The U.S. government ordered a temporary halt to drilling at 33 deepwater exploration rigs on Thursday, part of a broader response to the massive BP oil spill that threatens efforts to tap offshore fields seen as crucial to increased U.S. oil output.

The move may potentially delay project development plans by companies like Chevron Corp in the Gulf of Mexico, where rising production has helped offset shrinking domestic onshore supply.

Unlike the administration’s six-month extension of its ban on new deepwater drilling permits, and cancellation of a much-anticipated lease sale offshore Virginia, the pause for existing deep-sea exploratory rigs threatens to affect proven oil discoveries rather than untested areas.