5. Property, Plant and Equipment

Property, plant and equipment at December 31 were as follows:

	2017		2016
	(In millions)
Exploration and Production
Unproved properties	$	520	$	710
Proved properties		3,162		4,249
Wells, equipment and related facilities		25,550		38,250
		29,232		43,209
Midstream		3,219		3,598
Corporate and Other		53		100
Total — at cost		32,504		46,907
Less: Reserves for depreciation, depletion, amortization and lease impairment		16,312		23,312
Property, Plant and Equipment — Net	$	16,192	$	23,595

Capitalized Exploratory Well Costs: The following table discloses the amount of capitalized exploratory well costs pending determination of proved reserves at December 31, and the changes therein during the respective years:

	2017			2016			2015
	(In millions)
Balance at January 1	$	597		$	1,415		$	1,416
Additions to capitalized exploratory well costs pending the determination of proved reserves		116			79			424
Reclassifications to wells, facilities and equipment based on the determination of proved reserves		(165	)		—			(72	)
Capitalized exploratory well costs charged to expense		(268	)		(897	)		(356	)
Dispositions and other		24			—			3
Balance at December 31	$	304		$	597		$	1,415
Number of Wells at December 31		12			17			35

Additions to capitalized exploratory well costs primarily related to drilling activity at the Stabroek license offshore Guyana in 2017, 2016 and 2015, and the Gulf of Mexico in 2016 and 2015. Reclassifications to wells, facilities and equipment based on the determination of proved reserves primarily related to the sanction of the first phase of Liza Field development, offshore Guyana in 2017 and Equatorial Guinea in 2015.

Capitalized exploratory well costs charged to expense include the following:

2017: In Ghana, at the Hess operated offshore Deepwater Tano/Cape Three Points license (Hess 50% license interest), management determined in the fourth quarter of 2017 that it would not develop the previously discovered fields. As a result, we recorded a charge of $268 million to write-off previously capitalized exploration wells. See Note 24, Subsequent Events.

2016: At the Hess-operated Equus natural gas project, offshore the North West Shelf of Australia in the fourth quarter of 2016, we terminated a joint front-end engineering study with a third-party natural gas liquefaction joint venture and notified the Australian government of our intent to defer the project. As a result, we expensed all well costs associated with the project, including an exploration well completed in the second quarter of 2016, totaling $830 million. These properties were sold in 2017. In the second quarter of 2016, we expensed costs associated with two exploration wells at the non-operated Sicily project in the Gulf of Mexico where hydrocarbons were encountered but we decided not to pursue the project due to the low commodity price environment and the limited time remaining on the leases. We also expensed the cost of an unsuccessful exploration well at the non-operated Melmar project in the Gulf of Mexico, where noncommercial quantities of hydrocarbons were encountered.

2015: At the Dinarta Block in the Kurdistan Region of Iraq, we expensed an exploration well resulting from our and our partners’ decision to cease further drilling and relinquish the block. At the Deepwater/Tano Cape Three Points block, offshore Ghana, we expensed well costs primarily related to natural gas discoveries where we were unable to sufficiently progress appraisal negotiations with the regulator. We also expensed three wells with discovered resources offshore Australia that we determined would not be included in the development concept for the Equus project.

The preceding table excludes exploratory dry hole costs of $167 million in 2016 and $54 million in 2015, which were incurred and subsequently expensed in the same year.

Exploratory well costs capitalized for greater than one year following completion of drilling were $170 million at December 31, 2017, separated by year of completion as follows (in millions):

2016	$	—
2015		166
2014		—
2013		4
	$	170

Gulf of Mexico: Approximately 70% of the capitalized well costs in excess of one year relates to the appraisal of the northern portion of the Shenzi Field (Hess 28% participating interest) in the Gulf of Mexico, where hydrocarbons were encountered in the fourth quarter of 2015. The operator is conducting appraisal activities on adjacent acreage and is evaluating plans for development of the northern portion of the Shenzi Field.

JDA: Approximately 20% of the capitalized well costs in excess of one year relates to the JDA in the Gulf of Thailand (Hess 50%) where hydrocarbons were encountered in three successful exploration wells drilled in the western part of Block A-18. The operator is currently conducting subsurface evaluations and pre-development planning to facilitate commercial negotiations with the regulator for an extension of the existing gas sales contract to include development of the western part of the block area.