UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13G

(Amendment No. 2)

Under the Securities Exchange Act of 1934

PAMPA ENERGÍA S.A.

(Name of Issuer)

PAMPA ENERGY INC

(Translation of Issuer’s Name into English)

American Depositary Shares, representing Common Shares, Ps. 1.00 Par Value Per Share

(Title of Class of Securities)

697660207

(CUSIP Number)

December 31, 2012

(Date of Event which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule if filed:*

¨ Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

This Amendment No. 2 to Schedule 13G (“Amendment No. 2”) amends and supplements the statement on Schedule 13G filed on June 1, 2011 (the “Original Filing”), with respect to Barnston, Inc. (“Barnston”), Cambria, Inc. and Joseph Lewis, as amended on January 3, 2012 (“Amendment No. 1”) with respect to Barnston, Labmex International S.À R.L (“Labmex”) and Joseph Lewis. This Amendment No. 2 constitutes an initial statement on Schedule 13G for Berkley Capital Management Ltd. (“Berkley Capital”).

Pampa Energía S.A.

Translation of the issuer’s name into English:

Pampa Energy Inc.

Ortiz de Ocampo 3302

Building #4

C1425DSR

Buenos Aires, Argentina

This statement is filed jointly by Labmex, Barnston, Berkley Capital and Joseph Lewis (together with Labmex, Barnston and Berkley Capital the “Reporting Persons”) pursuant to Rule 13d-1(k)(1). Joseph Lewis is the sole indirect owner of, and controls each of Labmex, Barnston and Berkley Capital.

The principal business address of Barnston, Berkley Capital and Joseph Lewis is:

c/o Cay House

P.O. Box N-7776

E.P. Taylor Drive

Lyford Cay, New Providence, Bahamas

The principal business address of Labmex is:

56, rue Charles Martel,

Luxembourg L-2134

Labmex is a private limited liability company (société à responsabilité limitée) organized and existing under the laws of Luxembourg. Barnston and Berkley Capital are international business companies organized under the laws of the Bahamas. Joseph Lewis is a permanent resident of the Bahamas and a citizen of the United Kingdom.

American Depositary Shares, representing Common Shares.

697660207

Not applicable.

457,459 ADS, which is equivalent to 11,436,475 Common Shares.*

0.9% of the outstanding Common Shares.*

0*

457,459 ADS, which is equivalent to 11,436,475 Common Shares.*

0*

457,459 ADS, which is equivalent to 11,436,475 Common Shares.*

As of January 31, 2013, pursuant to the terms of the Subscription Agreement, Labmex has the right to acquire on the Redemption Date, the 3,150,000 ADS held by Pampa Holdings, which represents 787,500,000 Common Shares or 6.0%, based on 1,314,310,895 Common Shares stated to be outstanding as of September 30, 2012 in the Form 6-K filed with the SEC on November 23, 2012. Therefore, as of January 31, 2013, Mr. Lewis beneficially owned 3,607,459 ADS which represents 90,186,475 or 6.9% of the outstanding Common Shares, based on 1,314,310,895 Common Shares stated to be outstanding as of September 30, 2012 in the Form 6-K filed with the SEC on November 23, 2012.Each ADS represents 25 Common Shares.

x

With respect to Barnston only, this statement is being filed to report the fact that as of the date hereof the Reporting Person, Barnston, has ceased to be the beneficial owner of more than 5 percent of the class of securities.

Pampa Holdings has the right to receive dividends and the proceeds from the sale of the Labmex Shares that Labmex may be deemed to beneficially own pursuant to the Subscription Agreement until the redemption date. See Item 4 above.

Not applicable.

Not applicable.

Not applicable.

(c) By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

Exhibits

PAMPA ENERGÍA S.A.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Exhibit 1

PAMPA ENERGÍA S.A.

JOINT FILING AGREEMENT

The undersigned hereby agree that the Statement on Schedule 13G, dated February 11, 2013 with respect to the shares of Common Stock of Pampa Energía S.A., is, and any amendments thereto executed by each of us shall be, filed on behalf of each of us pursuant to and in accordance with the provisions of Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, as amended, and that this Agreement shall be included as an Exhibit to the Schedule 13G and each such amendment. Each of the undersigned agrees to be responsible for the timely filing of the Schedule 13G and any amendments thereto, and for the completeness and accuracy of the information concerning itself contained therein. This Agreement may be executed in any number of counterparts, all of which taken together shall constitute one and the same instrument.

IN WITNESS WHEREOF, the undersigned have executed this Agreement as of the 11th day of February, 2013.