The biggest oil market risk for 2017 is the prospect of a market share war — or at least a showdown — between OPEC and U.S. shale drillers, said Kate Richard, CEO of energy investment firm Warwick Energy. Richard offered her view after U.S. crude futures ended Tuesday’s trading lower for a seventh straight session, hitting their weakest closing level since November, according to CNBC.

OPEC’s effort to draw down brimming global crude inventories through production cuts bolstered oil prices above $50 through last week. But that rebound made more high-cost U.S. output profitable, leading to a recovery in American drilling that threatens to spoil the cartel’s bid to balance an oversupplied market.

It also threatens the oil-dependent economies of the Organization of the Petroleum Exporting Countries.

“What OPEC has shown is that it clearly cannot stomach $40 to $50 oil, whereas the U.S. producers over the past two years have gotten more and more efficient,” Richard told CNBC’s “Power Lunch,” referring to American shale drillers who rely on expensive enhanced drilling methods to squeeze oil and gas from shale rock.

“We’ve retooled our cost structure. We’ve become cheaper. We’ve become more efficient, and we can now make really good money in the core plays in the U.S. at $40 to $50 oil.” Still, Richard noted there is a huge dispersion in potential returns between the haves and have-nots in America’s oil patch. Drillers with the best acreage in Texas’ Permian Basin and Oklahoma’s Scoop and Stack regions can make money with oil prices at $40 to $50 a barrel, while producers in the southern expanse of the Permian and parts of the Rockies will struggle at that level.