France has already decided to cease Iranian crude oil import and other EU members are likely to follow it. But France only imports about 50 thousand barrels per day of Iranian crude oil and purchases by Germany and U.K. are less than that. The EU's import ban could be effective if bigger buyers such as Italy and Spain participate in the movement.
China's Iranian crude oil imports in the first half of 2011 rose by 50% from a year earlier. It means that China has absorbed excess Iranian crude oil supply following the sanction by other nations.
Even if the EU and Japan reduce imports from Iran, China could take the excess oil.
Iran's crude oil production, however, is clearly decreasing. You can see the tendency when you compare the Iran's crude oil output to the OPEC total production. While entire crude oil production by OPEC 12 members increased this year in order to compensate for the supply shortage caused by the conflicts in Libya, Iran's production has reduced month by month.
The UN Security Council imposed sanctions for 4 times since December 2006. While the international society has blocked the nuclear development of Iran based on the sanction resolutions, the U.S. also performed extra sanctions.
Foreign companies that had developed energy resources in Iran were required to withdraw.
Japan gave up the development of huge Azadegan Oil field as well.
The withdrawal of foreign companies causes reduction of crude oil output in Iran. The country's crude oil production during the first six months in 2011 decreased by 9% on year.
Iran's crude oil production is estimated to shrink by 400-700 thousand bpd annually because of the stagnated development and less output from old wells. Actually reduced output over the past 12 months proves it.
While a slump in the production, Iran's crude oil export in this year shows the year-on-year increase. Thus crude oil supply for the domestic market is estimated to decrease sharply.
Iran's domestic crude oil refining capacity is also prevented from expanding due to the absence of foreign companies.
Gasoline consumption in the country has been reported declining due to the cut of subsidy to save the energy. I'm not sure how the less gasoline consumption affects economic activity in the country. But the inconvenience of daily life might fuel Iranian people's antipathy to the sanctions.