<PAGE>
 
                                                                   EXHIBIT 10.11



                           STOCK PURCHASE AGREEMENT

                                    BETWEEN

                         PATINA OIL & GAS CORPORATION

                                      AND

                                 JAY W. DECKER

                                MARCH 16, 1998
<PAGE>
 
     STOCK PURCHASE AGREEMENT, dated March 16, 1998 (this "Agreement") between
Patina Oil & Gas Corporation, a Delaware corporation (the "Company"), and Jay W.
Decker ("Purchaser").

                             W I T N E S S E T H:
                             - - - - - - - - - - 

     WHEREAS, Purchaser was elected President of the Company by the Board of
Directors effective March 16, 1998; and

     WHEREAS, as an inducement to Purchaser to accept employment by the Company,
the Board of Directors agreed to sell to Purchaser up to 100,000 shares of the
Company's common stock, par value $.01 per share ("Common Stock") for a price
per share equal to the closing price of the Common Stock on March 13, 1998 on
the New York Stock Exchange; and

     WHEREAS, the per share closing price of the Common Stock on the New York
Stock Exchange on March on March 13, 1998 was $6.875; and

     WHEREAS, the Company, as a further inducement to Purchaser to accept
employment with the Company, has agreed to loan to Purchaser, on a recourse
basis, 85% of the purchase price, such loan to bear interest at 8.50% per annum;
and

     WHEREAS, the Company as a further inducement to Purchaser to accept
employment with the Company, has agreed to grant to Purchaser, subject to the
Restricted Stock Agreement as herein defined, shares of Common Stock equal in
number to the number of shares of Common Stock Purchaser elects to purchase; and

     WHEREAS, Purchaser has elected to purchase 100,000 shares of Common Stock
from the Company for a total purchase price of $687,500 and will therefore be
granted 100,000 shares of Common Stock all being subject to the terms of this
Agreement, the Promissory Note, the Pledge Agreement and the Restricted Stock
Agreement as herein defined.

     NOW THEREFORE, in consideration of the premises and the representations,
warranties and agreements herein contained and intending to be legally bound
hereby, the parties hereby agree as follows:

                                  ARTICLE I.

                                  Definitions
                                  -----------

     Section 1.01. Definitions.  As used in this Agreement, the following terms
                   -----------                                                 
shall have the meanings set forth below:

     "Agreement" shall have the meaning set forth in the recitals hereto.
      ---------                                                          

     "Common Stock" shall have the meaning set forth in the recitals hereto.
      ------------                                                          

                                       2
<PAGE>
 
     "Company" shall have the meaning set forth in the recitals hereto.
      -------                                                          

     "Exchange Act" shall mean the Securities Exchange Act of 1934, as amended.
      ------------                                                             

     "Person" or "person" shall mean an individual, corporation, association,
      ------      ------                                                     
partnership, trust, joint venture, business trust or unincorporated
organization, or a government or any agency or political subdivision thereof.

     "Pledge Agreement" shall have the meaning set forth in Section 2.02(b).
      ----------------                                                      

     "Promissory Note" shall have the meaning set forth in Section 2.02(b).
      ---------------                                                      

     "Purchase Price" shall mean $6.875 per Common Share.
      --------------                                     

     "Securities Act" shall mean the Securities Act of 1933, as amended.
      --------------                                                    

     "SEC" shall mean the United States Securities and Exchange Commission.
      ---                                                                  


                                  ARTICLE II.

                       Sale and Purchase of Common Stock
                       ---------------------------------

     Section 2.01.  Sale and Purchase of Common Stock.  Subject to all of the
                    ---------------------------------                        
terms and conditions of this Agreement, and in reliance upon the representations
and warranties hereinafter set forth, the Company will sell Purchaser, and
Purchaser will purchase from the Company 100,000 shares of Common Stock at the
Purchase Price for a total of $687,500.

