Batch-autoclave experiments have been conducted to evaluate the potential use of synthetic fluid inclusions as a simultaneous temperature-pressure (and fluid sampling) logging tool in deep-seated-high-temperature (>350degreesC) geothermal systems. The application of synthetic fluid inclusions allows us to obtain information about thermal-pressure conditions in deep-seated geothermal systems. where conventional tools cannot be used because of the extreme temperature conditions. Fluid inclusions, up to 50 mum long. have been readily synthesized during 5-day autoclave experiments (conducted at 375-475 degreesC and 39-62 MPa) in pre-fractured, inclusion- and impurity-free artificial quartz. Inferred fluid inclusion (temperature-pressure) trapping conditions are calculated by deducing the intersection of isochores derived from microthermometric data for three sets of simultaneously trapped synthetic fluid inclusions in healed microfractures. Synthetic fluid inclusion logging offers a precise borehole temperature measurement technique without need of any pressure correction. Pressure estimates are less precise, although the method may be improved by using a combination of H2O-NaCl and H2O-KCl solutions/salinities, and fluid/quartz/amorphous silica systems that facilitate crack healing but trap fluids that do not homogenize at near-critical conditions. (C) 2004 CNR. Published by Elsevier B.V. All rights reserved.