Note 9. Asset Retirement Obligations

Asset retirement obligations (AROs) consist primarily of estimated costs of dismantlement, removal, site reclamation and similar activities associated with our oil and gas properties. Changes in AROs were as follows:



	Year Ended December 31,
(millions)	2017			2016
Asset Retirement Obligations, Beginning Balance	$	935		$	989
Liabilities Incurred	94			21
Liabilities Settled	(82		)	(120		)
Revision of Estimate	(65		)	(3		)
Reclassification to Liabilities Associated with Assets Held for Sale	(54		)	—
Accretion Expense	47			48
Asset Retirement Obligations, Ending Balance	$	875		$	935

Year Ended December 31, 2017 Liabilities incurred include $63 million related to the Clayton Williams Energy Acquisition and $31 million primarily for other US onshore wells and midstream facilities placed into service.

Liabilities settled include $43 million related to abandonment of US onshore properties, $19 million related to properties sold in the Greeley Crescent (DJ Basin) acreage divestiture, $12 million related to properties sold in the Marcellus Shale upstream divestiture and $8 million related to other offshore domestic and international properties.

Revisions of estimates include a $42 million decrease related to changes in cost and timing associated with the North Sea abandonment project and a $38 million decrease for US onshore and Gulf of Mexico properties, partially offset by an increase of $15 million for West Africa.

In 2017, we also transferred $42 million and $12 million of ARO liabilities associated with Southwest Royalties and Tamar field, offshore Israel, respectively, to liabilities associated with assets held for sale. Refer to Item 8. Financial Statements and Supplementary Data - Note 4. Acquisitions, Divestitures and Merger.

Year Ended December 31, 2016 Liabilities incurred were due to new wells and facilities placed into service for US onshore, Gulf of Mexico, and offshore Israel.

Liabilities settled were related to wells and facilities permanently abandoned at the end of their useful lives and to assets sold. Settlements included $65 million related to abandonment of Gulf of Mexico properties, $49 million related to US onshore properties abandoned or sold, $5 million related to offshore Israel properties and $1 million related to the North Sea.