     Section 2.02.  Closing.
                    ------- 

     (a)       Subject to the satisfaction or waiver of the conditions set forth
     in this Agreement, the sale and purchase of the Common Stock pursuant to
     Section 2.01 (the "Closing") shall take place at the offices of the Company
                        ------- 
     on March 18, 1998 (the "Closing Date"), or at such other time and place as
                             ------------
     may be mutually agreed upon by the Purchaser and the Company.

     (b)       At the Closing, or as soon thereafter as reasonably practicable:

               (i)  the Company will deliver to Purchaser a certificate or
               certificates for the Common Stock being sold to Purchaser in
               accordance with the provisions of Section 2.01, registered in the
               Purchaser's name;

               (ii) Purchaser will deliver to the Company in full payment for
               the Common Stock to be purchased pursuant to Section 2.01, (A) a
               Promissory Note dated the date of the Closing, in a principal
               amount of $584,375 and in substantially 

                                       3
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               the form attached hereto as Exhibit A ("Promissory Note"), and 
                                           ---------                         
               (B) a check in the amount of $103,125 for the remaining balance
               of the aggregate Purchase Price for the shares of Common Stock to
               be purchased by Purchaser,

               (iii)  a Pledge Agreement of Purchaser dated the date of the
               Closing, in substantially the form attached hereto as Exhibit B
               (each a "Pledge Agreement"); and
                        ---------------- 

               (iv)   each party shall take or cause to happen such other
               actions, and shall execute and deliver such other instruments or
               documents, as shall be required under Article V hereof.


                                 ARTICLE III.

                        Recommendations and Warranties
                        ------------------------------

     Section 3.01.  Representations and Warranties of the Company.  The Company
                    ---------------------------------------------              
represents and warrants to, and agrees with, the Purchaser as follows:

     (a)  Organization and Good Standing.  The Company is a corporation duly
          ------------------------------                                    
     organized, validly existing and in good standing under the laws of its
     jurisdiction of incorporation and has all requisite corporate power and
     authority to own, operate and lease its properties and to carry on its
     business as it is now being conducted.
 
     (b)  Authorization.  The Company has full corporate power and authority to
          -------------                                                        
     enter into this Agreement and to consummate the transactions contemplated
     hereby. The execution, delivery and performance of this Agreement and the
     consummation of the transactions contemplated hereby has been duly
     authorized by the Compensation Committee of the Board of Directors of the
     Company. No other corporate proceedings on the part of the Company are
     necessary to authorize the execution, delivery and performance of this
     Agreement and the transactions contemplated hereby. This Agreement has been
     duly and validly executed and delivered by the Company and this Agreement
     constitutes a valid and binding obligation of the Company enforceable in
     accordance with its terms, except as such enforceability may be limited by
     bankruptcy, insolvency, reorganization, moratorium and other similar laws
     relating to or affecting creditors' rights generally, by general equity
     principles (regardless of whether such enforceability is considered in a
     proceeding in equity or at law) and by an implied covenant of good faith
     and fair dealing.

     (c)  Newly Issued Shares of Common Stock.  The shares of Common Stock to be
          -----------------------------------                                   
     issued and sold hereunder have been duly authorized to be so issued and
     sold by all necessary corporate action. When issued and sold against
     receipt of the consideration therefor, the shares of Common Stock to be
     issued and sold hereunder will be validly issued, fully paid and
     nonassessable, will not subject the holders thereof to any personal
     liability and will not be subject to any preemptive rights of any other
     stockholder of the 

                                       4
<PAGE>
 
     Company. At the Closing, the Purchaser will receive valid title to the
     Common Stock to be purchased on such date, free and clear of any claim,
     lien, security interest or other encumbrance.
     
     Section 3.02.  Representations and Warranties of Purchaser.  Purchaser
                    -------------------------------------------            
represents and warrants to, and agrees with, the Company as follows:

     (a)  Legal Capacity.  Purchaser has the legal capacity to execute, deliver
          --------------                                                       
     and perform this Agreement.

