Refiners rise as Isaac nears Gulf Coast By: MarketWatch August 27, 2012 at 13:06 PM EDT Tropical Storm Isaac shuts about a quarter of the U.S. Gulf’s oil output. Valero shares surge after deadly blast at giant Venezuela refinery. Read More >> Related Stocks: BP Plc ADR Chevron Corp Marathon Petroleum PHLX Oil Service Sector Index Royal Dutch Shell Plc ADR Valero Energy