Water injected to the subsurface reservoir via water injectors to support reservoir pressure and improve oil displacement
It can be sourced form any part of Earth's hydrosphere: subsurface aquifer, ocean, sea, lake , river or it can be processed formation water after it is lift up on surface alongside with hydrocarbons.
See also
Natural Science / Physics / Chemistry / Chemical Substance / Water
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology / Petroleum reservoir fluids