The Alberta Energy Regulator said it would allow a restart of 40 of 95 pipelines closed at Nexen's Long Lake oil sands operations after reviewing maintenance and monitoring documentation.

"The remaining 55 pipelines affected by the order, which contain several products, including crude oil, natural gas, salt water, fresh water and emulsion, continue to be suspended," the regulator said in a statement.

"These pipelines ... will not return to service until Nexen can demonstrate that the pipelines can be operated safely and within all requirements."

The provincial regulator last month ordered Nexen to shut in the pipelines at the Long Lake facility as part of an investigation into one of the largest-ever oil-related pipeline spills on North American soil, discovered in July.

(Reporting by Jeffrey Hodgson; Editing by Gopakumar Warrier)