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Supplemental Coal Data (unaudited)
 12 Months Ended
Dec. 31, 2017
Mineral Industries Disclosures [Abstract]  
Supplemental Coal Data (unaudited)
Supplemental Coal Data (unaudited)
 
 
Millions of Tons
 
 
For the Year Ended December 31,
 
 
2017
 
2016
 
2015
 
2014
 
2013
Proven and probable coal reserves at beginning of period
 
2,361

 
3,047

 
3,238

 
3,032

 
4,229

Purchased reserves
 
—

 
—

 
24

 
—

 
1

Reserves sold in place
 
(16
)
 
(601
)
 
(43
)
 
(233
)
 
(1,199
)
Production
 
(26
)
 
(26
)
 
(29
)
 
(32
)
 
(55
)
Revisions and other changes
 
(21
)
 
(59
)
 
(143
)
 
471

 
56

Consolidated proven and probable coal reserves at end of period* (1)
 
2,298

 
2,361

 
3,047

 
3,238

 
3,032

______________
* Proven and probable coal reserves are the equivalent of “demonstrated reserves” under the coal resource classification system of the U.S. Geological Survey. Generally, these reserves would be commercially mineable at year-end prices and cost levels, using current technology and mining practices.
(1) 143.3 million tons of the Northern Appalachia product are controlled by CCC, a former subsidiary of ParentCo that was sold in December 2013. As of this filing, these tons are still controlled by CCC but are shown in CONSOL Energy's reserves due to a binding agreement that these tons will be released to CONSOL Energy upon the assignment of the underlying lease to CONSOL Energy.
CONSOL Energy's coal reserves are located in nearly every major coal-producing region in North America. Our estimate of proven and probable coal reserves has been determined by CONSOL Energy. At December 31, 2017, 227 million tons were assigned to mines either in production or temporarily idled. The proven and probable coal reserves at December 31, 2017 include 2,211 million tons of thermal coal reserves, of which approximately 2 percent has a sulfur content equivalent to less than 1.2 pounds sulfur dioxide per million British thermal unit (Btu), 8 percent has a sulfur content equivalent to between 1.2 and 2.5 pounds sulfur dioxide per million Btu, and 90 percent has a sulfur content equivalent to greater than 2.5 pounds sulfur dioxide per million Btu. The reserves also include 87 million tons of metallurgical coal in consolidated reserves, of which approximately 24 percent has a sulfur content equivalent to less than 1.2 pounds sulfur dioxide per million Btu and 76 percent has a sulfur content equivalent to between 1.2 and 2.5 pounds sulfur dioxide per million Btu.