees shall
incur or suffer (collectively, the "WEDGE Recoverable Losses"), arising out of
or resulting from (a) any misrepresentation by the Company, or (b) breach by the
Company of any (i) representation or warranty contained in Article III hereof,
(ii) agreement or covenant under or pursuant to this Agreement, including the
Registration Rights Agreement, or (iii) document, certificate, schedule or
instrument delivered by or on behalf of the Company pursuant hereto.

         6.4 Indemnity by WEDGE. WEDGE shall indemnify and hold harmless the
Company and the officers, directors, agents, affiliates and representatives of
the Company or any of them

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(the "Company Indemnitees") from and against, and shall reimburse the Company
Indemnitees for any loss, liability, damage or expense, including reasonable
attorneys' fees and cost of investigation incurred as a result thereof, that the
Company Indemnitees shall incur or suffer (collectively, the "Company's
Recoverable Losses") resulting from (a) any misrepresentation by WEDGE, or (b)
breach by WEDGE of any (i) representation or warranty contained in Article III
hereof, (ii) agreement or covenant under or pursuant to this Agreement, or (iii)
document, certificate, schedule or instrument delivered by or on behalf of WEDGE
in connection herewith.

         6.5 Limitation of Liability. Notwithstanding any liability which the
Company or WEDGE may incur in Sections 7.3 and 7.4, respectively, above, the
Company shall not be obligated for a WEDGE's Recoverable Loss, and WEDGE shall
not be obligated for a Company's Recoverable Loss, unless and until such loss,
individually, or in the aggregate, shall have exceed $100,000, in which case
such liability shall be for all amounts in excess thereof.

                                   ARTICLE VII
                              AFFIRMATIVE COVENANTS

         7.1 Approval of Shareholders. The Company agrees to immediately, but
not later than seven (7) days from the execution hereof, commence preparation
and procedures for a special meeting of its Shareholders be held no later than
June 30, 2001, for the purpose of causing the Shareholders of the Company to
approve the conversion of the Debenture into the Stock.

         7.2 Board Seats. So long as WEDGE shall own at least 10% of the capital
stock of the Company, the Board of Directors agrees to support and cause to be
placed on the ballot at each election of Directors not less than one name which
shall be provided by WEDGE and which shall be a WEDGE nominee to the Board of
Directors of the Company (the "WEDGE Board Nominee"). So long as WEDGE shall own
at least 25% of the capital stock of the Company, the Board of Directors agrees
to support and cause to be placed on the ballot at each election of Directors
not less than two names which shall be provided by WEDGE and which shall be
nominees to the Board of Directors of the Company. The Company shall take all
necessary steps to cause the immediate appointment of a second WEDGE Board
Nominee to the Board of Directors of the Company within fifteen (15) days from
the Closing Date. Additionally, at least one WEDGE Board Nominee shall be
appointed to serve on the audit committee and compensation committee of the
Board of Directors.

         7.3 Inspection of Property, Books and Records. The Company shall (a)
keep proper books of record and account in which full, true and correct entries
in conformity with generally accepted accounting principles shall be made of all
dealings and transactions in relation to its business activity, (b) permit
representatives of WEDGE to visit and inspect any of its properties and to
examine and make abstracts from any of its books and records at their customary
location during normal business hours or at such other times as WEDGE may
reasonably request, and as often as may be reasonably desired for use by WEDGE,
and to discuss the business, operations, properties and financial and other
condition of the Company with the Company's officers and employees.
Additionally, the Company will adhere to the requirements of Section 3.01 of the
Debenture by providing all financial and business information and statements
required therein within the time frames provided.

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                                  ARTICLE VIII
                               NEGATIVE COVENANTS

         8.1 Reorganization, Stock Dividends, Reclassification, Subdivision or
Stock Issuances. The Company will not (a) enter into a reorganization,
consolidation, merger, lease or sale with another entity in connection with the
sale, transfer or conveyance of its Common Stock or assets, (b) subdivide or
reclassify the outstanding Common Stock into a greater or lesser number of
shares, or (c) issue any additional shares of its capital stock, unless it shall
have given WEDGE at least fifteen (15) days' advance written notice.

                                   ARTICLE IX
                                     DEFAULT

         9.1 Event of Default. As used in this Agreement, the term "Event of
Default" shall mean any of the following:

                  (a) any default of any kind or nature under Section 8.01 of
         the Debenture;

                  (b) any breach of any representation or warranty contained in
         Article III herein in any material respect as of the date of issuance
         or making thereof; or

                  (c) any default in the observance of any affirmative covenant
         set forth in Article VI herein which is not remedied within 30 days
         after written notice thereof to the Company by WEDGE.

         9.2 Default Remedies. Upon the occurrence of an Event of Default, WEDGE
shall be entitled to enforce all rights and remedies provided in Section 8.02 of
the Debenture, in addition to all other rights and remedies it may be entitled
to at equity or under law.

                                    ARTICLE X
                                  MISCELLANEOUS

         10.1 Conflict of Agreement. The parties agree that in the event that
any of the terms or conditions of this Agreement shall conflict with the terms
or conditions of the Debenture, the Debenture shall govern.

         10.2 Counterparts. This Agreement may be executed in several
counterparts, each of which shall be an original and all of which together shall
constitute one agreement.

         10.3 Amendments. This Agreement may be amended, modified or terminated
only by a written instrument signed by the parties hereto.

         10.4 Governing Law. This Agreement shall in all respects be governed by
and shall be construed in accordance with the laws of the State of Texas.

         10.5 Severability. If any provision or part thereof of this Agreement
is found to be prohibited, unenforceable or invalid under the laws of any
jurisdiction, the provision or part thereof shall be ineffective only to the
extent of such prohibition, unenforceability or invalidity

                                      -8-
<PAGE>   12

under the applicable law without effecting the enforceability or validity of
such provision in any other jurisdiction and without invalidating the remainder
of such provision or other provisions in this Agreement.

         10.6 Injunctive Relief. The Company acknowledges that a breach of any
of the provisions hereof would cause irreparable harm to WEDGE and agrees that
in the event of any such threatened breach WEDGE shall be entitled to injunctive
relief and that it shall not be required to post any bond in excess of $1,000.

         10.7 Term. Defined terms not defined herein shall have the meaning
ascribed thereto in the Debenture.

         10.8 Notices. All notices, requests, consents and other communications
hereunder shall be in writing and shall be deemed to have been made when
delivered or mailed, first-class postage prepaid, by certified mail, return
receipt requested.

                  If to WEDGE:

                                    WEDGE Energy Services, L.L.C.
                                    1415 Louisiana
                                    Suite 3000
                                    Houston, Texas 77002
                                    Attention: President

                  with a copy to:

                                    WEDGE Services, L.L.C.
                                    1415 Louisiana
                                    Suite 3000
                                    Houston, Texas 77002
                                    Attention: General Counsel

                  with a copy to:

                                    Darryl M. Burman, Esq.
                                    DiCecco, Fant & Burman
                                    1900 West Loop South
                                    Suite 1100
                                    Houston, Texas 77027

                  If to the Company:

                                    South Texas Drilling & Exploration, Inc.
                                    9310 Broadway, Building I
                                    San Antonio, Texas 78217

                                      -9-
<PAGE>   13

                  with a copy to:

                                    John D. Fisch
                                    Matthews and Branscomb
                                    112 East Pecan Street
                                    Suite 1100
                                    San Antonio, Texas 78205

         10.9 Headings. The headings of the sections of this Agreement have been
inserted for convenience or reference only and shall in no way restrict or
otherwise modify any of the terms or provisions hereof.

         10.10 Arbitration.

Negotiation Period. Any dispute, controversy or claim arising out of or relating
to this Agreement, or any alleged breach hereof, will be subject to binding
arbitration in accordance with this Section 10.10. If such a dispute,
controversy or claim exists, the parties shall attempt for a 30-day period (the
"Negotiation Period") from the date any party gives any one or more of the other
parties notice (a "Dispute Notice") pursuant to this Section, to negotiate in
good faith, a resolution of the dispute. The Dispute Notice shall set forth with
specificity the basis of the dispute. During the Negotiation Period,
representatives of each party involved in the dispute who have authority to
settle the dispute shall meet at mutually convenient times and places and use
their best efforts to resolve the dispute.

Commencement of Arbitration. If a resolution is not reached by the parties prior
to the end of the Negotiation Period, either party may provide a written request
to the American Arbitration Association within ten (10) days from the end of
such period requesting the selection of three (3) arbitrators (the "Panel") to
arbitrate the parties' respective rights and obligations with respect to the
matters set forth in the Dispute Notice. Each arbitrator on the Panel shall be
experienced in the arbitration of complex commercial disputes. Discovery. Each
party to an arbitration shall be entitled to such discovery as the Panel shall
determine is appropriate.

Expenses of Arbitration. The expenses of the Panel shall be paid by the party
that does not substantially prevail on the merits in the arbitration (as
determined by the award of the Panel).

Location of Arbitration. The arbitration shall take place in Houston, Texas.

AAA Rules. Except as expressly provided in this Section 10.10, the Arbitration
shall be conducted in accordance with the Commercial Rules of the America
Arbitration Association as then in effect.

Attorneys' Fees and Expenses. The party that substantially prevails on the
merits of the arbitration (as defined by the Panel) shall be entitled to
reasonable attorneys' fees, costs, expenses, and necessary disbursements in
addition to any other relief to which such party may be entitled.

