The Energy Information Administration (EIA) has published a new drilling rig productivity report that you may find interesting. Among other things: "Increases in drilling efficiency and new well productivity, rather than an increase in the number of active rigs, have been the main drivers of recent growth in domestic oil and natural gas production. *** The Bakken and Eagle Ford regions together account for about 75% of current monthly oil production growth across the six regions tracked in the DPR. Over the past year, production in these two regions increased by nearly 700,000 barrels per day. The Permian region, which remains the biggest absolute oil producer, grew by about 93,000 barrels per day from last year's production level."
For an executive summary, see here. For the whole report, see here.