As source rocks for most conventional oil reservoirs, oil shale deposits are found in all world oil provinces, although most of them are too deep to be exploited economically. As with all oil and gas resources, analysts distinguish between oil shale resources and oil shale reserves. "Resources" refers to all oil shale deposits, while "reserves", represents those deposits from which producers can extract oil shale economically using existing technology. Since extraction technologies develop continuously, planners can only estimate the amount of recoverable kerogen.