The Kingdom of Saudi Arabia executed Shiite Muslim cleric Nimr al-Nimr on Saturday. Hours later, Iranian protestors set fire to the Saudi embassy in Tehran. On Sunday, the Saudi government, which considers itself the guardian of Sunni Islam, cut diplomatic ties with Iran, which is a Shiite Muslim theocracy.

To explain what’s going on, the New York Times provided a primer on the difference between Sunni and Shiite Islam, informing us that “a schism emerged after the death of the Prophet Muhammad in 632” — i.e., 1,383 years ago.

But to the degree that the current crisis has anything to do with religion, it’s much less about whether Abu Bakr or Ali was Muhammad’s rightful successor and much more about who’s going to control something more concrete right now: oil.

In fact, much of the conflict can be explained by a fascinating map created by M.R. Izady, a cartographer and adjunct master professor at the U.S. Air Force Special Operations School/Joint Special Operations University in Florida.

What the map shows is that, due to a peculiar correlation of religious history and anaerobic decomposition of plankton, almost all the Persian Gulf’s fossil fuels are located underneath Shiites. This is true even in Sunni Saudi Arabia, where the major oil fields are in the Eastern Province, which has a majority Shiite population.

As a result, one of the Saudi royal family’s deepest fears is that one day Saudi Shiites will secede, with their oil, and ally with Shiite Iran.

This fear has only grown since the 2003 U.S. invasion of Iraq overturned Saddam Hussein’s minority Sunni regime, and empowered the pro-Iranian Shiite majority. Nimr himself said in 2009 that Saudi Shiites would call for secession if the Saudi government didn’t improve its treatment of them.