     (b)  Authorization.  The execution, delivery and performance of this
          -------------                                                  
     Agreement and the consummation of the transactions contemplated hereby have
     been authorized by all necessary action on behalf of Purchaser. No other
     proceedings on the part of such Purchaser are necessary to authorize the
     execution, delivery and performance of this Agreement and the transactions,
     contemplated hereby. This Agreement has been duly and validly executed and
     delivered by Purchaser and this Agreement constitutes a valid and binding
     obligation of Purchaser enforceable in accordance with its terms, except as
     such enforceability may be limited by bankruptcy, insolvency,
     reorganization, moratorium and other similar laws relating to or affecting
     creditors' rights generally, by general equity principles (regardless of
     whether such enforceability is considered in a proceeding in equity or at
     law) and by an implied covenant of good faith and fair dealing.

     (c)  Securities Act.
          -------------- 

          (i)  Purchaser is acquiring the Common Stock solely for the purpose of
          investment and not with a view to, or for resale in connection with,
          any distribution thereof in violation of the Securities Act. Purchaser
          has been given the opportunity to obtain any and all relevant
          information and documents relating to the Common Stock and to ask
          questions and receive answers about the Company and its business which
          Purchaser deems necessary to evaluate the merits and risks related to
          an investment in the Common Stock, and Purchaser has relied solely on
          such information.

          (ii) Purchaser further represents and warrants that (A) his financial
          condition is such that he can afford to bear the economic risk of
          holding the Common Stock for an indefinite period of time and has
          adequate means for providing for his current needs and personal
          contingencies, (B) Purchaser can afford to suffer a complete loss of
          his investment in the Common Stock, (C) he understands and has taken
          cognizance of all risk factors related to the purchase of the Common
          Stock, and (D) Purchaser's knowledge and experience in financial and
          business matters are such that he is capable of evaluating the merits
          and risks of his purchase of the Common Stock as contemplated by this
          Agreement.

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<PAGE>
 
     (d)  Brokers and Finders.  Purchaser has not utilized any broker, finder,
          -------------------                                                 
     placement agent or financial advisor or incurred any liability for any fees
     or commissions in connection with any of the transactions contemplated
     hereby.


                                  ARTICLE IV.

                     Additional Agreements of the Parties
                     ------------------------------------

     Section 4.01.  Taking of Necessary Action.  Each of the parties hereto
                    --------------------------                             
agrees to use all reasonable efforts promptly to take or cause to be taken all
action and promptly to do or cause to be done all things necessary, proper or
advisable under applicable laws and regulations to consummate and make effective
the transactions contemplated by this Agreement.  Without limiting the
foregoing, the Company and the Purchaser will, and the Company shall cause its
subsidiaries to, each use all reasonable efforts to make all filings and obtain
all consents of governmental entities or other persons relating to such party
which may be necessary or, in the opinion of the Company or the Purchaser, as
the case may be, advisable for the consummation of the transactions contemplated
by this Agreement.

     Section 4.02.  Restrictions on Sale or Transfer; Legend:
                    ---------------------------------------- 

     (a)  The Purchaser hereby acknowledges and agrees that shares of Common
     Stock to be purchased by him hereunder will be, upon the sale and purchase
     of such shares in accordance with the terms hereof, "restricted securities"
     under the Securities Act.  The Purchaser agrees that he will not, directly
     or indirectly, offer, sell, transfer, assign or otherwise dispose of the
     beneficial ownership of (any such act, a "Transfer") any such shares of
                                               --------                     
     Common Stock, except as permitted under the Securities Act and the rules
     and regulations thereunder (including, without limitation, Transfers
     pursuant to Rule 144 under the Securities Act).

