NOVELTY - Controlled-release composition comprises at least one encapsulating compound, and at least one tracer and/or oil-field chemical, where the encapsulating compound is in the form of synthetic crystals having inclusions, and the tracer and/or oil-field chemical is encapsulated within the inclusions.    USE - The controlled-release composition is useful for: tracking the flow, pH and/or salinity of at least one fluid (including oil, gas and water) within a geothermal reservoir or a reservoir for petroleum production; and monitoring fluid flow within a fracture in the reservoir including a geothermal reservoir (all claimed).    ADVANTAGE - The controlled-release composition: can release tracers or other chemicals into a subterranean reservoir in a controlled and predictable manner; and is physically robust to pressure and/or temperature.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) tracking the flow, pH and/or salinity of at least one fluid within a geothermal reservoir or a reservoir for petroleum production, comprising applying the controlled-release composition to at least one known position in the reservoir and monitoring for the absence, presence and/or concentration of at least one of the tracers in fluid produced from at least one well in the reservoir;    (2) monitoring the integrity of an isolation plug in a petroleum reservoir or geothermal reservoir, comprising applying the controlled-release composition to upstream of the isolation plug and monitoring for the tracer comprised in the controlled-release composition in the produced fluid from the reservoir; and    (3) forming the controlled-release composition, comprising contacting solutions of a first and second salt in the presence of a diffusion medium containing at least one tracer, where upon contact the solutions of the first and second salts precipitate crystals of an encapsulation compound having solid or fluid inclusions of the tracer (either as tracer trapped in fluid inclusions and/or as tracer trapped in the mineral crystal structure).