Warren Buffett just disclosed his newest $4.5 billion investment pick

On Friday evening, billionaire investor Warren Buffett, the founder of Berkshire Hathaway, disclosed that he had purchased a massive stake in oil refinery Phillips 66 (PSX).

Buffett now owns 57,975,456 shares, or a 10% stake in the Texas-based energy company, according to a securities filing posted shortly after 8pm ET.

As of Friday's share price close of $77.23, Buffett's stake is valued at $4.47 billion.

Buffett had been aggressively buying shares of Phillips 66 since Wednesday of this week.

Earlier this month, Buffett had indicated in his 13-F filing that he was building a new position.

Here's a rundown of Buffett's purchases over the last few days.