<PAGE>   1
                                                                   EXHIBIT 10.26


                   FIRST AMENDMENT TO STOCKHOLDERS' AGREEMENT

                  This FIRST AMENDMENT TO STOCKHOLDERS' AGREEMENT, dated as of
June 1, 2000 (this "First Amendment"), is by and between BASIC ENERGY WELL
SERVICES, INC. (formerly named SIERRA WELL SERVICE, INC.), a Delaware
corporation (the "Company"), JOINT ENERGY DEVELOPMENT INVESTMENTS II LIMITED
PARTNERSHIP, a Delaware limited partnership ("JEDI II") and the "Other
Stockholders" listed on the signature page hereto (together with JEDI II, each a
"Stockholder" and collectively, the "Stockholders"), and amends certain
provisions of that Stockholders Agreement dated as of March 21, 2000 (the
"Agreement") between the Company and each of the Stockholders.

                                 R E C I T A L S

                  WHEREAS, the Company and each of the Stockholders are party to
the Agreement; and

                  WHEREAS, subsequent to the Agreement, the parties have agreed
to modify certain terms of the Agreement as set forth below; and

                  WHEREAS, in connection with the execution and delivery of this
First Amendment and the Agreement, the Company has commenced a firm commitment
underwritten public offering pursuant to a registration statement under the
Securities Act and desires to further amend certain terms of the Agreement and
the Certificates of Designations for the Preferred Stock (as defined below);

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

                  SECTION 1. Certain Defined Terms. As used in this First
Amendment, defined terms not otherwise defined herein shall have the meanings
ascribed thereto in the Agreement.

                  SECTION 2. Termination of Old Stockholders' Agreement. The
Company, JEDI II and each of the other Stockholders hereby agree that Section 2
of the Agreement is hereby amended and restated in its entirety as follows:

                  SECTION 2. Termination of Old Stockholders' Agreement. The
         Company, JEDI II and each of the other Stockholders hereby agree the
         Old Stockholders' Agreement shall be terminated and of no further force
         and effect immediately and without further action by the Company or any
         Stockholder at such time as the Company (1) has completed an Initial
         Public Offering by June 30, 2000, (2) repaid a portion of the
         Subordinated Debt, together with accrued and unpaid interest, such that
         not more than $10 million aggregate principal amount remains
         outstanding, and (3) the shares of Series A Preferred Stock are no
         longer outstanding.


<PAGE>   2

                  SECTION 3. Agreements relating to Series B Preferred Stock.
The Company, JEDI II and each of the other Stockholders hereby agree that
Section 3(a) of the Agreement is hereby amended and restated in its entirety as
follows:

                  SECTION 3. Agreements relating to Series B Preferred Stock.
         The Company and JEDI II hereby agree that notwithstanding the existing
         terms of the Series B Preferred Stock:

                  (a) JEDI II hereby elects to convert its shares of Series B
         Preferred Stock, and that all its shares of Series B Preferred Stock
         shall be converted immediately and without further action by JEDI II,
         otherwise in accordance with Section 8 of the Series B Designation,
         effective immediately at such time as the Company (1) has completed an
         Initial Public Offering by June 30, 2000, (2) repaid a portion of the
         Subordinated Debt, together with accrued and unpaid interest, such that
         not more than $10 million aggregate principal amount remains
         outstanding, and (3) the shares of Series A Preferred Stock are no
         longer outstanding; and

                  SECTION 4. Agreements relating to Series C Preferred Stock.
The Company, JEDI II and each of the other Stockholders hereby agree that
Section 4 of the Agreement is hereby amended and restated in its entirety as
follows:

                  SECTION 4. Agreements relating to Series C Preferred Stock.
         The Company and JEDI II hereby agree that notwithstanding the
         provisions set forth in the Series C Designation, the shares of Series
         C Preferred Stock shall be delivered to the Company and canceled at
         such time as the Company (1) has completed an Initial Public Offering
         by June 30, 2000, (2) repaid a portion of the Subordinated Debt,
         together with accrued and unpaid interest, such that not more than $10
         million aggregate principal amount remains outstanding, and (3) the
         shares of Series A Preferred Stock are no longer outstanding and (4)
         has paid to JEDI II an additional $1,000. JEDI II hereby agrees that it
         will not transfer the shares of Series C Preferred Stock prior to June
         30, 2000, other than to Affiliates of JEDI II who agree to be bound by
         the terms of this Agreement.

