A process comprising A) continuously introducing into a reaction zone i) ethylene, ii) an iron salt, iii) a pyridine bisimine, iv) an organoaluminum compound, and v) an organic reaction medium, and B) forming an oligomer product in the reaction zone, the reaction zone having i) an iron of the iron salt concentration in a range of 5×10−4 mmol/kg to 5×10−3 mmol/kg, ii) an aluminum of the organoaluminum compound to iron of the iron salt molar ratio in a range of 300:1 to 800:1, ii) an ethylene partial pressure in a range of 750 psig to 1200 psig, iv) an ethylene to organic reaction medium mass ratio in a range of 0.8 to 4.5, v) a temperature in a range of 75° C. to 95° C., and optionally vi) a hydrogen partial pressure of at least 5 psi.