By Ellen Chang Buffett’s Berkshire Hathaway now owns a 18.7% stake in Occidental. Billionaire Warren Buffett could be among a small group of bullish energy investors since the Chairman and CEO of Berkshire Hathaway ( (BRK.A) – Get Berkshire Hathaway Inc. Report) bulked up his stake in oil producer Occidental to 18.7% by purchasing another 12 million shares. The conglomerate bought the shares for $698 million on Tuesday and Wednesday, according to a U.S. Securities and Exchange Commission filing. Last week the company also acquired 9.9 million shares of Houston-based Occidental. Berkshire owns …