a family of organic tracers is proposed for inter - well measurement of residual oil in petroleum reservoirs , as is their use as partitioning tracers in a petroleum reservoir . the tracers consist of at least one benzyl alcohol of formula i ) wherein each of r 1 to r 5 is independently selected from h , f , ci , br , i , cf 3 cf 2 ci , cfci 2 and cci 3 and wherein at least one of r 1 to r 5 is not h .