Word of the Week – 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 occurrence of epidote commonly coincides with the top of a high-temperature geothermal reservoir where it is hosted by igneous rocks.

Word of the Week – Illite


A potassium-bearing mica-like clay mineral that is a product of hydrothermal alteration. It commonly replaces feldspars and other alumino-silicate minerals and it forms above ~220°C. It also reflects a weakly acidic pH condition in hydrothermal fluids.

Word of the Week – Propylitic Alteration

Propylitic Alteration

A term borrowed from studies of hydrothermal ore deposits that refers to an assemblage of secondary hydrothermal minerals which forms in igneous rocks. This assemblage is made up of chlorite, illite, feldspar (both Na and K-rich varieties), epidote, calcite and pyrite, imparting a dark green tint to the appearance of altered rocks. It is commonly developed in volcanic-plutonic sequences that host high-temperature geothermal reservoirs.

Word of the Week – Mineral Geothermometer

Mineral Geothermometer

A secondary mineral phase whose stability is defined by a lower and sometimes an upper temperature threshold. Common mineral geothermometers comprise clays, zeolites and epidote, with crystal structures that contain a hydroxyl (OH-) or water (H2O) molecule. The temperature ranges of mineral geothermometers are calibrated from investigations of a large number of drilled geothermal resources where mineral zonations and temperature gradients have been determined.

Word of the Week – Slotted Linear

Slotted Liner

In geothermal wells, this is a type of production casing that does not go to the surface, and instead with the aid of a “liner hanger” it extends below the casing shoe to protect the wall of the well. Slotted liner has pre-milled slots that allows for production or injection of fluids along the full length of the lined interval, wherever feed and injection points might occur in the rock formation hosting the reservoir.

Word of the Week – Casing Shoe

Casing Shoe

In geothermal wells, this is the bottom of the cemented casing string. Below the casing shoe, the well may be unprotected and open to the surrounding rock or lined with a slotted liner. The depth of the casing shoe generally coincides with the top of the production interval in the reservoir.