SAN FRANCISCO (AP) — Twenty percent of California’s oil production now comes through hydraulic fracturing, according to a study commissioned by the state and released Wednesday.

For the past decade, roughly half of the average of 300 wells installed in California each month were fracking wells, the California Council on Science and Technology said in the report, the first in a series of fracking reviews as the state sets up its first comprehensive regulatory framework for the extraction process.

California is the country’s No. 3 oil-producing state, with most coming from aging fields in the Central Valley’s Kern County. Fracking entails forcing fluid, sand and chemicals underground to break rock formations and extract oil and gas from tough oil reservoirs that defy more conventional drilling techniques.

California lawmakers in 2013 passed Senate Bill 4 to regulate fracking, which increasingly is the method of oil extraction most targeted by environmental groups. Environmental groups on Wednesday criticized the state for moving forward on a required state environmental review of fracking before the researchers’ state-commissioned studies are completed. Last summer, a fracking review by the same research group said there was little evidence that fracking in California was risky, although the report noted vast gaps in available data.

All but 5 percent of reported fracking operations in California are in the San Joaquin Valley, and most of those in four fields in Kern County, the council’s study said. Significant reserves remain in the San Joaquin basin despite more than a century of production in some of the oilfields there, and fracking is likely to keep expanding production in that part of the state, researchers said.

The study also noted other potential areas for oil and gas exploration in the future, including California’s Monterey Shale, believed to hold vast reserves in underground formations currently too difficult and expensive to drill. Billions of other barrels of oil may lie offshore, recoverable by conventional means, council scientists involved in the study said. Environmentalists strongly oppose developing both areas, both because of potential pollution from production and the added greenhouse gases that would be emitted by burning through those reserves.

Environmental groups have urged California Gov. Jerry Brown to follow the lead of New York Gov. Andrew Cuomo, who banned fracking in his state late last year. Researchers involved in the California studies have said fracking in California is different than fracking in most of the rest of the country, involving shallower and more vertical wells than elsewhere.

“If California decided to ban what New York banned, it would make no difference,” because of the difference in fracking techniques used here, Jane Long, one of the researchers directing the work, told reporters Wednesday.