Aggregate amounts of capitalized costs relating to oil, natural gas and NGL activities and the aggregate amount of related accumulated depletion and amortization as of the dates indicated are presented below (in thousands):

Included in the depletable basis of proved oil and gas properties is the estimate of the Company’s proportionate share of asset retirement costs relating to these properties which are also reflected as “Asset retirement obligations” in the accompanying Consolidated Balance Sheets. At December 31, 2020 and 2019, the Company’s liability for oil and gas asset retirement obligations totaled $442.3 million and $369.5 million, respectively.

The following table reflects the costs incurred in oil, natural gas and NGL property acquisition, exploration and development activities during the years indicated (in thousands). Costs incurred also include new asset retirement obligations established in the current year, as well as increases or decreases to the asset retirement obligations resulting from changes to cost estimates during the year.

	Year Ended December 31,
	2020		2019		2018
Property acquisition costs:
Proved properties	$	422,833	$	27,660	$	850,515
Unproved properties, not subject to amortization		95,242		16,062		65,063
Total property acquisition costs		518,075		43,722		915,578
Exploration costs⁽¹⁾		59,422		209,161		93,780
Development costs		362,011		292,547		215,467
Total costs incurred	$	939,508	$	545,430	$	1,224,825

The Company employs full-time experienced reserve engineers and geologists who are responsible for determining proved reserves in compliance with SEC guidelines. There are numerous uncertainties inherent in estimating quantities of proved reserves and projecting future rates of production and timing of development expenditures. The reserve data in the following tables only represent estimates and should not be construed as being exact. Engineering reserve estimates were prepared based upon interpretation of production performance data and sub-surface information obtained from the drilling of existing wells. The Company’s Director of Reserves, internal reservoir engineers and geologists analyzed and prepared reserve estimates on all oil and natural gas fields. All of the Company’s proved oil, natural gas and NGL reserves are located in the United States primarily offshore Gulf of Mexico.

At, December 31, 2020, 2019 and 2018, 100% of proved oil, natural gas and NGL reserves attributable to all of the Company’s oil and natural gas properties were estimated and compiled for reporting purposes by the Company’s reservoir engineers and audited by Netherland, Sewell & Associates, Inc. (“NSAI”), independent petroleum engineers and geologists.

The following table presents the Company’s estimated proved reserves at its net ownership interest:

	Oil (MBbls)			Gas (MMcf)			NGL (MBbls)			Oil Equivalent (MBoe)
Total proved reserves at December 31, 2017		72,804			127,656			6,547			100,625
Revision of previous estimates		2,595			(37,933	)		3,187			(539	)
Production		(11,771	)		(22,771	)		(1,176	)		(16,742	)
Purchases of reserves		44,788			95,661			2,074			62,806
Extensions and discoveries		4,123			8,411			64			5,589
Total proved reserves at December 31, 2018		112,539			171,024			10,696			151,739
Revision of previous estimates		(5,553	)		(15,898	)		(1,237	)		(9,440	)
Production⁽¹⁾		(13,844	)		(23,306	)		(1,228	)		(18,956	)
Purchases of reserves		2,094			2,626			130			2,662
Extensions and discoveries		11,518			21,552			620			15,730
Total proved reserves at December 31, 2019		106,754			155,998			8,981			141,735
Revision of previous estimates		(14,633	)		(56,358	)		(168	)		(24,195	)
Production		(13,665	)		(28,652	)		(1,559	)		(19,999	)
Purchases of reserves		26,903			181,872			3,528			60,743
Extensions and discoveries		3,948			4,348			76			4,749
Total proved reserves at December 31, 2020		109,307			257,208			10,858			163,033
Total proved developed reserves as of:
December 31, 2018		85,530			131,364			8,104			115,528
December 31, 2019		72,016			115,381			6,733			97,979
December 31, 2020		85,007			204,054			8,104			127,120
Total proved undeveloped reserves as of:
December 31, 2018		27,009			39,660			2,592			36,211
December 31, 2019		34,738			40,617			2,248			43,756
December 31, 2020		24,300			53,154			2,754			35,913

During 2020, proved reserves decreased by 21.3 MMBoe primarily due to a decrease of 20.0 MMBoe of production and revision to previous estimates of 24.2 MMBoe due to decrease in commodity prices and differentials. The decrease was partially offset by the addition of 60.7 MMBoe added through purchases from the ILX and Castex Acquisition, Castex Energy 2005 Acquisition and LLOG Acquisition as well as 4.7 MMBoe of estimated proved reserves from extensions and discoveries primarily from an evaluation of Green Canyon 18 and Claiborne Fields.

