Occidental Petroleum Corp. offered to buy Anadarko Petroleum Corp. for $38 billion, launching a potential bidding war for a company that agreed earlier this month to be purchased by Chevron Corp. for about $33 billion. The competition for Anadarko, which has prized assets from the heart of the West Texas oil boom to East Africa, is set to intensify as Chevron weighs whether it will make a counteroffer, a step some analysts and shareholders now expect. …