Four other shippers active in the Bakken region—Tesoro Refining and Marketing Co., Suncor Energy Marketing Co., Kodiak Oil & Gas Corp, and Connecticut-based Northern Tier Energy—also wrote FERC to support a reasonable connection agreement between the two pipeline companies.

Another Company Complains to FERC about Enbridge

Enbridge is fighting similar discrimination claims involving another segment of its Lakehead pipeline system. In that complaint, also filed with FERC in May, PBF Holding Co. accused Enbridge of favoring Canadian crude oil shipments over the light sweet crude the company needs for its Toledo refinery.

PBF declined to elaborate on its case with FERC. According to the complaint, PBF said that after Enbridge limited volume on a light crude-only line that had reached capacity, PBF tried to secure space on another Enbridge pipeline that can carry various types of oil but typically carries Canadian heavy crude.

Enbridge denied PBF's request, arguing that the line is dedicated to heavy crude and it would be unfair to accommodate PBF at the expense of existing shippers who have no alternate route. In this case, Enbridge has the backing of a host of refiners that get oil from both lines, but are pocketing big profits these days by using the much cheaper heavy crude imported from Canada.

The increasing competition for pipeline space was underscored in a June report, titled "Oil Sands Margins Squeezed by Bakken Boom," from London-based consultants Wood Mackenzie.

"The massive growth of North American tight oil, particularly North Dakota's Bakken play, is placing pressure, and competing directly with, Canadian barrels moving south," the report said. "This problem will only get worse."