In Darfur, where more than 200,000 people have perished in what the United States calls a genocide, the killing has been supported by profits from companies helping the government of Sudan tap its vast reservoirs of oil, according to services that research corporate conduct for investors. The firms include...Sinopec...Petronas and Schlumberger...whose investors include the Bill & Melinda Gates Foundation. The Gates Foundation's most significant connection to the Sudanese oil industry, however, is through Berkshire Hathaway...[which] holds a $3.3-billion stake in PetroChina Co., a subsidiary of the China National Petroleum Corp., or CNPC, the biggest player in Sudanese oil... Monica Harrington, a senior policy officer at the foundation, said: "Bill and Melinda have initiated a process to assess the asset trust investments in Sudan."... [In] a rare open letter to his shareholders in February,...Buffett deplored the conditions in Darfur, but he rejected the idea that divestment from PetroChina "would in any way have a beneficial effect on Sudanese behavior."... The holdings of Berkshire Hathaway...came in dead last by a wide margin in a ranking of oil and gas holdings among...investors...based on social, environmental and governance performance ratings... More than any other large investor, Berkshire bought into oil companies whose records on greenhouse gas emissions, safety, business ethics, human rights and other issues significantly lagged those of their peers. [also refers to Petrodar, Greater Nile Petroleum Operating Co., General Re, Altria, Wal-Mart]