                  SECTION 5. Removal of Legend. The Company, JEDI II and each of
the other Stockholders hereby agree that Section 5 of the Agreement is hereby
amended and restated in its entirety as follows:

                  SECTION 5. Removal of Legend. Each of the Stockholders hereby
                  directs the Company, and the Company hereby agrees, to remove
                  the current legend on any certificates relating to the
                  Company's securities required by the Old Stockholders'
                  Agreement as promptly as practicable following after such time
                  as the Company (1) has completed an Initial Public Offering by
                  June 30, 2000, (2) repaid a portion of the Subordinated Debt,
                  together with accrued and unpaid interest, such that not


                                       -2-
<PAGE>   3

                  more than $10 million aggregate principal amount remains
                  outstanding, and (3) the shares of Series A Preferred Stock
                  are no longer outstanding.

                  SECTION 6.  Miscellaneous.

                  (a) Entire Agreement. This First Amendment, together with the
Agreement, is intended by the parties to be a final expression of their
agreement and a complete and exclusive statement of their agreement and
understanding in respect of the subject matter contained herein and therein.
Except as expressly set forth herein, this First Amendment shall be governed by
the other terms and conditions of the Agreement, including, without limitation,
terms and conditions relating to notices, choice of law and arbitration.


                                       -3-
<PAGE>   4

                  IN WITNESS WHEREOF, the parties hereto have duly executed this
First Amendment in their individual capacity or caused it to be duly executed by
their respective authorized signatories thereunto duly authorized as of the date
first above written.


                                     BASIC ENERGY SERVICES, INC.
                                     (formerly named Sierra Well Service, Inc.)


                                     By /s/ Kenneth V. Huseman
                                        --------------------------------------
                                        Name: Kenneth V. Huseman
                                        Title: President


                                     JOINT ENERGY DEVELOPMENT INVESTMENTS II
                                     LIMITED PARTNERSHIP

                                     By:    Enron Capital Management II Limited
                                            Partnership, its General Partner

                                     By:    Enron Capital II Corporation,
                                            its General Partner

                                     By: /s/ Richard D. Bye
                                        --------------------------------------
                                        Name: Richard D. Bye
                                        Title: Executive Vice President


                                     Address:
                                     c/o Enron Corp.
                                     1400 Smith Street
                                     Houston, Texas 77002
                                     Attention: General Counsel


                                       -4-
<PAGE>   5

                                     Other Stockholders:

                                     SOUTHWEST ROYALTIES, INC.

                                     By:   /s/ H. H. Wommack, III
                                        --------------------------------------
                                        Name:  H. H. Wommack, III
                                        Title: President

                                     Address:
                                     406 North Big Spring
                                     Midland, Texas 79701
                                     Attention: President

                                     SOUTHWEST PARTNERS II, L.P.

                                     By:    Southwest Royalties, Inc., its
                                            General Partner

                                     By:   /s/ H. H. Wommack, III
                                        --------------------------------------
                                        Name:  H. H. Wommack, III
                                        Title: President

                                     Address:
                                     406 North Big Spring
                                     Midland, Texas 79701
                                     Attention: General Partner

                                     SOUTHWEST PARTNERS III, L.P.

                                     By:    Southwest Royalties, Inc.,
                                            its General Partner

                                     By:   /s/ H. H. Wommack III
                                        --------------------------------------
                                        Name:  H. H. Wommack III
                                        Title: President

                                     Address:
                                     406 North Big Spring
                                     Midland, Texas 79701
                                     Attention: General Partner


                                       -5-
<PAGE>   6
                                     /s/ Joey D. Fields
                                     -----------------------------------------
                                     JOEY D. FIELDS

                                     Address:

                                     406 North Big Spring
                                     Midland, Texas 79701


                                     /s/ Dub W. Harrison
                                     -----------------------------------------
                                     DUB W. HARRISON

                                     Address:

                                     406 North Big Spring
                                     Midland, Texas 79701


                                     /s/ Kenneth V. Huseman
                                     -----------------------------------------
                                     KENNETH V. HUSEMAN

                                     Address:

                                     406 North Big Spring
                                     Midland, Texas 79701


                                       -6-