**EXHIBIT    99.F** **EXCHANGE    AGENT AGREEMENT** **          ,    2007**     The Bank of New York Trust Company, N.A.c/o The    Bank of New York    Corporate Trust Operations    Reorganization Unit    101 Barclay Street — 7 East    New York, New York 10286    Attn: Corporate Reorganization Unit     Ladies and Gentlemen:     Southern National Gas Company, a Delaware corporation (the    “Company”), proposes to make an offer (the    “Exchange Offer”) to exchange an aggregate principal    amount of up to $500,000,000 of its registered 5.90% Notes    due 2017 (the “New Notes”) for a like principal amount    of its outstanding 5.90% Notes due 2017 (the “Old    Notes”). The terms and conditions of the Exchange Offer as    currently contemplated are set forth in a prospectus (the    “Prospectus” included in the Company’s    registration statement on    Form S-4    (Registration    No. 333-146158)    as amended (the “Registration Statement”)) filed with    the Securities and Exchange Commission (the “SEC”),    and proposed to be distributed to all record holders of the Old    Notes. The Old Notes and the New Notes are collectively referred    to in this Exchange Agent Agreement (this “Agreement”)    as the “Notes” or the “Securities.”    Capitalized terms used herein and not defined shall have the    respective meanings ascribed to them in the Registration    Statement or the accompanying letter of transmittal (the    “Letter of Transmittal”).     The Company hereby appoints The Bank of New York    Trust Company, N.A. to act as exchange agent (the    “Exchange Agent”) in connection with the Exchange    Offer. References hereinafter to “you” shall refer to    The Bank of New York Trust Company, N.A.     The Exchange Offer is expected to be commenced by the Company on    or    about          ,    2007. The Letter of Transmittal accompanying the Registration    Statement is to be used by the holders of the Old Notes to    tender into the Exchange Offer, and contains instructions with    respect to the delivery of certificates for Old Notes tendered    in connection therewith. The Exchange Agent’s obligations    with respect to receipt and inspection of the Letter of    Transmittal in connection with the Exchange Offer shall be    satisfied for all purposes hereof by (1) inspection of the    electronic message transmitted to the Exchange Agent by Exchange    Offer participants in accordance with the Automated Tender Offer    Program (“ATOP”) of the Depositary Trust Company    (“DTC”), and by otherwise observing and complying with    all procedures established by DTC in connection with ATOP, to    the extent that ATOP is utilized by Exchange Offer participants,    or (2) by inspection of the Letter of Transmittal by each    respective holder of Old Notes.     The Exchange Offer shall expire at 5:00 p.m., New York City    time,    on          ,    2007, or on such later date or time to which the Company may    extend the Exchange Offer (the “Expiration Date”).    Subject to the terms and conditions set forth in the    Registration Statement, the Company expressly reserves the right    to extend the Exchange Offer from time to time and may extend    the Exchange Offer by giving oral (confirmed in writing) or    written notice to you at any time before 9:00 a.m., New    York City time, on the business day following the previously    scheduled Expiration Date, and in such case the term    “Expiration Date” shall mean the time and date on    which the Exchange Offer as so extended shall expire.     The Company expressly reserves the right, in its sole    discretion, to delay, amend or terminate the Exchange Offer, and    not to accept for exchange any Old Notes not theretofore    accepted for exchange, in among other cases upon the occurrence    of any of the conditions of the Exchange Offer specified in the    Registration Statement under the caption “The Exchange    Offer — Expiration Date; Extensions; Termination;    Amendments.” The Company will give to you as promptly as    practicable oral notice (confirmed in writing) or written notice    of any delay, amendment, termination or non-acceptance.

 
    In carrying out your duties as Exchange Agent, you are to act in    accordance with the following instructions:     1. You will perform such duties and only such duties    (i) as are specifically set forth (a) herein,    (b) in the section of the Registration Statement captioned    the “The Exchange Offer” and (c) in the Letter of    Transmittal accompanying the Registration Statement,    (ii) which are necessarily incidental thereto and    (iii) as may be subsequently agreed to in writing by you    and the Company; provided, however, that    notwithstanding any provision hereof to the contrary, in no way    will your general duty to act in good faith be discharged by the    foregoing.     2. You will establish a book-entry account with respect to    the Old Notes at The Depository Trust Company (the    “Book-Entry Transfer Facility”) for purposes of the    Exchange Offer within two business days after the date of the    Registration Statement, and any financial institution that is a    participant in the Book-Entry Transfer Facility’s systems    may make book-entry delivery of the Old Notes by causing the    Book-Entry Transfer Facility to transfer such Old Notes into    your account in accordance with the Book-Entry Transfer    Facility’s procedure for such transfer.     3. You are to examine each of the Letters of Transmittal    and certificates for Old Notes (or confirmation of book-entry    transfers into your account at the Book-Entry Transfer Facility)    and any other documents delivered or mailed to you by or for    holders of the Old Notes to ascertain whether: (i) the    Letters of Transmittal, certificates and any such other    documents are duly executed and properly completed in accordance    with instructions set forth therein and in the Registration    Statement and that such book-entry confirmations are in due and    proper form and contain the information required to be set forth    therein, and (ii) the Old Notes have otherwise been    properly tendered. In each case where the Letter of Transmittal    or any other document has been improperly completed or executed    or where book-entry confirmations are not in due and proper form    or omit certain information or any of the certificates for Old    Notes are not in proper form for transfer or some other    irregularity in connection with the acceptance of the Exchange    Offer exists, you will endeavor to inform the presenters of the    need for fulfillment of all requirements and to take any other    action as may be necessary or advisable to cause such    irregularity to be corrected.     4. With the approval of the Chief Financial Officer,    Secretary or any Vice President of the Company (such approval,    if given orally, promptly to be confirmed in writing) or any    other party designated by such officer in writing (each a    “Designated Officer”), you are authorized to waive any    irregularities in connection with any tender of Old Notes    pursuant to the Exchange Offer. You are not otherwise authorized    to waive any such irregularities.     5. Tenders of Old Notes may be made only as set forth in    the Letter of Transmittal and in the section of the Registration    Statement captioned “The Exchange Offer —    Procedures for Tendering Old Notes” and Old Notes shall be    considered properly tendered to you only when tendered in    accordance with the procedures set forth therein.     Notwithstanding the provisions of this paragraph 5, Old    Notes that any Designated Officer shall approve as having been    properly tendered shall be considered to be properly tendered    (such approval, if given orally, shall be promptly confirmed in    writing).     6. You shall promptly advise the Company with respect to    any Old Notes delivered subsequent to the Expiration Date and    accept its instructions with respect to disposition of such Old    Notes.     7. You shall accept tenders:     (a) in cases where the Old Notes are registered in two or    more names only if signed by all named holders;     (b) in cases where the signing person (as indicated on the    Letter of Transmittal) is acting in a fiduciary or a    representative capacity only when proper evidence of his or her    authority so to act is submitted; and     (c) from persons other than the registered holder of Old    Notes, provided that customary transfer requirements,    including any payment of applicable transfer taxes, are    fulfilled.     You shall accept partial tenders of Old Notes where so indicated    and as permitted in the Letter of Transmittal and return any    untendered Old Notes to the holder (or such other person as may    be designated in the Letter of Transmittal) as promptly as    practicable after expiration or termination of the Exchange    Offer. A new certificate for the remainder of the principal    amount of the Old Notes will be sent to the holders of Old Notes    unless otherwise indicated by partial tendering holders of Old    Notes.