An estimated $27 billion has been poured into two fossil-fuel stocks by Warren Buffet's company.

In the first three months of the year, the investor's conglomerate spent $17 billion quadrupling its stake in Chevron to 8%, and has spent $10 billion to purchase more than 20% of the oil company.

He has shared his views on the oil industry in the past. He invested in an oil producer to bet on the price of oil and downplayed his own track record in the space. He believes oil will be replaced by a cleaner energy source within a century.

Here are 8 of Buffett's best quotes about oil, lightly edited for length and clarity:

This is the first thing. The price of oil is the most important factor when buying into an oil company. It's not going to be any of those things. The price of oil affects the investment.

There are two A lot of straws have been stuck into the Earth. I can almost assure you that oil will sell for more in the future. Buffett was talking about the large number of oil rigs and wells around the world.

There are three. We've made a small amount of money in the oil-and-gas field. We won't be buying oil-and-gas stocks a lot. We might not have bought the last one. Buffett was referring to the ill-advised bets made by the company.

There are four. It doesn't mean that the price of oil is going up if we are in an oil stock. We bought oil futures if we thought oil would go up. In the late 1990s, Berkshire bought about 46 million barrels of oil from derivatives.

There are five. There is no reason why oil executives should be paid more if oil goes from $30 to $60 a barrel. They were not able to get it to $60 a barrel. I would pay a lot for that if they have low finding costs. A person who finds and develops oil at $6 a barrel is worth more than someone who finds and develops oil at $10 a barrel.

There are six. A management that discovers and develops oil at a lower-than-average unit cost is what you want if you have an oil- producing company. I would pay the people that did the best job at the company. They are creating wealth for me and I would pay them very well.

There are seven. The prosperity of the world is a result of it. For the next 100 years, the world won't depend on that particular windfall. There will be more than one free lunch. Buffett was talking about the importance of oil in the economy.

There are eight. It wasn't that long ago that anyone could produce a barrel of oil. Try not to have 11 million barrels of oil a day. Nobody knows what would happen if we tried to change over the course of a few years. The chances of it working are very low. Most people think it's better to have oil in this country than not.