The decree is yet another roadblock to Black Hills subsidiary Mallon Oil, which won access to the acreage over a decade ago, but has not signed a final agreement for the blocks.

Just last month, Costa Rica’s national environmental technical secretariat Setena ordered Mallon to specify areas in which it planned to investigate and undertake activities in the six-block area.

Mallon, which was acquired by Black Hills in 2003, was to also carry out an environmental study of the areas.

“For years it did not appear the concession contract would be executed, but...