View of three oil wells and flaring of natural gas on The Fort Berthold Indian Reservation near New Town, ND on August 13, 2014.

A further slump in oil prices may dampen shale drilling's profitable run, according to a report from Goldman Sachs.

In the past four weeks, global oil prices plunged eight percent. And a barrel in the U.S. is below $90, the first time in two years. On Thursday, shares of companies centered in North Dakota's Bakken Shale dropped more than 5 percent.

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If prices drop any further, the Wall Street Journal reports, drilling activity would slow down drastically. The key issue lies in the overabundance of oil, with sluggish global demand to match it. Texas, Colorado and North Dakota shale-drilling has increased U.S. production by nearly three million barrels a day since 2011.

