Interpretation of porosity and LWD resistivity from the Nankai accretionary wedge in light of clay physicochemical properties: Evidence for erosion and local overpressuring

Abstract

[1] In this study, we used porosity to assess the compaction state of the Nankai accretionary wedge sediments and any implications for stress and pore pressure. However, hydrous minerals affect porosity measurements, and accounting for them is essential toward defining the interstitial porosity truly representative of the compaction state. The water content of sediments was measured in core samples and estimated from logging data using a resistivity model for shale. We used the cation exchange capacity to correct the porosity data for the amount of water bound to clay minerals and to correct the porosity estimates for the surface conductivity of hydrous minerals. The results indicate that several apparent porosity anomalies are significantly reduced by this correction, implying that they are in part artifacts from hydrous minerals. The correction also improves the fit of porosity estimated from logging-while-drilling (LWD) resistivity data to porosity measured on cores. Low overall porosities at the toe of the accretionary wedge and in the splay fault area are best explained by erosion, and we estimated the quantity of sediments eroded within the splay fault area by comparing porosity-effective stress relationships of the sediments to a reference curve. Additionally, a comparison of LWD data with core data (resistivity and P wave velocity) obtained at Site C0001 landward of the mega-splay fault area, suggested a contribution from the fracture porosity to in situ properties on the formation.

Hydro-bio-geomechanical properties of hydrate-bearing sediments from Nankai Trough

“…(7) is often used to predict conductivities in marine sediment, and can be used here to anticipate the measured conductivity drop as the probe passes from the salt water surrounding the specimen and into the sediment itself. Taking s f ¼ 3.09 S/m as measured for the pressurizing water via a YSI conductivity meter, and the Archie parameters a ¼ 1 and either m ¼ 2e2.4 from IODP studies of the Nankai Trough (Conin et al, 2011; or m~2 based on estimates for hydrate-bearing sediments (Spangenberg, 2001), the predicted s s for core 8P, with a measured porosity, n, of~0.49, is 0.56e0.74 S/m. For core 10P, with a measured porosity n of~0.46, the anticipated specimen s s is 0.48e0.66 S/m.…”

Site C0018

“…Site C0008 results showed the utility of using the ages of MTDs to reconstruct slope failure activity and to relate it to megasplay fault movements (Strasser et al, 2009). Apart from the deepest section, Site C0008 lacks clear evidence for MTDs because of a significant hia-tus, suggesting erosion related to a prominent slope collapse structure seaward of the megasplay fault (Strasser et al, 2011;Conin et al, 2011). Site C0018 is located within a lower slope basin that (1) better represents the depocenter for downslope mass transport, (2) is clearly characterized by stacked MTDs as seismically imaged by acoustically transparent to chaotic bodies with ponded geometries (Fig.…”

Quantification of free gas in the Kumano fore-arc basin detected from borehole physical properties: IODP NanTroSEIZE drilling Site C0009

“…The water bound to the hydrated minerals is a function of the number of cations present between the layers of hydrated minerals [Ransom and Hegelson, 1994]. The number of cations can be obtained from cation exchange capacity (CEC) measurements [Henry, 1997;Bourlange et al, 2003;Conin et al, 2008]. The effective porosity (' e ) is expressed as…”

“…where a and m are chosen to equal 1 and 2, respectively, based on those computed for similar sediments drilled in the fore-arc basin at IODP Site C0002 [Conin et al, 2008]. The resistivity of the invaded zone is measured with the wireline MCFL™ tool, but unfortunately poor borehole conditions prevent its use in Unit IV.…”