| Abstract |
It is well known from isotopic studies (Craig, 1963) that almost all fluids discharged by geothermal systems originate from infiltrating meteoric waters. This implies that meteoric waters move at various levels towards a geothermal reservoir where hot fluids are discharged at the surface. Such a simple circulation model implies that some redistribution of terrestrial heat should occur since some heat will be transferred laterally from the recharge area into the reservoir. For high temperature systems, fluid movement in the recharge area causes some cooling, which is indicated by 1011 apparent heat floll arid which has been observed away from high temperature systems in the Taupo Volcanic Zone .in Nell Zealand (Studt and Thompson, 1969), and also in Japan; for example, in Hokkaido(Yuhara, 1973). For low temperature systems, regional transfer of heat by deep secular convection might constitute the only heat input, as has been postulated for some 1011 temperature systems in the North Island of New Zealand (Siswojo et al., 1985) and in the U.S.A. (Reed,1983). Very little has been done to asses the redistribution of crustal heat in recharge areas of geothermal systems since most model studies have been concerned with modelling the fluids floll inside geothermal reservoirs starting lIith a constant heat input at an inferred base of the reservoir (for example, Kassoy and Zeib, 1978). Recently, attempts have been made to model flUid floll patterns of low temperature systems which not only consider flow beneath the discharge area but also heat and mass transfer for the whole recharge area down to crustal depths of 5 to 10 km. The deep temperature field beneath a sedimentary trough underlain by a basement aquifer in Switzerland has been modelled by numerical simulation to explain high heat flail anomaly near the margin of the trough (Rybach et al. ,1987). Numerical simulation has also been used to model apparent high and low heat floll anomalies over an extensive basement aquifer system in China (Hochstein and Yang Zhongke, 1988). However, no attempt has yet been made to simulate the regional heat transfer in the recharge area of the most common type of low temperature geothermal systems, namely, the fracture zone system, where heat is swept by deep circulating meteoric waters tOllards a deep-reaching, highly permeable fracture zons(or intersection of fracture zones) in IIhich thermal fluids ascend slowly to the surface. In this study the results of numerical modelling of two fracture zone systems are described: the Waillera system in New Zealand and the Fuzhou system in China. The study allolls a quantitative assesment of the redistribution of terrestrial heat for both systems. Reservoir characteristics of both systems have been described recently (Hochstein,19 88). In the following lie use the term "terrestrial heat flow· to describe the undisturbed conductive heat flow which 1I0uld be observed outside the systems in the absence of any fluid flow, and below the level where .no fluids move. The disturbed heat flow in the recharge or discharge areas lIill be referred to as "apparent heat flow" or "local heat flow". |