<PAGE>
 
                                                                    EXHIBIT 12.3
                            Offer to Acquire Notes
                            ----------------------
                                February 12,1999

     Western Gas Resources, Inc., and Guarantors (collectively "Company")
pursuant to that certain Note Purchase Agreement dated as of April 1, 1993, as
amended to date, by and among the Company and the Purchasers named therein
relating to the purchase and sale of the Company's 7.65% Senior Notes due 
April 30, 2003 (the "Agreement") hereby offers to acquire Notes as
described in and pursuant to the terms of Paragraph 7.16 of the Agreement
under the following terms and conditions. Unless otherwise defined herein, the
terms defined in the Agreement shall be used herein as therein defined.

     This offer is being made to Purchasers for payment of the Notes in full at
par value of the principal amount with interest on such amount being paid as
accrued to the "Payment Date" as defined below. The principal amount due to each
Purchaser under this Offer to Acquire Notes is as per Schedule I attached
hereto. This Offer to Acquire Notes is subject to the approval of the majority
lenders of the Company's Loan Agreement with NationsBank of Texas, N.A. as
agent, and will expire at 8:00 am MST on Tuesday, February 16, 1999 ("the
Expiration"). The "Payment Date" hereunder shall be a date not later than
March 5, 1999. In the event that the undersigned Purchaser has accepted this
Offer to Acquire Notes, and such Purchaser does not receive payment in full of
all its Notes in accordance with the terms hereof on or before March 5, 1999,
such Purchaser's acceptance of the terms of this Offer to Acquire Notes shall be
null and void and of no force and effect.

     Upon timely receipt of full payment of its Notes in accordance with the
foregoing terms of this Offer to Acquire Notes by each Purchaser which has
accepted this offer, such Purchaser's Notes shall be surrendered to the Company
for cancellation and shall not be reissued, and no Notes will be issued in
substitution thereof.

     From the date of acceptance of this Offer to Acquire Notes until the timely
receipt of full payment of said Notes in accordance with the foregoing terms of
this offer, but no later than the Payment Date, each Purchaser accepting this
offer agrees to waive compliance with Section 7.11 of the Agreement for the
fiscal quarter ended December 31, 1998; provided however, that for the period of
four consecutive fiscal quarters ended on December 31, 1998, Income Available
for Fixed Charges shall be at least two hundred forty percent (240%), of Fixed
Charges.

     Based upon the foregoing terms and conditions contained in this offer to
acquire Notes, please indicate below your acceptance or nonacceptance of the
terms hereof and return by facsimile transmission to Company. Your failure to
respond prior to the Expiration shall be deemed to constitute your decision to
decline this Offer to Acquire Notes.
<PAGE>
 
Offer to Acquire Notes
February 12, 1999
Page 2

           X    The undersigned Purchaser ACCEPTS the offer to acquire Notes 
         -----
                under the terms and conditions of this offer by Company;


                The undersigned Purchaser DECLINES the offer to acquire Notes 
         -----
                under the terms and conditions of this offer by Company;


                Dated this day of 11th day of February, 1999
                                  ----        -------- 

Western, Gas Resources, Inc.


  /s/  Vance S. Blalock
---------------------------
By:
Name:  VANCE S. BLALOCK
Title: TREASURER
Fax:   (303) 254-9794
<PAGE>
 
Offer to Acquire Notes
February 12, 1999
Page 3

ACKNOWLEDGED AND ACCEPTED:

Connecticut General Life Insurance Company* 
Connecticut General Life Insurance Company,
   on behalf of one or more separate accounts 
Life Insurance Company of North America* 
Insurance Company of North America*

BY:  CIGNA Investments, Inc.,

     By:   /s/ [ILLEGIBLE] 
        --------------------------------

     * This entity is either the registered owner of one or more of the
     securities pertaining hereto or is a beneficial owner of one or more of
     such securities owned by and registered in the name of a nominee for that
     entity.


BY:  THE CANADA LIFE ASSURANCE COMPANY

     By:   /s/ [ILLEGIBLE]    
        --------------------------------
               CUMMINGS & CO.
               PARTNER  
 
BY:  CANADA LIFE INSURANCE COMPANY OF AMERICA

     By:   /s/ [ILLEGIBLE]    
        --------------------------------
               CUMMINGS & CO.
               PARTNER  


BY:  CANADA LIFE INSURANCE COMPANY OF NEW YORK

     By:   
        --------------------------------

BY:  THE FRANKLIN LIFE INSURANCE COMPANY

     By:   
        --------------------------------

BY:  ROYAL MACCABEES LIFE INSURANCE COMPANY

     By:   /s/ Leonard D. [ILLEGIBLE]       
        --------------------------------
               VICE PRESIDENT
<PAGE>
 
Offer to Acquire Notes 
February 12, 1999 
Page 4

SCHEDULE I

$27,857,142.86    Connecticut General Life Insurance Company* 
                  Connecticut General Life Insurance Company,
                     on behalf of one or more separate accounts 
                  Life Insurance Company of North America*
                  Insurance Company of North America 
                  *This entity is either the registered owner of one or more of
                  the securities pertaining hereto or is a beneficial owner of
                  one or more of such securities owned by and registered in the
                  name of a nominee for that entity.
                                 

$ 3,571,428.58    The Canada Life Assurance Company
                  Canada Life Insurance Company of America
                  Canada Life Insurance Company of New York  


$ 2,142,857.16    The Franklin Life Insurance Company
                 

$ 2,142,857.16    Royal Maccabees Life Insurance Company