Businesses around the world are preparing for the largest geopolitically triggered financial bonanza since the 1991 Soviet breakup: Iran’s reopening for business.

Look for large new oil and other commercial deals, and lower global oil prices.

The last stage of talks between Tehran and international negotiators, known as the P5+1, begins June 27 in Vienna. The two sides are working against a June 30 deadline, but most observers think the talks will go on for up to another week.

Notwithstanding last-minute tough talk by Iranian leader Ali Khamenei, the prevailing expectation is that the talks will succeed. In an interview today with Quartz, Tor Soltvedt, an analyst with the political risk firm Maplecroft, put the chances of a deal at 60%.

Western business representatives of all types are pouring into Iran ahead of this settlement. In such chaotic situations, there is as much room for fraud as riches. But the sentiment is that one must at least try or lose out.

Among those first in line are major oil companies, given potential access to the world’s fourth-largest petroleum reserves for the first time in three and a half decades.

Already this week, two European oil majors—Shell and Italy’s Eni—have acknowledged holding preliminary discussions with Tehran. BP is not admitting it publicly, but it, too, has held such discussions, Quartz has been told by a knowledgeable source.

Given a much stricter compliance regime in the US, American companies are far more cautious. A spokesman for ExxonMobil said it has held no such talks because of sanctions. Chevron was less categorical—in response to a query, a spokeswoman did not say specifically whether it has or has not held such discussions, but only that the company “always act(s) in compliance with current laws and regulations.”

Whatever the case, one can assume that, whatever spade work any of the oil majors has accomplished up till now, they will be in Tehran in full force as soon as a nuclear deal makes it possible.

Oil prices will probably fall immediately, and could plunge. This is because Tehran is likely to rapidly begin to unload 30 million barrels of its oil floating in storage off its shores.

But how fast will Iran and foreign oil companies manage to pump more oil out of the country’s fields? In a report today, Wood Mackenzie, an oil industry research firm, said it expects the process to go slow. By the end of 2017, Iran will have restored some 600,000 barrels a day of production, bringing its output to about 3.2 million barrels a day from about 2.6 now.

Wood Mac expects the Iranian oil to have no significant affect on global prices. But that appears to play down what we’ve observed over the last year, which is how a big new supply of oil can cause the market to overshoot and puncture prices. The same dynamic would be at play–all else being equal–with the added Iranian barrels.