Oct. 28 (Bloomberg) -- SemGroup Corp., the oil and natural- gas pipeline company that is the target of a $1 billion buyout attempt by Plains All American Pipeline LP, adopted a plan designed to thwart the unwanted bid.

If a buyer acquires 10 percent or more of Tulsa, Oklahoma- based SemGroup’s shares without approval of SemGroup’s board, all other owners of the company’s stock will have the right to buy additional shares at a discount, making the acquisition more expensive, according to a company statement on Business Wire. SemGroup’s board also revised its bylaws governing buyouts, the statement said.

Plains, based in Houston, disclosed on Oct. 24 that it had made an offer of $24 a share for SemGroup. SemGroup, which owns 2,800 miles (4,500 kilometers) of pipelines along with crude oil storage tanks, rejected the bid, saying it undervalued the company.

--Editors: Susan Warren, Steven Frank

To contact the reporter on this story: Mike Lee in Dallas at mlee326@bloomberg.net

To contact the editor responsible for this story: Susan Warren at susanwarren@bloomberg.net -0- Oct/28/2011 21:12 GMT