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



	2013			2012				2011
	(In thousands)
Revenues:
Oil and natural gas	$	649,718		$	567,944			$	514,614
Contract drilling	479,091			579,368				536,872
Elimination of inter-segment revenue	(64,313		)	(49,649		)		(52,221		)
Contract drilling net of inter-segment revenue	414,778			529,719				484,651
Gas gathering and processing	378,397			290,773				284,248
Elimination of inter-segment revenue	(91,043		)	(73,313		)		(76,010		)
Gas gathering and processing net of inter-segment revenue	287,354			217,460				208,238
Total revenues	$	1,351,850		$	1,315,123			$	1,207,503
Operating income:
Oil and natural gas	$	239,219		$	(77,221	)	⁽³⁾	$	199,993
Contract drilling	96,304			159,188				135,085
Gas gathering and processing	10,757			5,780			⁽⁴⁾	17,278
Total operating income ⁽¹⁾	346,280			87,747				352,356
General and administrative expense	(38,323		)	(33,086		)		(30,055		)
Gain (loss) on disposition of assets	17,076			253				(595		)
Interest expense, net	(15,015		)	(14,137		)		(4,167		)
Gain (loss) on derivatives not designated as hedges and hedge ineffectiveness, net	(8,374		)	(1,243		)		1,702
Other income (expense), net	(175		)	(132		)		(239		)
Income before income taxes	$	301,469		$	39,402			$	319,002
Identifiable assets:
Oil and natural gas	$	2,441,792		$	2,214,029			$	1,820,492
Contract drilling	1,042,661			1,079,736				1,118,666
Gas gathering and processing	473,717			413,708				247,763
Total identifiable assets ⁽²⁾	3,958,170			3,707,473				3,186,921
Corporate assets	64,220			53,647				69,799
Total assets	$	4,022,390		$	3,761,120			$	3,256,720
Capital expenditures:
Oil and natural gas ⁽⁵⁾	$	531,233		$	1,145,337			$	588,158
Contract drilling	64,325			77,520				162,208
Gas gathering and processing	96,085			183,162				79,355
Other ⁽⁵⁾	4,483			11,083				2,688
Total capital expenditures	$	696,126		$	1,417,102			$	832,409
Depreciation, depletion, amortization, and impairment:
Oil and natural gas
Depreciation, depletion and amortization	226,498			211,347				183,350
Impairment of oil and natural gas properties	—			283,606			⁽³⁾	—
Contract drilling	71,194			81,007				79,667
Gas gathering and processing	33,191			24,388			⁽⁴⁾	16,101
Other	3,024			2,279				1,333
Total depreciation, depletion, amortization, and impairment	$	333,907		$	602,627			$	280,451

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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 June 2012 and December 2012, due to low 12-month average commodity prices, we incurred non-cash ceiling test write downs of our oil and natural gas properties of $115.9 million pre-tax ($72.1 million net of tax) and $167.7 million pre-tax ($104.4 million net of tax), respectively.


(4)	Depreciation, depletion, amortization, and impairment for gas gathering and processing includes a $1.2 million write down of our Erick system.


(5)	Reclassified salt water disposal capital expenditures out of other and into oil and natural gas of $16,988 and $8,103 for 2012 and 2011, respectively.