{HEAD ~ Oil} is extracted from the oil recovery zone through an outlet opening at the intermediate level of {TAIL ~ the oil stratum} in the zone 
{ HEAD ~ Oil } is extracted from the oil recovery zone through an outlet opening at the intermediate level of { TAIL ~ the oil stratum } in the zone
HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH