A process of making observations of a subterranean reservoir penetrated by a wellbore uses distinguishable sets of tracer particles and comprises steps of: (i) delivering a plurality of sets of tracer particles to respective subterranean locations via the wellbore, the particles in each set comprising a tracer substance which distinguishes that set form the other sets; (ii) causing or allowing the tracer substances to flow out from the tracer particles whilst the particles are at the respective subterranean locations; (iii) causing or allowing production of fluid out of said reservoir via the wellbore; and (iv) detecting the presence or absence of the tracer substances in the produced fluid. The tracer substances are sufficiently distinguishable from each other to enable a tracer substance detected in the produced fluid to identify the set of tracer particles from which it has come and hence identify the location from which it has come. The process may be used in conjunction with hydraulic fracturing, placing sets of particles at different locations within a fracture and/or in different fractures extending from a single wellbore.