Abstract: A well packer or layer-separating device adapted to seal off a stringer against a deep-well casing comprises a polished sleeve against a shoulder at the upper end of which a sealing ring is compressible to seal the device against the inner wall of the casing. Below this seal there are provided two rows of keys engageable with the casing to hold the device in place. The upper row is forced outwardly by a downwardly divergent wedging member coupled to an anchoring sleeve which is held by an upper locking device against the polished sleeve. The lower end of the polished sleeve is formed as a piston which cooperates with a cylinder driving an upwardly converging wedge member beneath the lower row of keys and, via shear screws causing the anchor sleeve to ride upwardly.