Scientific Drilling, Special Issue No.1, 2007 71 Part 3: Fault Zone Structure, Composition, and Physical Properties

Several techniques have been applied to obtain infor-mation on the composition of gases and fluids in drill holes, such as downhole fluid sampling, gas and fluid analysis during pumping tests, or in vacuo degassing of drill core samples. All these techniques can yield valuable information on the geochemistry of gases, but they are less useful for obtaining a continuous dataset on the distribution and composition of gases at depth. To reveal such data during drilling, we have developed a technique where gas is extracted from circulating drill mud with an agitator and continuously piped into a nearby laboratory trailer for on-line analysis. In addition, we routinely determine electrical conductivity, pH, and temperature continuously during the drilling operation within the mud pit at 10-min intervals. Hydrocarbons, helium, radon, and carbon dioxide are the most suitable gases for the real-time detection of fluid-bearing horizons such as shear zone, open fractures, sections of enhanced permeability, and occurrences of permafrost methane hydrate. Gas analysis is carried out with a quadrupole mass spectrometer for N2, O2, Ar, CO2, CH4, He, H2, and H 2S; a gas chromatograph (GC) equipped with a flame ionization detector (FID) for hydrocarbons (CH4, C2H6, C3H8, n-C4H10, and i-C4H10), and a custom-built Lucas cell alpha-detector for 222Rn activity. When significant amounts of non-atmospheric gases are detected, off-line gas samples are automatically collected from the gas line for further investigations on isotopes (e.g. noble gases, carbon, and hydrogen) to reveal information on the origin and evolution of deep-seated fluids. The technique has been applied successfully on severa