Sechin is a leading member of the siloviki clan of officials with secret security backgrounds who believe in the supremacy of the state.
"These are not the sort of guys you want to go out drinking with. They're relatively humorless, and their patriotism may rub you up the wrong way," said Eric Kraus, chief strategist at Sovlink Securities.
Sechin, whose early career was as a translator, also does not have any known business experience.
"This guy is an administrator, he's there to keep control of the money," Kraus said.
"Sechin is kind of faceless, the market will have to find out a lot more about him."
The shock takeover of Yugansk assets by state oil firm Rosneft has cast an unwelcome spotlight on its chairman, Sechin, 44, believed to be the most influential member of President Vladimir Putin's inner circle.
"Sechin is our No. 2," said Alexander Shokhin, a former Russian chief debt negotiator.
"Sechin is one of the most powerful men not only in the political life of Russia, but also in our economic life."
Analysts and observers believe that Sechin masterminded the crackdown on Yukos and played a key hand in the state's takeover of the energy sector. Insiders say Sechin resented the defiance of authority of Mikhail Khodorkovsky, the founder of Yukos, who is now in jail.
In a bizarre twist, Sechin's daughter recently married the son of Prosecutor General Vladimir Ustinov, who is in charge of the Yukos case.
Sechin's appointment in July as chairman of Rosneft, which is due to merge with Gazprom, had already given the markets an indication of how the Kremlin intended to consolidate its control over Russia's riches.
Little is known about Sechin's career before he teamed up with Putin in the St. Petersburg city administration in the early 1990s. A graduate of Leningrad State University in 1984 with a degree in philology, he keeps such a low profile that even the Russian press calls him a "phantom."
Many observers and analysts believe Sechin is a graduate of the KGB, to which Putin belonged.
Shokhin said Sechin had worked in Angola and Mozambique and specialized in Portuguese-speaking countries in his former job, although his official biography makes no reference to these assignments.
Russian media reports say Sechin in the mid-1980s worked as a translator in Mozambique for the Soviet state company Tekhnoexport, which was responsible for arms supplies, before serving as a military translator in Angola.
Analysts say Sechin is not a man who flies solo and everything he does reflects Putin's will.
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