STANDARDIZED MEASURE OF DISCOUNTED FUTURE NET CASH FLOWS RELATING TO PROVED OIL, NATURAL GAS AND NATURAL GAS LIQUIDS RESERVES (UNAUDITED) (Tables)

12 Months Ended

Dec. 31, 2019

Standardized Measure of Discontinued Future Net Cash Flows Relating to Proved Oil, Natural Gas and Natural Gas Liquids Reserves [Abstract]

Standardized Measure of Discounted Future Net Cash Flows Relating to Estimated Proved Oil, Natural Gas and Natural Gas Liquids Reserves

The standardized measure of discounted future net cash flows relating to estimated proved oil, natural gas and natural gas liquids reserves is presented below for the years ended December 31 (in thousands):

	2019		2018		2017
Future cash inflows	$	613,120	$	2,710,234	$	2,778,662
Future production and development costs		(387,175)		(1,622,040)		(1,603,373)
Future income tax expenses		(9,266)		(167,692)		(6,022)
Future net cash flows		216,679		920,502		1,169,267
10% annual discount for estimated timing of cash flows		(109,743)		(484,069)		(589,862)
Standardized measure of discounted future net cash flows	$	106,936	$	436,433	$	579,405

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

The principal sources of changes in the standardized measure of future net cash flows are as follows for the years ended December 31 (in thousands):

	2019		2018		2017
Standardized measure at beginning of period	$	436,433	$	579,405	$	371,117
Sales and transfers of oil, natural gas and natural gas liquids
produced, net of production costs		(28,428)		(126,181)		(111,118)
Net changes in prices and production costs		(125,166)		48,322		202,810
Extensions, discoveries and improved recovery, less related costs		46		3,148		5,451
Development costs incurred during the period		—		16		2,261
Revisions and other		(54,980)		88,587		13,516
Accretion of 10% timing discount		50,963		58,222		37,356
Changes in income taxes		69,678		(70,390)		(371)
Changes in estimated future development costs		(5,502)		3,174		20,177
Changes in timing and other		(6,185)		21,551		8,302
Purchase of minerals in place		—		—		32,949
Sales of minerals in place		(229,923)		(169,421)		(3,045)
Standardized measure of discounted future net cash flows	$	106,936	$	436,433	$	579,405