Congressman Edward Markey has asked the Commerce Department for more details about companies applying to ship U.S. crude oil out of the country, after Royal Dutch Shell (RDSa.L) confirmed last week it is seeking permission to export oil.

A rapid rise in U.S. oil and gas production from unconventional shale sources has led to questions about whether the United States should keep its newfound energy bounty to itself, or allow for more exports.

Federal law restricts the export of domestically produced crude oil, requiring that companies receive a special license from the Commerce Department before sending oil to foreign countries.

Markey, the top Democrat on the House Natural Resources committee, called on Commerce to turn over all applications to export U.S. crude oil, as well as any supporting documents.

"It is imperative to ensure that the House Natural Resources Committee is aware of and can evaluate the nature of these applications and what they are proposing with respect to the export of crude oil produced in the United States," Markey said in a letter Friday to acting Commerce Secretary Rebecca Blank.

Markey has been a vocal critic of efforts to export excess natural gas, while also calling for limits on shipping coal and petroleum products abroad.

The growth in U.S. oil production from shale formations in North Dakota and Texas has pushed U.S. oil output to its highest level since 1995, but also led to a glut of light, sweet crude oil.

Since the Gulf coast refining hub is more suited To process heavier crudes, analysts say selling the light, sweet crude on international markets could make economic sense for companies.

Royal Dutch Shell confirmed last week that it had applied for a license to export crude oil, but the company would not comment on likely export destinations or the amount of crude involved.

A source also told Reuters last week that BP Plc (BP.L) had received a license to export crude to certain Canadian refineries.