     (b)  The Purchaser acknowledges and agrees that as of the date hereof the
     shares of Common Stock issued and sold hereunder have not been and will not
     be registered under the Securities Act or the securities laws of any state
     and that they may be sold or otherwise disposed of only in one or more
     transactions registered under the Securities Act (and, where applicable,
     such laws) or as to which an exemption from the registration requirements
     of the Securities Act (and where applicable, such laws) is available.  The
     Purchaser acknowledges that, except as provided in this Agreement, he has
     no right to require the Company to register the shares of Common Stock
     purchased hereunder.  The Purchaser further acknowledges and agrees that
     each certificate for the Common Stock purchased hereunder shall bear the
     following legend:

     THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
UNDER THE SECURITIES ACT OF 1993, AS AMENDED, AND MAY NOT BE TRANSFERRED, SOLD
OR OTHERWISE DISPOSED OF EXCEPT WHILE SUCH A REGISTRATION IS IN EFFECT UNDER
SUCH ACT AND APPLICABLE STATE 

                                       6
<PAGE>
 
SECURITIES LAWS OR PURSUANT TO AN EXEMPTION FROM REGISTRATION UNDER SUCH ACT OR
SUCH LAWS. THIS CERTIFICATE IS ISSUED PURSUANT TO AND SUBJECT TO THE
RESTRICTIONS ON TRANSFER, VOTING AND OTHER PROVISIONS OF A STOCK PURCHASE
AGREEMENT DATED MARCH 16, 1998 BETWEEN THE COMPANY AND THE PURCHASER, A COPY OF
WHICH IS ON FILE WITH THE COMPANY. EXCEPT AS PROVIDED IN SUCH STOCK PURCHASE
AGREEMENT, THE SECURITIES REPRESENTED BY THIS CERTIFICATE ARE NOT TRANSFERABLE
AND ANY PURPORTED TRANSFER IN VIOLATION OF THE PROVISIONS OF SUCH STOCK PURCHASE
AGREEMENT SHALL BE VOID AND OF NO FORCE AND EFFECT.
 
     Section 4.03.  New York Stock Exchange Listing.  As promptly as practicable
                    -------------------------------                             
following the Closing Date, the Company will apply to the New York Stock
Exchange to list the shares of Common Stock issued and sold hereunder, and the
Company will use its reasonable efforts to cause such shares to be listed on the
New York Stock Exchange as promptly thereafter as practicable.
 
     Section 4.04.  Piggyback Registration Rights.  If the Company proposes to
                    -----------------------------                             
file a registration statement under the Securities Act with respect to an
offering for its own account of any class of its equity securities (other than a
registration statement on Form S-8 (or any successor form) or any other
registration statement relating solely to director and/or employee benefit plans
or filed in connection with an exchange offer, a transaction to which Rule 145
(or any successor rule) under the Securities Act applies or an offering of
securities solely to the Company's existing stockholders), then the Company
shall in each case give written notice of such proposed filing to the Purchaser
as soon as practicable before the anticipated filing date, and such notice shall
offer the Purchaser the opportunity to register such number of shares of Common
Stock purchased hereunder and such number of shares of Common Stock which have
vested pursuant to the Restricted Stock Agreement, in each case as the Purchaser
may request; provided that the Company shall not be required to deliver such
             --------                                                       
notice to the Purchaser at the time such notice is to be delivered if he is able
to transfer, pursuant to Rule 144 under the Securities Act, all the shares of
Common Stock owned by him that could otherwise be included in the Company's
registered offering.  If the Purchaser desires to have his shares included in
such registration statement, he shall so advise the Company in writing within
five business days after the date of the Company's notice, setting forth the
amount of his shares for which registration is requested.  If the Company's
offering is to be an underwritten offering, the Company shall use its reasonable
efforts to cause the managing underwriter or underwriters of a proposed
underwritten offering to permit the Purchaser's shares to be included in the
registration for such offering to include such securities in such offering on
the same terms and conditions as any similar securities of the Company included
therein, provided that:
         --------      
 
     (i)  if the registration of which the Company gives notice involves an
     underwriting, the right of the purchaser to registration pursuant to this
     Section shall, unless the Company otherwise agrees, to be conditioned upon
     the Purchaser's participation as a seller in such underwriting and
     execution of an underwriting agreement with the managing underwriter or
     underwriters selected by the Company, and

                                       7
<PAGE>
 
     (ii) if the managing underwriter or underwriters of such offering informs
     the Company that the success of the offering would be materially and
     adversely affected by the inclusion of the Purchaser's shares requested to
     be included, then the Company shall not be required to include such shares
     in such offering.