During 2019, proved reserves decreased by 10.0 MMBoe primarily due to a decrease of 19.0 MMBoe of production and revision to previous estimates of 9.7 MMBoe due to the Phoenix and Ram Powell Fields. The decrease was partially offset by the addition of 15.7 MMBoe of estimated proved reserves from extensions and discoveries primarily from an evaluation of Green Canyon 21, Pompano, and Ewing Bank 305 as well as 3.0 MMBoe added through purchases from the Gunflint Acquisition.

During 2018, the Company added 51.1 MMBoe of estimated proved reserves, which included 62.8 MMBoe added through purchases of 59.3 MMBoe from the Stone Combination and 3.5 MMBoe from the Whistler Acquisition. The Company also added 5.6 MMBoe of estimated proved reserves from extensions and discoveries primarily from an evaluation of Green Canyon Block 18. The increase was partially offset by a decrease of 16.7 MMBoe of production.

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

The following table reflects the standardized measure of discounted future net cash flows relating to the Company’s interest in proved oil, natural gas and NGL reserves (in thousands):

	Year Ended December 31,
	2020			2019			2018
Future cash inflows	$	4,927,497		$	7,151,875		$	8,654,631
Future costs:
Production		(1,105,211	)		(1,633,432	)		(1,740,850	)
Development and abandonment		(1,236,874	)		(1,464,270	)		(1,349,005	)
Future net cash flows before income taxes		2,585,412			4,054,173			5,564,776
Future income tax expense		(141,515	)		(662,317	)		(862,473	)
Future net cash flows after income taxes		2,443,897			3,391,856			4,702,303
Discount at 10% annual rate		(538,963	)		(854,261	)		(1,362,057	)
Standardized measure of discounted future net cash flows	$	1,904,934		$	2,537,595		$	3,340,246

Future cash inflows are computed by applying SEC Pricing to year-end quantities of proved reserves. The discounted future cash flow estimates do not include the effects of derivative instruments. See the following table for base prices used in determining the standardized measure:

Future net cash flows are discounted at the prescribed rate of 10%. Actual future net cash flows may vary considerably from these estimates. Although the Company’s estimates of total proved reserves, development costs and production rates were based on the best information available, the development and production of oil and gas reserves may not occur in the periods assumed. Actual prices realized, costs incurred and production quantities may vary significantly from those used. Therefore, such estimated future net cash flow computations should not be considered to represent the Company’s estimate of the expected revenues or the current value of existing proved reserves.

Principal changes in the standardized measure of discounted future net cash flows attributable to the Company’s proved oil, natural gas and NGL reserves are as follows (in thousands):

	Year Ended December 31,
	2020			2019			2018
Standardized measure, beginning of year	$	2,537,595		$	3,340,246		$	1,807,669
Sales and transfers of oil, net gas and NGLs produced during the period		(339,557	)		(665,226	)		(727,969	)
Net change in prices and production costs		(1,468,304	)		(849,696	)		1,578,330
Changes in estimated future development costs		32,589			(75,564	)		32,328
Previously estimated development costs incurred		46,143			117,049			45,937
Accretion of discount		299,302			392,526			180,767
Net change in income taxes		361,875			129,590			(585,017	)
Purchases of reserves		730,611			75,009			943,519
Extensions and discoveries		71,589			306,515			148,068
Net change due to revision in quantity estimates		(309,338	)		(199,576	)		190,853
Changes in production rates (timing) and other		(57,571	)		(33,278	)		(274,239	)
Standardized measure, end of year	$	1,904,934		$	2,537,595		$	3,340,246