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doi:10.2204/iodp.proc.314315316.114.2009

Physical properties

Physical properties analyzed at Site C0002 include thermal neutron porosity (TNPH), IDRO, five different resistivity logs (bit; ring; and shallow, medium, and deep button), and sonic P-wave velocity. Porosities were also derived by several different methods. A series of cross-plots between different physical properties were produced to compare and reveal relationships between properties.

Density

The density log in Hole C0002A is based on IDRO, which is derived by extracting the highest value from the 16 azimuthal measurements of density at a given depth.

Log density in Hole C0002A generally increases with depth (Fig. F17). In logging Unit I, the density log increases rapidly from 1.15 g/cm³ at the seafloor to 1.68 g/cm³ at 64.0 m LSF. From 64.0 m LSF to the bottom of Unit I (135.5 m LSF) density data are affected by hole conditions. The density log remains nearly constant at ~1.8 g/cm³ from ~136 to ~300 m LSF with some negative spikes. The density log shifts to higher values (~1.9 g/cm³) between 299.3 and 343.8 m LSF with minor fluctuations and then remains constant (~1.93 g/cm³) to 481.6 m LSF. No large fluctuations in density are observed in Zone A (hydrate zone) (see “Log characterization and lithologic interpretation”), whereas resistivity data show high variability (see “Resistivity and estimated porosity”). The density log shows a gradual increase to ~2.0 g/cm³ at the base of logging Unit II. In Zone B (potential gas zone, 481.6–547.1 m LSF) density exhibits large variations associated with fluctuations in the caliper, which can also be seen on the resistivity log (see “Resistivity and estimated porosity”). This variability may correspond to a turbidite-rich layer. At the boundary of Units II and III (830.4 m LSF) density shows a negative peak at 1.83 g/cm³. In logging Unit III, density decreases from 2.10 g/cm³ at 832.6 m LSF to 2.00 g/cm³ at the bottom of Unit III (935.6 m LSF). The data are much more scattered in logging Unit IV as a result of hole conditions. Density shows an increasing trend with depth, reaching a value of ~2.3 g/cm³ at 1366.4 m LSF.

Density-derived porosity

A porosity profile was calculated from the bulk density log (IDRO) (Fig. F18), using a constant grain density (ρ_g) of 2.65 g/cm³ and a water density (ρ_w) of 1.024 g/cm³ (see the “Expedition 314 methods” chapter). The density-derived porosity shows higher values than the neutron porosity in logging Unit I. The density-derived porosity and neutron porosity show almost the same value from 131 to 319 m LSF, and the density-derived porosity is ~10% lower than the neutron porosity to the bottom of the hole. The difference in values is especially marked in Unit II below Zone A (Zone B excepted) and in Unit III, which is clay-rich.

The density-derived porosity log is generally slightly less scattered than the neutron porosity log, except for intervals with large caliper measurements (i.e., 135.5–344.5, 481.6–547.1, and 935.6 m LSF to the bottom of the log). The discrepancy between the two porosities is also demonstrated in the cross-plot of density-derived porosity versus neutron porosity, which shows an overall positive correlation between the density-derived porosity and neutron porosity (Fig. F19). High porosity data mainly lie above the diagonal line and relatively low porosity data (≤55%) mainly lie below the diagonal line.

Neutron porosity

The TNPH log in Hole C0002A shows large scatter throughout the entire depth range (Fig. F20); therefore, the porosity data were smoothed using a 4.5 m running average to reduce the scatter. In logging Unit I, the porosity data show relatively high porosity (60%–75%) with high-frequency fluctuation. In logging Unit II, the porosity log shows a stepwise decrease from 63.3% to 49.2%. At the first step, the porosity log drops from 63.3% at 135.5 m LSF to 56.0% at 146.0 m LSF; thereafter, the log stays constant at ~56%. At the second step, the porosity log drops to 54% at ~262 m LSF and remains nearly constant until 547.1 m LSF (54.3%). The porosity log decreases from 54.3% to 50.0% across the 46.8 m interval from 547.1 to 593.9 m LSF then remains nearly constant at ~50% to the bottom of Unit II, with only small fluctuations around this value. Porosity slightly increases from 49.2% to 55.0% in Unit III (830.4–935.6 m LSF). In logging Unit IV, TNPH data show significant scatter and are affected by the hole conditions. The porosity log shows a decreasing trend with depth and reaches ~37.2% at the bottom of the log.

Resistivity and estimated porosity

Resistivity logs

Figure F10 shows a comparison of the caliper (CCAV) log, smoothed logs of the five different resistivity measurements (ring; bit; and shallow, medium, and deep button resistivity), and the smoothed result of the difference between shallow and deep button resistivity. A moving average using a 21-point (~3 m interval) window was used to smooth the resistivity values. Superposition of the deep, medium, and shallow button resistivity measurements shows generally good agreement among them. The logs show increasing resistivity values from the seafloor to the bottom of Zone A followed by a slight decreasing trend of resistivity to the top of logging Unit IV and an increasing trend to the bottom of the hole.

