Ethylene oligomerization process comprising, at a temperature between 90 and 190°C, simultaneously bringing ethylene into contact with the following components: - a metallic precursor of chromium - a pyrrole derivative corresponding to the general formula in which - R 1 is chosen independently from a C 1 -C 15 alkyl group, a C R' group, a COOR' ' group, CCl 3, CF 3, R' being chosen from H, a C 1 -C 6 alkyl, a chlorine, a bromine, a fluorine; R" being chosen from H, a C 1 -C 6 alkyl; - m is an integer between 0 and 4; - X is chosen from a hydrogen, a lithium, a sodium, a potassium, a caesium, or an aluminium atom, - an aluminium-based compound of general formula AlR 2 R 3 R 4 in which the groups R 2, R 3 and R 4, which may be identical or different, are chosen independently from a hydrogen and C 1 -C 20 alkyl, C 1 -C 20 alkoxy and C 5 -C 30 aryloxy groups, - at least one halogenated aluminium compound corresponding to the general formula Al n R 5 o Y p, in which R 5 is a C 1 -C 20 alkyl group, Y is a halogen, n is an integer from 1 to 2, o is an integer from 1 to 3 and p is an integer from 1 to 3; - an aromatic additive, said components forming in situ a catalytic composition, wherein the molar ratio between the total number of halogen atoms provided by the components of the catalytic composition and the sum of the aluminium atoms, denoted Altot, provided by the aluminium-based compound and the halogenated aluminium compound, denoted halo/Altot, is between 0.10 and 3.0. - a metallic precursor of chromium - a pyrrole derivative corresponding to the general formula in which - R 1 is chosen independently from a C 1 -C 15 alkyl group, a C R' group, a COOR' ' group, CCl 3, CF 3, R' being chosen from H, a C 1 -C 6 alkyl, a chlorine, a bromine, a fluorine; R" being chosen from H, a C 1 -C 6 alkyl; - m is an integer between 0 and 4; - X is chosen from a hydrogen, a lithium, a sodium, a potassium, a caesium, or an aluminium atom, - an aluminium-based compound of general formula AlR 2 R 3 R 4 in which the groups R 2, R 3 and R 4, which may be identical or different, are chosen independently from a hydrogen and C 1 -C 20 alkyl, C 1 -C 20 alkoxy and C 5 -C 30 aryloxy groups, - at least one halogenated aluminium compound corresponding to the general formula Al n R 5 o Y p, in which R 5 is a C 1 -C 20 alkyl group, Y is a halogen, n is an integer from 1 to 2, o is an integer from 1 to 3 and p is an integer from 1 to 3; - an aromatic additive, - R 1 is chosen independently from a C 1 -C 15 alkyl group, a C R' group, a COOR' ' group, CCl 3, CF 3, R' being chosen from H, a C 1 -C 6 alkyl, a chlorine, a bromine, a fluorine; R" being chosen from H, a C 1 -C 6 alkyl; - m is an integer between 0 and 4; - X is chosen from a hydrogen, a lithium, a sodium, a potassium, a caesium, or an aluminium atom, - an aluminium-based compound of general formula AlR 2 R 3 R 4 in which the groups R 2, R 3 and R 4, which may be identical or different, are chosen independently from a hydrogen and C 1 -C 20 alkyl, C 1 -C 20 alkoxy and C 5 -C 30 aryloxy groups, - at least one halogenated aluminium compound corresponding to the general formula Al n R 5 o Y p, in which R 5 is a C 1 -C 20 alkyl group, Y is a halogen, n is an integer from 1 to 2, o is an integer from 1 to 3 and p is an integer from 1 to 3; - an aromatic additive,