Tar sands mining has been credited as the largest industrial project on the planet, and comes with extreme costs to the region's people, forests, waterways, animal species, and the broader specter of global warming. If you missed it, National Geographic published a great piece on the controversies of tar sand operations.
As noted by DeSmog, Valero faces declining oil sources from Mexico and a shifty political scene in Venezuela, turning to the tar sands craze in Canada to secure more access to oil. The rapid development of tar sands extraction has helped secure Canada as the United States' top supplier of oil--we import almost twice as much oil from Canada as from Saudi Arabia, and half of Canada's oil is sourced from tar sands mining.
As the refining of bitumen from tar sands mines creates particularly dirty fuel, Valero and the other oil companies crawling around northern Alberta aren't happy to see California's Global Warming Solutions Act survive Proposition 23. Pullman notes:
As tar sands oil has a much larger carbon footprint than conventional oil, climate change legislation targeted by Prop 23 would limit California's imports of high-carbon fuels -- fuels that would likely include toxic tar sands oil from Alberta. Valero's Texas refineries may be halfway across the United States, but industry worries about the 'domino effect' of climate change legislative efforts and how they may be adopted elsewhere.
While Prop 23 flopped, Proposition 26 did pass, to the delight of some of its most philanthropic financiers. Chevron spent almost $4 million on the initiative, and ConocoPhillips, Exxon, Shell, and Occidental Petroleum added another $1,125,000.
All of these giants are involved in tar sands production, and have just as much motivation as Valero to roadblock a low carbon fuel standard.
Be sure to check out Pullman's full article on Valero's mischief.