SAN FRANCISCO (AP) – California’s top oil regulator announced his resignation on Monday 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 chief, is quitting to return to the Lawrence Livermore National Laboratory research center, according to a statement from Brown’s office.

The statement did not give a reason for Bohlen’s resignation, but it said Bohlen had been on loan from his work at the research center. Officials of his Division of Oil, Gas and Geothermal Resources did not immediately return The Associated Press’s calls seeking comment.

The AP reported earlier this month that Brown directed Bohlen in June 2014, days after appointing Bohlen to the job, to investigate and map out the oil, gas and mineral potential and history of the Brown family ranch in Northern California.

Bohlen and Brown aides defended the public work for Brown family private property, saying it was legal and normal. The governor’s spokesmen said the governor was not interested in possibly drilling on the family property.

The oil regulator who prepared the map for Brown filed a whistleblower complaint over being made to do the work, her lawyer confirmed. 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 said 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 status 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. Environmental Protection Agency stepped up criticism that the agency was failing to enforce federal laws meant to protect public health and the environment from oilfield pollution. 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 accused her of demanding rigorous environmental reviews.

Brown’s office did not immediately respond to the AP’s request for comment about turnover at the agency.

Copyright 2015 The Associated Press.