http://www.wsj.com/articles/in-west-texas-oil-drillers-keep-pumping-1446254165

When are the variable costs lower:  when "drillers can use one rig to bore vertically down through as many as four layers of oil-and-gas-rich rock and then horizontally through each layer in succession to stretch out the productive life of a well" or when they need a rig for each layer?

Why does the Permian continue to show strength even though the number of oil rigs drilling in the Permian has plunged by about 60% so far this year. What does this imply about marginal costs, average variable cost, and average total cost?