Hydrological features across the Japan Trench, derived from pressure while drilling of expedition IODP 343 (J-FAST)

The J-FAST expedition was a fast-drilling response to the devastating M9 2011 Tohoku earthquake. The expedition successfully drilled and cored through the plate boundary fault zone of the Japan Trench at 820 meter below sea floor.

We reprocess here the data of annular pressure while drilling (APWD) acquired during the drilling of the LWD hole. Several corrections need to be applied before interpreting the APWD data as a fluid influx/outflux event between the formation and the wellbore.

We first corrected the hydrostatic effect due to the density of the mud column in the matrix, with the additional effect of the cuttings. The density was estimated with two methods; either using up-down movements of the drillstring in the absence of pumping, or by using theoretical formula estimating the volume of cuttings generated by drilling.

Second, the effect of pressure loss induced by fluid circulation along the annulus was considered. The hydrodynamic correction was found to be better fitted with a quadratic model, consistent with a turbulent circulation within the annulus.

What remains of APWD is assumed to be reflective of fluid influx/outflux. However, the results show that only influxes from the formation into the wellbore occur within the borehole. This is consistent with the fact that drilling was riserless and that seawater with very little additives has been used as drilling fluid.

The residual APWD due to influx increases with depth. Within the shallow logging unit 1, the residual is almost null. Between 150 and 600 mbsf, the residual oscillates between null and 150 kPa, and then levels at 200 kPa at 800 mbsf. Below 800 mbsf, APWD increases again to 400 kPa and even more when reaching the chert formation in the footwall section.

No peculiar changes are associated with the décollement e.g. no anomaly was observed in estimated influx rate, but the décollement itself is located within a zone of high pressure. This high pressure likely stems from hydrogeological processes in the footwall where the formation fluids migrate slowly upwards through the low permeability fault.