Why have large declines in oil prices and in the rig count not triggered a more dramatic decline in production? At what price level would a large share of U.S. shale oil production lose economic viability?

The spot price of crude oil declined from $106.07 per barrel on June 30, 2014 to $53.45 per barrel on December 31, 2014, representing a 50% decline within six months. By December 31, 2015, the spot price had declined further, to $37.13, and it remains substantially below its level during 2011...