                                      -10-
<PAGE>   14

         IN WITNESS WHEREOF, the parties have executed this Agreement on the
date first set forth above.

SOUTH TEXAS DRILLING                             WEDGE ENERGY SERVICES, L.L.C.
 & EXPLORATION, INC.

By: /s/ William Stacy Locke                      By: /s/ Richard E. Blohm, Jr.
   -------------------------                        ---------------------------

Name:                                            Name:
      ----------------------                          ------------------------

Title:
       ---------------------                     -----------------------------

                                      -11-<PAGE>   1
                                                                     EXHIBIT 4.6

                               DEBENTURE AGREEMENT

      4.86% CONVERTIBLE SUBORDINATED DEBENTURE, SERIES A DUE MARCH 29, 2002
                      ORIGINAL PRINCIPAL AMOUNT $9,000,000

         THIS DEBENTURE AGREEMENT (the "Agreement") is made and entered into on
this 30th day of March, 2001, by and between WEDGE Energy Services, L.L.C., a
Delaware limited liability company ("Holder") and South Texas Drilling &
Exploration, Inc., a Texas corporation (the "Company").

                                    RECITALS

         WHEREAS the Company is issuing this 4.86% Convertible Subordinated
Debenture, Series A due March 29, 2002 in the original principal amount of
$9,000,000 (the "Debenture" or the "Note") pursuant to the payment of $9,000,000
cash by Holder to the Company and as set forth in that certain Debenture
Purchase Agreement of even date herewith (the "Purchase Agreement").

         WHEREAS the Company is now issuing to the Holder and the Holder is
receiving such Debenture from the Company; and

         WHEREAS the parties hereto wish to set forth the terms and conditions
of such Debenture;

         NOW, THEREFORE, in consideration of the premises and of the mutual
agreements hereinafter set forth, the parties hereto agree as follows:

                                   ARTICLE I

                                   DEFINITIONS

         In addition to the terms defined elsewhere in this Agreement, the
following terms shall have the meanings set forth below:

         Section 1.01 Closing Date. The term Closing Date shall mean March 30,
2001.

         Section 1.02 Common Stock. The term "Common Stock" shall mean the
Common Stock, par value $.10 per share, of the Company.

         Section 1.03 Conversion Price. The conversion price per share of Common
Stock into which the Debenture is convertible, which price is $3.75 per share,
as such conversion price may be adjusted and readjusted from time to time in
accordance with the terms of Section 7.05 hereof.

         Section 1.04 Debenture. The term "Debenture" shall mean the Debenture
issued by the Company and concurrently herewith being acquired by the Holder, in
the form set forth on Exhibit 1.09 attached hereto, as originally executed or as
may from time to time be supplemented or amended pursuant to its provisions or
the provisions hereof. If the Holder purchases or

<PAGE>   2

otherwise becomes the owner of more than one Debenture, the term "Debentures"
shall include all of the Debentures owned by the Holder taken as a whole. The
term "Debentures" shall mean all of the Debentures issued by the Company and
governed by this Agreement and other Debenture Agreements of like tenor.

         Section 1.05 Event of Default. The term "Event of Default" shall mean
an Event of Default as defined in Section 8.01 hereof.

         Section 1.06 Indebtedness. The term "Indebtedness" shall mean (a)
indebtedness for money borrowed and deferred payment obligations representing
the unpaid purchase price of property or stock, other than normal trade credits,
which would be included in determining total liabilities shown on the liability
side of a consolidated balance sheet of the Company and its Subsidiaries; (b)
guarantees and endorsements of obligations of others, directly or indirectly,
including obligations under industrial revenue and pollution control bonds, and
all other repurchase agreements and indebtedness in effect guaranteed through an
agreement, contingent or otherwise, to purchase such indebtedness, or to
purchase or sell property, or to purchase or sell services, primarily for the
purpose of enabling the debtor to make payment of the indebtedness or to assure
the owner of the indebtedness against loss, or to supply funds to or in any
manner invest in the debtor, or otherwise (but excluding guarantees and
endorsements of notes, bills and checks made in the ordinary course of
business); (c) indebtedness secured by any mortgage, lien, pledge, conditional
sale agreement, title retention agreement, or other security interest or
encumbrance upon property owned by the Company, or its Subsidiaries, even though
such indebtedness has not been assumed; and (d) amounts due under capitalized
leases as reflected on the balance sheet.

         Section 1.07 Interest Rate. The term "Interest Rate" shall mean an
interest rate payable on the Debentures of 4.86% per annum and as set forth on
the face of the Debenture.

         Section 1.08 Issuance Date. The term "Issuance Date" shall mean the
date of issuance of the Debenture of March 30, 2001, as set forth on the face of
the Debenture.

         Section 1.09 Material Adverse Effect. For purposes of this Debenture
the term "Material Adverse Effect" shall mean an event, circumstance, loss,
development or effect (individually or in the aggregate) when considered in
light of the total operations of the Company, would prohibit the Company from
engaging in any material aspect of its business or result in a material adverse
change in the business, operations, properties, prospects or assets of the
Company, or if measured monetarily, would exceed $100,000.

         Section 1.10 Maturity Date. The term "Maturity Date" shall mean March
29, 2002.

         Section 1.11 Person. The term "Person" shall mean an individual,
partnership, corporation, trust or unincorporated organization, and a government
or agency or political subdivision thereof.

         Section 1.12 Stock. The term "Stock" shall mean those shares of Common
Stock received by Holder upon conversion as provided in Section 7.

                                      -2-
<PAGE>   3

         Section 1.13 Superior Indebtedness. The term "Superior Indebtedness"
shall mean the indebtedness owed by the Company to the Frost National Bank and
American Bank as described in the subordination agreements of even date herewith
between Holder and each Bank.

         Section 1.14 Subsidiary. The term "Subsidiary" shall mean any
corporation of which more than 80% (by number of votes) of the voting stock is
owned by the Company or another Subsidiary.

                                   ARTICLE II

                                  THE DEBENTURE

         Section 2.01 Debenture. This Debenture is in the principal amount of
$9,000,000 and is being issued by the Company to the Holder pursuant to the
payment of Nine Million Dollars ($9,000,000) cash by Holder to the Company. The
Holder hereby agrees to receive such Debenture from the Company pursuant to the
terms of this Agreement, and the Company hereby agrees to issue, convey,
transfer, and assign to the Holder, the Debenture free and clear of all liens,
options, claims, and encumbrances of any kind or character whatsoever, except
for applicable transfer restrictions required by federal and state securities
laws. The Debenture may have such notations or legends as are required by
applicable law. The Debenture shall be executed on behalf of the Company by its
president or any vice president and attested to by its secretary or any
assistant secretary. The Debenture shall recite upon its face the principal
amount of indebtedness evidenced by the Debenture, the rate at which interest is
payable on the Debenture, and the terms of repayment.

         Section 2.02 Acquisition Price. If the Debenture is being received from
the Company upon issuance, the acquisition price for the Debenture shall be the
aggregate principal amount thereof. No original issue discount is contemplated
by the issuance of these Debentures.

         Section 2.03 [Intentionally Omitted]

         Section 2.04 Registration. The Debentures shall be registered in the
Debenture records of the Company as follows: The Company shall maintain a
register of the issuance of the Debentures by recording the issuance date, the
face amount, and the name and address of the initial holder and, upon transfer
in accordance with Article X of this Agreement, each transferee of each of the
Debentures upon the books of the Company. The Company shall be entitled to
recognize the person registered in the register as the exclusive owner of a
Debenture for the purposes of payment of principal and interest thereon, and the
Company shall not be bound to recognize any equitable or other claim to or
interest in such Debenture on the part of any other person, whether or not the
Company has express notice thereof, except as otherwise provided by applicable
law.

         Section 2.05 Interest on Debenture. Interest shall be payable on the
outstanding principal amount of the Debenture at the Interest Rate. Interest on
the Debenture shall be calculated on the basis of a 360-day year of twelve
30-day months. Interest shall be calculated and paid through the last such date
prior to the Maturity Date or conversion.

                                      -3-
<PAGE>   4

         Section 2.06 Lost or Stolen Certificates. In the event the certificate
representing the Debenture is destroyed, misplaced, or stolen, the Holder shall
promptly notify the Company of such loss. In its discretion, the Company may, as
a condition precedent to reissuing a new Debenture certificate, require the
Holder to do one or more of the following things:

         (a) Deliver a notice to the Company in the form prescribed by the
         Company requesting the Company to stop transfer of such lost Debenture
         certificate;

         (b) Execute and deliver to the Company an affidavit of the facts
         covering the loss of the Debenture certificate; and

         (c) Execute and file any form required by any state or federal
         regulatory authority in connection with the loss of the Debenture
         certificate.

After the Holder has complied with such requirements as the Company deems
necessary and appropriate, the Company shall cancel the lost certificate in its
register and shall issue a new Debenture certificate to the Holder with terms
and provisions identical to those contained in the lost certificate.

         Section 2.07 Governmental Charges. For any transfer of a Debenture or
exchange of a Debenture for Debentures of another denomination, the Company may
require from the Holder the payment of a sum sufficient to reimburse it for any
stamp tax or other governmental charge incidental thereto.

         Section 2.08 Prepayment of Debenture. The Company shall not be entitled
to prepay any of the outstanding amount of this Debenture.

