The Krafla geothermal field is located on the neovolcanic zone in north-eastern Iceland. drilled at the the presence of high temperature (up to 350'C) geothermal reservoirs. The subsurface rocks are mostly basaltic, with tuffs dominating in the uppermost 800 meters, and subarea1 lavas and intrusions dominating below 800 m. In the "old" wellfield (west of the gully Hveragil) there are two reservoirs. Below a 200 m thick caprock, there is a compressed liquid reservoir extending to a depth of approximately 1000 m. This reservoir (upper reservoir) contains fluids with temperatures of 200 - 220'C (Figure 2). Below the upper reservoir there is a confining layer at depths of 1000 - 1500 m; the confining layer increases in thickness to the west. The lower reservoir underlies the confining layer and extends to a depth in excess of 2200 m (the depth of the deepest well). It contains a gas-rich stemwater mixture.

In the new wellfield (to the east) the To date, over 20 wells have been site (Figure 11, revealing two-phase zone extends practically to the surface, and the temperatures follow the boiling curve with depth.

Prior to the exploitation of a geothermal system, the mass and heat flows in the reservoir are controlled by natural driving forces. The mass transfer is generally dominated by upflow zones, where hot fluids emerge from depth, and by lateral flows that result from the regional groundwater pressure gradients. Geochemical evidence has indicated the presence of three major upflow zones at Krafla, the first one in the Leirhnjukur area (west of the old wellfield), the second one in the "old" wellfield close to Hveragil, and the third one in the "new" wellfield (Armannsson, Gislason and Hauksson, 1982).

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