A federal judge on Tuesday blocked the U.S. Bureau of Land Management from opening more than 1 million acres in Central California to oil drilling because the agency did not properly explore the potential dangers of fracking.

U.S. District Judge Michael Fitzgerald sided with environmentalists who argued that the bureau should have addressed the possible impacts of hydraulic fracturing in an environmental impact statement issued as part of the formal process of opening public lands to drilling.

Instead, the 1,073-page impact statement mentioned fracking only three times and never discussed the controversial practice in depth, according to the judge.

He ordered the bureau to prepare a supplemental impact statement that includes fracking before the bureau moves forward on oil and gas development in the area, which includes federal properties in Fresno, Kern, Kings, Madera, San Luis Obispo, Santa Barbara, Tulare and Ventura counties.

Environmentalists who consider fracking a threat to California’s strained groundwater supplies hailed the ruling.

“The Obama administration must get the message and end this reckless rush to auction off our public land to oil companies,” said Brendan Cummings, conservation director for the Center for Biological Diversity, one of two environmental groups that sued the bureau. “As California struggles against drought and climate change, we’ve got to end fracking and leave this dirty oil in the ground.”

A bureau spokeswoman could not be reached for comment.

Hydraulic fracturing uses a high-pressure blend of water, sand and chemicals to crack underground rocks, releasing oil or natural gas. Combined with horizontal drilling, fracking has created a boom in U.S. oil and gas production, one that has helped push down oil prices worldwide. But questions about its environmental effects have dogged the practice for years, even as the oil industry and its political supporters insist the practice is safe.

Fitzgerald noted in his decision that by the bureau’s own estimate, fracking would probably be used in 25 percent of wells drilled in the area.

In addition, the bureau had commissioned a survey from the California Council on Science and Technology in 2014 about research on fracking’s potential dangers. And yet the bureau’s environmental impact statement avoided discussing the topic in any detail, the judge wrote.

“To be clear, the act of commissioning the CCST Report itself does not satisfy the Bureau’s obligations to take a ‘hard look’ at the potentially adverse effects of fracking,” Fitzgerald wrote.

David R. Baker is a San Francisco Chronicle staff writer. Email: dbaker@sfchronicle.com

Twitter: @DavidBakerSF