A 1984 study estimated the EROEI of the various known oil-shale deposits as varying between 0.7–13.3.[21] More recent studies estimates the EROEI of oil shales to be 1–2:1 or 2–16:1 – depending on whether self-energy is counted as a cost or internal energy is excluded and only purchased energy is counted as input.[22] Royal Dutch Shell reported an EROEI of three to four in 2006 on its in situ development in the "Mahogany Research Project."[23][24]