Lester Logan Geochemistry plays an essential role in all stages of geothermal exploration and development. At early stages in the exploration process, geochemical studies provide information on resource size, shape, temperatures and compositions. Chemical data collected during early development of a system are utilized to assess production characteristics, the potential for scaling, and the potential for mineral recovery. Subsequent monitoring of the production chemistry provides information on processes occurring in the reservoir and on the health of the field. Advances in Isotope Geochemistry, seeks to provide in-depth reviews of isotopie systems, methods and applications to a degree which is not possíble within journal articles. Methods are described in detail, from sample collection and preparation to the fine tuning and subtleties of the mass spectrometric methods, data reduction and interpretation. Following important advances in identification of biological markers in sediments and crude oils, 1 se urkers are used for oil-source rock correlation, and proposed for recostruct on ! Japositional environment and subsequent thermal evolution. The importae of a hydrocarbon-phase migration is widely recognized. Overpressurig i for te fluids is mainly responsible for expulsion of petroleum. Hope this boor will be helpful to the students, research scholars and those who have interest in this field very helpful. Lester Logan, PhD, is Research Chair in Chemistry and Associate Professor in the Denartment of Chemistry of a University, He did post-doctoral research as a