Warren Buffett's holding company Berkshire Hathaway has significantly increased its stake in Occidental Petroleum, underscoring the investment guru's big bet on oil as global prices continue to surge to all-time highs.

A filing with the Securities and Exchange Commission submitted by the company showed that it had acquired an additional 18.1 million shares of Occidental, worth nearly $1 billion, earlier in the week.

Warren Buffett's Berkshire Hathaway purchased an additional 18.1 million shares of Occidental Petroleum for almost $1 billion over the first three days of this week. Berkshire Hathaway purchased the shares at a weighted average of $54.41 each, costing it around $985 million. Based on Occidental's share price as of Wednesday's close, Berkshire's purchased shares are now worth around $959 million.

Berkshire Hathaway now owns a total of 136.4 million common shares in the oil giant, valued at an estimated $7.2 billion. The stock Berkshire holds accounts for more than 13% of Occidental's outstanding stock.

Berkshire also has warrants to purchase almost 84 million more shares at a price of little under $60 per share. They were gained as part of Occidental's 2019 transaction to assist the company's acquisition of Anadarko. Berkshire's investment in the company equates to about 22% of its total when the warrants are included. Berkshire acquired all 136 million common shares it presently owns over the previous 13 trading days.

Occidental's stock price has risen more than 80% this year as oil prices have reached multi-year highs in the wake of the pandemic and the crisis in Ukraine. On Wednesday, Berkshire Hathaway's Class A shares surpassed $500,000 for the first time.

Buffett previously mentioned that his company had started purchasing Occidental shares in late February. He added that he instructed Berkshire to purchase all the shares that it could. Since then, Berkshire has added 45 million additional shares to the 91 million it acquired the first week.

Berkshire's purchases have pushed its shareholding, including the warrants, beyond the 10% threshold that necessitates it disclosing any new purchases within two business days.

It's unclear if Berkshire is funding its recent purchases with cash on hand or by selling other equities. Berkshire Hathaway had $146.7 billion in cash and equivalents at the end of 2021. In 2021, it was a net seller of stocks.

Following the news of the share acquisition, Occidental Petroleum Corp's stock surged by more than 8.5% in New York. The stock was trading as high as $57.50 per share in early trading. As recently as October 2020, during the height of the pandemic, the stock was trading below $9 per share.