Exxon Mobil Corp. is the latest company to raise concerns that a stockpile of U.S. government crude is tainted with poisonous gas.  The American energy giant said some of the oil it purchased last year from the Energy Department’s Strategic Petroleum Reserve, or SPR, contained “extremely high levels” of hydrogen sulfide, according to emails obtained by Bloomberg under the Freedom of Information Act. In some cases, the gas level was 250 times higher than government safety standards allow.

“The Department of Energy takes safety, security and environmental impacts involving SPR activities very seriously,” agency spokeswoman Jess Szymanski said. “Last fall, an SPR cargo received by Exxon Mobil was found to contain higher-than-expected levels of hydrogen sulfide. Since then, the Department has worked with Exxon to resolve this concern, and find alternate options for the cargo’s delivery.”

Analysts have pointed to the stockpile as a safeguard against tightening crude supplies after U.S. sanctions on Iran and Venezuela curbed their oil exports. But Exxon’s discovery, which follows complaints by Royal Dutch Shell Plc, Macquarie Group Ltd and PetroChina Co., suggest that the reserve may not offer refiners as much insurance against diminishing volumes of higher sulfur, or sour, crude as previously thought.

The Energy Department disputed claims that it repeatedly sold tainted crude, saying that some companies’ high hydrogen sulfide readings were “spurious” or the result of contamination during shipping. In PetroChina’s case, however, the agency acknowledged spending around $1 million to clean up a contaminated cargo.

The prospect of tainted crude in the reserve complicates future sales of U.S. oil, a key tool for funding government programs. A 5 million-barrel sale is planned for 2019, and 221 million barrels of oil are planned for sale from 2020 to 2027.