Disclosures about oil and natural gas activities (unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Oil and Gas Exploration and Production Industries Disclosures [Abstract]

Summary of changes in quantities of proved oil and natural gas reserves

A summary of the changes in our quantities of proved oil and natural gas reserves for the three years ended December 31, 2019 are as follows:

	Oil (MBbls)		Natural gas (MMcf)		Natural gas liquids (MBbls)		Total (MBoe)
Proved developed and undeveloped reserves
As of January 1, 2017	96,621		135,449		12,105		131,301
Sales of minerals in place	(74,918	)	(1,663	)	(46	)	(75,241	)
Extensions and discoveries	8,957		39,843		5,442		21,040
Revisions (1)	3,515		11,135		2,216		7,586
Production	(4,571	)	(14,598	)	(1,395	)	(8,399	)
Balance at December 31, 2017	29,604		170,166		18,322		76,287
Sales of minerals in place	(2,422	)	(14,184	)	(1,374	)	(6,160	)
Extensions and discoveries	6,545		69,189		9,329		27,406
Revisions (1)	1,254		12,596		1,411		4,764
Production	(2,684	)	(17,549	)	(1,881	)	(7,490	)
Balance at December 31, 2018	32,297		220,218		25,807		94,807
Sales of minerals in place	—		—		—		—
Extensions and discoveries	4,766		48,967		8,343		21,271
Revisions (1)	(6,703	)	(26,340	)	1,166		(9,927	)
Production	(3,111	)	(22,095	)	(2,799	)	(9,593	)
Balance at December 31, 2019	27,249		220,750		32,517		96,558
Proved developed reserves:
January 1, 2017	28,590		108,800		9,352		56,076
December 31, 2017	18,301		123,451		11,858		50,734
December 31, 2018	18,051		135,425		14,846		55,468
December 31, 2019	18,447		152,187		20,949		64,761
Proved undeveloped reserves:
January 1, 2017	68,031		26,649		2,753		75,225
December 31, 2017	11,303		46,715		6,464		25,553
December 31, 2018	14,246		84,793		10,961		39,339
December 31, 2019	8,802		68,563		11,568		31,797

(1) The revisions in 2019 and 2018 were primarily due to changes in pricing during the respective periods. The upward revision in 2017 was primarily due to changes in pricing and costs.

Standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves

The standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves are as follows:

	December 31,
	2019			2018			2017
Future cash flows	$	2,424,620		$	3,255,771		$	2,331,940
Future production costs	(1,040,314		)	(1,187,071		)	(899,380		)
Future development and abandonment costs	(304,229		)	(450,220		)	(336,828		)
Future income tax provisions	—			—			—
Net future cash flows	1,080,077			1,618,480			1,095,732
Less effect of 10% discount factor	(565,874		)	(932,114		)	(597,859		)
Standardized measure of discounted future net cash flows	$	514,203		$	686,366		$	497,873

Changes in standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves

The changes in the standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves are as follows:

	For the year ended December 31,
	2019			2018			2017
Beginning of year	$	686,366		$	497,873		$	528,781
Sale of oil and natural gas produced, net of production costs	(170,255		)	(175,199		)	(175,246		)
Net changes in prices and production costs	(332,546		)	95,430			125,795
Extensions and discoveries	114,199			192,105			136,887
Improved recoveries	—			—			—
Changes in future development costs	116,677			(2,424		)	(4,879		)
Development costs incurred during the period that reduced future development costs	38,270			6,277			37,912
Revisions of previous quantity estimates (1)	(8,152		)	79,192			68,428
Purchases and sales of reserves in place, net	—			(45,222		)	(238,445		)
Accretion of discount	58,668			36,386			24,267
Net change in income taxes	—			—			—
Changes in production rates and other	10,976			1,948			(5,627		)
End of year	$	514,203		$	686,366		$	497,873

(1) Amounts in 2019 and 2018 are primarily the result of changes in pricing. Amounts in 2017 are primarily the result of increased volumes due to changes in pricing and costs.

Prices used In determining future net revenues related to the standardized measure of discounted future cash flows

The following prices for oil, natural gas, and natural gas liquids before field differentials were used in determining future net revenues related to the standardized measure calculation.

	2019		2018		2017
Oil (per Bbl)	$	55.69	$	65.56	$	51.34
Natural gas (per Mcf)	$	2.58	$	3.10	$	2.98
Natural gas liquids (per Bbl)	$	16.21	$	25.56	$	24.17