INCOME TAXES
 9 Months Ended
Sep. 30, 2016
Income Tax Disclosure [Abstract]  
INCOME TAXES
INCOME TAXES
Our effective tax rate is impacted primarily by the amount of taxable income associated with each jurisdiction in which our income is subject to income tax, permanent differences between the financial statement carrying amounts and tax bases of assets and liabilities.
A summary of the provision for income taxes is as follows (in thousands):
 
 
Three Months Ended September 30,
 
Nine Months Ended September 30,
 
 
2016
 
2015
 
2016
 
2015
Current portion of income tax expense (benefit)
 
$
1

 
$
39

 
$
4

 
$
5

Deferred portion of income tax (benefit) expense
 
—

 
(7,891
)
 
—

 
(5,136
)
Total income tax expense (benefit)
 
$
1

 
$
(7,852
)
 
$
4

 
$
(5,131
)
Effective tax rate
 
—
%
 
49.2
%
 
—
%
 
44.0
%

During the three- and nine-month periods ended September 30, 2016, our effective tax rate differed from the statutory rate primarily as a result of the impact of recording a valuation allowance to offset the amount of additional deferred tax asset generated during the periods. For the three- and nine-month periods ended September 30, 2015, our effective tax rate differed from the statutory rate primarily as a result of the benefit from estimated depletion deductions that exceed the tax basis in the mineral reserves.
The blended state tax rate applied to the deferred tax calculation is subject to change due to changes in state laws and changes in the mix of our business and the states in which we have a taxable relationship. This creates fluctuations in the value of our net deferred tax asset.
As of September 30, 2016, we do not believe it is more likely than not that we will fully realize the benefit of our deferred tax assets. As such, we increased the valuation allowance related to our deferred tax assets by $20.7 million for the nine months ended September 30, 2016. We recognized a full valuation allowance against our net deferred tax assets as of September 30, 2016, and December 31, 2015.
A summary of our valuation allowance activity is as follows (in thousands):
 
 
Nine Months Ended September 30,
 
 
2016
 
2015
Valuation allowance, beginning of period
 
$
300,601

 
$
268

Additions
 
20,740

 
—

Reversals
 
—

 
—

Valuation allowance, end of period
 
$
321,341

 
$
268