Property, Plant and Equipment (Tables)

6 Months Ended

Jun. 30, 2018

Property, Plant And Equipment [Abstract]

Net Changes in Capitalized Exploratory Well Costs

(Thousands of dollars)	2018		2017
Beginning balance at January 1	$	175,640	148,500
Additions pending the determination of proved reserves		30,731	48,764
Reclassifications to proved properties based on the determination of proved reserves		–	(13,370)
Capitalized exploratory well costs charged to expense		–	(8,360)
Balance at June 30	$	206,371	175,534

Aging of Capitalized Exploratory Well Costs

		June 30,
	2018				2017
(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	$	34,779	2	2	$	57,900	3	3
One to two years		35,934	2	1		53,023	3	3
Two to three years		50,272	2	2		–	–	–
Three years or more		85,386	7	1		64,611	6	–
	$	206,371	13	6	$	175,534	12	6