Several of the world’s biggest climate-focused mutual funds have stakes in traditional oil and gas companies mixed in with their solar, wind and clean tech holdings, a potential shock to green investors seeking to avoid fossil fuels altogether.

Six of the 20 biggest funds that mention "climate change" or "global warming" in their titles or marketing materials have oil and gas stocks, including two funds with investments in companies proposing new pipelines from Canada's carbon-intensive oil sands reserves, according to Reuters data.

"This really underscores that prospective investors should carefully read a fund's details and not just its label," said Meg Voorhes, head of research at the Forum for Sustainable and Responsible Investment in Washington, D.C.

Investing in climate risk is an important theme as world leaders gather in Paris to hash out an agreement to rein in global warming. Climate funds are a relatively new tool for environmentally minded investors, and remain a tiny part of the $30 trillion-plus mutual fund universe. 

A climate fund offered to European investors by HSBC, the HSBC GIF Global Equity Climate Change fund, holds stock in U.S. oil producer and refiner Chevron Corp, South African energy company SASOL Ltd and British oil and gas producer BG Group Plc.

All three companies are large carbon emitters but have launched efforts in recent years to mitigate or cut their emissions impact under pressure from environmental groups and politicians seeking to counter the risks of climate change.

"It is not so much about avoiding companies that are carbon emitters, but about investing in companies that have sought to adapt their businesses to meet the challenge," said HSBC spokesman Charles Clarke of the fund's holdings.

A prospectus for HSBC fund says it invests in "companies that aim to be the market-leaders in their respective sectors at managing their businesses in the face of climate change."

Deutsche Bank's $75 million Global Warming Prevention Equity Fund offered to Asian investors - the second biggest climate-focused fund in the field - held stock this year in U.S. oil and gas driller Helmerich and Payne Inc, according to fund disclosures.

A Deutsche Bank spokeswoman on Tuesday cited the drilling company's focus on natural gas, which burns cleaner than oil or coal, and its hydraulic fracturing or fracking technology, which had increased its productivity. She added the fund had since sold off the stock.