Six Months Ended

June 30,

2012

2011

Asset retirement obligation, beginning of period

$

30,427

$

24,592

Liabilities incurred

840

100

Liabilities settled

(2,450

)

(513

)

Accretion expense

1,017

1,179

Revisions in estimated cash flows

(42

)

—

Asset retirement obligation, end of period

29,792

25,358

Less: current portion of asset retirement obligation

(1,034

)

(674

)

Long-term asset retirement obligation

$

28,758

$

24,684