Vanadium and nickel distributions in selective-separated n-heptane asphaltenes of heavy crude oils

•Volatile and non-volatile V and Ni compounds were separated by extrography.•Qualitative analysis of the spent silica gel contained adsorbed Ni and V porphyrins.•Acetone asphaltene fractions contain a large amount of distillable V (62 %wt.).•Acetone asphaltenes have higher vanadyl porphyrins content than heptol and TTM.