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A slickline cable used for logging an oil&gas well that has a smooth, round outer surface that allows for a seal to be used to separate the pressure from the well from atmosphere.  This contrasts with a wireline cable which typically has two layers of wires stranded around a core which, due to this structure, has many interstitial paths that wellbore fluids and gases can migrate up through and get to atmosphere.  As some of these fluids and gases can be literally lethal, a method of sealing is necessary.  This is typically done with a grease injector which uses high pressurized grease to create a barrier between wellbore fluids and gases and the atmosphere.