Oil and natural gas activities

12 Months Ended

Dec. 31, 2013

Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Abstract]

Our oil and natural gas activities are conducted entirely in the United States. Costs incurred in oil and natural gas producing activities are as follows for the years ended December 31:



	2013		2012		2011
Property acquisition costs
Proved properties	$	69,962	$	1,108	$	1,024
Unproved properties	145,853		46,895		15,795
Total acquisition costs	215,815		48,003		16,819
Development costs	376,394		409,429		250,182
Exploration costs(1)	77,507		54,432		57,016
Total	$	669,716	$	511,864	$	324,017

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(1)	Includes $37,344, $52,188, and $33,030 of EOR costs in 2013, 2012, and 2011, respectively.

Depreciation, depletion, and amortization expense of oil and natural gas properties was $179,734, $154,788, and $132,307 for the years ended December 31, 2013, 2012, and 2011, respectively. The average depreciation, depletion and amortization rate per equivalent unit of production was $18.45, $16.98, and $15.29 for the years ended December 31, 2013, 2012, and 2011, respectively.

Oil and natural gas properties not subject to amortization consist of the cost of unevaluated properties and seismic costs associated with specific unevaluated properties. Of the $321,477 of unproved property costs at 2013 being excluded from the amortization base, $169,044, $114,406, and $9,372 were incurred in 2013, 2012, and 2011, respectively, and $28,655 was incurred in prior years. We expect to complete our evaluation for the majority of these costs relating to non-EOR properties within the next two to five years.