Under FASB guidance exploratory well costs should continue to be capitalized when the well has found a sufficient quantity of reserves to justify its completion as a producing well and the Company is making sufficient progress assessing the reserves and the economic and operating viability of the project.

As of March 31, 2021, the Company had total capitalized exploratory well costs for continuing operations pending the determination of proved reserves of $182.4 million. The following table reflects the net changes in capitalized exploratory well costs during the three-month periods ended March 31, 2021 and 2020.


(Thousands of dollars)	2021		2020
Beginning balance at January 1	$	181,616	217,326
Additions pending the determination of proved reserves	785		816

Capitalized exploratory well costs charged to expense	—		(39,709)
Balance at March 31	$	182,401	178,433

The capitalized well costs charged to expense during 2020 represent a charge for asset impairments (see below).

The following table provides an aging of capitalized exploratory well costs based on the date the drilling was completed for each individual well and the number of projects for which exploratory well costs have been capitalized. The projects are aged based on the last well drilled in the project.


	March 31,
	2021				2020

(Thousands of dollars)	Amount		No. of Wells	No. of Projects	Amount	No. of Wells	No. of Projects
Aging of capitalized well costs:
Zero to one year	$	—	—	—	24,637	3	3
One to two years	23,514		3	3	31,541	2	2
Two to three years	30,562		2	2	—	—	—
Three years or more	128,325		6	—	122,255	6	—
	$	182,401	11	5	178,433	11	5

Of the $182.4 million of exploratory well costs capitalized more than one year at March 31, 2021, $90.6 million is in Vietnam, $45.9 million is in the U.S., $25.7 million is in Brunei, $15.4 million is in Mexico, and $4.8 million is in Canada. In all geographical areas, either further appraisal or development drilling is planned and/or development studies/plans are in various stages of completion.

During the first quarter of 2021, the Company recorded an impairment charge of $171.3 million for Terra Nova due to the current status, including agreements with the partners, of operating and production plans.

In the first quarter of 2020, declines in future oil and natural gas prices (principally driven by increased supply from foreign oil producers and reduced demand in response to the COVID-19 pandemic) led to impairments in certain of the Company’s U.S. Offshore and Other Foreign properties. The Company recorded pretax noncash impairment charges of $967.5 million to reduce the carrying values to their estimated fair values as of March 31, 2020 at select properties.

The fair values were determined by internal discounted cash flow models using estimates of future production, prices, costs and discount rates believed to be consistent with those used by principal market participants in the applicable region.

The following table reflects the recognized impairments for the three months ended March 31, 2021 and 2020.


	Three Months Ended March 31,
(Thousands of dollars)	2021		2020
U.S.	$	—	927,821
Canada	171,296		—
Other Foreign	—		39,709

	$	171,296	967,530

On March 17, 2021, the King’s Quay FPS was sold to ArcLight Capital Partners, LLC (ArcLight) for proceeds of $267.7 million, which reimburses the Company for previously incurred capital expenditures.