Pore Pressure Profile within Hole C0024A, at the Toe of the Nankai Accretionary Prism, using Borehole Geophysical and Core Petrophysical Data from Expedition IODP 358 (NanTroSEIZE Stage 4)

Abstract:

The Nankai subduction zone is subject to infrequent M8+ earthquakes, threatening the South-Eastern Japan coast. Consequently, it had been the focus of large scale NanTroSEIZE project which involved 13 IODP expeditions. The last expedition 358, notably drilled the plate boundary fault at 813 mbsf (see figure). In this study we utilised drilling data to estimate pore pressure profiles along the C0024 hole, applying empirical models from the industry to show that the pore pressure is above hydrostatic within the borehole, especially when crossing the décollement.

The D-exponent corrected (Dxc) method delineates the empirical relationship between rock strength, bit size, formation drill-ability, Rate of Penetration (ROP) and ECD (Equivalent Circulating Density) of the mud. The ECD had to be recomputed because the operators did not initially consider the riserless nature of well C0024A.

In situ lithological physical properties were compared with NCTL and overburden pressure (OBP) in Eaton Computation. The normal compaction trend line (NCTL) was enhanced with median filtering of Dxc. OBP was computed by integrating the bulk density of overlaying rocks while the NCTL was evaluated considering sediment compaction trend over the 3 main official logging units: (1) slope basin facies (<112 mbsf), (2) accretionary trench-wedge facies and (3) the Shikoku Basin hemipelagic-pyroclastic facies (>555 mbsf).

The pore pressure follows hydrostatics until the top of the hemipelagites with local pore pressure gradients rising up to 1.33g/cm³. The variation of empirical n-exponent in Dxc equation is key to change the amplitude of the pore pressure gradient, so we checked the robustness of the prediction by varying the value between 0.9-2.0. Corresponding increase in n-exponent leads to increase in pore pressure gradient. In this computation pore pressure is always smaller than the mud pressure and the elevated pore pressure is consistent with large positive excursion of in situ Downhole Annular Pressure acquired in real-time near the drill bit. They confirm that pore pressure is up to 14% above hydrostatic at the locus of the décollement.