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doi:10.2204/iodp.proc.314315316.116.2009

Physical properties

This section presents the measurements available for physical property analyses at Site C0004. They include five different sets of resistivity logs (bit; ring; and shallow, medium, and deep button) and sonic log (DTCO) measurement for sonic P-wave velocity analysis. Since no radioactive source was available, no neutron porosity or density data were recorded.

As at previous sites, additional analyses were conducted to produce different porosities derived from resistivity. Accordingly, estimations of temperatures and formation factors were carried out.

Resistivity and estimated porosity

Resistivity logs

Figure F9 shows the caliper log (CCAV; average of the four caliper measurements made at the same depth); ring and bit resistivity measurements; smoothed logs of shallow, medium, and deep button resistivity; and the 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. It appears that the range of variation of resistivity is small (from 0.6 to 1.6 Ωm) (Fig. F9, F18). The superposition of the deep, medium, and shallow button resistivity measurements shows very good agreement between medium and deep button values. The shallow button measurement departs significantly from the two other button resistivities. A more detailed comparison shows that the deep resistivity is consistently greater than the shallow resistivity and the difference increases inside the fractured zones (Fig. F9, F19).

Based on bit resistivity, logging Unit I resistivity values gradually increase from 0.6 to 1.1 Ωm. The increase is steeper in logging Subunit IIA, where the resistivity value reaches 1.45 Ωm at the bottom of the zone. The slightly decreasing trend of resistivity values in Subunit IIB, from 1.4 to 1.3 Ωm, is followed by a slight increase of resistivity values at the bottom (from 140 to 160 m LSF the values vary from 1.2 to 1.4 Ωm). In Subunit IIC, resistivity generally increases from 1.3 to 1.5 Ωm, but the signal contains high-amplitude fluctuations from 1.4 to 1.2 Ωm with three zones of lower resistivity from 165 to 175, 182 to 189, and 210 to 221 m LSF, partly correlated with fractured zones. In Subunit IID the trend of resistivity decreases from 1.5 to 1.2 Ωm with a low-resistivity peak at 311 m LSF and two zones of lower resistivity values from 252 to 265 and 286 to 292 m LSF corresponding to two major fractured zones. In logging Unit III the resistivity trend slightly decreases and is affected by low-resistivity peaks that correlate well with single fractures at 349, 355, 376, and 390 m LSF. (Fig. F9, F20).

Estimation of temperature profile

The downhole temperature profile was estimated from a regional surface heat flow of 60 mW/m² (Kinoshita et al., 2003), and assuming 1 W/(m·K) thermal conductivity for the upper 68 m LSF and 1.3 W/(m·K) below and 2°C surface temperature. The estimated temperature is 21°C at 399 m LSF.

Estimation of porosity from resistivity

Bit and ring formation factors have been calculated from resistivity logs and temperature-corrected seawater electrical resistivity. They were converted to porosity using Archie’s law. In the absence of neutron porosity data to calibrate Archie’s law parameters, the same values of a = 1 and m = 2.4 as at the previous sites were used. It should be noted that lithologic variations not taken into account in this estimation could affect the resulting porosity, as could the choice of Archie’s law constants.

The general trend of the resistivity-derived porosity log presents several intervals where the average value remains fairly constant (Fig. F20). Resistivity-derived porosity decreases from 65% at 6 m LSF to 58% at 54 m LSF. From 54 to 95 m LSF, resistivity-derived porosity decrease is steeper, from 58% to 52%. From 95 to 142 m LSF, resistivity-derived porosity remains nearly constant at 52%. Bit resistivity–derived porosity variations around this average value are <1% in amplitude (2.5% for ring resistivity–derived porosity). From 142 to 191 m LSF, the average resistivity-derived porosity value decreases to ~49%. The moderately fractured interval 170–184 m with conductive fractures corresponds to a low-porosity zone of ~51%. From 191 m LSF to the bottom of the hole, resistivity-derived porosity seems to fluctuate around an average value of 49%. Two intervals of anomalous resistivity-derived porosity can be observed from 207 to 264 m LSF: (1) a 23 m high-porosity interval with a maximum value of 50% at 217 m LSF and (2) a 32 m low-porosity interval with a minimum value of 47% at 238 m LSF. A 50% high-porosity peak anomaly at 289 m LSF corresponds with a major fracture zone. From 292 to 311 m LSF, another low-porosity zone (48%) is present. Between 311 and 313 m LSF, a sharp high-porosity anomaly reaches 52% for the bit resistivity, which may correspond to a sand layer (see “Log characterization and lithologic interpretation”). Below 316 m LSF, four high resistivity-derived porosity peaks occur at 349, 368, 389, and 394 m LSF.

