The following table sets forth the capitalized costs under the successful efforts method for our oil and natural gas properties as of:

	September 30, 2019			December 31, 2018
	(in thousands)
Oil and natural gas properties, proved:
Turkey	$	169,758		$	162,494
Bulgaria		479			512
Total oil and natural gas properties, proved		170,237			163,006
Oil and natural gas properties, unproved:
Turkey		14,911			14,965
Bulgaria		–			730
Total oil and natural gas properties, unproved		14,911			15,695
Gross oil and natural gas properties		185,148			178,701
Accumulated depletion		(103,235	)		(100,582	)
Net oil and natural gas properties	$	81,913		$	78,119

At September 30, 2019 there were no amounts excluded from the depletion calculation for proved development wells currently in progress and for costs associated with fields currently not in production. At December 31, 2018, we excluded $0.5 million, from the depletion calculation for proved development wells currently in progress and for costs associated with fields currently not in production.

At September 30, 2019, the capitalized costs of our oil and natural gas properties, net of accumulated depletion, included $5.5 million relating to acquisition costs of proved properties, which are being depleted by the unit-of-production method using total proved reserves, and $63.8 million relating to well costs and additional development costs, which are being depleted by the unit-of-production method using proved developed reserves.

At December 31, 2018, the capitalized costs of our oil and natural gas properties included $6.5 million relating to acquisition costs of proved properties, which are being amortized by the unit-of-production method using total proved reserves, and $53.4 million relating to well costs and additional development costs, which are being amortized by the unit-of-production method using proved developed reserves.

Proved oil and natural gas properties are reviewed for impairment when events and circumstances indicate the carrying value of such properties may not be recoverable. We primarily use Level 3 inputs to determine fair value, including but not limited to, estimates of proved reserves, future commodity prices, the timing and amount of future production and capital expenditures and discount rates commensurate with the risk reflective of the lives remaining for the respective oil and natural gas properties.

During the three months ended September 30, 2019 and 2018, we recorded $0.5 million of exploratory dry-hole costs and $0.2 million of impairment of proved properties and exploratory well costs, respectively, which are primarily measured using Level 3 inputs.

During the nine months ended September 30, 2019 and 2018, we recorded $6.3 million of exploratory dry-hole costs and $0.4 million of impairment of proved properties and exploratory well costs, respectively, which are primarily measured using Level 3 inputs.

As of September 30, 2019, and December 31, 2018 there were no exploratory well costs greater than one year.

The historical cost of equipment and other property, presented on a gross basis with accumulated depreciation, is summarized as follows:

	September 30, 2019			December 31, 2018
	(in thousands)
Other equipment	$	1,176		$	1,240
Land		139			149
Inventory		5,858			6,791
Gas gathering system and facilities		181			194
Vehicles		316			336
Leasehold improvements, office equipment and software		5,421			5,698
Gross equipment and other property		13,091			14,408
Accumulated depreciation		(5,411	)		(5,268	)
Net equipment and other property	$	7,680		$	9,140

At September 30, 2019, and December 31, 2018, in addition to the above, we have classified $4.8 million and $5.2 million, respectively, of inventory as a current asset, which represents our expected inventory consumption during the next twelve months. We classify our remaining materials and supply inventory as a long-term asset because such materials will ultimately be classified as a long-term asset when the material is used in the drilling of a well.

At September 30, 2019 and December 31, 2018, we excluded $11.7 million and $12.0 million of inventory, respectively, from depreciation as the inventory had not been placed into service.