Pioneer Natural Resources and Enterprise Products Partners were authorized last month by the U.S. Bureau of Industry and Security, a division of the Commerce Department, to export so-called condensate.
Crude oil exports are banned under legislation enacted in response to the 1970s oil embargo by Arab members of the Organization of Petroleum Exporting Countries. There are no such restrictions for petroleum products like gasoline and other fuel products.
"The Commerce Department does not appear to possess the authority to issue exemptions for condensates or some subset of condensates from the crude export restrictions," they said in a letter sent Wednesday.
The Commerce Department in June said the decision on condensates does not represent a major shift in U.S. export policy.
Pioneer and Enterprise have been quiet on the decision.