Intangible assets

12 Months Ended

Dec. 31, 2017

Intangible Assets [Abstract]

Intangible assets

Intangible assets

											$ million
					2017						2016
	Exploration and appraisal expenditurea		Other intangibles		Total		Exploration and appraisal expenditurea		Other intangibles		Total
Cost
At 1 January	18,524		4,035		22,559		19,856		4,055		23,911
Exchange adjustments	—		197		197		—		(149	)	(149	)
Acquisitions	—		41		41		—		15		15
Additions	2,128		310		2,438		2,896		251		3,147
Transfers	(451	)	—		(451	)	(1,629	)	—		(1,629	)
Deletions	(2,315	)	(95	)	(2,410	)	(2,599	)	(137	)	(2,736	)
At 31 December	17,886		4,488		22,374		18,524		4,035		22,559
Amortization
At 1 January	1,564		2,812		4,376		2,570		2,681		5,251
Exchange adjustments	—		107		107		—		(96	)	(96	)
Charge for the year	1,603		335		1,938		1,274		351		1,625
Impairment losses	—		—		—		62		—		62
Transfers	—		—		—		(5	)	—		(5	)
Deletions	(2,307	)	(95	)	(2,402	)	(2,337	)	(124	)	(2,461	)
At 31 December	860		3,159		4,019		1,564		2,812		4,376
Net book amount at 31 December	17,026		1,329		18,355		16,960		1,223		18,183
Net book amount at 1 January	16,960		1,223		18,183		17,286		1,374		18,660

^a For further information see Intangible assets within Note 1 and Note 6.