HR: 13:55h
AN: T23E-02    [Abstracts]
TI: Real-Time Fluid and Gas Monitoring During Drilling of the SAFOD Main Hole in Parkfield, CA.
AU: * Wiersberg, T
EM: wiers@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Erzinger, J
EM: erz@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AB: Little is known about the role and origin of fluids and gases associated with the San Andreas Fault zone (SAF). To gain information on fluids and gases at depth, we performed real-time mud gas monitoring during drilling of the SAFOD (San Andreas Fault Observatory at Depth) Pilot Hole (PH) and Main Hole (MH). Gas extracted from returning drill mud was piped into a nearby laboratory trailer and analyzed on-line. Permanent gases were detected using a portable mass spectrometer, hydrocarbons with a gas chromatograph, and the ²²²Rn-activity with a Lucas-Cell detector. When significant amounts of non-atmospheric gases were detected, off-line gas samples were collected from the gas line for further isotope studies. The SAFOD PH and MH were drilled in only a few meter distance, but in contrast to the straight PH, which penetrates through 768 m of sediments into granites down to 2168 m target depth (TD), the nearby MH is deviated towards the SAF and returns into sedimentary strata below 1930 m. The MH drilled sedimentary rocks down to 3987 m TD, approximately 45 m northeast of the surface trace of the SAF. From surface to 1930 m, the depth distribution of gas is similar for SAFOD PH and MH. Shear zones, identified by geophysical logging, are often characterized by elevated concentrations of CH₄, CO₂, H₂, Rn, and He. The same gases were found in the MH below 1930 m, but their concentrations were, with the exception of He, significantly higher: CH₄, CO₂, and H₂ sometimes reach several volume percent. Generally, the gas composition is partly controlled by the lithology. Variation in the methane concentration in several depth intervals reflects the changes in lithology from low gas abundance in clays and silts to more gas rich shales, which are the source rocks for hydrocarbons. Highly porous and permeable sandstone yield the highest concentrations of hydrocarbons (up to 15 vol% methane), and may be regarded as reservoir rocks. We interpret high radon activities in mud gas as indicator for circulating fluids entering the borehole via fractures. These fluids are also rich in hydrocarbons, carbon dioxide, and hydrogen, but only low concentrated in helium. Such intervals could be identified in several depth intervals (2675-2750 m, 2825-2900 m, and 3550-3650 m depth, and below 3700 m). The hydrocarbons in the surrounding rocks show a similar composition as those associated with fault zones. In addition to the low helium concentration, these results demonstrate fluid migration from the nearby with only little evidence for gas migration from a deeper source. A striking observation is the high amount of hydrogen found in these intervals. We can exclude a significant contribution of artificial hydrogen (drilling artifact) and mantle hydrogen. From soil gas studies, it is known that fault zones sometimes show enhanced concentration of hydrogen. As a possible source of hydrogen, the interaction of water with freshly ground rock, caused by fault zone movement, is discussed. Isotopic studies on hydrogen in combination with laboratory experiments are ongoing to test hydrogen synthesis by rock-water interaction. First isotopic studies on δ¹³C of methane indicate mixing of microbial methane with only small amounts of methane generated by thermal degradation of organic matter in the shallower depth (down to ~2500 m). Below this depth, the concentration of heavy hydrocarbons increases. CH₄/(C₂H₆+C₃H₈) significantly drops from >100 to values <30 towards the bottom of the MH, and, methane becomes isotopically heavier, which is more typical for thermogenic hydrocarbons.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3653 Fluid flow
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8194 Instruments and techniques
SC: Tectonophysics [T]
MN: Fall Meeting 2005