Activity Index and Thermal History of Geothermal Systems

Abstract

An activity index which indicates the degree of potential of a geothermal systems or a geothermal well is proposed here as follows: AI=(1-(TB-Tm)/(Tb-Tg)x100, where Tm is the maximum temperature measured or estimated, Tb is the saturated temperature corresponding to the integrated hydrostatic pressure at the depth yielding the highest temperature, and Tg is the assumed temperature at the above depth when the geothermal gradient is the same as the average value for the world, 3°C/100m. Based on this activity index, a geothermal system or a geothermal well can be objectively evaluated and classified in one of the following seven categories: AA above 100, A 100-80, B 80-60, C 60-40, D 40-20, E 20-0, and F below 0. Very active ones to Type B or C. However, the fluid inclusion study has revealed that some of the latter were of Type A in the past, and their thermal histories have been traced by a combination of the fission track data.