Large blocks of investors in the two biggest U.S. oil companies on Wednesday demanded more disclosure about the environmental risks of extracting oil and gas through hydraulic fracturing. ExxonMobil Corp defended the practice at its annual shareholder meeting on Wednesday, even as investors peppered Chief Executive Rex Tillerson with concerns and questions about it. A proposal requiring more disclosure by Exxon...received about 30 percent of the votes...At rival Chevron Corp,...41 percent of shareholders backed a similar resolution. "Breaking 40 percent on a first year resolution has only happened a few times in the last few decades, so it shows how seriously the company's shareholders are taking this issue," said Michael Passoff,...[of] As You Sow...Environmentalists say [hydraulic fracturing] can contaminate groundwater with dangerous chemicals. The industry insists it is safe, and Tillerson said there were claims about the 50-year-old technology that had no basis in fact...While acknowledging the risks, Tillerson said Exxon works to bring together regulators in states with shale drilling to examine current rules and determine which are most effective. [also refers to XTO Energy (now part of ExxonMobil)]