Geothermal handbook: PlanninG and Financing power Generation t e c h n I c a L r e p o r t 2 / 2

F I g u r E 1 . 5
Schematic Figure of a Sedimentary Basin with a Geothermal Reservoir at 2-4 km Depth 
Source | Saemundsson, Axelsson, and Steingrímsson 2011. 
 
 
 
 
 
Borehole
km
0
1
2
3
4
5
0 
50 
100 
T [
o
C]
Fault
Recharge
Permeable Layer
The temperature profile to the right shows a typical sedimentary geothermal gradient profile.
The temperature profile to the right represents the central part of the model.

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G e o t h e r m a l H a n d b o o k : P l a n n i n g a n d F i n a n c i n g P o w e r G e n e r a t i o n 
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Factors that determine the Likely use of a geothermal resource
The use of geothermal resources is strongly influenced by the nature of the system that produces 
them. Broadly speaking, the resources of hot volcanic systems are utilized primarily for electric power 
generation, whereas the resources of lower temperature systems are utilized mostly for space heating 
and other direct uses.
Consideration of a number of factors is required to determine the optimal use of a geothermal 
resource. These include the type (hot water or steam), rate of flow, temperature, chemical composition, 
and pressure of the geothermal fluid, and depth of the geothermal reservoir. Geothermal resources 
vary in temperature from 50° to 350°C, and can either be dry, mainly steam, a mixture of steam 
and water or just liquid water. Hydrothermal fields are often classified into high, medium, and 
low temperature fields. This division is based on inferred temperature at a depth of 1 km; high 
temperature fields are those where a temperature of 200°C or more is reached at a depth of 1 km; 
and low temperature fields are those in which the temperature is below 150°C at the same depth. 
permeability or are virtually impermeable; therefore, they cannot be exploited in a
conventional way. However, experiments have been conducted in a number of locations to
use hydro-fracturing, also known as “fracking,” to try to create artificial reservoirs in such
systems, or to enhance already existent fracture networks. Such systems will mostly be used
through production or reinjection doublets.
7 
B O x 1 . 1
What is a Geothermal System (as Opposed to a Reservoir, or Field)?