Application of soil He surveys to the mapping of underlying geological structures in the Dalby Area, Queensland

Abstract

Elevated He is commonly associated with hydrocarbon pools and U-bearing deposits. Being chemically inert, He tends not to be absorbed by mineral material and is highly mobile. Helium movement through sedimentary rocks and regolith is largely controlled by permeability but may be affected by the amount of pore water or soil moisture and other climatic factors such as air pressure. Faults and fractures commonly act as preferential pathways for the migration of He. Soil He concentrations have been mapped across a 7.2 km2 test site near Dalby, SE Queensland, to further establish the use of this technique to map the location of hydrocarbon deposits and major structures. The region contains a series of hydrocarbon deposits (mainly coal bed methane) that are hosted in slightly deformed sedimentary rock units. The test site is contained within an agricultural area and is overlain by alluvium with depths up to 50 m. In-situ measurement of He in near-surface air and subsoil (70–80 cm depth) gas were conducted using a (portable) Varian PHD-4 detector on a 200 x 200 m grid with a higher density (50 m spacing) traverse. Laboratory testing of the PHD-4 device indicated a near-linear correlation between actual and measured He values between 5 and 500 ppm, and minimal effect from other gases (CO2, ethanol vapour, methane, SO2) except for H2S at concentrations >375 ppm. Helium values across the site range from ~5 ppm (average global background) to >500 ppm. The upper limit to regional He background is 15 ppm and 60 ppm as local background. The peak values are amongst the highest recorded for soils of the many hydrocarbon exploration surveys conducted using the PHD-4. The spatial patterns suggest He is leaking (vertically) through the alluvium along conjugate fault structures. There is high correlation between near-surface air and subsoil gas He values, though the latter is typically higher by a factor of ~2. Irrigation of the fields suppressed the He soil concentrations, but did not affect the general trends in the He patterns.