HR: 0800h
AN: H11B-0486    [Abstracts]
TI: Characterization of gas From Seismogenic Depths of the San Andreas Fault at SAFOD
AU: Erzinger, J
EM: erz@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473, Germany
AU: * Wiersberg, T
EM: wiers@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473, Germany
AB: We present data on the molecular composition of drill-mud gas from the lower sedimentary section (1800 - 3987 m) of the SAFOD (San Andreas Fault Observatory at Depth) Main Hole (MH), two sidetracks and from drill core samples thereof. Selected gas samples from drill mud and drill core have been analysed off-line for carbon and hydrogen isotopes; drill mud gas samples also for noble gases. Hydrocarbons, hydrogen, and CO₂ are the most abundant non-atmospheric gases at seismogenic depths. The SAF (San Andreas Fault), encountered between ~3100 m and 3450 m borehole depth, is generally low in gas, but is encompassed by two gas-rich zones (2700 - 2900 m and below 3550 m) at the fault margins with enhanced ²²²Rn activities and distinct gas compositions. Helium isotope composition and the molecular composition of formation gas are distinct on both sides of the fault, implying that the SAF is a barrier for horizontal gas and fluid migration. Furthermore, the overall low contribution of mantle helium (0.4 Ra on the Pacific Plate and 0.9 Ra on the North American Plate) is minimum within the SAF, suggesting that the fault core does not act as a conduit for overpressured fluids from greater depth. Gas influx into the well at depth is related to the lithology and permeability of the drilled strata in general: larger formation gas influx was detected when drilling through organic-rich shale and permeable sandstone. The isotopic composition indicates an organic source of hydrocarbons and CO2 in the entire sedimentary section of the well. Hydrocarbons in sedimentary strata are partly of microbial origin down to ~2500 m borehole depth. The contribution of thermogenic gas increases between ~2500 m and 3100 m. Below 3100 m, carbon isotopes suggest that hydrocarbons almost exclusively derive from thermal degradation of organic matter. Although low in gas, two sections in the fault (3150 - 3200 m and 3310 - 3340 m depth of the MH) are enriched in hydrocarbons with ratios of CH₄/(C₂H₆+C₃H₈) lower than in the surrounding strata.
DE: 3653 Fluid flow
DE: 8106 Continental margins: transform
SC: Hydrology [H]
MN: 2007 Fall Meeting