With Trump having started the 6 month process of pulling out from the Iranian nuclear deal (or rather as Steven Mnuchin admitted, Trump's true intention is merely renegotiating the existing deal and "entering a new agreement") the biggest concern among traders and analysts is what impact the Trump decision will have on Iran's oil exports.
As a reminder, some such as Barclays have suggested that Iran's oil production may not be affected at all; others such as UBS predict the sanctions could lead to the reduction of oil exports by 200-500kb/d over the next 6 months. Meanwhile, Deutsche Bank notes that because of the 180-day wind down period, neither Iranian oil production nor exports will drop before the 5 November 2018 effective date. In fact, if behavior follows the example from 2012, there is the possibility of a short spike in Iranian exports just before the effective date, after which a slow decline may set in.
As Goldman explains this morning, the final impact on Iran oil will likely be somewhere inbetween, with the ultimate impact on Iran production rather negligible for the foreseeable future. The reason for that is that following the announcement, other signatories of the deal reiterated their support for the agreement as well as their desire to revisit it. President Macron said that France, Germany and the UK regretted the decision and the EU vowed to uphold the Iran nuclear accord. Russia announced that the US alone would not be able to overturn the deal and its Deputy Foreign Minister said it was willing to support France's proposal for new negotiations. At the same time, Iran announced that it will remain in the nuclear deal and will start talks with European nations, China, and Russia.
So with the support of the other deal signatories in place, Goldman's Damien Courvalin writes that the impact on Iranian production may be more limited than implied by the US secondary sanctions, and certainly less than the 1mmb/d decline seen in 2012-15 which many use a benchmark for what happens next.
After all, as shown in the chart below, the bulk of Iranian exports is shipped to Asian countries - most of whom have already said they will continue importing Iranian oil - while the handful of European nations that received Iran crude will likely continue to do so in the future, once they request, and are granted, sanctions waivers.
Here is another breakdown, courtesy of Bloomberg:
For Iran's clients what happens this time will likely echo the last episode earlier this decade, when Iran was also sanctioned by the US. Back then countries were given exemptions by the U.S. - reviewed every 180 days - if they "significantly" reduced imports from the Islamic Republic. While a specific quantity of reductions that would make buyers eligible for waivers wasn't announced, a slew of nations including China, India and South Korea received them.