<Page>
                                                         Exhibit 10.21(a)


                                 FIRST AMENDMENT

                                       TO

                          LIMITED PARTNERSHIP AGREEMENT

                                       OF

                         CEDAR BAYOU FRACTIONATORS, L.P.
                        (A Delaware Limited Partnership)

THIS FIRST AMENDMENT to Limited Partnership Agreement is entered into as of,
and is effective, the 23rd day of December 1998 for the purpose of amending
that certain "Limited Partnership Agreement of Cedar Bayou Fractionators,
L.P. ("CBF") between Dynegy Midstream Services, Limited Partnership (formerly
known as Warren Petroleum Company, Limited Partnership) ("Dynegy") and Amoco
MB Fractionation Company ("Amoco") effective January 1, 1998 (the
"Agreement"). Said Agreement provided for the creation of a limited
partnership between the parties, for the purpose of owning certain
partnership interests in and to a Delaware limited partnership to own and
operate a fractionation facility situated in Mont Belvieu, Texas.

WHEREAS, effective the same date as the above effective date of this
Amendment simultaneously with the execution of this First Amendment, Dynegy
and Amoco have executed a certain Letter Agreement with Williams Midstream
Natural Gas Liquids, Inc. ("Williams") regarding "Option to Acquire interest
in Cedar Bayou Fractionators, L.P." wherein Williams was granted an option to
acquire a five percent (5%) limited liability interest in CBF. Additionally,
Dynegy has granted an additional option to Williams to acquire certain
portions of Dynegy's  limited partnership interests in CBF (both of said
options being referred to as the "Options"); and

WHEREAS, Dynegy and Amoco have agreed that certain amendments to the Agreement
would be desirable and are required to accommodate the addition of a third owner
in CBF and to reflect other matters for which Dynegy and Amoco have agreed
amendments are appropriate.

NOW, THEREFORE, Amoco and Dynegy hereby agree to amend the Agreement in the
following manner:

1.  GLOBAL CHANGES. To reflect the change of names of the below listed entities,
    all references to the below terms in the Agreement are amended to the below
    listed references, each as of the date indicated:

      a)  All references to "Warren Petroleum Company, Limited Partnership"
          and the defined term "Warren" applicable to said entity (when
          appearing by itself in the Agreement), are


                                      -1-
<Page>


          amended to be "Dynegy Midstream Services, Limited Partnership" and
          "Dynegy," respectively, effective June 9, 1998.

      b)  All references to "Warren Petroleum G.P., Inc." are amended to be
          "Dynegy Midstream G.P., Inc." effective June 14, 1998.

2.    SECTION 1, PARTNERSHIP FORMATION, SECTION 1.6, TERM, is amended by
      deleting the reference in the first sentence thereof to "eighty-eight
      percent (88%)" and substituting therefor "eighty-six and nine hundred
      twenty-eight thousandths percent (86.928%)."

3.    SECTION 2, DEFINED TERMS, is amended by adding the following new
      definitions and by amending certain existing definitions as indicated
      below:

      a)  NEW DEFINITIONS:

          "ADDITIONAL LIMITED PARTNER" means a Person other than the Initial
          Limited Partners who has acquired a Percentage Interest in the
          Partnership either from all of the Partners or from another Partner
          in accordance with Section 11 of this Agreement, which Percentage
          Interest, as well as the other Partners after the transfer, will be
          reflected in such Additional Limited Partner's Admission Agreement.

          "ADMISSION AGREEMENT" means the agreement between an Additional
          Limited Partner and the Partnership described in Section 11.5 and
          referenced in Section 3.11.

      b)  AMENDMENTS TO EXISTING DEFINITIONS:

          In the first line of the definition of "EFFECTIVE DATE," delete the
          words "first Section" and replace them with "first paragraph."

          "INITIAL LIMITED PARTNERS" are DEVCO, Dynegy and Amoco, as
          identified in the preamble hereof.

          In the definition of "PERCENTAGE INTERESTS," insert in the third
          line before the word "Partners'" the words "Initial Limited", delete
          the last sentence of the definition and replace it with the
          following: "Any Persons who become Additional Limited Partners
          hereunder after the Effective Date of this Agreement in compliance
          with the provisions of this Agreement governing transfers of
          Partnership interests shall hold such Percentage Interests as are
          designated in the transfer to them and as described in their
          Admission Agreement and the Percentage Interests of the Initial
          Limited Partners or other Partners, as applicable, transferring such
          interest to such an Additional Limited Partner shall be adjusted
          accordingly and shall also be described in said Admission Agreement.

4.    Section 3.6, INCREASE IN AMOCO INTEREST., Delete the date "December 31,
      2002" and replace it with "October 1, 2004."


                                      -2-
<Page>


5.    Add a new Section 3.11 as follows:

      "3.11  ADDITIONAL LIMITED PARTNERS. Each Additional Limited Partner shall
      make Capital Contributions described in its Admission Agreement. The
      amount of the Additional Limited Partner's Capital Contribution, the time
      for making such contributions, and any change in other Limited Partners'
      Capital Accounts and Percentage Interests that may result, shall be set
      forth in such Admission Agreement."

6.    The following amendments are each agreed to be effective only if and when
      Williams acquires a limited partnership interest in CBF:

      a)  SECTION 8.3, VOTING PROCEDURES, is amended by adding the following
          sentence at the end thereof:

          "Provided that proper notice is given or duly waived by the
          Management Committee representatives for all Partners entitled to
          vote at a meeting, the presence of at least two (2) Management
          Committee members, representing Partners which collectively hold
          more than eighty-six and nine hundred twenty-eight thousandths
          percent (86.928%) of the total Percentage Interests in the
          Partnership, shall be sufficient to constitute a quorum for the
          transaction of business."

      b)  SECTION 8.4, VOTE REQUIRED, is amended by changing the reference
          therein to "eighty-eight percent (88%)" to "eighty-six and nine
          hundred twenty-eight thousandths percent (86.928%).

      c)  SECTION 8.6, MATTERS REQUIRING UNANIMOUS APPROVAL, delete the section
          title and introductory sentence and replace them with the following:

          "MATTERS REQUIRING SUPER-MAJORITY APPROVAL. Approval of the
          following matters by the Management Committee shall require a vote
          of members having authority to vote ninety percent (90%) or more of
          the total Percentage Interests of all Partners entitled to vote:"

      d)  SECTION 8.92, REMOVAL OF MANAGING GENERAL PARTNER, is amended by
          changing the reference therein to "eighty-eight percent (88%)" to
          "eighty-six and nine hundred twenty-eight thousandths percent
          (86.928%)."

Except as amended by this First Amendment all other terms and conditions of the
Agreement effective January 1, 1998, shall remain unchanged and in full force
and effect.


                                      -3-
<Page>


      IN WITNESS WHEREOF, this First Amendment is entered into and executed as
of the date first written above.

DYNEGY MIDSTREAM SERVICES, LIMITED PARTNERSHIP
as Initial Limited Partner
By: Dynegy Midstream G.P., Inc.,
            its General Partner

/s/  STEPHEN A. FURBACHER
---------------------------------------
Name:  Stephen A. Furbacher
Title: President


AMOCO MB FRACTIONATION COMPANY, as Initial Limited Partner

By:  /s/  A. BOYD ANDERSON
---------------------------------------
Name:  A. Boyd Anderson
Title: President


DOWNSTREAM ENERGY VENTURES CO., L.L.C., as General Partner

By:  /s/  WILLIAM E. PUCKETT
---------------------------------------
William E. Puckett, Vice President




                                      -4-