The total amount of capitalized costs relating to oil and natural gas producing activities and the total amount of related accumulated depreciation, depletion and amortization is as follows at the dates indicated.

Costs Incurred in Oil and Natural Gas Property Acquisition, Exploration and Development Activities

Costs incurred in property acquisition, exploration and development activities were as follows for the periods indicated:

	For the Year Ended
	December 31,
	2020			2019
	(In thousands)
Property acquisition costs, proved	$	42		$	150,871
Property acquisition costs, unproved		(49,307	)		7,674
Exploration		—			—
Development		29,543			71,460
Total	$	(19,722	)	$	230,005

As required by the FASB and SEC, the standardized measure of discounted future net cash flows presented below is computed by applying first-day-of-the-month average prices, year-end costs and legislated tax rates and a discount factor of 10 percent to proved reserves. We do not believe the standardized measure provides a reliable estimate of the Company’s expected future cash flows to be obtained from the development and production of its oil and gas properties or of the value of its proved oil and gas reserves. The standardized measure is prepared on the basis of certain prescribed assumptions including first-day-of-the-month average prices, which represent discrete points in time and therefore may cause significant variability in cash flows from year to year as prices change.

Users of this information should be aware that the process of estimating quantities of “proved” and “proved developed” oil and natural gas reserves is very complex, requiring significant subjective decisions in the evaluation of all available geological, engineering and economic data for each reservoir. The data for a given reservoir may also change substantially over time as a result of numerous factors including, but not limited to, additional development activity, evolving production history and continual reassessment of the viability of production under varying economic conditions. As a result, revisions to existing reserve estimates may occur from time to time. Although every reasonable effort is made to ensure reserve estimates reported represent the most accurate assessments possible, the subjective decisions and variances in available data for various reservoirs make these estimates generally less precise than other estimates included in the financial statement disclosures.

Proved reserves are those quantities of oil and natural gas that by analysis of geoscience and engineering data can be estimated with reasonable certainty to be economically producible — from a given date forward, from known reservoirs, and under existing economic conditions, operating methods and government regulations — prior to the time at which contracts providing the right to operate expire, unless evidence indicates that renewal is reasonably certain, regardless of whether deterministic or probabilistic methods are used for the estimation. The project to extract the hydrocarbons must have commenced or the operator must be reasonably certain that it will commence the project within a reasonable time.

We engaged CG&A to prepare our reserves estimates for all of our estimated proved reserves at December 31, 2020 and 2019. All proved reserves are located in the United States and all prices are held constant in accordance with SEC rules.

The weighted-average benchmark product prices used for valuing the reserves are based upon the average of the first-day-of-the-month price for each month within the period January through December of each year presented:

	2020		2019
Oil ($/Bbl):
WTI (1)	$	39.57	$	55.69

NGL ($/Bbl):
WTI (1)	$	39.57	$	55.69

Natural Gas ($/MMbtu):
Henry Hub (2)	$	1.99	$	2.58

The following tables set forth estimates of the net reserves for the periods indicated:

	For the Year Ended December 31, 2020
	Oil			Gas			NGLs			Total
	(MBbls)			(MMcf)			(MBbls)			(MBoe)
Proved developed and undeveloped reserves:
Beginning of year		70,772			377,869			29,252			163,002
Extensions and discoveries		291			655			61			461
Production		(3,887	)		(27,473	)		(1,725	)		(10,190	)
Revision of previous estimates		(20,500	)		(76,912	)		(6,104	)		(39,424	)
End of year		46,676			274,139			21,484			113,849

Proved developed reserves:
Beginning of year		53,476			320,731			23,646			130,577
End of year		35,613			252,218			19,009			96,658

Proved undeveloped reserves:
Beginning of year		17,296			57,138			5,606			32,425
End of year		11,063			21,921			2,475			17,191

	For the Year Ended December 31, 2019
	Oil			Gas			NGLs			Total
	(MBbls)			(MMcf)			(MBbls)			(MBoe)
Proved developed and undeveloped reserves:
Beginning of period		69,624			293,959			21,572			140,189
Extensions and discoveries		301			576			44			441
Purchase of minerals in place		17,429			202,409			18,533			69,697
Production		(3,498	)		(26,489	)		(1,343	)		(9,256	)
Revision of previous estimates		(13,084	)		(92,586	)		(9,554	)		(38,069	)
End of period		70,772			377,869			29,252			163,002

Proved developed reserves:
Beginning of period		54,147			232,110			17,324			110,156
End of period		53,476			320,731			23,646			130,577

Proved undeveloped reserves:
Beginning of period		15,477			61,849			4,248			30,033
End of period		17,296			57,138			5,606			32,425

Noteworthy amounts included in the categories of proved reserve changes in the above tables include:

A variety of methodologies are used to determine our proved reserve estimates. The principal methodologies employed are reservoir simulation, decline curve analysis, volumetric, material balance, advance production type curve matching, petro-physics/log analysis and analogy. Some combination of these methods is used to determine reserve estimates in substantially all of our fields.

	For the Year Ended
	December 31,
	2020			2019
	(In thousands)
Future cash inflows	$	2,410,260		$	5,146,288
Future production costs (1)		(1,589,945	)		(2,917,262	)
Future development costs (1)		(375,146	)		(567,423	)
Future income tax expense		—			—
Future net cash flows for estimated timing of cash flows		445,169			1,661,603
10% annual discount for estimated timing of cash flows		(147,358	)		(745,042	)
Standardized measure of discounted future net cash flows	$	297,811		$	916,561

Changes in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Reserves

The following is a summary of the changes in the standardized measure of discounted future net cash flows for the proved oil and natural gas reserves during each of the years in the two year period presented:

	For the Year Ended
	December 31,
	2020			2019
	(In thousands)
Beginning of year	$	916,561		$	1,113,029
Sale of oil and natural gas produced, net of production costs		(47,687	)		(113,545	)
Purchase of minerals in place		—			408,370
Extensions and discoveries		3,687			5,334
Changes in prices and costs		(548,429	)		(623,592	)
Previously estimated development costs incurred		49,144			84,341
Net changes in future development costs		89,997			110,892
Revisions of previous quantities		(150,245	)		(183,300	)
Accretion of discount		91,657			92,998
Change in production rates and other		(106,874	)		22,034
End of year	$	297,811		$	916,561