                                  ARTICLE III

                   FINANCIAL STATEMENTS AND OTHER INFORMATION

         Section 3.01 Financial and Business Information. The Company agrees to
furnish to you so long as you or your nominee are the holder of any Debenture
and to each other holder of the then outstanding Debentures:

         (a) Quarterly Statements. Within 45 days after the end of each
         quarterly fiscal period (except the last) in each fiscal year of the
         Company, duplicate copies of:

                  (1) consolidated balance sheets of the Company as of the close
                  of such period,

                  (2) consolidated statements of income and retained earnings of
                  the Company for such quarterly fiscal period and for the
                  portion of the fiscal year ending with such period, and

                  (3) consolidated statements of cash flows of the Company for
                  the portion of the fiscal year ending with such period.

                                      -4-
<PAGE>   5

in each case (except (a)(1) above) setting forth in comparative form the figures
for the corresponding period of the preceding fiscal year, all in reasonable
detail and certified as having been prepared in accordance with generally
accepted accounting principles, but subject to changes resulting from year-end
adjustments, by an authorized financial officer of the Company.

         (b) Annual Statements. As soon as available and in any event within 90
         days after the close of each fiscal year of the Company, duplicate
         copies of:

                  (1) audited consolidated balance sheets of the Company as of
                  the close of such fiscal year, and

                  (2) audited consolidated statements of income and retained
                  earnings and changes in financial position of the Company for
                  such fiscal year.

In each case setting forth in comparative form the figures for the preceding
fiscal year, all in reasonable detail and accompanied, in the case of audited
statements, by an opinion thereon of a firm of independent public accountants of
recognized national standing selected by the Company to the effect that the
audited financial statements have been prepared in accordance with generally
accepted accounting principles consistently applied (except for changes in which
such accountants concur) and that the audit by such accountants in connection
with financial statements has been made in accordance with generally accepted
auditing standards.

         The financial statements delivered pursuant to paragraphs (a) and (b)
above shall set forth the amounts charged in each of the periods involved for
depreciation and amortization, and for interest expense.

         (c) Audit Reports. Promptly upon receipt thereof, one copy of each
         interim or special audit made by independent accountants of the books
         of the Company.

         (d) SEC and Other Reports. Promptly upon their becoming available, one
         copy of each financial statement, report, notice or proxy statement
         sent by the Company to stockholders generally, of each Form 8-K,
         10-KSB, and 10-QSB, or any successor forms, and any registration
         statement or prospectus filed by the Company with any securities
         exchange or with the Securities Exchange Commission, and of all press
         releases and other statements made available generally by the Company
         to the public concerning material developments in the business of the
         Company.

         (e) Together with each set of quarterly statements and annual
         statements pursuant to paragraphs (a) and (b) above, a certificate of
         an executive officer of the Company that such financial statements are
         true and correct and that the Company is not then in default under the
         terms of this Debenture.

                                   ARTICLE IV

                  REPRESENTATIONS AND WARRANTIES OF THE COMPANY

         The Company hereby represents and warrants to Holder as follows:

                                      -5-
<PAGE>   6

         Section 4.01 Organization and Existence. The Company is a corporation
duly organized and validly existing and in good standing under the laws of the
State of Texas and has all requisite corporate power to carry on its business as
now conducted and is qualified to do business in those jurisdictions where such
registration is required; its lease of property nor the conduct of its business
requires such qualification under the laws of any other jurisdiction, except
where the failure to do so would not have a Material Adverse Effect on the
financial condition or results of operations of the Company. The Company has
delivered to the Purchaser complete and correct copies of the Articles of
Incorporation and Bylaws of the Company as in effect on the date hereof.

         Section 4.02 Capitalization: Ownership of Stock: Authorization. The
Company has 30,000,000 authorized shares of its common stock $0.10 par value
(the "Common Stock") and 1,000,000 authorized shares of its preferred stock,
issuable in series (the "Preferred Stock"). As of March 26, 2001, the Company
had (a) 12,140,921 issued and outstanding shares of Common Stock; (b) 184,615
shares of issued and outstanding Series B Preferred Stock, $16.25 redemption and
liquidation value; and (c) no treasury shares. As of March 26, 2000, the Company
had granted stock options which, if all were exercised, would equal 2,072,500
shares of Common Stock. Other than the registration rights granted to Purchaser
in accordance with the transactions contemplated hereby, the Company has only
incidental registration rights to two (2) of its officers and directors, Wm.
Stacy Locke and Michael E. Little, and no other individual or entity has any
registration rights of any kind or nature (other than rights under Form S-8),
including incidental or demand registration rights. There is one outstanding
warrant held by a former employee of the Company in the amount of 8,000 shares
which has been exercised and an employee stock option for 100,000 shares will be
issued to Andrew Mills, an employee of Mustang Drilling, Ltd., upon the purchase
by the Company of its assets of Mustang Drilling, Ltd. Other than items referred
to herein, there are no other options, warrants, rights, conversion rights,
phantom rights, preemptive rights or any other rights by any party to receive
equity of the Company. Upon issuance of the Common Stock into which the
Debenture may be converted (the "Stock") to Purchaser, Purchaser will be the
record and beneficial owner of the Stock and the Stock will be duly authorized,
validly issued and outstanding, fully paid and nonassessable and would have been
issued in accordance with appropriate federal and state securities law. By
virtue of the conversion rights included in the Debenture, Purchaser shall
receive good and valid title to the Stock, free and clear of all liens,
encumbrances, pledges, options, claims, assessments and adverse charges. If the
Stock were issued on the Closing Date, Purchaser's ownership would constitute
approximately 49.9% of the Company's issued and outstanding shares of Common
Stock. As a result of the issuance of the Stock, the Company will not become
obligated to issue any additional shares of capital stock (preferred or common)
to any officer, director, shareholder or other party.

         The execution, delivery and performance of this Agreement and the
Registration Rights Agreement will not result in a violation or breach of any
term or provision of or constitute a default or accelerate the performance
required under the Articles of Incorporation or Bylaws of the Company or any
indenture, mortgage, deed of trust or other contract or agreement to which the
Company is a party or by which its assets are bound, or violate any order, writ,
injunction or decree of any court, administrative agency or governmental body.

                                      -6-
<PAGE>   7

         Section 4.03 Enforceability. The Company has full right, power, legal
capacity and authority to execute, deliver and perform this Agreement and the
Registration Rights Agreement and to consummate the transactions contemplated
hereby and thereby, and this Agreement and the Registration Rights Agreement are
valid and legally binding obligations enforceable in accordance with their
respective terms, except as enforcement may be limited by bankruptcy,
insolvency, moratorium or similar laws affecting the enforcement of creditors'
rights, by the availability of injunctive relief or specific performance and by
general principles of equity.

         Section 4.04 Securities and Exchange Commission. The Company has filed
all forms, reports and documents required to be filed by the Securities and
Exchange Commission, National Association of Securities Dealers, Inc. and the
American Stock Exchange and the state of Texas in order to comply with all
applicable laws, rules and regulations of the Securities Act of 1933, as amended
(the "Act"), the Securities and Exchange Act of 1934, as amended, and the
securities laws of the state of Texas. Notwithstanding the foregoing, Seller has
delivered to Purchaser a copy of that certain "no action" letter dated August
25, 1999 from the Securities and Exchange Commission regarding Seller's failure
to comply with Regulation S-X of the Act in connection with its acquisition of
assets of Howell Drilling, Inc.

         Section 4.05 Litigation; Contingencies. Except as described in the
Reports, there is no action, suit or proceeding pending or, to the knowledge of
the Company, threatened against the Company before any court, agency or
arbitrator which might result in any Material Adverse Effect in the business,
properties or condition (financial or otherwise) of the Company or which
question the validity of any action taken or to be taken pursuant to or in
connection with this Agreement, the Registration Rights Agreement or the Stock.

         Section 4.06 No Subsidiaries. Other than subsidiaries that have no
assets, liabilities or operations, the Company has no subsidiaries or any
interests in other corporations, partnerships or joint ventures except as
follows: The Company owns 100% of PDC Investment Corp., a Delaware corporation.
PDC Investment Corp. is the sole limited partner with a 99% partnership interest
in Pioneer Drilling Co., Ltd., a Texas limited partnership. The Company owns
100% of SOTEX Exploration Company, a Texas corporation. SOTEX Exploration
Company is the sole general partner of Pioneer Drilling Co., Ltd. and holds a 1%
partnership interest in such limited partnership. Pioneer Drilling Co., Ltd.,
holds substantially all of the operating assets of the consolidated group
consisting of the Company, PDC Investment Corp., Pioneer Drilling Co., Ltd., and
SOTEX Exploration Company.

         Section 4.07 Title to Assets (Personal Property).

         (a) The Company is the owner of, and has marketable title to, all of
         its assets, free and clear of all liens except those noted in the
         Company's December 31, 2000 financial statements and except for those
         assets leased under leases specifically noted in the Company's December
         31, 2000 financial statements. The assets referred to in the preceding
         sentence include, without limitation, all assets, properties and rights
         of the Company shown or reflected on the February 28, 2001 Balance
         Sheet or acquired by the Company since February 28, 2001, except only
         for (i) cash expended and (ii) inventories and other assets used or
         sold and receivables collected in the ordinary course of business since
         February 28, 2001. The Company has maintained all tangible assets
         material to the

                                      -7-
<PAGE>   8

         business in good repair, working order and operating condition, subject
         only to ordinary wear and tear, and all such tangible assets are
         suitable for the purposes for which they are presently being used.

