A growing number of American oil and gas giants are divesting themselves of their Canadian tar sands holdings.

In the latest move, Chevron is reportedly looking to sell its 20 percent stake in the Athabasca Oil Sands project, located in Alberta. The company has been in talks with investment banks about the sale, which could net Chevron $2.5 billion,a source told Reuters. Chevron is the second biggest oil producer in the U.S.

The month of March saw three companies -- two of them American -- divest some of their tar sands assets.

ConocoPhillips signed a deal worth $13.3 billion in March to divest a good part of its Canadian assets. The Houston-based oil and gas giant agreed to sell its interests in Foster Creek Christina Lake oil sands partnership in northeast Alberta and the majority of its gas assets in the Western Canada deep basin in Alberta to Cenovus, a Canadian company. The company will retain its interests in two other Alberta tar sands projects.

Divesting itself of part of its tar sands assets “improves our underlying financial and portfolio metrics, which will drive free cash flow generation and returns,” Ryan Lance, ConocoPhillips chairman and CEO, said of the deal during a press conference.

Marathon Oil is another American oil and gas giant to sell its interests in Canada’s tar sands. In early March, the company signed an agreement to sell its Canadian subsidiary, which includes its 20 percent interest in the Athabasca Oil Sands project, for $2.5 billion. At the same time, the company announced an agreement to acquire about 70,000 net surface acres in the Permian basin, located in Texas and New Mexico, for $1.1 billion.

Divesting of its tar sands assets while acquiring holdings in the Permian basin “are transformative milestones that will further align our portfolio with our strategy,” said Marathon Oil President and CEO Lee Tillman.

Shell also sold some of its tar sands assets in March, including reducing its interests in AOSP from 60 percent to 10 percent. The British–Dutch multinational sold part of its interests in the Athabasca project to a subsidiary of Canadian Natural Resources Ltd. for $8.5 billion.

Shell CEO Ben van Beurden characterized the announcement as “significant step in re-shaping Shell’s portfolio in line with our long-term strategy.”

ExxonMobil also wrote off one of its investments in a tar sands project this year. In a February statement, the company said that “as a result of very low prices during 2016, certain quantities of liquids and natural gas no longer qualified as proved reserves under SEC guidelines.” Exxon is the largest publicly-traded oil and gas company in the world. In its annual 10-K filing to the SEC, the company said it reduced its proven reserves by 3.3 billion oil-equivalent barrels last year compared to 2015.

As a result of the divestments American and European oil and gas giants have announced this year, Canadian-headquartered companies will own about 80 percent of tar sands assets, according to JWN Energy.

Tar sands oil are some of the dirtiest fossil fuels

Alberta is home to a boreal forest that provides 1.3 billion acres of habitat for a wide swath of species that include grizzly bears and wolves. That is an area the size of Florida. The trees and bogs capture carbon dioxide, and the wetlands act as a filter for millions of gallons of water.

First Nation communities, indigenous Canadians, rely on the boreal forest for hunting, fishing and trapping.

The boreal forest is also part of the Canadian tar sands region. If you're not familiar with it, the Natural Resources Defense Council (NRDC) describes tar sands as “a sludgy deposit of sand, clay, water and sticky, black bitumen used to make synthetic oil.”

The process to turn tar sands into oil is very energy-intensive and causes environmental damage to the boreal forest, where trees are clear-cut to make way for tar sands development. The cycle represents the fastest growing source of Canada’s greenhouse gas emissions, according to Greenpeace.

Despite the divestment from tar sands assets by foreign oil and gas giants, the tar sands will continue to be developed by Canadian companies, to the detriment of the boreal forest.

As Dan Woynillowicz, senior policy analyst with the Pembina Institute, based in Calgary, Alberta, wrote for World Watch, North America is “a critical juncture in its transportation fuel future.” Indeed it is, and the time is now to transition to clean and renewable fuels.

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