WASHINGTON (Reuters) - The Obama administration on Tuesday bowed to months of growing pressure over a 40-year-old ban on exports of most domestic crude, taking two steps expected to unleash a wave of ultra-light shale oil onto global markets.

An offshore oil platform is seen in Huntington Beach, California September 28, 2014. REUTERS/Lucy Nicholson

The Bureau of Industry and Security, or BIS, which regulates export controls, said it had granted permission to “some” companies to sell lightly treated condensate abroad. Condensate is a form of ultra-light crude.

Some two dozen energy companies had asked the agency for clarification on permissible exports earlier this year, but until Tuesday those requests had been put on indefinite hold.

The BIS also released guidance in the form of frequently asked questions, or FAQs, to explain what kind of oil was generally allowed under the ban, the first effort by the administration to clarify an issue that has caused confusion and consternation in energy markets for more than a year.

The two measures are clearest signs yet that the administration is ready to allow more of the booming U.S. shale oil production to be sold overseas, where drillers have said it can fetch a premium of $10 a barrel or more.

They follow a year of murky messages and widespread uncertainty over what is or is not allowed under a trade restriction that critics say is a relic of a bygone age, when oil was seen as scarce after the 1970s Arab oil embargo.

A domestic drilling boom of the past six years has transformed the United States into an energy powerhouse, boosting U.S. production by more than 50 percent and reversing decades of decline.

Output of very light oil has been especially strong, leading to a glut that threatens to overwhelm domestic demand. The constraints helped fuel bumper profits for refiners such as Valero Energy Corp VLO.N and PBF Energy Inc PBF.N, but angered drillers such as Hess Corp HES.N that say they were selling at a discount.

Jamie Webster, the senior director of oil markets at research firm IHS, said the FAQ “takes the leash off of (the U.S. Department of) Commerce” and signals it may take additional action on crude exports after several months of inaction.

While likely to draw broad support from many quarters, the measures also open the Obama administration to attack by environmentalists and Democrats who may see it encouraging more hydraulic fracking and as a sop to big oil companies.

STEPS TO CLARIFY

How the measures will affect flows of condensate is uncertain, particularly given the dramatic slump in global oil markets, where prices have nearly halved since the summer.

An administration official said that the oil market - not a “fairly arcane clarification” in guidelines - would ultimately determine how much oil is exported. That echoed the Obama administration’s policy on exports of liquefied natural gas, or LNG, which are also now generally allowed.

The steps on Tuesday were “certainly not designed to add or detract from what can be exported. We are trying to make the boundary line clearer,” said the official.

In its FAQ, which the agency has been working on for most of this year, the BIS confirmed or clarified a number of nuanced issues related to the rules, including:

* Confirmation that lease condensate processed through a distillation tower is considered a petroleum product, and therefore can be exported without constraint.

* Clarification of what constitutes “distillation” for export, including the fact that pressure reduction alone, and flash drums with so-called heater-treaters or separators, would not be sufficient to qualify oil for overseas sales.

* A reminder that most petroleum products may be “exported to most of the world without a license,” a message seen by many analysts as blessing the process of self-certification.

* And clarification that “a minimum amount of mixing” between exportable foreign crude and restricted domestic crude may be allowed, a note likely making it easier to ship Canadian crude through U.S. pipelines and ports.

(For the FAQ, see: here)

FRUSTRATION BUILDING

Uncertainty about what kind of petroleum can be shipped abroad has frustrated oil market players since the BIS, an office of the Commerce Department, quietly gave permission in 2013 to a small company, Peaker Energy, to export minimally treated light oil called condensate.

Last spring BIS gave permission to export treated condensate in private letters to two other companies, Pioneer Natural Resources Co PXD.N and Enterprise Products Partners LP EPD.N.

The private nature of the communications between the government and the three energy companies left a wide range of other drillers in the dark about investing in expensive infrastructure to process condensate.

One company, Australia's BHP BHP.AX, said last month it would press ahead with exports without having received a formal approval from the BIS, but other energy companies have been reluctant to follow suit without further guidance.

Domestic pressure has also grown. Several lawmakers in the House of Representatives and Senate have said that unless energy companies can export oil to Asia and Europe, the drilling boom will eventually choke on its own output.