Dozens of Greenpeace activists shut down Shell's sand oil mining fields at Alberta, Canada on Tuesday to draw attention to their protests. The company has started mining tar sands in the region for extracting oil. Activists say that the greenhouse gas emissions in keeping the project running far outweigh the benefits.

A Greenpeace protest shut down Royal Dutch Shell Plc’s massive oil sands mine on Tuesday on the eve of Prime Minister Stephen Harper’s visit with President Barack Obama.

Two dozen Greenpeace members chained themselves to giant earth-moving equipment, shutting down Shell’s massive Albian Sands oil sands mine in northwestern Alberta for several hours.

Shell temporarily suspended operations to ensure the safety of the activists and of Shell staff, while it negotiated with the protesters.

Alberta’s oil sands projects have been criticized as a growing source of greenhouse gas emissions.

Shell and the Royal Canadian Mounted Police were eager to see a peaceful end to the demonstration as it stretched on through most of the day. Shell spokesman Paul Hagel said the company invited the protesters to unchain themselves and come to a nearby office to discuss their concerns. Shell also offered them food, water and bug spray.

RCMP Constable Karolina Malik said the protesters were very polite, so there was no rush to cut the chains or make arrests.

But Greenpeace resisted Shell’s attempts to negotiate an end to the protest. Mike Hudema, one of the Greenpeace leaders who chained himself to a massive dump truck, said they wanted to send a message to world leaders on the environmental damage being caused by giant oil sands projects.

“To send a very strong message to President Obama and Prime Minister Harper that climate leaders don’t buy tarsands,” he said.

Harper visits the White House on Wednesday. He is eager to protect the oil sands industry, which has spurred a booming economy in western Canada in recent years.

Environmental groups want Obama to get tough with Canada over the issue, but the president wants to reduce America’s reliance on Middle East oil and has said he would like to work with Canada on developing carbon capture and storage to help deal with the massive emissions from the sands as well as the U.S. coal industry. The new, largely unproven technology would bury harmful emissions underground.

Harper said in Parliament on Wednesday that the U.S. coal industry is 40 times bigger than the output of the oil sands. Harper also noted the oil sands are a huge source of jobs in Canada and said they are working with the Obama administration on a clean energy dialogue.

Industry officials estimate the oil sands in northern Alberta could yield as much as 175 billion barrels of oil, making Canada second only to Saudi Arabia in crude oil reserves. But the extraction process produces a high amount of the greenhouse gases blamed for climate change and critics say the growing operations by also threaten Alberta’s rivers and forest.