Six Months

Ended June 30,

2011

2010

(In millions)

Asset retirement obligations as of beginning of period

$ 1,497

$ 1,513

Liabilities incurred

23

25

Liabilities settled

(39)

(71)

Revision of estimated obligation

16

194

Liabilities assumed by others

—

(256)

Accretion expense on discounted obligation

46

50

Foreign currency translation adjustment

28

(14)

Asset retirement obligations as of end of period

1,571

1,441

Less current portion

72

95

Asset retirement obligations, long-term

$ 1,499

$ 1,346