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Supplementary information
 6 Months Ended
Jun. 30, 2015
Oil and Gas Exploration and Production Industries Disclosures [Abstract]  
Supplementary information
Supplementary information
Costs incurred in oil and natural gas property acquisition, exploration and development activities
Costs incurred in the acquisition, exploration and development of oil and natural gas assets are presented below for the periods presented:
 
 
Three months ended June 30,
 
Six months ended June 30,
(in thousands)
 
2015

2014
 
2015
 
2014
Property acquisition costs:
 
 

 
 

 
 

 
—

Evaluated
 
$
—

 
$
3,848

 
$
—

 
$
3,873

Unevaluated
 
—


2,645

 
—

 
9,925

Exploration
 
3,841


8,143

 
8,354

 
16,642

Development costs(1)
 
110,518


220,240

 
317,190

 
408,553

Total costs incurred
 
$
114,359


$
234,876

 
$
325,544

 
$
438,993

____________________________________________________________________________
(1)
The costs incurred for oil and natural gas development activities include $0.5 million and $0.9 million in asset retirement obligations for the three months ended June 30, 2015 and 2014, respectively, and $1.0 million and $1.5 million for the six months ended June 30, 2015 and 2014, respectively.