The shares of Occidental Petroleum Corp. OXY rose by 3.6% in afternoon trading Tuesday, enough to pace the S&P 500's SPX, energy sector gainers, after Warren Buffett's Berkshire Hathaway Inc. BRK.B, boosted its stake in Occidental to more than 20%. In a late-Monday filing, Occidental stated that Berkshire Hathaway increased its investment in Occidental by 6.68 million shares, valued at $390.72 million, to 188.37 million shares, or 20.2% of the outstanding shares. Berkshire Hathaway's common stock stake is valued at $11.72 billion at current prices. According to a MarketWatch analysis of filing data, the shares were purchased in a series of transactions from Aug. 4 to Aug. 8, with a weighted average price of $58.4763. That price is 6.0% less than current levels. According to FactSet data, Berkshire Hathaway is the Occidental's largest shareholder, as the second-largest shareholder is Dodge Cox at 10.75%. Occidental's stock has soared 114.6% year to date, while the SPDR Energy Select Sector ETF XLE has gone up 34.3% and the S&P 500 SPX has shed 13.5%.