         (b) With respect to each lease of real or personal property of the
         Company: (i) the lease is valid and binding on the Company and in full
         force and effect, (ii) no rental payment is in default, (iii) the
         Company is in peaceable possession of the real property or personal
         property which is subject thereto, and (iv) the Company is not in
         default of any material provision thereof, and to the best knowledge of
         the Company, no event has occurred that with the giving of notice, the
         passage of time or both, would become a material default under any such
         lease.

         (c) The Company has all easements, rights-of-way and similar
         authorizations required for the use of the real property leased by the
         Company and in the conduct of the business as heretofore conducted,
         excluding immaterial easements (the "Easements"). To the best knowledge
         of the Company, no party thereto is in default of any material
         provision of any easement or any material covenant, restriction or
         other agreement encumbering any of the real property, and to the best
         knowledge of the Company, no event that with the giving of notice, the
         passage of time or both would become a material default, has occurred
         under any easement or any material covenant, restriction or other
         agreement encumbering any of the real property. Neither the whole nor
         any portion of any real property occupied by the Company has been
         condemned or otherwise taken by any public authority, and the Company
         has received no written notice that any such condemnation or taking is
         threatened or contemplated.

         (d) (i) Neither the properties owned or occupied by the Company nor the
         occupancy or operation thereof is in material violation of any law or
         any building, zoning or other ordinance, code or regulation; (ii) no
         notice from any governmental body has been served upon the Company or
         upon any property owned or occupied by the Company claiming any
         material violation of any such law, ordinance, code or regulation or
         requiring, or calling to the attention of the Company the need for, any
         work, repair, construction, alterations or installation on or in
         connection with any such properties which has not been complied with;
         and (iii) there is no material encroachment of the improvements located
         on the real property owned or occupied by the Company upon any
         adjoining property, or of improvements located on any adjoining
         property upon any property owned or occupied by the Company.

         Section 4.08 Consents. The Company is not required to obtain any
consent from or approval of any court, governmental entity or any other person
in connection with the execution, delivery or performance by it of this
Agreement or the Registration Rights Agreement and the transactions contemplated
hereby. The consummation of the transactions contemplated by this Agreement will
not require the approval of any entity or person in order to prevent the
termination of any material right, privilege, license or agreement of the
Company.

         Section 4.09 Proprietary Rights. All patents (pending or issued),
copyrights, trademarks, state, federal and foreign registrations and
applications and trade secrets of the Company are valid and in full force and
effect and are not subject to any taxes, maintenance fees,

                                      -8-
<PAGE>   9

or extension, renewal or continuation actions by the Company falling due within
90 days after the date hereof. There have not been any claims, actions or
judicial or other adversary proceedings involving the Company concerning any of
the Proprietary Rights and, no such action or proceeding is threatened. The
Company has the right and authority to use each item of the rights and property
referenced above in connection with the conduct of its business including all
patents, trademarks, computer hardware and software licenses; such use has not
and will not conflict with, infringe upon, or violate any patent or other
proprietary right of any other person, and the Company has not infringed and is
not now infringing any proprietary right belonging to any other person. There
are no outstanding nor threatened claims for breach, termination or penalty
payment with respect to any licenses or similar agreements. The Company has no
patents issued, pending or filed.

         Section 4.10 Disclosure. The Company represents and warrants that no
representation or warranty by the Company in this Agreement or in any of the
Exhibits or Schedules hereto, or certificate furnished to the Purchaser by or on
behalf of the Company in connection with the transactions contemplated hereby,
contains or will contain any materially untrue statement of a material fact; and
further, no Schedule omits or will omit any material item required to be
included in such Schedule. Any indemnification by the Company hereunder for a
breach of its representation and warranty in this Section 4.10 shall be made in
the manner applicable pursuant to Section 9 hereof to such representation and
warranty or to the provision of this Agreement to which such Exhibit, Schedule
or certificate relates.

         Section 4.11 Financial Statements. The Company has made available to
the Purchaser its Annual Report on Form 10K for the period ended March 31, 2000
and its Quarterly Reports on Form 10Q for the periods ended June 30, 2000,
September 30, 2000 and December 31, 2000, a report of Form 8K dated August 21,
2000 and filed September 1, 2000, a report on Form 8K/A filed October 31, 2000
and a report on Form 8K filed on March 22, 2001 and its unaudited interim
financial statements dated January 31, 2001 and February 28, 2001 (the
"Reports"). "Financial Statements" shall mean the unaudited balance sheet and
statements of operations, changes in equity and cash flows for the Company as of
and for the fiscal period ended February 28, 2001. Prior to Closing, the Company
shall also deliver to the Purchaser each monthly financial statement that is
produced by the Company during calendar year 2001 in its ordinary course of
business and such monthly financial statements shall be included in this
definition of Financial Statements. The Financial Statements have been prepared
in conformity with generally accepted accounting principles applied on a basis
consistent with prior periods. All of the Financial Statements present fairly
the financial position and the results of operations of the Company on the dates
and for the periods shown therein, and to the best knowledge of the Company,
there has been no Material Adverse Effect in the financial condition of the
Company since February 28, 2001.

         Except as disclosed in the Reports or the Financial Statements, the
Company has no debt, liability or obligation, contingent or otherwise, which
would have a Material Adverse Effect on the business or the assets of the
Company.

         Section 4.12 Compliance with Laws; OSHA. To the best of the knowledge
of the Company, the Company is in compliance with all applicable laws,
ordinances, statutes, rules, regulations and orders promulgated by any court or
federal, state or local governmental body or

                                      -9-
<PAGE>   10

agency relating to its assets and business the failure to comply with which
would cause a Material Adverse Effect. The Company has not received any notice,
citation, claim, assessment or proposed assessment as to or alleging any
violation of any federal, state or local Occupational Safety and Health laws and
no violations which materially, presently exist.

         Section 4.13 Labor Matters. There is no labor strike or labor
disturbance pending, or to the knowledge of the Company threatened, against the
Company nor is any arbitration concerning an employee grievance currently
pending against the Company. The Company has experienced no work stoppage or
other material labor disturbance within the past three years. The Company is not
a party to any collective bargaining agreement with respect to its employees
and, to the knowledge of the Company, there are no current attempts to organize
its employees.

         Section 4.14 ERISA. The Company has no pension, retirement, savings,
deferred compensation, and profit-sharing plan and each stock option, stock
appreciation, stock purchase, performance share, bonus or other incentive plan,
severance plan, health, group insurance or other welfare plan, or other similar
plan and any "employee benefit plan" within the meaning of Section 3(3) of the
Employee Retirement Income Security Act of 1974 ("ERISA"), under which the
Company has any current or future obligation or liability or under which any
employee or former employee (or beneficiary of any employee or former employee)
of the Company has or may have any current or future right to benefits on
account of employment with the Company (the term "plan" shall include any
contract, agreement, policy or understanding, each such plan being hereinafter
referred to individually as a "Plan").

         Section 4.15 Environmental Matters. The Company has obtained all
Environmental Permits that are required with respect to its business, operations
and properties, either owned or leased, and (ii) the Company and its properties
are in compliance with all terms and conditions of all applicable Requirements
of Environmental Law and Environmental Permits, the failure to comply with which
would cause a Material Adverse Affect. There are no Environmental Claims
pending, or, to the knowledge of the Company, threatened, against the Company.
The Company has not received any notice from any governmental authority of any
violation or liability arising under any Requirements of Environmental Law or
Environmental Permit in connection with the assets, the businesses or operations
of the Company.

                  "Environmental Claim" means any third party (including
         governmental agencies and employees) action, lawsuit, claim or
         proceeding (including claims or proceedings under the Occupational
         Safety and Health Act or similar laws relating to safety of employees)
         which seeks to impose liability for (i) pollution or contamination of
         the air, surface water, ground water or land; (ii) solid, gaseous or
         liquid waste generation, handling, treatment, storage, disposal or
         transportation; (iii) exposure to hazardous or toxic substances; (iv)
         the safety or health of employees or (v) the transportation,
         processing, distribution in Commerce, use, or storage of hydrocarbons
         or chemical substances. An Environmental Claim includes, but is not
         limited to, a common law action, as well as a proceeding to issue,
         modify or terminate an Environmental Permit.

                  "Environmental Permit" means any permit, license, approval or
         other authorization under any applicable law, regulation and other
         requirement of the United States or foreign country or of any state,
         municipality or other subdivision thereof

                                      -10-
<PAGE>   11

relating to pollution or protection of health or the environment, including
laws, regulations or other requirements relating to emissions, discharges,
releases or threatened releases of Pollutants, contaminants or hazardous
substances or toxic materials or wastes into ambient air, surface water, ground
water or land, or otherwise relating to the manufacture, processing,
distribution, use, treatment, storage, disposal, transportation, or handling of
hydrocarbons or chemical substances, pollutants, contaminants or hazardous or
toxic materials or wastes.

                  "Requirements of Environmental Law" means all requirements in
         effect on the Closing Date imposed by any law, rule, regulation, or
         order of any federal, foreign, state or local executive, legislative,
         judicial, regulatory or administrative agency, board or authority with
         jurisdiction over the Company or its properties or assets which relate
         to (i) pollution or protection of the air, surface water, ground water
         or land; (ii) solid, gaseous or liquid waste generation, treatment,
         storage, disposal or transportation; (iii) exposure to hazardous or
         toxic substances; (iv) the safety or health of employees or (v)
         regulation of the manufacture, processing, distribution in Commerce,
         use, or storage of chemical substances.

         Section 4.16 Permits and Licenses. The Company has all material
licenses, permits and other authorizations necessary for the conduct of its
business as it is currently being conducted. To the best of the Company's
knowledge, all of such Permits are adequate for the operation of the business of
the Company as it is presently being conducted.

         Section 4.17 Insurance. Purchaser has been provided with all insurance
policies (together with all riders and amendments) relating to the assets or the
business of the Company are sufficient to protect against any material claim for
casualty or property damage. Such insurance policies are in full force and
effect, all premiums due thereon have been paid or accrued on the books of the
Company and will not terminate as of the Closing or the consummation of the
transactions contemplated hereby. The Company has no reason to believe that such
insurance policies will be terminated or subject to non-renewal.

         Section 4.18 Taxes. The Company has filed all tax returns and reports
required by law to be filed, or filed extensions for any period in which a tax
return was due and has paid or accrued on the financial statements provided to
the Purchaser all taxes, assessments and other governmental charges that are due
and payable. The charges, accruals and reserves on the books of the Company in
respect of taxes for all fiscal periods are considered adequate by the Company,
and the Company knows of no assessment for additional taxes for any of such
fiscal years or any basis therefor. All tax returns and reports are complete. No
claim has ever been made to the Company's knowledge that the Company is subject
to a tax in any jurisdiction in which the Company has not filed a return which
remains unpaid as of the Closing Date. The Company has withheld and paid all
taxes required to have been withheld or paid in connection with amounts paid or
owing to any employee, independent contractor, creditor, stockholder or other
third party. The Company has not since 1990 been the subject of an audit and the
Company has not waived any statute of limitations or agreed to an extension of
time with respect to a tax assessment or deficiency.

                                      -11-
<PAGE>   12

         Section 4.19 Absence of Certain Developments. Since February 28, 2001,
there has been no change which would have a Material Adverse Effect,
individually or in the aggregate, in the assets, liabilities, condition
(financial or otherwise), operating results, business or prospects of the
Company, except changes in the ordinary course of business. The Company has not,
since the date of the Financial Statements, directly or indirectly, declared or
paid any dividend or ordered or made any other distribution on account of any
shares of any class of the capital stock of the Company. The Company has not,
since such date, directly or indirectly redeemed, purchased or otherwise
acquired any such shares or agreed to do so or set aside any sum or property for
any such purpose. Additionally, except for the previous sale of common stock of
the Company to Purchaser on or about May 11, 2000, and the issuance of 341,575
shares of Common Stock as partial consideration for the purchase of the stock of
Pioneer Drilling Co. in August of 2000 and the issuance of common stock incident
to the exercise of employee options there have been no other sales of securities
of any kind or nature by the Company.

         Section 4.20 Underground Storage Tanks. There are no underground
storage tanks on any of the Company's owned or leased real property.

                                   ARTICLE V

                    REPRESENTATIONS AND WARRANTIES OF HOLDER

         The Holder hereby represents and warrants to Company:

         Section 5.01 Power and Authority. The Holder has all requisite power
and authority to enter into this Agreement and to acquire the Debenture. No
provision of the Articles of Incorporation, Bylaws, or other governing,
instruments of the Holder would preclude any of the transactions contemplated by
this Agreement.

         Section 5.02 Authorization. The execution of this Agreement and the
consummation of the transactions contemplated herein have been duly approved by
all necessary action, corporate and otherwise, of the Holder.

         Section 5.03 Investment Intent. The Holder is acquiring the Debenture
solely for its own account and not with a view to, or for resale in connection
with, any distribution or public offering thereof, within the meaning of any
applicable securities laws and regulations.

                                   ARTICLE VI

                                  SUBORDINATION

         Section 6.01 Debentures Subordinated to Superior Indebtedness. Anything
in this Agreement or the Debentures to the contrary notwithstanding, the
indebtedness evidenced by the Debentures (such indebtedness being hereinafter
referred to as Subordinated Indebtedness) shall be subordinate and junior in
right of payment, to (but only to) all Superior Indebtedness (as defined herein)
of the Company.

                                      -12-
<PAGE>   13

         Section 6.02 Payments on Subordinated Indebtedness. No payments of any
type shall be made by the Company or collected by the Holder until the Superior
Indebtedness has been paid in full.

         Section 6.03 Insolvency, etc. In the event of (a) any insolvency,
bankruptcy, receivership, liquidation, reorganization, readjustment,
composition, or other similar proceeding relating to the Company or its
property, (b) any proceeding for the liquidation, dissolution, or other
winding-up of the Company, voluntary or involuntary, and whether or not
involving insolvency or bankruptcy proceedings, (c) any assignment by the
Company for the benefit of creditors, or (d) any distribution, division,
marshaling, or application of any of the properties or assets of the Company or
the proceeds thereof to creditors, voluntary or involuntary, and whether or not
involving legal proceedings, then and in such event:

                  (i) all Superior Indebtedness shall first be paid in full
         (including all principal, premium, if any, and interest, including
         interest accruing after the commencement of any such proceeding) before
         any payment or distribution of any character, whether in cash,
         securities, or other property (other than securities of the Company or
         any other corporation provided for by a plan of reorganization or
         readjustment or similar plan, the payment of which is subordinated, at
         least to the extent provided in this Article VI with respect to
         Subordinated Indebtedness, to the payment of all Superior Indebtedness
         at the time outstanding and to any securities issued in respect thereof
         under any such plan) is made in respect of any Subordinated
         Indebtedness;

                  (ii) all principal and premium, if any, and interest on the
         Subordinated Indebtedness shall forthwith become due and payable, and
         any payment or distribution of any character, whether in cash,
         securities, or other property (other than securities of the Company or
         any other corporation provided for by a plan of reorganization or
         readjustment or similar plan, the payment of which is subordinated, at
         least to the extent provided in this Article VI with respect to
         Subordinated Indebtedness, to the payment of all Superior Indebtedness
         at the time outstanding and to any securities issued in respect thereof
         under any such plan) which would otherwise (but for the terms hereof)
         be payable or deliverable in respect of any Subordinated Indebtedness,
         shall be paid or delivered directly to the holders of the Superior
         Indebtedness, until all Superior Indebtedness shall have been paid in
         full, the holders of the Subordinated Indebtedness at the time
         outstanding irrevocably authorize, empower, and direct all receivers,
         trustees, liquidators, conservators, fiscal agents, and others having
         authority in the premises to effect all such payments and deliveries;

                  (iii) each holder of the Subordinated Indebtedness at the time
         outstanding irrevocably authorizes and empowers each holder of the
         Superior Indebtedness or such holder's representative to demand, sue
         for, collect, and receive such holder's ratable share of all such
         payments and distributions and to receipt therefor, and to file and
         prove all claims therefor and take all such other action, in the name
         of such holder or otherwise, as such holder of the Superior

                                      -13-
<PAGE>   14

         Indebtedness or such holder's representative may determine to be
         necessary or appropriate for the enforcement of this Section 6.03; and

                  (iv) the holders of the Subordinated Indebtedness shall
         execute and deliver to each holder of the Superior Indebtedness or such
         holder's representative all such further instruments confirming the
         above authorization, and all such powers of attorney, proofs of claim,
         assignments of claim, and other instruments, and shall take all such
         other action as may be requested by such holder of the Superior
         Indebtedness or such holder's representative to enforce all claims upon
         or in respect of the Subordinated Indebtedness.

For all purposes of this Agreement, Superior Indebtedness shall not be deemed to
have been paid in full unless the holders thereof shall have received cash equal
to the amount of principal, premium, if any, and interest in respect of all
Superior Indebtedness at the time outstanding, and in case there are two or more
holders of the Superior Indebtedness any payment or distribution required to be
paid or delivered to the holders of the Superior Indebtedness shall be paid or
delivered to such holders ratably according to the respective aggregate amounts
remaining unpaid on the Superior Indebtedness of such holders.

         Section 6.04 Payments and Distributions Received. If any payment or
distribution of any character (whether in cash, securities, or other property)
or any security shall be received by any holder of any of the Subordinated
Indebtedness in contravention of any of the terms of this Article VI, and except
as permitted by Section 6.03 or Section 6.06, such payment or distribution or
security shall be held in trust for the benefit of, and shall be paid over or
delivered and transferred to, the holders of the Superior Indebtedness for
application to the payment of all Superior Indebtedness remaining unpaid, to the
extent necessary to pay all such Superior Indebtedness in full. In the event of
the failure of any holder of any of the Subordinated Indebtedness to endorse or
assign any such payment, distribution or security, any holder of the Superior
Indebtedness or such holder's representative is hereby irrevocably authorized to
endorse or assign the same.

         Section 6.05 Subrogation. In the event that cash, securities, or other
property otherwise payable and deliverable to the holders of the Subordinated
Indebtedness shall have been applied pursuant to Section 6.03 or Section 6.04 to
the payment of Superior Indebtedness in full, then and in each such case, the
holders of the Subordinated Indebtedness shall be subrogated to any rights of
any holders of Superior Indebtedness to receive further payments or
distributions in respect of or applicable to the Superior Indebtedness.

         Section 6.06 Acceleration of Subordinated Indebtedness. In case any
Subordinated Indebtedness is declared due and payable because of the occurrence
of an Event of Default with respect to the Subordinated Indebtedness under
circumstances when the terms of Section 6.03 are not applicable, the holders of
such Subordinated Indebtedness shall not be entitled to receive payment or
distribution in respect thereof until all Superior Indebtedness at the time
outstanding shall have been paid in full.

