The secret wealth of the world's richest oil billionaires

In Brunei, things may be about to change, and the Sultan may find his wealth considerably downsized. Oil production is down 40 percent since 2006, and what's left has lost a great deal of value due to low oil prices. Nearly 96 percent of Brunei's exports are oil, gas and related products—that tops even Saudi Arabia, Kuwait and the UAE. Brunei could run out of oil in just over 20 years, but then again, the Sultan is said to have massive real estate holdings to tide him over.

Angola's president is stepping down and the oil price crisis has hit him hard, but he'll still control the ruling party and a new president will defer to him (and his daughter).

In Kazakhstan, Nazarbayev is president for life. In Azerbaijan, the family elite is as strong as ever and will continue to be so through any means necessary. In Russia, sanctions simply haven't worked because they are designed to target those around Putin, and Putin appears to have designed it so they are always vulnerable to him first and foremost.

As Russian businessman and former Putin friend Sergei Pugachev notes to the Guardian, and as reported by U.S. News and World Report: "Everything that belongs to the territory of the Russian Federation Putin considers to be his. Everything – Gazprom, Rosneft, private companies. Any attempt to calculate it won't succeed. … He's the richest person in the world until he leaves power."