Berkshire Hathaway now owns just over 20% of Occidental Petroleum after about $400 million in fresh purchases of the energy company in recent days, according to a filing late Monday.

With a 20.2% stake, Berkshire Hathaway (ticker: BRK/A, BRK/B) should be able to include a proportionate amount of the earnings of Occidental Petroleum (OXY) in its financial results, lifting annual earnings by about $2 billion off a current base of more than $30 billion.

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That contrasts with the current treatment in which Berkshire only includes Occidental's dividends in its earnings. The dividend treatment applies to sub 20% stakes.

Berkshire CEO Warren Buffett has steadily accumulated Occidental stock this year and Berkshire now holds 188.4 million shares in the domestic-focused oil and gas producer, whose earnings have surged this year with oil and gas prices.

Berkshire bought about 6.7 million shares from Thursday through Monday, according to a Form 4 filing. As a 10% holder, Berkshire must file within two business days when it changes its position in Occidental.

Berkshire paid in the range of $57 to $60 for the fresh Occidental purchases. Buffett has rarely paid more than $60 for Occidental stock this year and took advantage of the selloff in energy stocks last week to buy more.

Occidental rose 1.8% Monday to $60.04 while Berkshire's Class A stock was about flat at $439,167.

There continues to be speculation that Berkshire may ultimately look to buy the remaining 80% of Occidental—which could cost $60 billion at a price of $80 a share. Berkshire already owns $10 billion of Occidental 8% preferred stock and warrants to buy 83.9 million shares at an exercise price of $59.62 a share.

Berkshire had no immediate comment earlier Monday on the buyout talk.

Buffett is a fan of Occidental CEO Vicki Hollub and likes U.S.-oriented companies. Occidental is one of the largest domestic producers of oil and gas.