The Company’s interests in oil and natural gas proved properties are located in the United States, primarily in the U.S. 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.

During the three months ended March 31, 2021 and 2020, the Company’s ceiling test computations did not result in a write-down of its U.S. oil and natural gas properties. At March 31, 2021, the Company’s ceiling test computation was based on SEC pricing of $39.49 per Bbl of oil, $2.15 per Mcf of natural gas and $11.19 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 discounted asset retirement obligations included in the Condensed Consolidated Balance Sheets in current and non-current liabilities, and the changes in that liability were as follows (in thousands):

	March 31, 2021
Beginning asset retirement obligations	$	442,269
Fair value of asset retirement obligations acquired		4
Obligations settled		(10,120	)
Fair value of asset retirement obligations divested		(176	)
Accretion expense		14,985
Obligations incurred		—
Changes in estimate		5,206
Ending asset retirement obligations	$	452,168
Less: Current portion		(45,478	)
Long-term portion	$	406,690