The following table provides certain information about the operations of each of our segments:



	2015				2014				2013
	(In thousands)
Revenues:
Oil and natural gas	$	385,774			$	740,079			$	649,718

Contract drilling	287,767				566,012				479,091
Elimination of inter-segment revenue	(22,099		)		(89,495		)		(64,313		)
Contract drilling net of inter-segment revenue	265,668				476,517				414,778

Gas gathering and processing	268,012				445,934				378,397
Elimination of inter-segment revenue	(65,223		)		(89,586		)		(91,043		)
Gas gathering and processing net of inter-segment revenue	202,789				356,348				287,354

Total revenues	$	854,231			$	1,572,944			$	1,351,850
Operating income (loss):
Oil and natural gas	$	(1,631,564	)	⁽⁴⁾	$	199,392		⁽⁴⁾	$	239,219
Contract drilling	44,811			⁽⁵⁾	41,896			⁽⁵⁾	96,304
Gas gathering and processing	(29,409		)	⁽⁶⁾	2,015			⁽⁶⁾	10,757
Total operating income (loss) ⁽¹⁾	(1,616,162		)		243,303				346,280
General and administrative	(35,345		)		(42,023		)		(38,323		)
Gain (loss) on disposition of assets	(7,229		)		8,953				17,076
Interest expense, net	(31,963		)		(17,371		)		(15,015		)
Gain (loss) on derivatives not designated as hedges and hedge ineffectiveness, net	26,345				30,147				(8,374		)
Other	45				(70		)		(175		)
Income (loss) before income taxes	$	(1,664,309	)		$	222,939			$	301,469
Identifiable assets:
Oil and natural gas	$	1,218,036			$	2,856,833			$	2,441,792
Contract drilling	993,015				1,059,980				1,042,661
Gas gathering and processing	478,661				500,255				473,717
Total identifiable assets ⁽²⁾	2,689,712				4,417,068				3,958,170
Corporate assets	118,797				56,660				64,220
Total assets	$	2,808,509			$	4,473,728			$	4,022,390
Capital expenditures:
Oil and natural gas	$	267,944			$	740,262			$	531,233
Contract drilling	84,802				176,683				64,325
Gas gathering and processing	63,476				79,268			⁽³⁾	96,085
Other	38,065				17,067				4,483
Total capital expenditures	$	454,287			$	1,013,280			$	696,126
Depreciation, depletion, amortization, and impairment:
Oil and natural gas:
Depreciation, depletion, and amortization	$	251,944			$	276,088			$	226,498
Impairment of oil and natural gas properties	1,599,348			⁽⁴⁾	76,683			⁽⁴⁾	—
Contract drilling:
Depreciation	56,135				85,370				71,194
Impairment of contract drilling equipment	8,314			⁽⁵⁾	74,318			⁽⁵⁾	—
Gas gathering and processing:
Depreciation and amortization	43,676				40,434				33,191
Impairment of gas gathering and processing systems	26,966			⁽⁶⁾	7,068			⁽⁶⁾	—
Other	3,075				3,051				3,024
Total depreciation, depletion, amortization, and impairment	$	1,989,458			$	563,012			$	333,907

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(1)	Operating income (loss) is total operating revenues less operating expenses, depreciation, depletion, amortization, and impairment and does not include general corporate expenses, gain (loss) on disposition of assets, gain (loss) on non-designated hedges and hedge ineffectiveness, net, interest expense, other income (loss), or income taxes.


(2)	Identifiable assets are those used in Unit’s operations in each industry segment. Corporate assets are principally cash and cash equivalents, short-term investments, corporate leasehold improvements, furniture, and equipment.


(3)	In 2014, we entered into capital leases for $28.2 million.


(4)	In 2015 and 2014, we incurred non-cash ceiling test write-downs of our oil and natural gas properties of $1.6 billion and $76.7 million pre-tax ($1.0 billion and $47.7 million, net of tax), respectively.


(5)	Impairment for contract drilling equipment for 2015 includes a $8.3 million pre-tax write-down and 2014 includes a $74.3 million pre-tax write-down for 31 drilling rigs, some older top drives, and certain drill pipe removed from service.


(6)	Impairment for gas gathering and processing systems for 2015 includes a $27.0 million pre-tax write-down for three of our systems, Bruceton Mills, Midwell, and Spring Creek. For 2014, it includes a $7.1 million pre-tax write-down for three of our systems, Weatherford, Billy Rose, and Spring Creek.