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.

The fund had also previously owned large stakes in oil companies Noble Energy and pipeline company Kinder Morgan Inc, according to disclosures.

Nomura's $13.5 million Nomura Global Climate Change Fund, launched in 2007 and ranked 16th-largest in the field, was among the most heavily invested in oil and gas at about 5.8 percent of its portfolio. Among the stock's holdings was a small stake in TransCanada Corp.

TransCanada became a symbolic enemy of the U.S. green movement in recent years over worries its Keystone project would have led to an increase in oil sands production. The project was rejected by the Obama administration on Nov. 6 on environmental grounds after a prolonged review.

A spokesman for Nomura said the fund was small and available only to investors in Taiwan. He did not respond to requests for comment about the fund's holdings.

Nomura's fund also held stock in Australia’s top oil and gas producer, Woodside Petroleum Ltd, and Norway's Statoil.

Another Taiwan-based fund, the Jih Sun Anti-Global Warming Fund, held a small stake in Enbridge Inc, a Canadian company that has proposed an oil sands pipeline to Canada's West Coast, a potential Plan B to the failed Keystone XL project.

The best performers in the climate fund field so far this year were portfolios that avoided fossil fuels entirely. A global glut in oil and gas supplies has crushed prices since late 2014 and in turn the share prices of producers.

They include Schroder’s $196 million ISF Glo Climate Change Equity fund, which was up 13.7 percent, and the $11.1 million Jupiter JGF Global Ecology Growth fund, which was up 10.8 percent – eye-popping returns in a lackluster overall market.

A spokeswoman for Schroder did not comment.

Among the biggest holdings in the five largest climate-focused portfolios as measured by assets under management were U.S. design and manufacturing firm Danaher Corp, U.S. wind energy and aerospace materials maker Hexcel Corp, and wind and solar power company NextEra Energy Inc.

U.S. investment company Etho Capital launched one of the few climate-focused funds available in the United States earlier this year, the Etho Climate Leadership exchange-traded fund. The fund holds no oil or gas stocks and tracks the performance of some 400 U.S. companies "that are leaders in their industry with respect to their climate impact."

That fund is up 14 percent since its launch.