Occidental Petroleum shares have dropped 29% since mid-April, impacting Warren Buffett's stake in the company. Berkshire Hathaway's $13 billion stake in Occidental Petroleum may be underwater, based on estimates. The price decline in Occidental shares has coincided with a 23% decline in crude oil prices since mid-April. AdvertisementTo be clear, the average price Berkshire Hathaway paid for its Occidental Petroleum stake is only known by Berkshire Hathaway itself. Another sign that Berkshire Hathaway's Occidental Petroleum bet is souring is based on the warrants it owns to purchase additional shares.

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