SC 14D1/A

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                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

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                               AMENDMENT NO. 1 TO
                                 SCHEDULE 14D-l
                   TENDER OFFER STATEMENT PURSUANT TO SECTION
                 14(d)(1) OF THE SECURITIES EXCHANGE ACT OF 1934


                              SHERIDAN ENERGY, INC.
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                            (Name of Subject Company)

                               CPN SHERIDAN, INC.
                               CALPINE CORPORATION
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                                    (Bidder)

                     COMMON STOCK, PAR VALUE $.01 PER SHARE
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                         (Title of Class of Securities)

                                   823764 10 5
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                      (CUSIP Number of Class of Securities)

                                  ANN B. CURTIS
                               CPN SHERIDAN, INC.
                             C/O CALPINE CORPORATION
                           50 WEST SAN FERNANDO STREET
                               SAN JOSE, CA 95113
                            TELEPHONE: (408) 995-5115
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            (Name, Address and Telephone Number of Person Authorized
           to Receive Notices and Communications on Behalf of Bidder)

                                   COPIES TO:
                            WILLIAM R. COLLINS, ESQ.
                            HOWARD, SMITH & LEVIN LLP
                           1330 AVENUE OF THE AMERICAS
                               NEW YORK, NY 10019
                            TELEPHONE: (212) 841-1000
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                                 AUGUST 31, 1999
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                       (Date Tender Offer First Published,
                       Sent or Given to Security Holders)




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            Calpine Corporation ("Calpine") and its wholly owned subsidiary,
CPN Sheridan, Inc., hereby amend and supplement their Tender Offer Statement on
Schedule 14D-1, originally filed on August 31, 1999 (the "Statement"), with
respect to an offer to purchase all outstanding shares of Common Stock, par
value $.01 per share, of Sheridan Energy, Inc. as set forth in the Statement.
Capitalized terms not defined in this Amendment have the meanings assigned to
them in the Statement.


ITEM 11.    MATERIAL TO BE FILED AS EXHIBITS.

(a)(10)     Text of letter sent by Calpine and CPN Sheridan, Inc. to holders of
            TGX Corporation Series A Preferred Stock Certificates on September
            2, 1999.



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                                    SIGNATURE

            After due inquiry and to the best of my knowledge and belief, the
undersigned certifies that the information set forth in this statement is true,
complete and correct.

Dated:  September 8, 1999



                               CPN SHERIDAN, INC.


                               By:/s/ Ann B. Curtis
                                  ---------------------------------------------
                                Name:   Ann B. Curtis
                                Title:  Vice President, Chief Financial Officer
                                        and Secretary




                               CALPINE CORPORATION


                               By:/s/ Ann B. Curtis
                                  ---------------------------------------------
                                Name:  Ann B. Curtis
                                Title: Executive Vice President



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                                  EXHIBIT INDEX

Exhibit
Number      Exhibit Name
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(a)(10)     Text of letter sent by Calpine and CPN Sheridan, Inc. to holders of
            TGX Corporation Series A Preferred Stock Certificates on September
            2, 1999




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EX-99.A.10

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                                                                 Exhibit (a)(10)



To the Holders of TGX Corporation Series A Senior Preferred Stock Certificates:

            The records of the Transfer Agent of Sheridan Energy, Inc. (the
"Company") indicate that you have not previously exchanged your TGX Corporation
("TGX") Series A Senior Preferred Stock (the "TGX Preferred Stock")
certificate(s) for certificates representing shares of common stock of the
Company (the "Common Stock"), in accordance with the June 12, 1997 merger
between TGX and the Company (the "TGX Merger"). Pursuant to the TGX Merger,
shares of TGX Preferred Stock were exchanged for shares of Common Stock of the
Company at a ratio of 2 for 1.

            Enclosed you will find documents relating to the Calpine Corporation
("Calpine") offer to purchase for cash all shares of Common Stock of the Company
at a purchase price of $5.50 per share, net to the seller in cash (the "Offer").
The Offer is being made pursuant to the Agreement and Plan of Merger among the
Company, Calpine and CPN Sheridan, Inc. "CPN", dated as of August 25, 1999,
which is described in the enclosed materials.

            You may participate in this Offer by tendering your TGX Preferred
Stock certificate(s) and completing the Letter of Transmittal along with any
other required documents specified in the enclosed materials.

            Please note that share amounts represented on the TGX Preferred
Stock certificates will be converted by the Depositary, American Stock Transfer
& Trust Company, at the TGX Merger exchange ratio of 2 for 1, to determine the
cash purchase price to be received by a validly tendering holder of TGX
Preferred Stock certificates. For example, if you hold a TGX Preferred Stock
certificate for 100 shares, the Depositary will convert that to 50 shares of
Common Stock of the Company, entitling you to receive $275.00 in cash from CPN
based on the $5.50 net per share Offer from CPN.

            The Offer is fully explained in the enclosed materials. Should you
have any questions, however, you may call D.F. King & Co., Inc. who is acting as
information agent for the Offer at (800) 848-3094 (Toll Free) or (212) 269-5550
(Call Collect).