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Accounting for Suspended Exploratory Wells (Tables)
 12 Months Ended
Dec. 31, 2017
Accounting for Suspended Exploratory Wells [Abstract]  
Changes in company's suspended exploratory well costs
The following table indicates the changes to the company’s suspended exploratory well costs for the three years ended December 31, 2017:
 
2017

2016

2015

Beginning balance at January 1
$
3,540

$
3,312

$
4,195

Additions to capitalized exploratory well costs pending the determination of proved reserves
323

465

869

Reclassifications to wells, facilities and equipment based on the determination of proved reserves
(113
)
(119
)
(164
)
Capitalized exploratory well costs charged to expense
(39
)
(118
)
(1,397
)
Other reductions*
(9
)
—

(191
)
Ending balance at December 31
$
3,702

$
3,540

$
3,312

*    Represents property sales.
Aging of capitalized well costs and number of project
The following table provides an aging of capitalized well costs and the number of projects for which exploratory well costs have been capitalized for a period greater than one year since the completion of drilling.
 
At December 31
 
 
2017

2016

2015

Exploratory well costs capitalized for a period of one year or less
$
307

$
445

$
489

Exploratory well costs capitalized for a period greater than one year
3,395

3,095

2,823

Balance at December 31
$
3,702

$
3,540

$
3,312

Number of projects with exploratory well costs that have been capitalized for a period greater than one year*
32

35

39

*    Certain projects have multiple wells or fields or both.
Aging of Costs on Well and Project Basis
The $3,395 of suspended well costs capitalized for a period greater than one year as of December 31, 2017, represents 158 exploratory wells in 32 projects. The tables below contain the aging of these costs on a well and project basis:
Aging based on drilling completion date of individual wells:
Amount

 
 
Number of wells

1998-2006
$
318

 
 
29

2007-2011
879

 
 
50

2012-2016
2,198

 
 
79

Total
$
3,395

 
 
158

 
 
 
 
 
Aging based on drilling completion date of last suspended well in project:
Amount

 
 
Number of projects

2003-2009
$
344

 
 
5

2010-2013
367

 
 
6

2014-2017
2,684

 
 
21

Total
$
3,395

 
 
32