The Company’s interests in oil and natural gas proved properties are located in the United States, primarily in the Gulf of Mexico deep and shallow waters. The Company follows the full cost method of accounting for its oil and natural gas exploration and development activities.

Pursuant to SEC Regulation S-X, Rule 4-10, under the full cost method of accounting, the Company’s capitalized oil and natural gas costs are limited to a ceiling based on the present value of future net revenues from proved reserves, computed using a discount factor of 10%, plus the lower of cost or estimated fair value of unproved oil and natural gas properties not being amortized less the related tax effects. The Company performs this ceiling test calculation each quarter utilizing SEC pricing. During 2020, 2019 and 2018, the Company’s ceiling test computations resulted in a write-down of its U.S. oil and natural gas properties of $267.9 million, nil and nil, respectively. At December 31, 2020, its ceiling test computation was based on SEC pricing of $39.47 per Bbl of oil, $1.97 per Mcf of natural gas and $9.89 per Bbl of NGLs.

Unproved capitalized costs of oil and natural gas properties excluded from amortization relate to unevaluated properties associated with acquisitions, leases awarded in the U.S. Gulf of Mexico federal lease sales, certain geological and geophysical costs, expenditures associated with certain exploratory wells in progress and capitalized interest. Unproved properties also include expenditures associated with exploration and appraisal activities in Block 7 and Block 31 located in the shallow waters off the coast of Mexico’s Veracruz and Tabasco states.

The following table sets forth a summary of the Company’s oil and natural gas property costs not being amortized at December 31, 2020, by the year in which such costs were incurred (in thousands):

			Year Ended December 31,
	Total		2020		2019		2018		2017 and Prior
Acquisition United States	$	80,799	$	61,315	$	3,268	$	16,216	$	—
Exploration United States		52,470		12,714		32,698		5,761		1,297
Exploration Mexico		121,725		14,811		61,809		14,362		30,743
Total unproved properties, not subject to amortization	$	254,994	$	88,840	$	97,775	$	36,339	$	32,040

The excluded costs will be included in the amortization base as properties are evaluated and proved reserves are established or impairment is determined.

The Company’s evaluation of unproved property located in Block 2 offshore Mexico, specifically future exploratory drilling opportunities, results from exploratory wells drilled during the second quarter of 2019 and the Block 2 production sharing contract’s expiration date resulted in the Company recording a non-cash impairment presented as “Write-down of oil and natural gas properties” on the Consolidated Statements of Operations. For the years ended December 31, 2020, 2019 and 2018, the Company recorded an impairment of $0.1 million, $12.2 million and nil, respectively.

The discounted asset retirement obligations included in the Consolidated Balance Sheets in current and non-current liabilities, and the changes in that liability during each of the years ended December 31, 2020 and 2019 were as follows (in thousands):

	Year Ended December 31,
	2020			2019
Asset retirement obligations at January 1	$	369,478		$	382,817
Fair value of asset retirement obligations acquired⁽¹⁾		44,311			5,047
Obligations settled		(43,933	)		(75,331	)
Fair value of asset retirement obligations divested		(185	)		(5,450	)
Accretion expense		49,741			34,389
Obligations incurred		4,511			4,111
Changes in estimate		18,346			23,895
Asset retirement obligations at December 31	$	442,269		$	369,478
Less: Current portion		(49,921	)		(61,051	)
Long-term portion	$	392,348		$	308,427