Diamond Shamrock is likely to fight a merger bid.

The Dallas-based energy concern is the object of an unsolicited takeover bid by Texas oilman T. Boone Pickens Jr., whose Mesa Limited Partnership has offered to buy Diamond Shamrock through an exchange of securities valued at $2 billion. But some industry analysts said they expect Diamond Shamrock to fight the bid. Headed by Chairman and Chief Executive William H. Bricker, Diamond Shamrock has yet to formally respond to Pickens’ offer.