More systematic comparisons between resistivity logs were made through cross-correlations. Figure F21 shows bit and ring resistivity and shallow and deep button resistivity cross-plots. Contrary to Hole C0001D, the ring resistivity is generally lower than the bit resistivity. The difference between bit and ring measurements is ~0.3–0.5 Ωm for all the resistivity values.

The cross-plot between shallow and deep button measurements indicates that deep resistivity measurements are generally higher than shallow resistivity measurements. Figure F10 shows that the zones where deep resistivity values are significantly higher than shallow resistivity values correspond in most cases to intervals with large caliper values identified as possibly sand-rich layers (see “Log characterization and lithologic interpretation”). At the bottom of Zone A, some notable peaks showing relatively high deep resistivity values imply the presence of gas hydrate.

In logging Unit I, resistivity values gently increase from 0.8 to 1.1 Ωm. A stepwise increase from 1.1 to 1.4 Ωm is observed at the logging Unit I/II boundary. The top section of logging Unit II, above Zone A, shows relatively constant resistivity values of ~1.5 Ωm, reduced at depths between 200 and 218 m LSF. The resistivity values increase in Zone A from 1.5 to 2.6 Ωm, with a lower resistivity value excursion from 280 to 320 m LSF. It is noted that this lower resistivity zone is between (below and above) spikes of high resistivity values. Similar high resistivity spikes (up to 50 Ωm) are found at the bottom of Zone A and are interpreted as the signature of gas hydrate–rich intervals (see “Log characterization and lithologic interpretation”). The bottom of Zone A is marked by a net decreasing trend of resistivity values from 2.6 to 1.7 Ωm. The resistivity values are nearly constant in Zone B and the deepest part of logging Unit II, below Zone B (1.5 and 1.6 Ωm, respectively). The ring and the shallow, medium, and deep button resistivity signals become very noisy in Zone B and are scattered in patches below Zone B to the logging Unit II/III boundary. This scattering is likely to be linked to the presence of sand-rich layers. It is noted that most spikes in Zone B are negative (conductive), which is typical for sandy layers, whereas spikes in Zone A are mostly positive (resistive), representing a gas hydrate–dominated character. From 530 to 700 m LSF a decreasing trend of resistivity from 1.7 to 1.3 Ωm is observed followed by an increasing trend of resistivity from 1.3 to 1.5 Ωm. Logging Unit III is characterized by a slightly decreasing trend of resistivity from 1.5 to 1.4 Ωm and by a stable ring resistivity signal. The logging Unit III/IV boundary is marked by a decrease in resistivity from 1.4 to 1.1 Ωm. The upper section of logging Unit IV (from 936 to 1080 m LSF) is characterized by the general increase of resistivity from 1.4 to 2.1 Ωm. Below that section, the resistivity measurement is relatively constant at ~1.9 Ωm except for one zone of lower resistivity values (1.7 Ωm) between 1080 and 1140 m LSF and two zones of higher resistivity values (2.2 and 2.3 Ωm, respectively) at ~1220 and ~1300 m LSF.

Estimation of temperature profile

The temperature gradient was integrated from porosity-dependent rock thermal conductivity and estimation of temperature at the base of the gas hydrate zone.

The depth-dependent thermal conductivity (K) at this site was inferred using a geometric mean model, as follows:

where K_g is the grain thermal conductivity (2.85 W/[m·K], from Leg 190 Site 1173) (Moore, Taira, Klaus, et al., 2001), K_w is the water thermal conductivity (0.60 W/[m·K]), and ϕ is the porosity. TNPH was used to estimate ϕ. Calculated K is ~1.25 W/(m·K) from seafloor to ~950 m LSF and ~1.5 W/(m·K) from ~950 m LSF to the bottom of the hole.

The temperature at the base of the gas hydrate zone (400.4 m LSF) was estimated at 22°C from the stability fields of gas hydrate (Kvenvolden, 1988).The resulting regional surface heat flow (Q) is 60 mW/m², as obtained from the integration of the thermal resistance between the surface and the base of the hydrate zone:

Surface temperature (T₀ = 2°C) was assumed. The estimated heat flow is consistent with surface heat flow data. The in situ temperature profile was calculated using the same equation assuming the heat flow estimated above. The resulting temperature profile reaches 67°C at 1370.7 mbsf.

Estimation of porosity from resistivity

Seawater electrical resistivity was calculated using the temperature profile estimated above (see the “Expedition 314 methods” chapter) and was used to evaluate formation factor from both ring and resistivity logs. They are normally related to porosity by Archie’s law. The Archie’s law parameters that best fit the lower bound of the density-derived porosity are a = 1 and m = 2.4 (Figs. F22, F23, F24). It should be noted that lithologic variations not taken into account in this estimation could affect the resulting porosity, as well as the choice of Archie’s law constants.