Bit resistivity–derived porosity anomalies do not clearly coincide with the position of the fractured intervals. High ring resistivity–derived porosity seems to coincide more clearly with such intervals, even if the relation is not systematic.

Estimation of density

Because of the absence of bulk density measurements, we estimated bulk density from the resistivity-derived porosity (Fig. F20). Resistivity-derived porosity was converted to density using standard methods (see “Physical properties” in the “Expedition 314 methods” chapter).

P-wave velocity

The sonic P-wave velocity log in Hole C0004B is of good quality (see “Data and log quality”). In this hole, the sonic response appears to reflect well the effects of fracture zones and lithology variation.

Figure F21 shows the P-wave velocity log juxtaposed with logging units and major and minor fracture zones. Below the first 60 m jet-in interval, in which only mud arrivals are detected, the log begins to respond to actual formation velocity, sharply increasing with depth. At the lower portion of logging Subunit IIA, a low-velocity zone forms, attributable to several possible reasons, such as increasing breakout widths and/or decreasing resistivities with depth. It is not clear whether the low-velocity zone is only due to hole condition or a result of low formation velocity. However, it should be noted that no clear variation in gamma ray values, as well as no clear fractures, are associated with this low-velocity zone (Fig. F1), which implies that the low velocity might be an artifact caused by increasing breakout width (Fig. F22). At the boundary between logging Subunits IIA and IIB, a notable jump in velocity (from 1580 m/s at 92 m LSF to 1826 m/s at 99 m LSF) is detected. It is followed by a gradual increase in velocity, although a minor fracture zone is encountered at the interval 96–112 m LSF. A slight decrease in velocity with depth from 112 to 133 m LSF appears to be related to lithology because subtle changes in gamma ray values and resistivity are also observed at this depth interval. Thereafter, velocity increases steadily throughout logging Subunit IIB.

The upper part of logging Subunit IIC (0–213 m LSF) is characterized by a nearly constant or decreasing velocity. The main control over such velocity behavior seems to be major and minor fracture zones (170–184 and 208–213 m LSF, respectively) and associated borehole breakouts. For the rest of logging Subunit IIC, velocity increases because hole condition was nearly intact.

Logging Subunit IID, including the lowest formations of the thrust sheet and the major fault and fracture zones, exhibits a drastic velocity variation. Velocity decreases almost continually through logging Subunit IID as depth approaches the main fault zone (~290 m LSF). Velocity then increases abruptly across the fault, from 1909 m/s at 291 m LSF to 2115 m/s at 293 m LSF. Immediately below the fault, velocity remains high, averaging ~2120 m/s in a depth interval between 293 and 311 m LSF. A minor fracture interval from 308 to 324 m LSF appears to cause a velocity decrease in this zone.

Logging Unit III (underthrust sediments) is characterized by a general increase in velocity with depth. However, it is also affected by several velocity lows, which could correspond to either fractures, lithologic variations, or possibly fluid overpressures.

Comparisons between P-wave velocity and other physical properties

Comparisons of P-wave velocity with resistivity (Fig. F23) and with resistivity-derived porosity (Fig. F24) are made using cross-plots between these properties. Note that resistivity-derived porosity is, in principle, a modified expression of resistivity, although it incorporates the estimated pore fluid temperature as an independent parameter.

Figure F23 shows two cross-plots between P-wave velocity and bit and ring resistivity. Individual logging units and subunits show unique velocity-resistivity relations. Logging Unit I and Subunit IIA have positive relations in which both velocity and resistivity increase (albeit at different slopes). The rest of logging Unit II (Subunits IIB, IIC, and IID) show constant increasing velocity with depth, with resistivity remaining at an average of ~1.3 Ωm. Logging Unit III is characterized by a lower resistivity and higher velocity than in logging Unit II, resulting in a reversal in the data trend in the cross-plot. Such behavior is more clearly visible in the cross-plot with bit resistivity.

In the cross-plot between velocity and ring resistivity-derived porosity (Fig. F24), there is a consistent negative relationship between the two properties with increasing slope with increasing velocity.

In all cross-plots shown in Figures F23 and F24, logging Subunit IIA appears to be a transition between the upper slope sediments (Unit I) and the thrust sheet (Subunits IIB, IIC, and IID). Also note that the two clusters of data for logging Subunit IID and Unit III are not clearly differentiated, even though there is a major structural and lithologic change between these groups of data.

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