Gas concentrations in fluids encountered during drilling of geothermal fields range from 0.05 wt% (Wairakei, Ahuachapan) up to about 1 wt% (Ngawha, Broadlands). We discovered in Chapter 2 that carbon dioxide is the dominant gas in geothermal systems and, as we shall see later, plays an important role in controlling the pH of the aquifer fluid. The ratios of the principal gases (e.g., CO₂, H₂, CH₄) are controlled by reactions such as and may therefore be used as geothermometers in the same way as we have used alkali ion ratios. The development of gas geothermometers is discussed in a later chapter; at this stage we will examine the behaviour of gases when phase separation occurs from an initially singlephase geothermal fluid. This is important when we consider the recalculation of analyses of steam samples separated at the surface to determine aquifer dissolved gas compositions.

Gas pressures are also important in reservoir modelling studies as well as in a number of engineering problems associated with geothermal field development; in studies of fossil hydrothermal systems — ore deposits — the constraints imposed by gas contents are just as important and deserve much more attention.