44 Geophysics in Geothermal Exploration Figure 1.13 Geophysical methods. The concept of hydrothermal play is central to understand and explore geothermal resources. A geothermal play refers to a naturally occurring geological system with conditions favorable for the accumulation, heating and circulation of fluids. Heat is stored in the reservoir host rocks. Water is the vector to transfer energy to the surface. Successful geothermal exploration relies on a detailed understanding of these plays, as they dictate where, how, and to what extent geothermal energy can be effectively extracted. Hydrothermal plays, like other geological plays (such as petroleum systems), are complex systems that depend on a specific set of geological conditions. Four main elements define a hydrothermal play: • Heat Source: The heat source in a hydrothermal play can be due to natural geothermal gradients (the Earth’s internal heat flow), magmatic intrusions, or tectonic activity. In volcanic regions, for example, magma intrusion degassing and cooling represent the prime heat source for geothermal developments. • Reservoir: The reservoir is the underground porous and/or fractured formation where the hot geothermal fluid is stored. The host rock can be of sedimentary, magmatic, volcanic or metamorphic origin. The quality of a geothermal reservoir depends on the rock’s porosity, permeability and productive thickness, which determine how much fluid it can store and how easily fluids can flow. • Fluid (Hydrothermal System): Geothermal energy depends on the presence of fluids that absorb heat from the reservoir rock and can be brought to the surface. In hydrothermal plays, the water is meteoric water or seawater that has infiltrated into the Earth’s crust, circulated, chemically reacted with host rock and been heated. Fluid is essential to transport heat from the reservoir to the surface, either naturally or through engineered systems. • Caprock or Seal: A caprock is an impermeable layer that helps trap geothermal fluids within the reservoir, preventing them from migrating to the surface or into
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