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Asset Retirement Obligations
 6 Months Ended
Jun. 30, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations
9. 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)
2017
 
2016
Asset Retirement Obligations, Beginning Balance
$
935

 
$
989

Liabilities Incurred
82

 
3

Liabilities Settled
(32
)
 
(38
)
Revision of Estimate
(15
)
 
4

Accretion Expense (1)
23

 
25

Asset Retirement Obligations, Ending Balance
$
993

 
$
983


(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, 2017 Liabilities incurred include $59 million related to the Clayton Williams Energy Acquisition and $23 million primarily for other US onshore wells and facilities placed into service. Liabilities settled primarily related to US onshore property abandonments, as well as $12 million related to properties sold in the Marcellus Shale upstream divestiture. Revisions of estimates related to decreases in cost and timing estimates of $30 million for US onshore and Gulf of Mexico, partially offset by an increase of $15 million for West Africa.
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.