Royal Dutch Shell is looking to add more green to its portfolio.

The oil titan has quadrupled its budget for renewable-energy projects this year, Bloomberg reported Thursday.

Graeme Sweeney, an executive vice president for Shell, did not disclose how much the company would spend on renewable energy this year, but said it has spent a total of $1 billion in the past five years on projects to cut greenhouse-gas emissions.

It's the latest in a series of steps oil companies are taking toward investing more in greentech.

BP and ConocoPhillips Co. have invested in university-led biofuels research, for example, and ConocoPhillips is developing a material to reduce the cost of lithium-ion batteries used in hybrid and electric vehicles.

ConocoPhillips and Shell also are working to reduce emissions from coal-fired power plants, although some environmentalists disagree about whether those technologies count as green.

While some greentech industry watchers welcome oil companies' green efforts as positive for renewable energy, they also view them with skepticism.

Matt Horton, a principal with greentech venture firm @Ventures, said major oil players could help greentech grow.

"The scale of the problems we are trying to tackle really requires companies of magnitude to bring the resources to bear," he said. "It would be naïve of us as venture capitalists and cleantech entrepreneurs to think that that we will reshape the energy industry without the participation of the world's major energy companies."

But Horton warned against cheering too much when a big oil company makes a green play.

"I think it's important to keep in mind the size of commitment they are making relative to their overall business," he said.

During its second quarter, Shell's net profit rose 33 percent to $11.56 billion from $8.67 billion in the same period last year.

Rick Kment, a biofuels analyst at DTN, added that Big Oil's green investments could be more of a marketing move than an attempt to bring about real change. For example, "it's politically unpopular not to be involved in biofuel-development research right now," he said.

Both Horton and Kment said they aren't sure just how serious Big Oil is about green investments. But Horton said he does think companies like Shell have the capital to dominate a greentech sector if they decide to make it a priority, which could squeeze out startups.

On the flip side, Horton said big energy companies could open the door for new partnerships with cleantech startups or for acquisitions.