5. Impairments

The following summarizes impairments by segment for the years ended December 31:



millions	2013		2012		2011
Oil and gas exploration and production
Long-lived assets held for use
U.S. onshore properties	$	142	$	259	$	1,063
Gulf of Mexico properties	562		104		162
Cost-method investment	11		13		91
Midstream
Long-lived assets held for use	79		13		458
Impairments	$	794	$	389	$	1,774

In 2013, certain Gulf of Mexico properties were impaired due to a reduction in estimated future net cash flows per barrel and downward revisions of reserves that the Company no longer plans to develop. Also in 2013, certain U.S. onshore properties and related midstream assets were impaired due to downward revisions of reserves that the Company no longer plans to develop. In addition, a midstream property was impaired during 2013 due to a reduction in estimated future cash flows. In 2012, certain U.S. onshore and midstream properties were impaired primarily due to lower natural-gas prices and Gulf of Mexico properties were impaired primarily as a result of downward reserves revisions for a property that was near the end of its economic life. In 2011, certain U.S. onshore and midstream properties were impaired primarily due to decreases in natural-gas prices, and Gulf of Mexico properties were impaired due to declines in estimated recoverable reserves. Impairments of the Company’s Venezuelan cost-method investment were due to declines in estimated recoverable value.

The following summarizes the post-impairment fair value of the above-described assets, all of which were measured using the income approach and Level 3 inputs:



millions	2013		2012
Long-lived assets held for use	$	548	$	103
Cost-method investment ⁽¹⁾	32		34

__________________________________________________________________


⁽¹⁾	This represents the Company’s after-tax net investment.

Impairments of Unproved Properties Impairments of unproved properties are included in exploration expense in the Company’s Consolidated Statements of Income. In 2012, the Company recognized a $721 million impairment of unproved Powder River coalbed methane properties primarily due to lower natural-gas prices. Also in 2012, the Company recognized a $124 million impairment of an unproved Gulf of Mexico natural-gas property that the Company does not expect to develop under the forecasted natural-gas price environment.