The following table presents a summary of the Company’s property and equipment balances as of December 31, 2017 and 2016 (in thousands).



	December 31,
	2017			2016
Oil and natural gas properties
Evaluated (subject to amortization)	$	3,004,770		$	2,408,305
Unproved and unevaluated (not subject to amortization)	637,396			479,736
Total oil and natural gas properties	3,642,166			2,888,041
Accumulated depletion	(2,021,169		)	(1,850,882		)
Net oil and natural gas properties	1,620,997			1,037,159
Midstream and other property and equipment
Midstream equipment and facilities	258,725			145,662
Furniture, fixtures and other equipment	6,109			5,487
Software	7,942			3,206
Land	2,892			1,437
Leasehold improvements	5,428			5,003
Total midstream and other property and equipment	281,096			160,795
Accumulated depreciation	(20,637		)	(13,429		)
Net midstream and other property and equipment	260,459			147,366
Net property and equipment	$	1,881,456		$	1,184,525

The following table provides a breakdown of the Company’s unproved and unevaluated property costs not subject to amortization as of December 31, 2017 and the year in which these costs were incurred (in thousands).



Description	2017		2016		2015		2014 and prior		Total
Costs incurred for
Property acquisition	$	213,076	$	125,689	$	222,912	$	45,809	$	607,486
Exploration wells	16,688		988		547		—		18,223
Development wells	11,396		272		19		—		11,687
Total	$	241,160	$	126,949	$	223,478	$	45,809	$	637,396

Property acquisition costs primarily include leasehold costs paid to secure oil and natural gas mineral leases, but may also include broker and legal expenses, geological and geophysical expenses and capitalized internal costs associated with developing oil and natural gas prospects on these properties. Property acquisition costs are transferred into the amortization base on an ongoing basis as these properties are evaluated and proved reserves are established or impairment is determined. Unproved and unevaluated properties are assessed for possible impairment on a periodic basis based upon changes in operating or economic conditions.

Property acquisition costs incurred that remain in unproved and unevaluated property at December 31, 2017 are related almost entirely to the Company’s leasehold and mineral acquisitions in the Wolfcamp and Bone Spring plays in the Delaware Basin in Southeast New Mexico and West Texas during the past five years. These costs are associated with acreage for which proved reserves have yet to be assigned. A significant portion of these costs are associated with properties that are held by production or have automatic lease renewal options. As the Company drills wells and assigns proved reserves to these properties or determines that certain portions of this acreage, if any, cannot be assigned proved reserves, portions of these costs are transferred to the amortization base.

Costs excluded from amortization also include those costs associated with exploration and development wells in progress or awaiting completion at year-end. These costs are transferred into the amortization base on an ongoing basis as these wells are completed and proved reserves are established or confirmed. These costs totaled $29.9 million at December 31, 2017. Of this total, $18.2 million was associated with exploration wells and $11.7 million was associated with development wells. The Company anticipates that most of the $29.9 million associated with these wells in progress at December 31, 2017 will be transferred to the amortization base during 2018.