California's chief oil regulator announced his resignation after 17 months as head of the embattled agency, including a criticized episode in which he directed state workers to investigate the oil and gas potential of Gov. Jerry Brown's family ranch.

Steve Bohlen, the state's oil and gas supervisor, is leaving the Division of Oil, Gas and Geothermal Resources to return to the Lawrence Livermore National Laboratory research center, Brown's office said in a statement.

State officials did not give a reason for Bohlen's resignation, which was announced Monday. But the governor's statement said Bohlen had been on loan from his work at the research center.

Bohlen took the job at the state's oil and gas division, which is charged with protecting the public and environment from oilfield pollution, at a time when the agency already was under scrutiny from the U.S. Environmental Protection Agency for failing to fully enforce federal safe-drinking water laws on oilfield operations.

Additionally, days after Brown appointed Bohlen in June 2014, the governor and the governor's aides asked Bohlen to have oil and gas regulators research and map out the oil, gas and mineral potential and history of the Brown family ranch in Northern California, The Associated Press reported earlier this month.

More On This...

Bohlen and Brown aides this month defended the oil agency's work for Brown family private property, saying it was legal and normal, and that Brown had no interest in drilling on his family land.

However, the oil regulator who prepared the map for Brown filed a whistleblower complaint over being made to do the work, her lawyer confirmed to the AP. Former oil regulators and oil-industry veterans told the AP the state oil work for Brown was unique because of the custom map with drilling information and color-coded geological records and legends, and because of the report's conclusion by state regulators that the area of Brown's family ranch was unlikely to warrant any drilling or mining in the future.

Bohlen told The Los Angeles Times this month that Brown also directed him in June 2014 to keep the personal work done for Brown out of email, citing open-records laws.

State officials were unable to produce any other examples of state regulators mapping out the petroleum, mineral and geology of land for personal purposes, and records showed the state oil agency specifically rejecting requests for maps from a state lawmaker's office and from a private individual.

The state's oilfield regulation agency has had a series of top-level turnovers at least since 2011, when the U.S. EPA began stepping up pressure on the state's oil regulators to enforce rules meant to prevent oilfield pollution of underground water reserves. Brown, who has sought to boost the state's oil production while encouraging some of the most ambitious conservation programs in the country, boasted in 2012 of speeding up the agency's oil permitting by firing one of Bohlen's predecessors, after oil companies said she demanded rigorous environmental reviews.

An oil-industry group, the Western States Petroleum Association, on Monday praised Bohlen as a professional whose tenure it said had left California, the country's No. 3 oil-producing state, with some of the toughest oilfield regulations in the world.

However, Bohlen's tenure was marked by other crises, including an oil spill in May off Santa Barbara and an ongoing natural-gas leak at one of the country's largest natural-gas storage facilities, in Southern California.

Environmental groups said Brown needed to take responsibility for the state's oversight of the oil industry.

Bohlen's departure leaves the agency still too cozy with the industry it is supposed to regulate, said Hollin Kretzmann, an attorney with the Center for Biological Diversity environmental group. "California regulators have prioritized oil company profits and the governor's personal requests at the expense of our air, water and health."

Brown's statement said that Ken Harris, a longtime state water-quality official, would succeed Bohlen.

Bohlen's resignation was first reported by the Sacramento Bee.