High Fluid Pressures and High Fluid Flow Rates From a Zone of Natural Hydrofractures Associated with a Major Out-of-Sequence Thrust Zone, Convergent Margin, SW Japan

Moore, James C.; Barrett, Myles; Thu, Moe Kyaw

The convergent margin of SW Japan is a fold and thrust belt characterized by active in-sequence and out-of-sequence thrusts. The margin is composed of deep water siliciclastic deposits. This margin has been transected by 11 drilling sites including 9 riserless and 2 riser sites during the IODP Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). NanTroSEIZE is designed to analyze the complete architecture of a convergent margin while studying and instrumenting faults that produce the great earthquakes of this region. Because it strongly influences fault behavior, multiple efforts have been made to measure fluid pressure, including packer measurements, leakoff tests, longterm instrumentation, and measurements of annular pressure while drilling.

Annular pressure while drilling data shows high fluid overpressures at Site C0001 in part of the out-of-sequence thrust zone. Mostly normal pressures occur at three other sites, including two penetrating major faults. The two holes at Site C0001 show a step up to lithostatic fluid pressure at about 500 mbsf (meters below seafloor), following initial indicators of overpressure at about 375 mbsf. The fluid pressure remains high and increasing to total depth of 1000 mbsf. The pressure curves resemble those associated with shallow water-flow in the Gulf of Mexico. Seismic lines through the site show bright reflectors in the zone of initial fluid pressure increase. Borehole images, sonic velocities, and resistivity all suggest a zone of fractures in mudstones at around 500 mbsf and below. A hydraulic model of the fluid system explains the observed pressures by influx of formation fluid at about 500 mbsf. The combination of a natural influx of 3300 l/m plus 2200 l/m from the drilling system can explain the observed fluid pressures. The overpressured Site C0001 occurs in the probable shallow extension of the fault zone of the 1944 great earthquake and may represent a related but incompletely healed fracture system.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013