This invention provides a method of controlling water in a subterranean formation, said method comprising contacting said formation with a water control treatment agent which comprises an organosilane in an amount effective to reduce the water permeability of at least part of said formation. At the same time, any decrease in the overall hydrocarbon (e.g. oil) permeability of said formation is preferably minimized. The invention further provides a method of plugging or sealing a subterranean formation.