Note 20 Property, plant and equipment

Amounts in US$'000	Oil &; gas properties			Furniture, equipment and vehicles		Production facilities and machinery		Buildings and improvements		Construction in progress		Exploration and evaluation assets^(b)			Total
Cost at 1 January 2015		749,947			12,057		111,646		9,527		59,425		140,444			1,083,046
Additions		(4,640)	^(a)		954		-		272		36,543		12,299			45,428
Currency translation differences		(27,522)			(182)		(2,577)		(92)		-		(1,510)			(31,883)
Disposals		(241)			(13)		(1,685)		(84)		-		-			(2,023)
Write-off / Impairment loss		(128,956)			-		(13,242)		-		(7,376)		(30,084)	^(c)		(179,658)
Transfers		60,404			929		30,690		895		(58,769)		(34,149)			-
Cost at 31 December 2015		648,992			13,745		124,832		10,518		29,823		87,000			914,910
Additions		(3,531)	^(a)		406		466		-		20,322		18,181			35,844
Currency translation differences		16,132			126		2,077		35		73		790			19,233
Disposals		-			(22)		-		-		-		-			(22)
Write-off / Impairment reversal		5,664			-		-		-		-		(31,366)	^(d)		(25,702)
Transfers		24,984			102		5,038		-		(17,292)		(12,832)			-
Cost at 31 December 2016		692,241			14,357		132,413		10,553		32,926		61,773			944,263
Additions		7,997	^(a)		954		-		-		66,953		49,455			125,359
Currency translation differences		(1,142)			(12)		(147)		(3)		(62)		(104)			(1,470)
Disposals		-			(112)		-		(189)		-		-			(301)
Write-off / Impairment reversal		-			-		-		-		-		(5,834)	^(e)		(5,834)
Transfers		77,408			211		25,130		-		(61,827)		(40,922)			-
Cost at 31 December 2017		776,504			15,398		157,396		10,361		37,990		64,368			1,062,017

Depreciation and write-down at 1 January 2015		(240,439)			(4,449)		(45,147)		(2,244)		-		-			(292,279)
Depreciation		(84,849)			(2,850)		(15,467)		(874)		-		-			(104,040)
Disposals		-			8		-		15		-		-			23
Currency translation differences		4,115			(26)		-		(92)		-		-			3,997
Depreciation and write-down at 31 December 2015		(321,173)			(7,317)		(60,614)		(3,195)		-		-			(392,299)
Depreciation		(61,080)			(2,702)		(10,788)		(920)		-		-			(75,490)
Disposals		-			8		-		-		-		-			8
Currency translation differences		(2,486)			(38)		(296)		(16)		-		-			(2,836)
Depreciation and write-down at 31 December 2016		(384,739)			(10,049)		(71,698)		(4,131)		-		-			(470,617)
Depreciation		(57,725)			(1,948)		(14,558)		(844)		-		-			(75,075)
Disposals		-			73		-		38		-		-			111
Currency translation differences		930			8		24		5		-		-			967
Depreciation and write-down at 31 December 2017		(441,534)			(11,916)		(86,232)		(4,932)		-		-			(544,614)

Carrying amount at 31 December 2015		327,819			6,428		64,218		7,323		29,823		87,000			522,611
Carrying amount at 31 December 2016		307,502			4,308		60,715		6,422		32,926		61,773			473,646
Carrying amount at 31 December 2017		334,970			3,482		71,164		5,429		37,990		64,368			517,403

^(a) Corresponds to the effect of change in estimate of assets retirement obligations.

^(b) Exploration wells movement and balances are shown in the table below; seismic and other exploratory assets amount to US$ 53,764,000 (US$ 53,523,000 in 2016 and US$ 64,094,000 in 2015).

Amounts in US$ '000	Total
Exploration wells at 31 December 2015		22,906
Additions		15,088
Write-offs		(19,949)
Transfers		(9,795)
Exploration wells at 31 December 2016		8,250
Additions		35,299
Write-offs		(3,664)
Transfers		(29,281)
Exploration wells at 31 December 2017		10,604

As of 31 December 2017, there were two exploratory wells that have been capitalised for a period less than a year amounting to US$ 4,488,000 and two exploratory wells that have been capitalised for a period over a year amounting to US$ 6,116,000.

^(c) Corresponds to the cost of two unsuccessful exploratory wells in Colombia (one well in CPO4 Block and one well in Llanos 32). The charge also includes the loss generated by the write-off of the seismic cost for Flamenco Block in Chile generated by the relinquishment of 143 sq km in November 2015 and the write off of two wells drilled in previous years in the same block for which no additional work would be performed.

^(d) Corresponds to the write-off of five wells drilled in previous years in the Chilean blocks for which no additional work would be performed, the loss generated by the write-off of the seismic cost for Llanos 62 Block in Colombia generated by the relinquishment of the area in September 2016. In addition, during September 2016, five blocks in Brazil were relinquished so the associated investment was written off.

^(e) Corresponds to five unsuccessful exploratory wells, one well drilled in Colombia (Llanos 34 Block), one well drilled in Brazil (REC-T-94 Block) and three non-operated wells drilled in Argentina (Puelen and Sierra del Nevado Blocks) in 2017. The charge also includes the loss generated by the write-off of the seismic cost for Campanario and Isla Norte Blocks in Chile generated by the relinquishment of 327 sq km in 2017.