Drill Core Intact cylindrical shaped sample of rock obtained during drilling with a special coring bit. Although commonly narrow in diameter (e.g., 1-4" or 25-100 mm), it preserves a...

Drill Cuttings Small chips of rocks (e.g., <1/8 inch or <3 mm) obtained during drilling with a rotary bit that are continuously returned to the surface by the circulation of mud....

Epidote Alumino-silicate hydrothermal mineral having a distinctive yellow-green color that is used a mineral geothermometer, forming at temperatures above ~230°C. The first down hole...

Felsic A geological term referring to igneous rocks that are equivalent to rhyolite and granite in composition.

Flowback Return flow of slickwater that was injected during a stimulation. This is a highly controlled process in which well head pressure is reduced to allow managed fluid recovery. Fluids...

Fumarole A steam vent and surface thermal feature that is commonly associated with steaming ground and mud pots in hydrothermally and volcanically active areas.

Geochemical Tracer An environmentally benign chemical that is injected to trace fluid movement through the geothermal reservoir and that can be easily detected in produced hot fluid.

Geothermal Gradient The increase in temperature with depth below the surface, usually reported in units of °C/km.

Geyser A rare type of hot spring that periodically discharges a fountain of hot water and steam (or sometimes just gas).

Gneiss Metamorphic rock that forms under intense pressure and high temperature and that is made of quartz, feldspar, amphibole and mica. Gneiss represents the highest grade of metamorphic...

Granite Light colored, coarse grained, intrusive rock with a felsic (silica-rich) composition that is mostly made of quartz, sodium and potassium-bearing feldspar, and plagioclase. Minor...