                                  ARTICLE V.

                             Conditions Precedent
                             --------------------

     Section 5.01.  Conditions to the Purchaser's Obligations.  The obligation
                    -----------------------------------------                 
of the Purchaser to consummate the Closing hereunder shall be subject to the
satisfaction on the Closing Date of each of the following conditions:
 
     (a)  Representations and Warranties.  The representations and warranties of
          ------------------------------                                        
     the Company contained in this Agreement which are qualified as to
     materiality shall be true and correct, and which are not so qualified shall
     be true and correct in all material respects, in each case, as of the date
     of this Agreement and on and as of the Closing Date with the same effect as
     though made on and as of such dates.

     (b)  Covenants.  The Company shall have performed in all material respects
          ---------                                                            
     all obligations and complied with all agreements, undertakings, covenants
     and conditions required by it to be performed at or prior to the Closing.

     (c)  Restricted Stock Agreement.  The Company shall have executed and
          --------------------------                                      
     delivered to the Purchaser the Restricted Stock Agreement.

     (d)  Additional Certificates, Etc.  The Company shall have executed and
          -----------------------------                                     
     delivered, or caused to be executed and delivered to the Purchaser the
     certificates and other documents related to the consummation of the
     transactions contemplated hereby as may be reasonably requested by the
     Purchaser.

     Section 5.02.  Conditions to the Company's Obligations.  The obligation of
                    ---------------------------------------                    
the Company to consummate the Closing hereunder shall be subject to the
satisfaction on the Closing Date of each of the following conditions:
 
     (a)  Representations and Warranties.  The representations and warranties of
          ------------------------------                                        
     the Purchaser contained in this Agreement which are qualified as to
     materiality shall be true and correct, and which are not so qualified shall
     be true and correct in all material respects, in each case, as of the date
     of this Agreement and on and as of the Closing Date with the same effect as
     though made on and as of such dates.

     (b)  Covenants. The Purchaser shall have performed in all material respects
          ---------  
     all obligations and complied with all agreements, undertakings, covenants
     and conditions required by it to be performed at or prior to the Closing.

                                       8
<PAGE>
 
     (c)  Promissory Notes and Pledge Agreements.  Purchaser shall have executed
          --------------------------------------                                
     and delivered to the Company a Promissory Note and a Pledge Agreement, in
     each case substantially in the form of Exhibits A and B attached hereto,
                                            ----------------                 
     respectively.

     (d)  Additional Certificates, Etc.  Purchaser shall have executed and
          -----------------------------                                   
     delivered, or caused to be executed and delivered, to the Company such
     certificates and other documents related to the consummation of the
     transactions contemplated hereby as may be reasonably requested by the
     Company.


                                  ARTICLE VI.

                                     Term
                                     ----

     Section 6.01.  Termination.  This Agreement may be terminated on or any
                    -----------                                             
time prior to the Closing by the mutual written consent of the Purchaser and the
Company.

     Section 6.02.  Effect of Termination.  In the event of the termination of
                    ---------------------                                     
this Agreement as provided herein, this Agreement shall forthwith become void
except for the provisions set forth in Article VII and there shall be no
liability or obligation on the part of the parties hereto except as otherwise
provided in this Agreement.


                                 ARTICLE VII.

                                 Miscellaneous
                                 -------------

     Section 7.01.  No Survival of Representations and Warranties  No
                    ---------------------------------------------    
representation or warranty made herein or in any certificates delivered in
connection with the Closing shall survive the Closing.

