Annual report pursuant to Section 13 and 15(d)

Supplemental Information on Oil and Natural Gas Properties (unaudited) (Proved Developed and Undeveloped Oil and Gas Reserve Quantities) (Details)

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Supplemental Information on Oil and Natural Gas Properties (unaudited) (Proved Developed and Undeveloped Oil and Gas Reserve Quantities) (Details)
MMcf in Thousands, MBoe in Thousands, MBbls in Thousands
 12 Months Ended
Dec. 31, 2017
MBoe
MBbls
MMcf
Dec. 31, 2016
MBoe
MBbls
MMcf
Dec. 31, 2015
MBoe
MBbls
MMcf
Proved undeveloped reserves:      
Beginning of period, MBOE proved developed | MBoe 43,232 28,617 18,201
End of period, MBOE proved developed | MBoe 69,318 43,232 28,617
Beginning of periodc, MBOE proved developed | MBoe 48,348 25,654 14,623
End of period, MBOE proved developed | MBoe 67,656 48,348 25,654
Oil      
Proved developed and undeveloped reserves:      
Beginning of period | MBbls 71,145 43,348 25,733
Revisions to previous estimates | MBbls (5,171) (5,738) (1,632)
Purchase of reserves in place | MBbls 8,388 25,054 2,932
Sale of reserves in place | MBbls 0 (1,718) (23)
Extensions and discoveries | MBbls 39,267 14,479 19,127
Production | MBbls (6,557) (4,280) (2,789)
End of period | MBbls 107,072 71,145 43,348
Proved undeveloped reserves:      
Beginning of period, proved developed | MBbls 32,920 22,257 14,006
End of period, proved developed | MBbls 51,920 32,920 22,257
Beginning of period, proved undeveloped | MBbls 38,225 21,091 11,727
End of period, proved undeveloped | MBbls 55,152 38,225 21,091
Natural gas      
Proved developed and undeveloped reserves:      
Beginning of period | MMcf 122,611 65,537 42,548
Revisions to previous estimates | MMcf 6,336 13,929 4,870
Purchase of reserves in place | MMcf 12,711 36,474 2,915
Sale of reserves in place | MMcf 0 (2,765) (105)
Extensions and discoveries | MMcf 48,648 17,194 19,621
Production | MMcf (10,896) (7,758) (4,312)
End of period | MMcf 179,410 122,611 65,537
Proved undeveloped reserves:      
Beginning of period, proved developed | MMcf 61,871 38,157 25,171
End of period, proved developed | MMcf 104,389 61,871 38,157
Beginning of period, proved undeveloped | MMcf 60,740 27,380 17,377
End of period, proved undeveloped | MMcf 75,021 60,740 27,380