         Section 6.07 Notice. In the event that any Subordinated Indebtedness
shall become due and payable before its expressed maturity on demand of the
holder thereof as the result of the

                                      -14-
<PAGE>   15

occurrence of a default or event of default, the Company will give prompt notice
in writing of such happening to each holder of Superior Indebtedness.

         Section 6.08 Subordination Not Affected, etc. The terms of this Article
VI, the subordination effected hereby, and the rights of the holders of the
Superior Indebtedness shall not be affected by (a) any amendment of or addition
or supplement to any Superior Indebtedness or any instrument or agreement
relating thereto, (b) any exercise or non-exercise of any right, power, or
remedy under or in respect of any Superior Indebtedness or any instrument or
agreement relating thereto, or (c) any waiver, consent, release, indulgence,
extension, renewal, modification, delay, or other action, inaction or omission,
in respect of any Superior Indebtedness or any instrument or agreement relating
thereto or any security therefor or guaranty thereof, whether or not any holder
of any Subordinated Indebtedness shall have had notice or knowledge of any of
the foregoing. In addition, in the event that any holder or prospective holder
of Superior Indebtedness reasonably requires a modification or amendment of the
terms of this Article VI with respect to the subordination of the Subordinated
Indebtedness, the Holder agrees to execute any such modification or amendment to
this Article VI with respect thereto.

         Section 6.09 Obligations Unimpaired. The holder of Superior
Indebtedness shall not be prejudiced in the right to enforce subordination of
the Subordinated Indebtedness by any act or failure to act on the part of the
Company. The provisions of this Article VI are solely for the purpose of
defining the relative rights of the holders of Superior Indebtedness on the one
hand and the holders of Subordinated Indebtedness on the other hand, and nothing
in this Article VI shall (a) impair as between the Company and the holder of any
Subordinated Indebtedness the obligation of the Company, which is unconditional
and absolute, to pay to the holder thereof the principal, premium, if any, and
interest thereon in accordance with the terms thereof, or (b) prevent the holder
of any Subordinated Indebtedness from exercising all remedies otherwise
permitted by applicable law under this Agreement, subject to the rights, if any,
under this Article VI of the holders of Superior Indebtedness.

         Section 6.10 Securities Subordinate to Debenture. All equity securities
of the Company shall be subordinate and junior in right of payment as to
dividends, and on liquidation, to the rights of the Debenture to payment of
principal and interest and on liquidation.

                                  ARTICLE VII

                            CONVERSION OF DEBENTURES

         Section 7.01 Conversion Privilege. Upon satisfaction of the condition
precedent set forth in Section 7.09 below, the unpaid principal amount of any
Debenture may, at the election of either the Company or the Holder thereof, at
any time, be convened into shares of Common Stock at the conversion price per
share of Common Stock of Three Dollars and Seventy-Five Cents ($3.75) (such
conversion price, as so adjusted and readjusted and in effect at any time, being
herein called the "Conversion Price"), into the number of fully paid and
non-assessable shares of Common Stock determined by dividing (x) the aggregate
principal amount of the Debentures to be so converted by (y) the Conversion
Price in effect at the time of such conversion.

                                      -15-
<PAGE>   16

         Section 7.02 Manner of Conversion; Partial Conversion, etc.

         (a) Any Debenture must be converted in whole by the holder thereof by
         surrender of such Debenture, accompanied by a written statement
         designating the principal amount of such Debenture to be converted and
         stating the name and address of the person in whose name certificates
         for shares of Common Stock are to be registered, at the office of the
         Company specified in or pursuant to Section 14.01. The conversion shall
         be deemed to have been effected as of the close of business on the date
         on which such Debenture shall have been so surrendered to such office,
         and at such time the rights of the holder of such Debenture as such
         shall, to the extent of the principal amount thereof converted, cease,
         and the person or persons in whose name or names any certificate or
         certificates for shares of Common Stock shall be issuable upon such
         conversion shall be deemed to have become the holder or holders of
         record thereof.

         (b) The Company shall pay all cash interest on any Debenture
         surrendered for conversion to the date of such conversion.

         Section 7.03 Delivery of Stock Certificates. As promptly as practicable
after the conversion of any Debenture in full, and in any event within 20 days
thereafter, the Company at its expense (including the payment by it of any
applicable issue taxes) will issue and deliver to the holder of such Debenture,
or as such holder (upon payment by such holder of any applicable transfer taxes)
may direct, a certificate or certificates for the number of full and fractional
shares of Common Stock issuable upon such conversion.

         Section 7.04 Shares to be Fully Paid; Reservation of Shares. The
Company covenants and agrees that all shares of Common Stock which may be issued
upon conversion of the Debentures will, upon issuance, be fully paid and
non-assessable and free from all taxes, liens, and charges with respect to the
issue thereof; and without limiting the generality of the foregoing, the Company
covenants and agrees that it will from time to time take all such action as may
be requisite to assure that the par value (if any) per share of the Common Stock
is at all times equal to or less than the then effective purchase price per
share of the Common Stock issuable upon conversion of the Debentures. The
Company further covenants and agrees that the Company will at all times have
authorized, and reserved for the purpose of issue or transfer upon the
conversion of the Debentures, a sufficient number of shares of its Common Stock
to provide for the conversion of the Debentures.

         Section 7.05 Conversion Price Adjustments. The Conversion Price shall
be subject to adjustment from time to time as follows:

         (a) Stock Dividends, Subdivisions, Reclassifications or Combinations.
         If the Company shall (i) declare a dividend or make a distribution on
         its Common Stock in shares of its Common Stock, (ii) subdivide or
         reclassify the outstanding shares of Common Stock into a greater number
         of shares, or (iii) combine or reclassify the outstanding Common Stock
         into a smaller number of shares, the Conversion Price in effect at the
         time of the record date for such dividend or distribution or the
         effective date of such subdivision, combination or reclassification
         shall be proportionately adjusted so that the holder of any Debentures
         surrendered for conversion after such date shall be entitled to receive
         the

                                      -16-
<PAGE>   17

         number of shares of Common Stock which he would have owned or been
         entitled to receive had such Debentures been converted immediately
         prior to such date. Successive adjustments in the Conversion Ratio
         shall be made whenever any event specified above shall occur.

         (b) Other Distributions. In case the Corporation shall fix a record
         date for the making of a distribution to all holders of shares of its
         Common Stock (i) of shares of any class other than its Common Stock or
         (ii) of evidences of indebtedness of the Corporation or any Subsidiary
         or (iii) of assets (excluding cash dividends or distributions, and
         dividends or distributions referred to in subparagraph 7.05(a) above),
         or (iv) of rights or warrants, in each such case of (i) through (iv)
         the Conversion Price in effect immediately prior thereto shall be
         immediately thereafter proportionately adjusted for such distribution
         so that the holder of Debentures would be entitled to receive the fair
         market value (as determined by the Board of Directors, whose
         determination in good faith shall be conclusive) of what a Holder would
         have been entitled to receive had such Debentures been converted prior
         to such distribution. Such adjustment shall be made successively
         whenever such a record date is fixed. In the event that such
         distribution is not so made, the Conversion Price then in effect shall
         be readjusted, effective as of the date when the Board of Directors
         determines not to distribute such shares, evidences of indebtedness,
         assets, rights or warrants, as the case may be, to the Conversion Price
         which would then be in effect if such record date had not been fixed.

         (c) Consolidation, Merger, Sale, Lease or Conveyance. In case of any
         consolidation with or merger of the Corporation with or into another
         corporation, or in case of any sale, lease or conveyance to another
         corporation of the assets of the Corporation as an entirety or
         substantially as an entirety, the Debentures shall after the date of
         such consolidation, merger, sale, lease or conveyance be convertible
         into the number of shares of stock or other securities or property
         (including cash) to which the shares of Common Stock issuable (at the
         time of such consolidation, merger, sale, lease or conveyance) upon
         conversion of such Debenture would have been entitled to upon such
         consolidation, merger, sale, lease or conveyance; and in any such case,
         if necessary, the provisions set forth herein with respect to the
         rights and interests thereafter of the holders of the Debentures shall
         be appropriately adjusted so as to be applicable, as nearly as may
         reasonably be, to any shares of stock or other securities or property
         thereafter deliverable on the conversion of the Debentures.

         Section 7.06 Statement Regarding Adjustments. Whenever the Conversion
Price shall be adjusted as provided in Section 7.05, the Company shall forthwith
file, at the principal office of the Company, a statement showing in detail the
facts requiring such adjustment and the Conversion Price that shall be in effect
after such adjustment, and the Company shall also cause a copy of such statement
to be sent by mail, first class postage prepaid, to each holder of Debentures,
at its address appearing on the Company's records. Where appropriate, such copy
may be given in advance and may be included as part of a notice required to be
mailed under the provisions of Section 7.07.

         Section 7.07 Notice to Holders. In the event the Company shall propose
to take any action of the type described in Section 7.05, the Company shall give
written notice to each holder

                                      -17-
<PAGE>   18

of Debentures, in the manner set forth in Section 7.06, which notice shall
specify the record date, if any, with respect to any such action and the
approximate date on which such action is to take place. Such notice shall also
set forth such facts with respect thereto as shall be reasonably necessary to
indicate the effect of such action (to the extent such effect may be known at
the date of such notice) on the Conversion Price and the number, kind or class
of shares which shall be deliverable upon conversion of Debentures. In the case
of any action which would require the fixing of a record date, such written
notice shall be given at least 15 days prior to the taking of such action.
Failure to give such written notice, or any defect therein, shall not affect the
legality or validity of any such action.

