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 June 30, 2021, the Company had total capitalized exploratory well costs for continuing operations pending the determination of proved reserves of $197.5 million. The following table reflects the net changes in capitalized exploratory well costs during the six-month periods ended June 30, 2021 and 2020.


(Thousands of dollars)	2021		2020
Beginning balance at January 1	$	181,616	217,326
Additions pending the determination of proved reserves	15,921		2,328

Capitalized exploratory well costs charged to expense	—		(39,519)
Balance at June 30	$	197,537	180,135

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.


	June 30,
	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	$	13,881	3	3	24,429	3	3
One to two years	23,811		3	3	30,691	2	2
Two to three years	30,562		2	2	—	—	—
Three years or more	129,283		6	—	125,015	6	—
	$	197,537	14	8	180,135	11	5

Of the $183.6 million of exploratory well costs capitalized more than one year at June 30, 2021, $91.5 million is in Vietnam, $46.2 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 status, including agreements with the partners, of operating and production plans.

In 2020, declines in future oil and natural gas prices (principally driven by reduced demand from 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 $987.1 million to reduce the carrying values to their estimated fair values 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 six months ended June 30, 2021 and 2020.


	Six Months Ended June 30,
(Thousands of dollars)	2021		2020
U.S.	$	—	947,437
Canada	171,296		—
Other Foreign	—		39,709

	$	171,296	987,146

During the first quarter of 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.