Abstract: Geology in Geothermal Energy Exploration
L. T. Grose
A basic difference between the geologic localizations of geothermal energy and those of oil, gas, and nearly every other energy and mineral commodity is rooted in the fact that geothermal energy sources--shallow heat concentrations--are geologically modern or young and transient "deposits," seldom older than late Miocene and usually of late Quaternary age. Therefore, knowledge of currently active mantle-crustal processes of heat generation and concentration is central to geothermal-exploration thinking.
The source, reservoir, and trap concepts so dominant in petroleum exploration also apply generally to geothermal exploration, although in a more complex, diversified, and obscure manner. Sources of heat for geothermal concentration are (1) magma, (2) high heat flow through conductive rock and/or through convective systems without a magmatic source, and (3) combinations of these conditions. The reservoir may be a porous and permeable water-bearing rock mass that is thermally insulated and relatively static, i.e., a system "closed" for a long enough time for heat to accumulate, or a concentration of heat in fluids in a fracture or porous stratum-controlled dynamic aquifer, i.e., an open system not strictly a reservoir. The heat trap is usually a low conductivity and low permeability, th ck sedimentary and/or volcanic sequence, or a self-sealed envelope formed by mineral deposition and alteration.
Exploration should concentrate in young, tectonically extensional areas associated with plate margins. Studies of late Quaternary tectonics and late Cenozoic sedimentation and volcanism will guide the utilization of more conventional exploration methods toward greater success.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC