heavy mineral oil fractions are selectively hydrocracked to produce mainly middle distillate oils boiling in the 300 °- 700 ° f range . certain novel , highly active and selective catalysts are utilized comprising molybdenum and / or tungsten plus nickel and / or cobalt supported upon certain heterogeneous composites of a silica - alumina cogel or copolymer dispersed in a matrix consisting essentially of alumina gel .