High fluid pressures and high fluid flow rates in the Megasplay Fault Zone, NanTroSEIZE Kumano Transect, SW Japan

Abstract

Annular pressure while drilling data shows high fluid overpressures at Site C0001 in part of the megasplay fault zone of the NanTroSEIZE transect across the subduction zone of SW Japan. Mostly standard annular pressures while drilling occur at three other sites, including two penetrating major faults. The two holes at Site C0001 show a step up to lithostatic annular pressure at about 500 mbsf, following initial indicators of overpressure at about 375 mbsf (meters below seafloor). The annular pressure remains high and increasing to total depth of 1000 mbsf. Seismic lines through the site show bright reflectors in the zone of initial annular pressure increase. Borehole images, sonic velocities, and resistivity all suggest a zone of fractures, from about 490 to 630 mbsf. 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 annular pressures. The highly fractured zone that bleeds fluids to the borehole may be sealed by a localized zone of compressive stress or by overlying gas hydrates.