What happened

Shares of ExxonMobil (XOM 0.94%) tumbled 10% in May, according to data provided by S&P Global Market Intelligence, wiping out more than $30 billion of the oil giant's market value. The main culprit was a sell-off in the price of crude oil, which plunged due to increasing concerns that the global economy could start slowing and sap demand for oil.

So what

The global oil benchmark, Brent, slumped 11% in May, closing at its lowest level since mid-February. Crude prices were under pressure on concerns that the continued trade war between the U.S. and China would impact economic growth and weaken demand for oil. On top of that, President Trump announced plans to impose new tariffs on Mexico, which further spooked the oil market given that the country is one of America's key energy trading partners.

Barrels of oil with a red arrow and a calculator.

Image source: Getty Images.

That sell-off in the oil market weighed on most energy stocks, including shares of oil giant ExxonMobil since lower oil prices will negatively impact the company's earnings and cash flow. However, it wasn't the only issue affecting the stock last month. Rising tensions in the Middle East forced Exxon to evacuate its staff from Iraq -- where it operates one of that country's largest oil fields -- and that could impact production. In addition, the head of Guyana's anti-corruption agency said he is investigating how that country awarded exploration rights to its deepwater areas offshore, including the Stabroek block, where Exxon has discovered more than 5 billion barrels of oil. The concern is that this investigation could influence Exxon's development activities in the region. Finally, political tensions in Papua New Guinea last month could delay the start of a major natural gas project Exxon is developing there.

Now what

Exxon faced a barrage of negativity in May as not only did oil prices tumble, but the company experienced some potential setbacks in several of its international operations. However, its longer-term outlook remains promising, given its prime position in the Permian Basin. That fast-growing oil field is a key fuel in an expansion plan that could see the oil giant more than double earnings by 2025 even if oil prices remain weak. Thus, last month's sell-off seems like an excellent buying opportunity for long-term investors.