Abstract: A screw-type pumping unit (51) for treatment of fluids in several phases comprises a case (54) which contains a pumping chamber (62), inside which there are accommodated at least two screws (70a, 70b), one of which (70a) is a drive screw, and supports at one of its ends a first gear (108a), which engages with a second gear (108b), which in turn is supported at one end of a driven screw (70b). The first and the second gear (108a, 108b) are contained in a casing (110) which is fitted onto them, and is provided with at least two through holes (112, 114) produced in two substantially opposite portions, through one of which oil can be admitted into the casing (110), and wherein the oil can be discharged through the other through hole (114), such that the casing (110), together with the gears (108a, 108b), forms a gear pump which can keep the lubrication oil of the pumping unit (51) circulating.