HOUSTON (Reuters) - Shares in U.S. oil producer Occidental Petroleum ( OXY ) fell to $56.17 on Tuesday, below a level that has routinely triggered purchases by its biggest holder, billionaire investor Warren Buffett's Berkshire Hathaway ( BRK/A ).

Past multimillion-share purchases were so routinely timed to drops below $60 that Wall Street analysts called it "the Berkshire put," for setting a price floor on the oil firm's shares.

But Occidental has traded below that price all month, the longest period since a swoon in January that ended after Berkshire acquired 4.3 million shares in early February.

The lack of purchases may reflect the Omaha, Nebraska, investor's decision not to add to his nearly 30% stake, analysts said. Berkshire is the largest owner of Occidental shares with a stake worth $16.1 billion, and holds U.S. regulatory approval to buy up to 50% of the firm.

Spokespeople for Berkshire and Occidental did not reply to requests for comment.

Occidental shares are off 12.3% in the last 52 weeks compared with a flat performance by the XLE fund that tracks the overall energy sector.

The stock is under pressure after CrownRock LP investors this month filed to sell 29.6 million Occidental shares acquired in Occidental's $12 billion deal for the Midland, Texas, oil producer.

Prior drops have routinely triggered big purchases. In June, Berkshire acquired 2.56 million shares at prices between $59.86 and $59.75 apiece. It bought nearly $590 million in Occidental shares after the price fell last December on the debt required in the CrownRock deal. The trend dates to fall 2022 with large Berkshire share acquisitions between $57.91 and $61.38.

In addition to its common shares, Berkshire owns warrants to purchase 83.5 million shares of Occidental at $59.62 per share, and holds preferred stock in the Houston-based company.