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                                                           Exhibit 10.11(a)

                            WARREN PETROLEUM COMPANY,
                               LIMITED PARTNERSHIP
                       13430 NORTHWEST FREEWAY, SUITE 1200
                            HOUSTON, TEXAS 77040-6095

                                October 14, 1996

Mr. R. D. Harrison
Chevron Chemical Company
1301 McKinney
Houston, Texas 77002

          RE:  CCC Product Sale and Purchase Agreement
               Article XII, Section 12.1 - Measurement and Analysis

Dear Mr. Harrison:

          Please reference Section 12.1 of the above referenced agreement which
addresses measurement techniques for Products that Warren Petroleum Company,
Limited Partnership ("Warren LP"), through its affiliate, Warren Gas Liquids,
buys and/or sells from and/or to Chevron Chemical Company ("Chevron Chemical").
As written, this section prescribes mass measurement for Ethane and E/P Mix
Products and volumetric measurement for other Products.

          Prior to the combination of NGC Corporation with the Warren Petroleum
Company ("Warren") and Natural Gas Business Unit divisions of Chevron U.S.A.
Inc., mass measurement was used to measure all Products sold to and/or purchased
from Chevron Chemical. It was not intended that the measurement techniques
previously used by Warren would change once the merger was completed.

          Keeping with Warren's prior practice, Warren LP and Chevron Chemical
mutually agree that the volume of all Products sold to Chevron Chemical shall be
measured on a mass basis. For invoice purposes, the mass quantity shall be
converted to a volumetric basis (gallons) per latest applicable API/GPA
standards or publications.

          If you agree with the foregoing, please evidence your agreement by
executing both of the originals of this letter enclosed herewith, keep one for
your records and return the other to me.

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For the purpose of this letter agreement, all capitalized terms as used herein
shall have the same meanings as set forth in the CCC Product Sale and Purchase
Agreement dated effective as of September 1, 1996 entered into between Warren LP
and Chevron Chemical.

                                             Very truly yours,

                                             /s/ Hunter Battle

                                             Hunter Battle
                                             Director- Liquid Assets

AGREED THIS 21 DAY OF OCT, 1996:

CHEVRON CHEMICAL COMPANY

By: /s/ R. D. Harrison
   ---------------------
   R. D. Harrison<Page>

                                                           Exhibit 10.11(b)

"Pages where confidential treatment has been requested are marked 'Confidential
Treatment Requested.' The redacted material has been separately filed with the
Commission, and the appropriate section has been marked at the appropriate
place and in the margin with a star (*)."

                        'Confidential Treatment Requested'

                                  AMENDMENT TO
                                CCC PRODUCT SALE
                             AND PURCHASE AGREEMENT

     This Amendment is executed this ____ day of _______________, 1999, to be
effective as of the Effective Date provided herein, by and between CHEVRON
CHEMICAL COMPANY ("CCC") and DYNEGY MIDSTREAM SERVICES, LIMITED PARTNERSHIP
("Dynegy"), formerly known as Warren Petroleum Company, Limited Partnership.

     WHEREAS, CCC and Dynegy entered into that certain CCC Product Sale and
Purchase Agreement, effective as of September 1, 1996 (the "Agreement"),
pursuant to which CCC agreed to purchase and sell certain Products from and to
Dynegy and Dynegy agreed to sell and Purchase certain Products to and from CCC;
and

     WHEREAS, CCC and Dynegy now wish to amend the Agreement as set forth
herein;

     NOW THEREFORE, in consideration of the premises and other good and valuable
consideration, the receipt and sufficiency of which is hereby acknowledged, CCC
and Dynegy agree as follows:

     1.   For the purpose of this Amendment, capitalized terms used herein shall
have the same meaning as set forth in the Agreement.

     2.   Section 5.1 of Article V, PRICE, shall be revised to read as follows:

               5.1   (a)   Except as otherwise provided herein, effective as of
          September 1, 1996, CCC shall pay Dynegy for (i) the Base Volumes
          purchased hereunder, excluding Natural Gasoline, a price equal to
*         [REDACTED] as quoted by the Oil Price Information Service ("OPIS")
          for Mont Belvieu, Texas (Non-TET), and for Natural Gasoline, shall
*         pay a price equal to [REDACTED] Mont Belvieu, Texas under the category
          "Other Non-TET N. Gasoline" and (ii) any Additional Volumes purchased
          hereunder, a price mutually agreed to by the Parties, and if the
*         Parties are unable to agree upon a price, a price equal to [REDACTED]
          as quoted by OPIS for Mont Belvieu, Texas (Non-TET) and/or (Other
          Non-TET), as applicable, for the period of time (Days) during each
          Delivery Month such Additional Volumes are delivered to CCC.

