Asphaltene produced during the production of hydrocarbons in an underground reservoir may be reduced and decomposed by introducing into the underground reservoir a fluid having a catalyst of from about 3 to about 7% Ni with a magnesium oxide support or a catalyst of from about 15 to about 25% tungsten oxide with a zirconium oxide support or a mixture thereof. The viscosity of heavy oil within the underground reservoir is reduced in the presence of the catalyst.