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, 2018

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:

	2018		2017		2016
Future cash inflows	$	2,710,234	$	2,778,662	$	2,261,520
Future production and development costs		(1,622,040)		(1,603,373)		(1,480,900)
Future income tax expenses		(167,692)		(6,022)		(5,442)
Future net cash flows		920,502		1,169,267		775,178
10% annual discount for estimated timing of cash flows		(484,069)		(589,862)		(404,061)
Standardized measure of discounted future net cash flows	$	436,433	$	579,405	$	371,117

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:

	2018		2017		2016
Standardized measure at beginning of period	$	579,405	$	371,117	$	536,450
Sales and transfers of oil, natural gas and natural gas liquids
produced, net of production costs		(126,181)		(111,118)		(71,939)
Net changes in prices and production costs		48,322		202,810		(83,146)
Extensions, discoveries and improved recovery, less related costs		3,148		5,451		1,712
Development costs incurred during the period		16		2,261		2,065
Revisions and other		88,587		13,516		(23,328)
Accretion of 10% timing discount		58,222		37,356		53,998
Changes in income taxes		(70,390)		(371)		1,093
Changes in estimated future development costs		3,174		20,177		3,976
Changes in timing and other		21,551		8,302		(26,920)
Purchase of minerals in place		—		32,949		—
Sales of minerals in place		(169,421)		(3,045)		(22,844)
Standardized measure of discounted future net cash flows	$	436,433	$	579,405	$	371,117