                     (b)   Notwithstanding the provisions of Section 5.1(a)
          above, effective February 1, 1999, the Price to be paid by CCC to
*         Dynegy for Normal Butane shall be equal to [REDACTED] for Mont
          Belvieu, Texas under the category "Other Non-TET N. Butane" for the
*         applicable period ([REDACTED])

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                        'Confidential Treatment Requested'

          depending on whether such volumes are Base Volumes or Additional
          Volumes.

                     (c)   In addition to the foregoing, for all Products
          delivered hereunder (including the Base Volumes and the Additional
          Volumes), CCC shall pay to Dynegy a delivery fee (the "Delivery Fee")
*         [REDACTED] per Barrel. For all purposes hereof, delivery of Products
          shall be deemed to occur at the point where title and risk of loss
          passes to CCC as provided herein.

     3.   Except as herein amended, all other terms and provisions of the
Agreement shall remain in full force and effect.

     IN WITNESS WHEREOF, CCC and Dynegy have each caused their duly authorized
representative to execute this Amendment as of the date set forth above.

DYNEGY MIDSTREAM SERVICES,                  CHEVRON CHEMICAL COMPANY
LIMITED PARTNERSHIP
By Dynegy Midstream G.P., Inc.,
its General Partner

By: /s/ Ron Logan                           By:
   --------------------------------------      --------------------------------
Name: /s/ Ron Logan                         Name:
     -------------------------------------       ------------------------------
Title: Director Supply + Term Sales         Title:
     -------------------------------------        -----------------------------

                                      2<PAGE>

                                                                Exhibit 10.12(a)

                                                                October 21, 1997

"Pages where confidential treatment has been requested are marked 'Confidential
Treatment Requested.' The redacted material has been separately filed with the
Commission, and the appropriate section has been marked at the appropriate place
and in the margin with a star (*)."

                  FIRST AMENDMENT TO CCC/WPC SERVICES AGREEMENT

     THIS First Amendment to CCC/WPC Services Agreement (the "Amendment") is
made and entered into this ______ day of _________________, 1997, by and between
CHEVRON CHEMICAL COMPANY, a Delaware corporation, hereinafter referred to as
"CCC" and WARREN PETROLEUM COMPANY, LIMITED PARTNERSHIP, a Delaware limited
partnership, hereinafter referred to as "WPC". CCC and WPC may be referred to
collectively as the "Parties" or singularly as a "Party."

     WHEREAS, CCC and WPC entered into that certain CCC/WPC Services Agreement
(the "Services Agreement") effective as of September 1, 1996, under which CCC
and WPC agreed to work together to fulfill commercial needs previously performed
by and between CCC and CUSA's Warren Petroleum Company division ("Warren") to
operate facilities previously owned and operated by CCC and Warren; and

     WHEREAS, CCC and WPC have agreed to amend and supplement certain portions
of the Services Agreement and now desire to enter into this Agreement to
evidence such agreement and to incorporate such changes into Exhibit "A-4" to
the Services Agreement;

     NOW THEREFORE, in consideration of the mutual covenants and agreements
herein contained, WPC and CCC agree as follows:

     1.   For the purpose of this Amendment, capitalized terms used herein shall
have the same meaning as set forth in the Services Agreement.

     2.   Effective September 1, 1997, Exhibit "A-4" to the Services Agreement
is amended and revised by Exhibit "A-4, First Amendment, Mont Belvieu
Terminalling" attached to this Amendment.

     3.   Except as specifically set forth herein, the Services Agreement
remains in full force and effect according to its terms.

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     IN WITNESS WHEREOF, the Parties hereto have cause this Amendment to be
executed by their duly authorized representatives.

