In a parched and dusty patch of land in California’s central valley, a series of wells has been pumping out some of the state’s dirtiest crude oil. Almost a century after the Chico-Martinez oilfield was first opened about 145 miles north of Los Angeles, it is still producing heavy crude. But to do it, drillers have turned to techniques — like injecting steam from gas-fired boilers to loosen the heavy crude deep underground — that have boosted carbon emissions 14-fold over the past decade. The oil produced there has been emitting almost four times more carbon per barrel than the average of all crude used in California’s refineries, more than the oil dug out of Canada’s notorious oil sands and more than the oil extracted from the frozen tundra of Alaska’s arctic, according to state data. Just a few years ago, it was among the cleanest in the state.

While Chico-Martinez generates a fraction of California’s output, it illustrates a dilemma for a state striving to be a leader in slashing carbon emissions. Increasingly, California’s aging oilfields require ever more energy-intensive drilling methods, spurring environmental concerns and boosting fuel prices under a decade-old state program.

“They are a little bit chasing their tails on where to go while maintaining their climate leadership,” Deborah Gordon, senior fellow at the Watson Institute for International & Public Affairs, said by phone. “We do know that as oil fields age, they tend to get more carbon intense.”