The Nankai Trough is a locus of slow slip, low frequency earthquakes and Mw>8 classical earthquakes. It is assumed that high pore pressure contributes substantially to earthquake dynamics. Hence, a full understanding of the hydraulic regime of the Nankai accretionary prism is needed to understand this diversity of behaviors. We contribute to this understanding by innovatively integrating the drilling and logging data of the NanTroSEIZE project. We focus on the toe of the accretionary prism by studying data from Hole C0024A drilled and intersected the décollement at 813 mbsf about 3km away from the trench. Down Hole Annular Pressure was monitored during drilling. We perform a careful quantitative reanalysis of its variation and show localized fluid exchange between the formation and the borehole (excess of 0.05m3/s), especially in the damage zones at the footwall of the décollement. Pore pressure was estimated using Eaton’s method on both drilling and sonic velocity data. The formation fluids are getting significantly over-pressurized only a few hundred meters from the toe of the accretionary prism near the décollement with excess pore-pressure (P*≈0.04–4.79MPa) and lithostatic load (λ≈88-0.96 & λ*≈0.1-0.62 ) contributing to maximum 62% of the overburden stress. The hydraulic profile suggests that the plate boundary acts as a barrier inhibiting upward fluid convection, as well as a lateral channel along the damage zone, favouring high pore pressure at the footwall. Such high pressure at the toe of the subsection zone makes high pressure probable further down in the locus of tremors and slow slip events.

The Nankai Trough is a locus of slow slips, low frequency, and large-magnitude classical earthquakes. It is assumed that high pore pressure contributes substantially to earthquake dynamics. Hence, a full understanding of the hydraulic regime of the Nankai accretionary prism is needed to understand this diversity of behavior.We contribute to understanding the full hydraulic regime within the Nankai accretionary prism by innovatively integrating the drilling and logging data of the NanTroSEIZE project. We focus on the toe of the Nankai accretionary prism by studying data from hole C0024A drilled during IODP expedition 358. This drilled hole intersected the Nankai décollement at 813 mbsf, about 3 km from the trench. Pore pressure was first estimated using Eaton’s method on both drilling and sonic velocity data. Both results show that pore pressure follows hydrostatics until the top of the hemipelagites, with local pore pressure rising up to 38% above hydrostatic especially crossing the décollement. Downhole Annular Pressure was also monitored during drilling, and a careful reanalysis of its variation shows the occurrence and the locus of fluid flow between the formation and the borehole. Primarily, there are two identified fluid flow anomalies intervals: (1) at the shallow depths <100 mbsf with loose coarse sediments, which could be related to erosional unloading, landslide, slope instability. (2) Below the décollement (<813 mbsf) at the two asymmetric damage zones. The damage zones at the footwalls of the major faults are predominantly permeable with significant porosity and permeability values with orders of magnitude between ∼10−16 to 10−17 m2 as quantitatively estimated using the Hvorslev equation for a fully penetrating well in a confined aquifer. Our results show that the formation fluids are getting significantly over-pressurized only a few hundred meters from the toe of the décollement. The décollement is already impermeable across the fault, and the fluid flow is channelized along the damage zones. The impermeable décollement acts as a hydraulic barrier inhibiting fluid flow upward, keeping high pore pressure at the footwall and increasing the structural weakness of the lithologies. It’s therefore probable that high pressure is also expected further down in the locus of tremors and slow slip events.

Despite exploration and production success in Niger Delta, several failed wells have been encountered due to overpressures. Hence, it is very essential to understand the spatial distribution of pore pressure and the generating mechanism in order to mitigate the pitfalls that might arise during drilling. This research provides estimates of pore pressure along three offshore wells using the Eaton’s transit time method. An accurate normal compaction trend was estimated using the Eaton’s exponent (m=3). Our results show that there are three pressure magnitude regimes: normal pressure zone (hydrostatic pressure), Transition pressure zone (slightly above hydrostatic pressure), and over pressured zone (significantly above hydrostatic pressure). The top of the geopressured zone (2873 mbRT or 9425.853 ft) averagely marks the onset of overpressurization with the excess pore pressure ratios above hydrostatic pressure varying averagely along the three wells between P * = 1.06 − 24.75 MPa and the lithostatic load range is λ = 0.46 − 0.97 and λ * = 0.2 − 0.9. The parametric study shows that the value of Eaton’s exponent (m = 3-6) need to be applied with caution based on the dominant pore pressure generating mechanism in the Niger Delta. The generating mechanisms responsible for high pore pressure in the Offshore Niger Delta are disequilibrium compaction, unloading (fluid expansion) and shale diagenesis.