An apparatus for downhole separation of an oil/water fluid and the injection of the separated water into the formation includes a casing in a borehole, a tubing string in the casing, first perforations in the casing at a production zone in the borehole for admitting the mixture into an annulus between the casing and the tubing string, second perforations in the tubing string for admitting production fluid from the casing into the tubing string, third perforations in the casing for discharging separated water from the annulus into the formation, a packer in the annulus separating the first perforations from the third perforations, a first check valve in the tubing string above the packer for admitting the mixture from the tubing string into the annulus where water separates by gravity from the hydrocarbons for discharge through the third perforations, and a second check valve in the tubing string above the first check valve for admitting separated hydrocarbons from the annulus into the tubing string for passage to the top of the borehole.