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



	2014				2013			2012
	(In thousands)
Revenues:
Oil and natural gas	$	740,079			$	649,718		$	567,944

Contract drilling	566,012				479,091			579,368
Elimination of inter-segment revenue	(89,495		)		(64,313		)	(49,649		)
Contract drilling net of inter-segment revenue	476,517				414,778			529,719

Gas gathering and processing	445,934				378,397			290,773
Elimination of inter-segment revenue	(89,586		)		(91,043		)	(73,313		)
Gas gathering and processing net of inter-segment revenue	356,348				287,354			217,460

Total revenues	$	1,572,944			$	1,351,850		$	1,315,123
Operating income:
Oil and natural gas	$	199,392		⁽⁵⁾	$	239,219		$	(77,221	)	⁽⁵⁾
Contract drilling	41,896			⁽⁶⁾	96,304			159,188
Gas gathering and processing	2,015			⁽⁷⁾	10,757			5,780			⁽⁷⁾
Total operating income ⁽¹⁾	243,303				346,280			87,747
General and administrative	(42,023		)		(38,323		)	(33,086		)
Gain on disposition of assets	8,953				17,076			253
Interest expense, net	(17,371		)		(15,015		)	(14,137		)
Gain (loss) on derivatives not designated as hedges and hedge ineffectiveness, net	30,147				(8,374		)	(1,243		)
Other	(70		)		(175		)	(132		)
Income before income taxes	$	222,939			$	301,469		$	39,402
Identifiable assets:
Oil and natural gas	$	2,856,833			$	2,441,792		$	2,214,029
Contract drilling	1,059,980				1,042,661			1,079,736
Gas gathering and processing	500,255				473,717			413,708
Total identifiable assets ⁽²⁾	4,417,068				3,958,170			3,707,473
Corporate assets	56,660				64,220			53,647
Total assets	$	4,473,728			$	4,022,390		$	3,761,120
Capital expenditures:
Oil and natural gas ⁽³⁾	$	740,262			$	531,233		$	1,145,337
Contract drilling	176,683				64,325			77,520
Gas gathering and processing	79,268			⁽⁴⁾	96,085			183,162
Other ⁽³⁾	17,067				4,483			11,083
Total capital expenditures	$	1,013,280			$	696,126		$	1,417,102
Depreciation, depletion, amortization, and impairment:
Oil and natural gas
Depreciation, depletion, and amortization	$	276,088			$	226,498		$	211,347
Impairment of oil and natural gas properties	76,683			⁽⁵⁾	—			283,606			⁽⁵⁾
Contract drilling	159,688			⁽⁶⁾	71,194			81,007
Gas gathering and processing	47,502			⁽⁷⁾	33,191			24,388			⁽⁷⁾
Other	3,051				3,024			2,279
Total depreciation, depletion, amortization, and impairment	$	563,012			$	333,907		$	602,627

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(1)	Operating income 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, 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 2013, we reclassified salt water disposal capital expenditures out of other and into oil and natural gas of $17.0 million for 2012.


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


(5)	In 2014 and 2012, we incurred non-cash ceiling test write-downs of our oil and natural gas properties of $76.7 million and $283.6 million pre-tax ($47.7 million and $176.5 million, net of tax), respectively.


(6)	Depreciation, depletion, amortization, and impairment for contract drilling includes a $74.3 million pre-tax write-down for 31 drilling rigs, some older top drives, and certain drill pipe removed from service.


(7)	Depreciation, depletion, amortization, and impairment for gas gathering and processing for 2014 includes a $7.1 million pre-tax write-down for three of our systems, Weatherford, Billy Rose, and Spring Creek and for 2012 includes a $1.2 million write-down of our Erick system.