The Bakken shale, which spans parts of North Dakota, Montana and Canada, has been a significant source of recent oil production growth for the US (though far from the only source, as many readers would point out). The bulk of that production is in North Dakota, which is producing over 900,000 barrels per day as of August, up from 330,000 bbl/d in the same month of 2010.

North Dakota production dipped late last year, and output remained largely flat from December 2012 to May 2013,  “driven in part by a major shift from drilling to hold leases to pad drilling, as well as inclement weather that slowed activity in April and May”, investment bank Simmons & Co wrote this morning in a note to clients.

But recent production figures indicate that it is back on a strong upward trajectory. North Dakota Bakken oil output averaged 847,000 barrels per day in August, up from 746,000 bbl/d in May, and 638,000 bbl/d the previous August. Simmons released a forecast on September 23 that said Williston Basin oil production would rise by around 19% year-over-year in 2013, and has since concluded that that may be too low.

“Based on actual production data released by the state of North Dakota since we published this report, North Dakota Bakken (and by extension total Williston Basin) production is growing at a faster rate than anticipated,” the bank said. “Should North Dakota Bakken production continue to grow at a pace similar to that seen in the last few months, our total US and Williston Basin oil production forecasts could prove conservative.”