A semi-continuous multi-stage catalytic technique for converting lower olefinic feedstock to heavier liquid hydrocarbon product. The invention provides methods and means for: (a) contacting alkene-rich feedstock at high space velocity elevated temperature in a continuous primary stage reaction zone with shape selective medium pore zeolite oligomerization catalyst particles to convert at least a portion of the lower olefinic components to intermediate olefinic hydrocarbons, said primary stage catalyst having an average acid cracking activity of about 0.1 to 20; (b) cooling primary stage oligomerization reaction effluent from the primary stage reaction zone to condense at least a portion of the intermediate hydrocarbons, separating the cooled and partially condensed primary reactor effluent stream in a primary phase separation zone into a light gas phase stream comprising light hydrocarbons and a condensed liquid intermediate hydrocarbon stream; and (c) contacting at least an intermediate effluent portion from the primary stage with shape selective medium pore zeolite oligomerization catalyst in a high pressure fixed bed secondary stage distillate mode catalytic reactor system at elevated temperature and high pressure to provide a heavier hydrocarbon effluent stream comprising distillate hydrocarbons, said secondary stage catalyst having an acid cracking activity of at least 5. A simplified second stage reactor system permits the use of one or more fixed bed reactors to be operated intermittently.