A method for separating material leaving a well wherein an upwardly extending and elongate separator conduit disposed in a water body is supplied with an oil containing fluid mixture at a fluid entrance zone. A plurality of relatively closely spaced inclined baffles spaced along the length of the separator conduit induce separation of oil from that fluid. The baffles define a plurality of quiescent zones for accumulating oil, with the quiescent zones communicating with a tortuous flowing zone established generally centrally of the separator conduit. Oil removal conduits in the form of risers communicate with the quiescent zones and transport the accumulated oil toward a remote location. Provision is made for intermittently impeding the downward velocity of fluid in the flowing zone for a waiting time period sufficient to permit oil particles of a predetermined size to rise from the flowing zone into the quiescent zones.The waiting time may be provided by controlling an exit valve at the exit zone of the separator conduit in response to sensing of the level of fluid in the conduit with a level control assembly. Alternatively, the exit zone of the separator conduit may be in continuous communication with the water body and the waiting time may be provided by interrupting the supply of fluid to the separator conduit utilizing a level control at a supply tank.Where the separator conduit is in continuous communication with the water body, potential adverse effects of wave action or tidal changes are minimized by controlling withdrawal of oil from an upper oil layer in the separator conduit in response to sensing, below the mean water level, the interface between that layer and the remaining fluid therebelow.