     Section 7.02.  Notices.  All notices and other communications hereunder
                    -------                                                 
shall be in writing and shall be deemed to have been duly given, if delivered
personally, by telecopier or sent by overnight courier as follows:
 
     (a)  If to the Purchaser, to the Purchaser, c/o the Company at its address
     set forth below or such other address as set forth in writing by Purchaser;
     and

     (b)  If to the Company, to:
          Patina Oil & Gas Corporation
          1625 Broadway, Suite 2000
          Denver, Colorado 80202
          Phone:  303-389-3600
          Fax:  303-595-7408
          Attention:  General Counsel
 

                                       9
<PAGE>
 
or to such other address or addresses as shall be designated in writing.  All
notices shall be effective when received.

     Section 7.03.  Entire Agreement; Amendment.  This Agreement and the
                    ---------------------------                         
documents described herein and therein or attached or delivered pursuant hereto
or thereto set forth the entire agreement among the parties hereto with respect
to the transactions contemplated by this Agreement.  Any provision of this
Agreement may be amended or modified in whole or in part at any time by an
agreement in writing among the parties hereto executed in the same manner as
this Agreement.

     Section 7.04.  Counterparts.  This Agreement may be executed in one or more
                    ------------                                                
counterparts, each of which shall be deemed to constitute an original, but all
of which together shall constitute one and the same documents.

     SECTION 7.05.  GOVERNING LAW.  THIS AGREEMENT SHALL BE GOVERNED BY, AND
                    -------------                                           
INTERPRETED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF DELAWARE APPLICABLE TO
CONTRACTS MADE AND TO BE PERFORMED IN THAT STATE.

     Section 7.06.  Successors and Assigns.  This Agreement shall inure to the
                    ----------------------                                    
benefit of and be binding upon the parties hereto and their respective
successors and assigns.  Neither this Agreement nor any of the parties' rights,
interests or obligations hereunder shall be assignable by any party hereto
without the prior written consent of the other parties hereto.  No assignment
shall relieve the assigning party of any of its obligations hereunder.  Any
attempted assignment of this Agreement in breach of this provision shall be void
and of no effect.

     Section 7.07.  No Third Party Rights.  Nothing in this Agreement, expressed
                    ---------------------                                       
or implied, shall or is intended to confer upon any person other than the
parties hereto or their respective successors or assigns any rights or remedies
of any nature or kind whatsoever under or by reason of this Agreement.

     Section 7.08.  Captions.  The captions contained in this Agreement are for
                    --------                                                   
reference purposes only and are not part of this Agreement.

     Section 7.09.  Severability.  Should any part of this Agreement for any
                    ------------                                            
reason be declared invalid, such decision shall not affect the validity of any
remaining portion, which remaining portion shall remain in full force and effect
as if this Agreement had been executed with the invalid portion thereof
eliminated, and it is hereby declared the intention of the parties hereto that
they would have executed the remaining portion of this Agreement without
including therein any such part or parts which may, for any reason, be hereafter
declared invalid.

     Section 7.10.  Mutual Waiver of Jury Trial.  Because disputes arising in
                    ---------------------------                              
connection with complex financial transactions are most quickly and economically
resolved by an experienced and expert person and the parties wish applicable
state and federal laws to apply (rather than 

                                       10
<PAGE>
 
arbitration rules), the parties desire that their disputes be resolved by a
judge applying such applicable laws. Therefore, to achieve the best combination
of the benefits of the judicial system and of arbitration, the parties hereto
waive all right to trial by jury in any action, suit or proceeding brought to
enforce or defend any rights or remedies under this Agreement.

     IN WITNESS WHEREOF, this Agreement has been executed by the parties hereto
or by their respective duly authorized officers, all as of the date first above
written.

                                   PATINA OIL & GAS CORPORATION


                                   By:  /s/ Brian J. Cree
                                        -------------------------------------
                                        Brian J. Cree
                                        Executive Vice President


                                   PURCHASER

 
                                   By:  /s/ Jay W. Decker
                                        -------------------------------------
                                        Jay W. Decker

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