A group of shareholders who focus on the environment said on Tuesday they are targeting companies operating in the Marcellus Shale to ensure development of natural gas does not pollute or endanger human health. The shareholder proposal campaign, aimed at 12 companies including Chesapeake Energy Corp, EOG Resources Inc and Exxon Mobil Corp, was sparked by mounting worry about chemicals used in a process to extract gas from rock called hydraulic fracturing, the groups said... environmentalists and critics say the drilling chemicals have polluted aquifers in Pennsylvania and Colorado and can cause cancer and other serious illnesses...The oil and gas industry says the process is safe, and there has never been a documented case of ground water contamination because of hydraulic fracturing...Chesapeake holds the largest number of drilling leases in the Marcellus along with its partner Statoil ASA.