Fluid pressures and fluid flows from boreholes spanning the NanTroSEIZE transect through the Nankai Trough, SW Japan

IODP Expedition 314 acquired annular pressure while drilling measurements in six holes extending from the outer forearc basin to the base of the trench slope. These data provide pressures inside the borehole near the bit during drilling; the pressures reflect the cuttings load in the borehole, and the viscous resistance to flow of fluids up the borehole due to pumping from the rig floor and due to fluid exchange with the formation. The forearc basin site provides a reference locality that extends to 1402 m and whose pressure curve is apparently controlled dominantly by the cuttings load and the rig-floor pumping rate. All other holes show at least initial pressure curves that are very similar to that at the forearc basin site, despite pumping rates varying by about 20–30%. Some small variations in pressure curves can be positively correlated to the penetration rate that controls cutting load. Two holes crossing a fracture zone show pressure anomalies extending to above-lithostatic conditions, and modeled fluid influx rates of 3000 l/m or 150% of the rig-floor pumping rate. A third hole, probably in the same highly overpressured zone, shows a near lithostatic pressure value just prior to losing the bottom hole assembly due to stuck pipe. The extraordinarily high pressures occur in the upper portion of the hanging wall of the megasplay fault zone and may reflect residual conditions from the 1944 M 8.1 earthquake. A penetration of the lower boundary of the megasplay fault zone shows only minor fluid pressure anomalies around the fault surface. A hole through the frontal thrust records fluid pressure anomalies in sandy sediments of the footwall, apparently due to expulsion of fluids from loading by the hanging wall.

► Differing fluid pressures and fluid flow rates in accretionary prism fault zones
► Overpressured fracture zone near the 1944 Tonankai earthquake rupture zone
► Fluid pressures and fluid flow rates recorded in boreholes during drilling
► Measurement while drilling results from the NanTroSEIZE Transect SW Japan