Resistivity-derived porosity generally decreases with depth (Fig. F21). In logging Unit I, resistivity-derived porosity decreases from 70% at the surface to 54% at its base. Within a 10 m interval at the top of logging Unit II, the resistivity-derived porosity jumps from 54% to 50% then decreases gradually to 47% to the top of Zone A. Resistivity-derived porosity further decreases within this zone to 33% at its base. The high ring resistivity peaks in this zone apparently generate negative resistivity-derived porosity peaks interpreted as intervals of gas hydrate occupying the pore spaces of the formation. At the base of Zone A the resistivity-derived porosity jumps from 33% to 40% within <7 m, corresponding to the BSR visible on the seismic sections. This apparent increase is almost certainly an artifact of the resistivity difference between hydrate-filled pore space and pore space filled with free gas rather than an actual change in porosity. From 406 m LSF to the base of logging Unit III (936 m LSF), resistivity-derived porosity decreases very slowly from 40% to 37%, corresponding to a gradient of ~0.6% per 100 m. Possible explanations for this very slow decrease could be the cementation of a somewhat sandy formation, the presence of fluid overpressure in an undrained material, increasing hydrous clay content downsection, or the result of lithologic variations. The resistivity-derived porosity gradient with depth increases to ~2.0% per 100 m in logging Unit IV, corresponding to the shallow part of the older accretionary prism. Resistivity-derived porosity reaches ~28% at the base of the hole.

P-wave velocity

The formations at this site can be divided into two main sections at 934 m LSF based on quality of the P-wave velocity log (Fig. F25): (1) upper formation of relatively good quality (quality level of 1–2) and (2) lower formation of relatively poor quality (quality level of 2–3) (see “Data and log quality”). There appears to be a significant discontinuity in physical properties at the boundary between the forearc basin (logging Units I–III) and the accretionary prism (logging Unit IV). The lower quality of the sonic logs in logging Unit IV can be attributed primarily to hole conditions.

No reliable formation velocity was measured in logging Unit I because of either mud arrival interaction (0–100 m LSF) or borehole washouts (~135 m LSF). Sonic P-wave velocity increases monotonically from 1600 m/s at the logging Unit I/II boundary to ~2000 m/s at ~380 m LSF. Velocity begins to decrease sharply at 380 m LSF through an interval below the BSR between 400 and 550 m LSF. Below this depth, velocity tends to gradually increase with depth with a few minor fluctuations. The low-velocity zones at 660 and 710 m LSF should be noted because sonic data at these depths are of high quality. Also noted is a nearly constant velocity (~2260 m/s) between 810 and 890 m LSF in logging Unit III, where muddy sediments are dominant. It is noteworthy that other physical properties such as resistivity, TNPH, and gamma ray values also remain almost constant in this depth interval. Directly above the logging Unit III/IV boundary, a notable low-velocity zone exists.

The quality of the sonic logs is poor in logging Unit IV. Thus, it is hard to define detailed characteristics of the sonic behavior in this unit. However, a strong increasing trend (1.33 m/s per meter) in V_P compared to that in the upper formations from 550 to 920 m LSF (0.73 m/s per meter) is noted. Values up to 3000 m/s or more were recorded in intervals of good data quality near the bottom of the hole.

Comparison of P-wave velocity with other properties

In this section, we compare V_P with resistivity (Fig. F26) and estimated porosity (Fig. F27). Figure F26A shows a cross-plot between V_P and bit resistivity. The entire data set in this cross-plot (except the mud arrival at 1500 m/s) produces three main subsets depending on the velocity-resistivity relation: (a) formations above the BSR (~400 m LSF, indicated by purple and light blue), (b) formations below the BSR (red and orange), and (c) accretionary prism (green). Note the remarkable difference in slopes between (a) and (c). The formations above the BSR (a) are characterized by a significant increase in resistivity but a minimal increase in velocity with depth, resulting in a gentle slope in the velocity-resistivity relation. In the prism (c), both velocity and resistivity increase with depth, resulting in a moderate slope in the velocity-resistivity relation. The data cluster for the formations below the BSR (b) has a negative slope resulting from the decrease in resistivity while velocity increases with depth, connecting the two subsets (a) and (c). Such a trend is more clearly visualized in Figure F26B. Both velocity and resistivity decrease with depth immediately below the BSR over a 100 m interval. Velocity then increases with depth, but resistivity continues to decrease and stays constant to the logging Unit III/IV boundary.

P-wave velocity and porosity relationships are presented in Figure F27. Four different porosity logs (TNPH, density-derived porosity, and bit and ring resistivity–derived porosities) are cross-plotted with P-wave velocity. Clearly, there are some differences in values between differently derived porosities. See relevant descriptions for individual porosities in “Physical properties” in the “Expedition 314 methods” chapter.

Whereas all four plots show a general agreement in trends of decreasing porosity with increasing velocity, there are slight differences in fitting data for individual formations. For example, the gas hydrate–bearing formations (indicated with light blue) tend to have different patterns of resistivity-derived porosities from those of other porosity data, such that the slope of the relationship is relatively low. It is mainly because resistivity-derived porosity is biased toward higher resistivity values in the gas hydrate–bearing formations. The velocity-porosity relationship for the accretionary prism is markedly scattered for density-derived porosity. The relationship for the prism is quite well constrained for both resistivity-derived porosities, simply because the resistivity-derived porosities themselves vary less than the noisy density-based values.

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