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