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doi:10.2204/iodp.proc.319.104.2010

Physical properties

Logging

Physical property analysis at Site C0010 utilized NGR and five sets of resistivity data from the GVR (see "Logging" in the "Methods" chapter). In addition, porosity is estimated from resistivity (for details, see "Physical properties" in the "Methods" chapter). This porosity is based on the total water content of the formation, which includes both pore water and water bound in hydrous minerals (e.g., clay minerals and biogenic opal). At Site C0010, however, this approach was hampered by the absence of core material, cuttings, or density log data to calibrate the transform from resistivity to porosity.

Resistivity logs

We acquired resistivity measurements at a spatial resolution of 0.1–0.15 m. These include bit resistivity, ring resistivity, and shallow-, medium-, and deep-button resistivity (Fig. F20; also see Table T3 in the "Methods" chapter). The buttons are longitudinally spaced along the LWD tool. Their spacing provides depths of investigation of ~1, 3, and 5 inches, respectively, and can be used to quantify invasion of drilling fluid. Depths of investigation of bit and ring resistivity are 7 and 12 inches, respectively.

Ring resistivity ranges between 0.7 and 0.9 Ωm in the uppermost portion of the logged succession within the slope sediments of logging Unit I (0–182.5 m LSF) (Fig. F20). Resistivity within the overridden slope apron deposits of Unit III (407–554 m LSF) ranges from 0.8 to 1.2 Ωm and exhibits a very gradual increase with depth (Fig. F20). In between, the resistivity of the thrust wedge (Unit II) is significantly higher. In its upper portion (~182.5–260 m LSF), below a ~10 m interval where resistivity values remain constant, resistivity increases linearly to values of ~1.6 Ωm. Below this, between 260 and 407 m LSF, resistivity is variable. Focussing on ring resistivity, values range from peak values of 2.5 Ωm to minimum values of ~1.5 Ωm over distances of ~10–20 m (Fig. F20). The two zones where resistivity varies significantly are delineated in gray on Figure F20.

The hole was abandoned at ~460 m LSF because of an incoming storm (see "Operations" and "Logging and data quality"). Then, when the hole was reentered, a section of the hole was relogged (red lines, Fig. F20). The resistivities acquired during the second logging run (Run 2) from 348 to 418 m LSF are different than those collected in Run 1 (see Fig. F20, compare black and red curves). In Run 2, focussing again on ring resistivity, between 348 and 370 m LSF, values scatter around 1.7–1.8 Ωm, whereas the zone from ~370 to 407 m LSF exhibits values around 1.2–1.3 Ωm (Fig. F20). Below ~407 m LSF, values drop back to ~1.0 Ωm, a value similar to those from Run 1 (Fig. F20). The data quality over the relogged interval is questionable (see below and "Logging and data quality").

Comparison of the different resistivity logs and qualitative estimation of invasion

The five resistivity measurements show the same overall trend with depth (Fig. F20). In general, the deep, medium, and shallow buttons integrate a smaller volume of rock than ring or bit resistivity. Hence, data from these buttons exhibit larger fluctuations than bit and ring measurements. Between 260 and 407 m LSF, the medium-button data appear very noisy and exhibit higher values than the deep-button data, which is the opposite of that expected; as a result, we believe the medium-button data are not reliable (Figs. F20, F21). Anomalous peaks in resistivity (>5 Ωm) are also observed in the deep-button curve from 350 to 390 m LSF. These excursions are not observed in the relogged data set (cf. Figs. F20, F21) and may be the consequence of bad hole conditions (e.g., cuttings caught between the sensors and the formation), consistent with high stick-slip during the first pass through that zone (Fig. F20). Overall, we observe that data from the entire 260–407 m LSF interval are significantly different for the shallow and deep buttons. Using the difference between the deep and shallow buttons as a proxy for invasion of seawater, and thus the permeability of the formation, we infer three zones of enhanced permeability: from 182.5 to 260 m LSF in Unit II, from 350 to 407 m LSF in Unit II, and from 522 to 530 m LSF in Unit III (Fig. F21). These intervals coincide with zones of lower overall resistivity, especially at the bottom of Unit II (Fig. F21).

In the relogged interval (348–418 m LSF in Run 2), where the deep–shallow difference is most pronounced, resistivity curves flatten as the depth of investigation decreases (namely from ring to shallow-button resistivity measurements; see Figs. F20, F21). We interpret this as the effect of deep invasion of cold seawater into the formation, which may have occurred during the ~2 days between Runs 1 and 2.

Estimation of porosity from resistivity

Estimation of bottom-hole temperature

We estimated a temperature profile in the formation using a surface heat flow of 53.6 mW/m² and thermal conductivity measurements from nearby Expedition 316 Holes C0004C and C0004D (Kimura et al., 2008). We assume a 2°C bottom water temperature and a steady-state conductive temperature profile. The estimated temperature at the bottom of the hole is 23.4°C.

Estimation of porosity

We calculated seawater electrical resistivity using the temperature profile estimated above and used it to evaluate the formation factor from the ring resistivity (see "Physical properties" in the "Methods" chapter). Formation factors were then converted to estimated porosity values using Archie's law (Archie, 1942). Because no other porosity measurements were available to constrain the Archie's law parameters, we used the same parameters obtained for Site C0004 (a = 1, m = 2.3) (Conin et al., 2008; Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009), where the sediments were drilled with LWD during Expedition 314 (Kinoshita et al., 2008) and cored during Expedition 316 (Kimura et al., 2008) (Fig. F22). It is important to recognize that the Archie's law parameters were defined at Site C0004, so porosity values estimated from resistivity in the thrust wedge at Site C0010, where resistivity and gamma ray values differ from those at Site C0010 (and thus lithology may be different), should be viewed with caution. However, even considering a large range of Archie's law parameters (m ranging from 2.1 to 2.6), the estimated porosities do not change dramatically (Fig. F23).

We used the ring resistivity measurement here because it has a penetration depth sufficiently large to exclude effects of seawater invasion. Estimated porosity decreases gradually from 74% to 63% in the uppermost part of the logged section (42–182.5 m LSF). The same gradual decrease continues below the Unit II/III boundary where computed porosity from Runs 1 and 2 range from 52% at the boundary to 48% at the bottom of the hole. If we assume that porosity decreases exponentially with depth because of vertical compaction (e.g., Athy, 1930), Units I and III can both be fit with the same compaction trend:

ϕ = 0.7 x exp(–z/1500),

where

• z = depth below seafloor (meters), and
• ϕ = fractional porosity.

Unit II clearly departs from this compaction trend, and based on the estimated porosities, most of the sediment in this section appears more consolidated than the slope sediments above and below. At the top of Unit II, computed porosity values drop sharply to 57% and gradually decrease to reach 38% at 340 m LSF (Fig. F22). In the central portion of Unit II (~260 to ~340 m LSF), estimated porosity fluctuates and exhibits two intervals with lower values (porosity of ~38% from ~265 to ~270 m LSF and ~289 to ~305 m LSF). These fluctuations are consistent with excursions in the gamma ray log (see "Lithology"). These apparent fluctuations may reflect differences in composition that reduce resistivity; alternatively, they may reflect true porosity differences related to variations in stress history, local changes in lithology, and/or the presence of fractures or faults.

From 348 to 407 m LSF, estimated porosity differs considerably between Runs 1 and 2 because the resistivity differs markedly. For data from Run 1, computed porosity increases gradually from 38% at 348 m LSF to 41% at 407 m LSF and includes two zones with higher values, the first between 373 and 385 m LSF (~46%) and the second between 390 and 395 m LSF (~42%). At the Unit II/III boundary, values increase sharply to 52%. The computed porosity profile for Run 2 approximately mirrors the trend of Run 1 but is shifted to higher values except for one interval where the two runs are in good agreement (402–417 m LSF, Fig. F22). This shift may result from deep invasion of the formation by seawater in the ~2 days between the two logging runs (see "Operations").

When comparing resistivity-derived porosities at Site C0010 (obtained assuming three different Archie's law parameters: m = 2.1, m = 2.3, and m = 2.6) (Fig. F23) to those at Site C0004, the estimated porosity of Unit II (thrust wedge) at Site C0010 is lower than that of the thrust wedge at Site C0004 (see Conin et al., 2008; Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009; Expedition 316 Scientists, 2009). This difference is driven by considerably higher resistivity in the thrust wedge at Site C0010 relative to Site C0004. In contrast, the compaction trend of the slope apron and overridden sediments at Site C0010 is similar to the one estimated for Site C0004. The lower computed porosity in the thrust wedge at Site C0010 bears some uncertainty because we used the same Archie's parameters as Site C0004, yet the lithologies likely differ between the two sites (see also "Lithology"). However, based on the fact that estimated porosity does not change substantially even when considering a wide range of values for the Archie parameter (cf. Fig. F23), we suggest that the higher resistivity at Site C0010 is likely to result, at least in part, from a difference in compaction state related to modern stress state or burial history.

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