5. Properties, Plant and Equipment

Oil and Gas Properties

The Company accounts for its oil and natural gas exploration and production activities under the successful efforts method of accounting. Oil and gas properties consisted of the following at March 31, 2018 and December 31, 2017:


	March 31,		December 31,
(in thousands of dollars)	2018		2017
Mineral interests in properties
Unproved	$	142,623	$	164,087
Proved		915,192		893,246
Wells and equipment and related facilities		1,496,286		1,434,383
		2,554,101		2,491,716
Less: Accumulated depletion and impairment		(936,441)		(894,676)
Net oil and gas properties	$	1,617,660	$	1,597,040

There were no exploratory wells drilled during the three months ended March 31, 2018 or 2017. As such, no associated costs were capitalized and no exploratory wells resulted in exploration expense during either period.

The Company capitalizes interest on expenditures for significant exploration and development projects that last more than six months while activities are in progress to bring the assets to their intended use. During the three months ended March 31, 2018, the Company capitalized less than $0.1 million associated with such in progress projects. The Company did not capitalize any interest during the three months ended March 31, 2017 as no projects lasted more than six months. Costs incurred to maintain wells and related equipment are charged to expense as incurred.

Depletion of oil and gas properties amounted to $41.1 million and $35.4 million for the three months ended March 31, 2018 and 2017, respectively.

The Company continues to monitor its proved and unproved properties for impairment. No impairments of proved or unproved properties were recorded during the three months ended March 31, 2018 and 2017.

Other Property, Plant and Equipment

Other property, plant and equipment consisted of the following at March 31, 2018 and December 31, 2017:


	March 31,		December 31,
(in thousands of dollars)	2018		2017
Leasehold improvements	$	1,186	$	1,186
Furniture, fixtures, computers and software		4,441		4,410
Vehicles		1,922		1,922
Aircraft		910		910
Other		211		210
		8,670		8,638
Less: Accumulated depreciation and amortization		(6,197)		(5,919)
Net other property, plant and equipment	$	2,473	$	2,719

Depreciation and amortization of other property, plant and equipment amounted to $0.3 million for each of the three months ended March 31, 2018 and 2017, respectively.