         Section 7.08 Costs. The Company shall pay all documentary, stamp,
transfer or other transactional taxes attributable to the issuance or delivery
of shares of Common Stock upon conversion of any Debentures; provided that the
Company shall not be required to pay any taxes which may be payable in respect
of any transfer involved in the issuance or delivery of any certificate for such
shares in a name other than that of the holder of the Debentures, in respect of
which shares are being issued.

         Section 7.09 Approval of Shareholders. All of the rights of Purchaser
under this Article VII are conditioned upon the approval of such rights by the
Shareholders of the Company at a meeting of Shareholders of the Company
satisfying the requirements of the American Stock Exchange regarding
shareholders' approval. The Company agrees to immediately, but not later than
seven (7) days from the execution hereof, to commence preparation and procedures
for a special meeting of its Shareholders be held no later than June 30, 2001,
for the purpose of approving the provisions of this Article VII.

                                  ARTICLE VIII

                              DEFAULT AND REMEDIES

         Section 8.01 Event of Default. As used in this Agreement and the
accompanying Debenture, the term "Event of Default" shall mean any one of the
following:

         (a) a default in the payment of interest on any Debenture when due and
         such default shall continue for more than fifteen (15) days from such
         due date;

         (b) a default in the payment of the principal of Debentures at maturity
         or at any date fixed in any notice for prepayment;

         (c) the Company sells or otherwise disposes of all or substantially all
         of its assets to any Person;

         (d) a default in the observance or performance of any covenant or
         provision of this Agreement or of the Purchase Agreement which is not
         remedied within fifteen (15) days after written notice thereof to the
         Company by the holder of any Debenture;

         (e) any representation or warranty made by the Company herein or in the
         Purchase Agreement, or made by the Company in any written statement or
         certificate furnished by the Company in connection with the
         consummation of the issuance and delivery of the

                                      -18-
<PAGE>   19

         Debentures or furnished by the Company pursuant hereto, is untrue in
         any material respect as of the date of the issuance or making thereof;

         (f) final judgment or Judgments for the payment of money aggregating in
         excess of $250,000 is or are outstanding against the Company or any
         Subsidiary or against any of the property or assets of the Company or
         any Subsidiary and any one of such judgments has remained unpaid,
         unvacated, unbonded or unstayed by appeal or otherwise for a period of
         thirty (30) days from the date of its entry;

         (g) the Company or any Subsidiary becomes insolvent or bankrupt, is
         generally not paying its debts as they become due or makes an
         assignment for the benefit of creditors, or the Company or any
         Subsidiary causes or suffers an order for relief to be entered with
         respect to it under applicable Federal bankruptcy law or applies for or
         consents to the appointment of a custodian, trustee or receiver for the
         Company or any Subsidiary or for the major part of the property of the
         Company or any Subsidiary;

         (h) a custodian, trustee or receiver is appointed for the Company or
         any Subsidiary or for the major part of the property of the Company or
         any Subsidiary and is not discharged within sixty (60) days after such
         appointment;

         (i) bankruptcy, reorganization, arrangement or insolvency proceedings,
         or other proceedings for relief under any bankruptcy or similar law or
         laws for the relief of debtors, are instituted by or against the
         Company or any Subsidiary and, if instituted against the Company or any
         Subsidiary, are consented to or are not dismissed within sixty (60)
         days after such institution; or

         (j) failure to convene a special meeting of stockholders of the Company
         to be held on or before June 30, 2001.

         Section 8.02 Default Remedies.

         (a) Upon the occurrence of an Event of Default, the Holder may, upon
         ten (10) days prior written notice to the Company, declare the
         Debenture to be, and the outstanding principal amount of the Debenture
         shall thereupon be and become, forthwith due and payable in cash,
         together with interest accrued thereon; and

         (b) If an Event of Default occurs, the Holder may proceed to protect
         and enforce its rights by a suit in equity, action at law, or other
         appropriate proceeding, whether for the specific performance of any
         agreement contained herein or for an injunction against a violation of
         any of the terms or provisions hereof, or in aid of the exercise of any
         power granted herein or by law.

         Section 8.03 Waiver of Events Default. The holders of 51 percent (51%)
of the aggregate principal amount of the Debentures outstanding may at any time
waive an existing Event of Default and its consequences.

                                      -19-
<PAGE>   20

                                   ARTICLE IX

                              TRANSFER OF DEBENTURE

         Section 9.01 Restriction on Transfer. In addition to any other
restrictions on transfer of the Debenture imposed by this Article IX, the Holder
may transfer or assign his, her, or its rights and obligations under this
Agreement only in conjunction with the transfer or assignment of the Debenture.

         Section 9.02 Requirements of Transfer. No transfer of the Debenture
shall be valid and effective unless and until (a) the transferor executes a
written assignment of the Debenture or executes a separate power of attorney
indicating his intent to transfer ownership, (b) the transferee executes a
Debenture Agreement, which shall be identical to this Agreement except for the
Holder's name and the date of execution, and (c) the transferor delivers written
transfer instructions (i) signed by the transferor and the transferee, (ii)
stating the name and mailing and residence address of the transferee, and (iii)
stating the desired effective date of such change of ownership. If the
transferee fails to execute a Debenture Agreement, the transferee's signature on
the instructions of transfer will be deemed to constitute the transferee's
assent to the terms of the Debenture and the Debenture Agreement.

         Section 9.03 Registration of Transfer. Transfer of the Debenture shall
be registered upon the Company's register of Debentures following the Company's
receipt of all documents necessary to effect transfer in accordance with Section
9.02. Such documents may be either personally delivered by the transferor or
transferee or mailed to the Company in accordance with Section 12.01 hereof.

         Section 9.04 Effective Date of Transfer. The effective date of the
transfer recorded on the Company's register of Debentures shall be the date
requested in the instructions of transfer; the effective date shall not,
however, precede the date of the most recent payment date of interest with
respect to such Debenture. In the event such date precedes the date of the most
recent payment of interest on the Debenture or if the desired date is omitted
from the instructions of transfer, the Company may in its discretion honor the
transfer, and, in such case, the effective date of transfer shall be the first
date at which the Company is in receipt of all of the items required by Section
9.02 hereof.

         Section 9.05 Transferee as Holder. Upon completion of a transfer in
accordance with the provisions provided in this Article X, such Transferee shall
be considered the Holder as if the transferee had been the original party to
execute this Agreement.

         Section 9.06 Issuance of New Certificates. Upon a transfer in
accordance with this Article X, and upon delivery by the transferor of his, her,
or its Debenture certificate representing the Debenture being transferred, the
Company shall cancel such Debenture certificate and shall issue a new
certificate in the transferee's name. Such new certificate shall be issued in
accordance with Article II hereof, and its provisions will be identical to those
of the old Debenture certificate except as to the Holder's name and the date of
execution, which date on the new certificate shall be the same as the effective
transfer date in accordance with Section 9.04 hereof.

                                      -20-
<PAGE>   21

         Section 9.07 Legend on Debenture. The Debenture shall bear the
following legend:

         "THIS DEBENTURE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
         1933, AS AMENDED (THE "ACT") OR THE SECURITIES LAWS OF ANY STATE, AND
         HAS BEEN ISSUED PURSUANT TO EXEMPTIONS FROM SUCH REGISTRATION AND
         QUALIFICATION REQUIREMENTS. THIS DEBENTURE MAY NOT BE SOLD,
         TRANSFERRED, OR ASSIGNED WITHOUT THE PERMISSION OF THE ISSUER AND
         UNLESS THIS DEBENTURE SHALL HAVE BEEN DULY REGISTERED UNDER THE ACT AND
         REGISTERED OR QUALIFIED UNDER APPLICABLE STATE SECURITIES LAWS, OR, IN
         THE OPINION OF COUNSEL SATISFACTORY TO THE ISSUER, REGISTRATION AND
         QUALIFICATION OF THE DEBENTURE SHALL NOT BE REQUIRED. THIS DEBENTURE IS
         SUBJECT TO AND ITS TRANSFER IS RESTRICTED BY THE TERMS AND PROVISIONS
         OF THAT CERTAIN DEBENTURE AGREEMENT, DATED MARCH 30, 2001, EXECUTED BY
         AND BETWEEN THE COMPANY AND THE HOLDER OF THIS DEBENTURE, A COPY OF
         WHICH IS ON FILE IN THE OFFICES OF THE COMPANY."

                                  ARTICLE X I

                               REGISTRATION RIGHTS

         Section 10.01 Registration on Request. Upon conversion of the Debenture
into Common Stock as provided in Article VII above, the Company and the Holder
shall agree to abide by and honor the terms of that certain Registration Rights
Agreement executed simultaneously herewith and attached hereto as Exhibit 10.01.

                                   ARTICLE XI

                      CONSOLIDATION, MERGER, AND CONVEYANCE

         Section 11.01 Continuation of Terms of Agreement. Nothing contained in
this Agreement or in the accompanying Debenture shall prevent any consolidation
or merger of the Company with or into any other corporation or association, or
any conveyance of the business, assets, and properties of the Company as a whole
or substantially as a whole, to any other corporation or other entity, provided
that all terms and conditions of this Agreement, including payment, to be
observed and performed by the Company shall be expressly assumed by the
successor entity formed by or resulting from any such merger or to which any
such conveyance shall have been made.