WARREN PETROLEUM COMPANY, LIMITED PARTNERSHIP
By:    Warren Petroleum G.P., Inc., its
       General Partner

By: /s/ William E. Puckett
   ------------------------------------------
Printed Name:      William E. Puckett
             --------------------------------
Title:     Vice President
      ---------------------------------------

CHEVRON CHEMICAL COMPANY

By:  /s/ Robert D. Harrison
   ------------------------------------------
Printed Name:      Robert D. Harrison
             --------------------------------
Title:  MGR - Supply & Logistics/OLEFINS
      ---------------------------------------

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                    'Confidential Treatment Requested'

                               Exhibit A-4
                             First Amendment
                         Mont Belvieu Terminalling

Effective September 1, 1997, Exhibit A-4 shall be amended by adding the
following terms and conditions:

1.0  Scope of Services.

                                      * * *

     1.4  WPC shall provide CCC exclusive use of the pipeline between CCC's
          ethylene storage and the Dow Chemical Pipeline interconnect at the WPC
          property limit. The pipeline shall be used solely for the delivery or
          exchange of ethylene between CCC and Dow Chemical (the Ethylene
          Line"). WPC shall be responsible for the operation and maintenance of
          the Ethylene Line for the fees specified in Section 2.0.

2.0  Fees.

     CCC shall pay fees to WPC for all directly attributable expenses to
     dedicated facilities (e.g., CCC well workover cost) plus a portion of the
     cost of facilities that are operated for the benefit of both WPC and CCC
     consisting of the following:

                                      * * *

     2.9  Ethylene Pipeline Connection.

          In addition to the directly attributable expenses associated with the
          Ethylene Line, CCC shall pay WPC a monthly fee based upon a rate of
*         [REDACTED] per year, prorated for a portion thereof, for the exclusive
          use of the Ethylene Line.

10.0 Indemnity

     Notwithstanding and in lieu of the provisions of Section 7.3 of the
     Services Agreement, the following terms and conditions shall apply to and
     only to the services under Section 1.4 of this Amendment and any reasonable
     future expansions of CCC's operations and facilities at the Mont Belvieu
     facility under Section 5.0 of Exhibit A-4:

     (a)  Except as provided in (b), WPC agrees to indemnify and hold harmless
          CCC, its officers, directors, employees, agents, contractors and
          Affiliates from all actual losses, costs, expenses, claims (including,
          without

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          limitation, personal injury and property damage claims), damages and
          causes of action, including, without limitation, reasonable attorneys'
          fees and costs of court incurred by CCC, or paid to third parties by
          CCC which are caused by (i) the negligence, dishonesty, willful
          misconduct or gross negligence of WPC, its officers, directors,
          employees, agents, contractors and Affiliates, and its or their
          performance of the Services and/or (ii) WPC's breach of this
          Agreement.

          Except as provided in (b), CCC agrees to indemnify and hold harmless
          WPC, its officers, directors, employees, agents, contractors and
          Affiliates from all actual losses, costs, expenses, claims (including,
          without limitation, personal injury and property damage claims),
          damages and causes of action, including, without limitation,
          reasonable attorneys' fees and costs of court incurred by WPC, or paid
          to third parties by WPC which are caused by (i) the negligence,
          dishonesty, willful misconduct or gross negligence of CCC, its
          officers, directors, employees, agents, contractors and Affiliates,
          and its performance of the Services and/or (ii) CCC's breach of this
          Agreement.

     (b)  CCC shall defend, indemnify and hold harmless WPC from and against any
          and all claims relating to this Agreement for injury to or death of an
          employee or representative of CCC or its subcontractors, whether or
          not WPC, its employees or representatives are claimed to be passively,
          concurrently or actively negligent, and regardless of whether
          liability without fault is imposed or sought to be imposed; provided
          this indemnity shall not apply when such injury or death is the result
          of the sole negligence or willful misconduct of WPC, its employees or
          representatives.

          WPC  shall defend, indemnify and hold harmless CCC from and against
          any and all claims relating to this Agreement for injury to or death
          of an employee or representative of WPC or its subcontractors,
          whether or not CCC, its employees or representatives are claimed to
          be passively, concurrently or actively negligent, and regardless of
          whether liability without fault is imposed or sought to be imposed;
          provided this indemnity shall not apply when such injury or death is
          the result of the sole negligence or willful misconduct of CCC, its
          employees or representatives.

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