<PAGE>

                                                                    EXHIBIT 10.6

                             AMENDATORY AGREEMENT

     This Amendment is entered into this 1st day of March, 2000, between
SOUTHERN NATURAL GAS COMPANY ("Company") and ATLANTA GAS LIGHT COMPANY
("Shipper").

                                  WITNESSETH

     WHEREAS, Company provides certificated firm transportation service to
Shipper for the following firm contract quantities and effective dates under the
indicated Rate Schedule(s) of Company's FERC Gas Tariff, First Revised Volume
No. 2A:

<TABLE>
<CAPTION>
-------------------------------------------------------------------------------
   Rate              Firm Contract Quantity         Date        Primary Term
 Schedule                  (Mcf/day)
-------------------------------------------------------------------------------
<S>                 <C>                          <C>            <C>
    STS               168,500 in Zone 3          June 1, 1979     3/31/2003
-------------------------------------------------------------------------------
    STS                 708 in Zone 2            June 1, 1979     3/31/2003
-------------------------------------------------------------------------------
    ST-2            708 on South Georgia         June 1, 1979     3/31/2003
                         Facilities
-------------------------------------------------------------------------------
</TABLE>

     WHEREAS, Shipper has chosen to support Company's rate case settlement filed
with the Federal Energy Regulatory Commission ("FERC") on March 10, 2000, in
Docket Nos. RP99-496-000 and RP99-496-001 ("Settlement") in order to receive the
benefit of the lower settlement rates effective March 1, 2000, and the other
benefits set forth in the settlement, in exchange for contract extensions of the
above-referenced firm contract(s) for the period(s) specified herein;

     NOW, THEREFORE, in consideration of the premises and the mutual benefits
and covenants contained herein, the parties agree as follows:

1.   The primary term(s) set forth in Section 4.1 of each firm Service Agreement
     below is hereby amended to extend the primary term(s) as follows:

<TABLE>
<CAPTION>
-------------------------------------------------------------------------------
   Rate              Firm Contract Quantity         Date        Primary Term
 Schedule                  (Mcf/day)
-------------------------------------------------------------------------------
<S>                 <C>                          <C>            <C>
    STS               168,500 in Zone 3          June 1, 1979     3/31/2006
-------------------------------------------------------------------------------
    STS                 708 in Zone 2            June 1, 1979     3/31/2006
-------------------------------------------------------------------------------
    ST-2            708 on South Georgia         June 1, 1979     3/31/2006
                         Facilities
-------------------------------------------------------------------------------
</TABLE>
<PAGE>

     Each primary term shall continue and remain in force and effect for
     successive terms thereafter as specified in the respective rate schedules.

2.   In the event the rate moratorium under the Settlement is terminated in
     accordance with the terms of Article IV of the Settlement, Shipper shall
     have the right to reduce its firm contract quantity under its firm
     transportation contracts during the extension period of any such contract
     and/or reduce the extension period, by giving no less than 90 days written
     notice to Company, to be given no later than one year after the effective
     date of Company's general rate change made pursuant to Article IV,
     paragraph 2, of the Settlement. Any reductions in TDs shall be pro rata
     based on TDs, among Shipper's full rate, discounted, and seasonal TDs, if
     any, and any reduction in the term of the extended contracts shall apply to
     all such contracts.

3.   This Amendment is subject to all applicable, valid laws, orders, rules and
     regulations of any governmental entity having jurisdiction over the parties
     or the subject matter hereof.

4.   This Amendment shall be binding on the parties' respective successors and
     assigns.

     WHEREFORE, the parties have executed this Amendment through their duly
authorized representatives to be effective as of the date first written above.


ATTEST:                            SOUTHERN NATURAL GAS COMPANY

By: __________________________     By: _____________________________________
Title: _______________________     Title: __________________________________


ATTEST:                            ATLANTA GAS LIGHT COMPANY

By: __________________________     By: _____________________________________
Title: _______________________     Title: __________________________________

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