OIL AND NATURAL GAS OPERATIONS (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Extractive Industries [Abstract]

Schedule of Capitalized Costs

The following table sets forth capitalized costs:

(in thousands)	December 31, 2017		December 31, 2016
Proved	$	8,466,708	$	7,543,464
Unproved	453,028		196,888
Total capitalized costs	8,919,736		7,740,352
Accumulated depreciation, depletion and amortization	4,200,797		3,723,669
Capitalized costs, net	$	4,718,939	$	4,016,683

Schedule of Cost Incurred in Property Acquisition, Exploration and Development Activities

The following table sets forth costs incurred in property acquisition, exploration and development activities and includes both capitalized costs and costs charged to expense during the year:

Years ended December 31, (in thousands)	2017		2016		2015
Property acquisition:
Proved*	$	9,889	$	4,066	$	1,866
Unproved	273,326		143,667		85,690
Exploration	676,357		349,463		649,764
Development	235,279		89,624		372,177
Total costs incurred	$	1,194,851	$	586,820	$	1,109,497

*Includes a $6.4 million noncash lawsuit settlement over certain leasehold interests.

Schedule of Results of Operations from Producing Activities

The following table sets forth results of Energen’s oil, natural gas liquids and natural gas operations from producing activities:

Years ended December 31, (in thousands)	2017		2016			2015
Gross revenues*	$	961,045	$	532,889		$	878,554
Production (lifting costs)	243,144		214,652			285,760
Exploration expense	10,075		5,415			14,877
Depreciation, depletion and amortization including asset impairments	480,491		663,659			1,880,190
Accretion expense	5,831		6,672			7,108
Income tax expense (benefit)**	79,815		(123,153		)	(469,362		)
Results of operations from producing activities	$	141,689	$	(234,356	)	$	(840,019	)

* The years ended December 31, 2017, 2016 and 2015 gross revenues include pre-tax non-cash mark-to-market losses on derivatives of $10.8 million, $71.2 million and $281.8 million, respectively.

**Income tax benefit does not reflect any impact due to the enactment of the Tax Cuts and Jobs Act in 2017.

Schedule of Proved Developed and Undeveloped Oil and Gas Reserves

Year ended December 31, 2017	Oil MBbl		NGL MBbl		Natural Gas MMcf		Total MMBOE
Proved reserves at beginning of period	199,575		58,046		352,248		316.3
Revisions of previous estimates	7,903		14,853		102,107		39.8
Purchases	179		37		201		0.2
Extensions and discoveries	66,304		23,098		156,461		115.5
Production	(16,951	)	(5,255	)	(33,528	)	(27.8	)
Proved reserves at end of period	257,010		90,779		577,489		444.0
Proved developed reserves at end of period	143,907		52,882		342,616		253.9
Proved undeveloped reserves at end of period	113,103		37,897		234,873		190.1

Year ended December 31, 2016	Oil MBbl		NGL MBbl		Natural Gas MMcf		Total MMBOE
Proved reserves at beginning of period	210,691		71,713		433,904		354.7
Revisions of previous estimates	(17,840	)	(6,800	)	(7,779	)	(26.0	)
Purchases	103		21		89		0.1
Extensions and discoveries	45,129		10,480		50,780		64.1
Production	(13,213	)	(3,892	)	(27,204	)	(21.6	)
Sales	(25,295	)	(13,476	)	(97,542	)	(55.0	)
Proved reserves at end of period	199,575		58,046		352,248		316.3
Proved developed reserves at end of period	101,202		29,767		187,117		162.1
Proved undeveloped reserves at end of period	98,373		28,279		165,131		154.2

Year ended December 31, 2015	Oil MBbl		NGL MBbl		Natural Gas MMcf		Total MMBOE
Proved reserves at beginning of period	181,227		73,463		707,926		372.7
Revisions of previous estimates	(39,537	)	(11,979	)	(44,176	)	(58.9	)
Purchases	2		1		2		0.0
Extensions and discoveries	83,319		25,530		143,022		132.6
Production	(14,023	)	(4,065	)	(35,604	)	(24.0	)
Sales	(297	)	(11,237	)	(337,266	)	(67.7	)
Proved reserves at end of period	210,691		71,713		433,904		354.7
Proved developed reserves at end of period	108,319		36,374		236,112		184.0
Proved undeveloped reserves at end of period	102,372		35,339		197,792		170.7

Schedule of Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves

Years ended December 31, (in thousands)	2017		2016		2015
Future gross revenues	$	15,531,237	$	9,191,808	$	11,714,729
Future production costs	4,467,989		3,126,153		4,353,974
Future development costs	2,077,918		1,632,577		1,961,661
Future income tax expense	1,381,999		762,921		1,065,887
Future net cash flows	7,603,331		3,670,157		4,333,207
Discount at 10% per annum	4,283,536		2,320,350		2,299,859
Standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves	$	3,319,795	$	1,349,807	$	2,033,348

Schedule of Principal Sources of Changes in Standardized Measure of Discounted Future Net Cash Flows

The following are the principal sources of changes in the standardized measure of discounted future net cash flows:

Years ended December 31, (in thousands)	2017			2016			2015
Balance at beginning of year	$	1,349,807		$	2,033,348		$	4,219,656
Revisions to reserves proved in prior years:
Net changes in prices and production costs	659,802			(343,839		)	(3,101,283		)
Net changes in future development costs	(86,642		)	122,200			239,692
Net changes due to revisions in quantity estimates	389,684			(167,188		)	(404,708		)
Development costs incurred, previously estimated	148,534			71,099			350,560
Accretion of discount	149,664			203,335			542,105
Changes in timing and other*	257,523			(100,742		)	(1,024,114		)
Total revisions	1,518,565			(215,135		)	(3,397,748		)
New field discoveries and extensions, net of future production and development costs	1,492,562			352,358			776,315
Sales of oil and gas produced, net of production costs	(788,130		)	(440,446		)	(514,380		)
Purchases	3,769			1,733			8
Sales	—			(235,222		)	(372,039		)
Net change in income taxes	(256,778		)	(146,829		)	1,321,536
Net change in standardized measure of discounted future net cash flows	1,969,988			(683,541		)	(2,186,308		)
Balance at end of year	$	3,319,795		$	1,349,807		$	2,033,348

*Amount represents changes in production timing and other. In 2015, the production timing is significantly affected by changes related to the delay of the drilling program.