HR: 1340h
AN: T23A-0565    [Abstracts]
TI: First seismic observation of a fluid pulse propagating along a fault
AU: * Haney, M M
EM: mhaney@dix.mines.edu
AF: Center for Wave Phenomena and Department of Geophysics, Colorado School of Mines, 1500 Illinois St., Golden, CO 80401 United States
AU: Sheiman, J L
EM: jonathan.sheiman@shell.com
AF: Shell International Exploration and Production Inc., 3737 Bellaire Blvd., Houston, TX 77025 United States
AU: Snieder, R K
EM: rsnieder@mines.edu
AF: Center for Wave Phenomena and Department of Geophysics, Colorado School of Mines, 1500 Illinois St., Golden, CO 80401 United States
AU: Losh, S
EM: losh@geology.geo.cornell.edu
AF: Department of Earth and Atmospheric Sciences, Cornell University, Snee Hall, Ithaca, NY 14853 United States
AB: Several mechanisms have been put forward to explain the lubrication of slipping faults. Among these, the presence of high pore pressure inside fault zones sparks considerable interest in both the fields of exploration and earthquake seismology. For instance, high pore pressure along the San Andreas fault can explain the apparent lack of heat produced at the slip plane. In sedimentary basins, growth faults cutting through young, poorly consolidated rocks provide a means for overpressured hydrocarbons generated in deep source areas to migrate into shallower, economically producible reservoirs during times of microearthquake activity. Though these two examples cover different spatial scales, a collaborative effort between exploration and earthquake seismologists can help to gain a better understanding of fault dynamics. Using seismic reflection data from the prolific South Eugene Island Block 330 field, offshore Louisiana, we study the possibility of geologically fast, pressure-driven fluid flow along growth faults. From the data, we find what is, to our knowledge, the first unequivocal image derived from seismic reflectivity of a fluid pulse inside a fault zone that, based on geochemical evidence and drilling data, is ascending the fault with time. The fluid pulse is confined to a growth fault known as the B-fault, for which there are associated fault-plane reflections in the seismic data. Though the fault zone may be relatively thin at the location of the fluid pulse, it is detectable because the fluid pulse is of high fluid pressure and, hence, extremely low P-wave velocity. We extract the amplitude of the fault-plane reflection from the B-fault by applying a processing technique known as dip-filtering to migrated 3D seismic data gathered by Shell in 1992. The reflectivity at the location of the fluid pulse is greater than at an unremarkable part of the B-fault where a sonic log passed through the fault in 1993. As a further test of the hypothesis that the high amplitude anomaly results from an ascending fluid pulse, we compare the reflections from the B-fault in data sets from 1985 and 1992. Areas of high reflectivity systematically move up the fault plane 950 m (for an average speed of 130 m/yr). In contrast, the reflectivity of the nearly horizontal layer boundaries do not change as significantly in the two data sets. The pulse speed we observe can be explained with a model of a permeable fault zone connecting the shallow, normally pressured sediments to a deep, overpressured compartment.
UR: http://www.mines.edu/~rsnieder/Publications.html
DE: 8010 Fractures and faults
DE: 8105 Continental margins and sedimentary basins
DE: 3022 Marine sediments--processes and transport
DE: 0935 Seismic methods (3025)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting