A pipeline owned by Nexen Energy ULC has spilled about 31,500 barrels of oil "emulsion" in northern Alberta, the province's energy watchdog says.

The Alberta Energy Regulator said late on Thursday that it was responding to a pipeline rupture about 36 kilometres southeast of Fort McMurray. Emulsion is a mixture of oil and water.

The regulator said the spill has affected about 16,000 square metres of land concentrated along the pipeline right-of-way. The area includes muskeg, but the spill has not flowed into a water body, the agency said. Cleanup is under way and the pipeline and a drilling site have been shut-in, stopping the release, according to the regulator.

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The rupture comes as the energy industry struggles to build support for a series of major pipeline proposals designed to move fast-growing oil sands production to new markets.

Nexen said the incident took place at its steam-driven Long Lake oil sands operation.

‎The leak was discovered Wednesday, prompting an emergency response. Nexen said the emulsion was a mixture of bitumen, produced water and sand. An investigation is under way into possible causes, the company said.

Nexen is a unit of China's CNOOC Ltd.