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Asset Retirement Obligations
 6 Months Ended
Jun. 30, 2016
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations
Asset Retirement Obligations
Asset retirement obligations (ARO) consist primarily of estimated costs of dismantlement, removal, site reclamation and similar activities associated with our oil and gas properties. Changes in ARO are as follows:
 
Six Months Ended
June 30,
(millions)
2016
 
2015
Asset Retirement Obligations, Beginning Balance
$
989

 
$
751

Liabilities Incurred
3

 
16

Liabilities Settled
(38
)
 
(15
)
Revision of Estimate
4

 
79

Accretion Expense (1)
25

 
21

Asset Retirement Obligations, Ending Balance
$
983

 
$
852

(1) 
Accretion expense is included in Depreciation, Depletion and Amortization (DD&A) expense in the consolidated statements of operations.
For the six months ended June 30, 2016 Liabilities incurred were due to new wells and facilities for onshore US. Liabilities settled primarily related to onshore US property abandonments.
For the six months ended June 30, 2015 Liabilities incurred were due to new wells and facilities for onshore US and deepwater Gulf of Mexico. Liabilities settled in 2015 relate primarily to non-core, onshore US properties sold.
Revisions were primarily due to changes in estimated costs for future abandonment activities and acceleration of timing of abandonment and included $43 million for Eastern Mediterranean and $28 million for DJ Basin.