A process for preparing a gas oil cut comprises the following steps in
succession: 1) oligomerizing an olefinic C2-C12 hydrocarbon cut,
preferably C.sub.3-C.sub.7 and more preferably C.sub.3-C.sub.5; 2)
separating the mixture of products obtained in step 1) into three cuts: a
light cut containing unreacted C4 and/or C5 olefinic hydrocarbons, an
intermediate cut having a T95 in the range 200-220.degree. C. and a heavy
cut comprising the complement; T95 being the temperature at which 95% by
weight of product has evaporated, as determined in accordance with
standard method ASTM D2887; 3) oligomerizing the intermediate cut
obtained in the separation step; characterized in that in step 3),
oligomerization is carried out in the presence of an olefinic C4 and/or
C5 hydrocarbon cut in a weight ratio of intermediate cut to olefinic
C.sub.4 and/or C.sub.5 cut in the range of 60/40 to 80/20.