         Section 11.02 Rights of Successor. If the Company or any successor
entity is consolidated or merged with or into, or shall make a conveyance to,
any other corporation or other entity, as permitted and upon the terms provided
in this Article XI, the entity formed by or resulting from such consolidation or
merger or to which such conveyance shall have been made shall succeed to and be
substituted for the Company, with the same force and effect as if it had

                                      -21-
<PAGE>   22

been named in, and had executed, this Agreement, and shall have and possess and
may exercise, subject to the terms and conditions of this Agreement, each and
every power, authority, and right herein reserved to or conferred upon the
Company.

         Section 11.03 Construction. For every purpose of this Agreement,
including the execution and issuance of the Debenture, the term "Corporation"
includes and means (unless the context otherwise requires) not only the
corporation that has executed this Agreement, but also any such successor entity
in accordance with the provisions of this Article XI.

                                  ARTICLE XII

                            MISCELLANEOUS PROVISIONS

         Section 12.01 Notices. Any notice or other communication required to be
given pursuant to this Agreement must be in writing and may be given by
registered or certified mail, and if given by registered or certified mail,
shall be deemed to have been given and received when a registered or certified
letter containing such notice, properly addressed with postage prepaid, is
deposited in the United States mail; and if given otherwise than by registered
or certified mail, it shall be deemed to have been given when delivered to and
received by the party to whom addressed. Such notices shall be given to the
parties hereto at the following addresses:

                  If to the Company:

                           South Texas Drilling & Exploration, Inc.
                           9310 Broadway, Building I
                           San Antonio, Texas 78217

                           with a copy to:

                           John D. Fisch
                           Matthews and Branscomb
                           112 East Pecan Street
                           Suite 1100
                           San Antonio, Texas 78205

                  If to the Holder:

                           WEDGE Energy Services, L.L.C.
                           1415 Louisiana
                           Suite 3000
                           Houston, Texas 77002
                           Attention: President

                           with a copy to:

                           WEDGE Group Incorporated
                           1415 Louisiana
                           Suite 3000
                           Houston, Texas 77002
                           Attention: General Counsel

                                      -22-
<PAGE>   23

                           with an additional copy to:

                           Darryl M. Burman
                           DiCecco, Fant & Burman
                           3D/International Tower
                           1900 West Loop South
                           Suite 1100
                           Houston, Texas 77027

or addressed to either party at such other address as such party shall hereafter
furnish to the other party in writing. The address for any purpose hereof may be
changed at any time and shall be the most recent address furnished in writing to
the other party.

         Section 12.02 Binding Agreement. This Agreement shall be binding upon
and inure to the benefit of the parties hereto and their respective heirs,
executors, administrators, legal representatives, successors, and assigns,
except as otherwise expressly provided herein.

         Section 12.03 Severability. If any one or more of the provisions
contained in this Agreement should for any reason be held to be invalid,
illegal, or unenforceable in any respect, such invalidity, illegality, or
unenforceability shall not affect any other provision hereof, and this Agreement
shall be construed as if such invalid, illegal, or unenforceable provision had
never been contained herein.

         Section 12.04 No Third Parties. Except as otherwise expressly provided
herein, nothing in this Agreement, expressed or implied, is intended or shall be
construed to confer upon or give to any person, firm, or corporation other than
the parties hereto and the holders from time to time of the accompanying
Debenture any security, rights, remedies, or claims, legal or equitable, under
or by reason of this Agreement, or under or by reason of any covenant,
condition, or stipulation herein contained; and this Agreement and all the
covenants, conditions, and provisions herein contained are and shall be held for
the sole and exclusive benefit of the parties hereto and the holders from time
to time of the accompanying Debenture.

         Section 12.05 Headings. The captions used in conjunction with this
Agreement are for convenience only, and shall not be deemed a part of this
Agreement or used to construe any provision hereof.

         Section 12.06 Survival of Representations, Warranties, and Covenants.
The representations, warranties, and covenants of the parties shall survive the
execution of this Agreement and the issuance of the Debenture and shall remain
in full force and effect thereafter.

         Section 12.07 Entire Agreement. This Agreement and the accompanying
Debenture constitute the sole and only agreements of the parties hereto and
supersede any prior understandings or written or oral agreements between the
parties respecting the subject matter within.

                                      -23-
<PAGE>   24

         Section 12.08 Inclusion of Debenture. Reference is made to the
accompanying Debenture. The provisions of such Debenture shall be deemed
incorporated into this Agreement for all purposes as though fully set forth on
the face hereof.

         Section 12.09 Governing Law. This Agreement and the Debenture shall be
governed by and construed in accordance with the laws of the State of Texas.

            [the remainder of this page is intentionally left blank]

                                      -24-
<PAGE>   25

         IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed as of the day and year first above written.

COMPANY:

SOUTH TEXAS DRILLING & EXPLORATION, INC.

By:  /s/ Wm. Stacy Locke
     ----------------------------
Name:    Wm. Stacy Locke
Title:   President

                                               HOLDER:

                                               WEDGE ENERGY SERVICES, L.L.C.

                                               By:  /s/ Richard E. Blohm, Jr.
                                                    ----------------------------
                                                    Name:  Richard E. Blohm, Jr
                                                    Title:    Vice President

            [The remainder of this page is intentionally left blank]

                                      -25-
<PAGE>   26

                                  EXHIBIT 1.09

                    SOUTH TEXAS DRILLING & EXPLORATION, INC.

               4.86% CONVERTIBLE SUBORDINATED DEBENTURE, SERIES A
                               DUE MARCH 29, 2002

NO. 01                                                           March 30, 2001

$9,000,000

         SOUTH TEXAS DRILLING & EXPLORATION, INC., a Texas corporation (the
"Company"), for value received, hereby promises to pay to the order of:

                          WEDGE ENERGY SERVICE, L.L.C.
                              or registered assigns
                         on the 29th day of March, 2002
                             the principal amount of

NINE MILLION DOLLARS ($9,000,000)

and to pay interest (computed on the basis of a 360-day year of twelve 30-day
months) on the principal amount from time to time remaining unpaid hereon at the
rate of 4.86% per annum from the date hereof until maturity, and in accordance
with the terms of that certain Debenture Agreement dated March 30, 2001,
calculated and payable at maturity. The Company agrees to pay interest on
overdue principal (including any overdue required or optional prepayment of
principal), and (to the extent legally enforceable) on any overdue installment
of interest, at the rate of ten percent (10%) per annum after maturity, whether
by acceleration or otherwise, until paid. Both the principal hereof and interest
hereon are payable at the principal office of the Company in San Antonio, Texas,
in coin or currency of the United States of America which at the time of payment
shall be legal tender for the payment of public and private debts.

         This Debenture is issued under and pursuant to the terms and provisions
of that certain Debenture Agreement dated March 30, 2001, entered into by the
Company with the original holder therein referred to, and this Debenture and the
holder hereof is entitled equally and ratably with the holders of all other
Debentures, if any, outstanding under the Debenture Agreement to all the
benefits provided for thereby or referred to therein, to which Debenture
Agreement reference is hereby made for the statement thereof.

         This Debenture and the other Debentures, if any, outstanding under the
Debenture Agreement may be declared due prior to their expressed maturity dates
and voluntary prepayments may be made thereon by the Company all in the events,
on the terms specified in the Debenture Agreement, and in the manner and amounts
as provided in the Debenture Agreement.

         This Debenture and the indebtedness evidenced hereby, including the
principal and interest, shall at all times remain junior and subordinate to any
and all Superior Indebtedness as

                                      -26-
<PAGE>   27

defined in the Debenture Agreement, all on the terms and to the extent more
fully set forth in the Debenture Agreement.

         This Debenture is registered on the books of the Company and is
transferable only by surrender thereof at the principal office of the Company
duly endorsed or accompanied by a written instrument of transfer duly executed
by the registered holder of this Debenture or its attorney duly authorized in
writing. Payment of or on account of principal, premium, if any, and interest of
this Debenture shall be made only to or upon the order in writing of the
registered holder.

                                        SOUTH TEXAS DRILLING
                                        & EXPLORATION, INC.

                                        By:
                                            -----------------------------------
                                            Wm. Stacey Locke, President

THIS DEBENTURE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED (THE "ACT") OR THE SECURITIES LAWS OF ANY STATE, AND HAS BEEN ISSUED
PURSUANT TO EXEMPTIONS FROM SUCH REGISTRATION AND QUALIFICATION REQUIREMENTS.
THIS DEBENTURE MAY NOT BE SOLD, TRANSFERRED, OR ASSIGNED WITHOUT THE PERMISSION
OF THE ISSUER AND UNLESS THIS DEBENTURE SHALL HAVE BEEN DULY REGISTERED UNDER
THE ACT AND REGISTERED OR QUALIFIED UNDER APPLICABLE STATE SECURITIES LAWS, OR,
IN THE OPINION OF COUNSEL SATISFACTORY TO THE ISSUER, REGISTRATION AND
QUALIFICATION OF THE DEBENTURE SHALL NOT BE REQUIRED. THIS DEBENTURE IS SUBJECT
TO AND ITS TRANSFER IS RESTRICTED BY THE TERMS AND PROVISIONS OF THAT CERTAIN
DEBENTURE AGREEMENT, DATED AS OF MARCH 30, 2001, EXECUTED BY AND BETWEEN THE
COMPANY AND THE HOLDER OF THIS DEBENTURE, A COPY OF WHICH IS ON FILE IN THE
OFFICES OF THE COMPANY.

                                      -27-
<PAGE>   28

                                  EXHIBIT 10.01

                          REGISTRATION RIGHTS AGREEMENT

                                      -28-

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