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doi:10.2204/iodp.proc.337.103.2013

Physical properties

At Site C0020, a series of physical properties measurements (moisture, density, P-wave velocity, electric resistivity, thermal conductivity, anelastic strain recovery [ASR], and vitrinite reflectance) were carried out using core samples and cuttings in the laboratory. All physical properties were measured at room temperature and atmospheric pressure conditions. Depth and lithologic variation effects on the physical properties were investigated. Physical properties of cuttings samples were also compared with those of core samples and with data from sequential core measurements in the multisensor core logger (MSCL).

Whole-round multisensor core logger

Whole-round multisensor core logger (MSCL-W) measurements were carried out on all sections from Units II–IV at Site C0020, with the exception of samples taken for interstitial water analysis. A sensor track of the split core multisensor core logger (MSCL-S) was used for Core 337-C0020A-8L of 10.8 cm industry-type LDC. It should be noted that data from whole-round cores (RCB) and from split cores (LDC) with differing diameters may not be directly comparable.

Results

The gamma ray attenuation (GRA) density values on the MSCL-W are mainly 1.8–2.23 g/cm³ above 1300 m CSF-B (Fig. F15). The main trend is that density decreases to 1.4–1.9 g/cm³ at ~1600 m CSF-B and increases again to 1.5–2.3 g/cm³ at ~1800 m CSF-B. In the interval from Cores 337-C0020A-15R through 25R (1921–2002 m CSF-B), density varies between 1.7 and 2.5 g/cm³. This may be due to variable lithology in this interval. Density of the coal portions in Cores 15R and 19R is 1.5–2.0 g/cm³ and 1.2–1.5 g/cm³, respectively, which is lower than in intervals represented by other lithologies, where typical values range from 1.7 to 2.3 g/cm³ with the exception of some minor deviations. After tight density measurements ranging between 1.7 and 2.1 g/cm³ in mudstone-dominated intervals of Cores 26R and 27R (2111–2204 m CSF-B), density values range from 1.2 to 2.3 g/cm³ to the lowermost part of the hole.

Peak magnetic susceptibility is generally 70 × 10^–5 to 100 × 10^–5 SI. There are some decreases, such as one drop to 11 × 10^–5 SI in Cores 9R and 10R at ~1630 m CSF-B and another drop to 12 × 10^–5 SI in Core 27R at ~2200 m CSF-B. Coal-bearing sediments had a low magnetic susceptibility of <11 × 10^–5 SI, which is lower than other sediment types.

Natural gamma radiation (NGR) in Unit II generally ranges from 20 to 40 counts per second (cps) and abruptly rises in Unit III, where NGR mainly ranges from 30 to 65 cps. Coal samples range between 6 and 47 cps. In Unit IV, NGR gradually increases from 30–45 cps in Core 28R at ~2300 m CSF-B to 40–62 cps in Core 32R at ~2460 m CSF-B.

Electrical resistivity in Unit II increases with depth from 0.4 to 2.0 Ωm, although Core 8L deviates from the RCB core trend. The resistivity of coal and other sediments is similar in the coaly interval of Core 15R, although coal shows higher resistivity than other sediments in Core 19R. Lower resistivity was observed in Cores 23R and 24R. A slight increase in resistivity was observed below Core 27R.

P-wave velocity was scattered from 1 to 2 km/s in whole-round core (WRC) sections, and lithologic and depth dependence on P-wave velocity is not clear. Deviation is small in Core 26R.

Data quality and error

Even though the data were scattered, distributions of GRA density, magnetic susceptibility, and NGR show certain trends. These trends depend on the lithology, mineral contents, degree of consolidation, and pore structures. However, data were also influenced by artificial disturbance during core processing.

Injection of drilling mud and fluid into cores and fracturing by drilling creates additional pore space, and it mainly reduces GRA density and resistivity. We rapidly took measurements with the MSCL-W and the MSCL-S without concern for thermal equilibrium to room temperature. Most of the temperatures measured at the top of each section, which were measured just before the MSCL measurements, were close to room temperature (the greatest observed difference was ~2 K). Therefore, the error caused by the temperature treatment is probably small, even though temperature dependence is high for electrical resistivity. Large deviation in P-wave velocity in a core, and with no distinct difference with depth, might be due to poor contact of the P-wave sensor with the core liner because the core was wrapped in an ESCAL bag. Expansion of the ESCAL bag potentially caused by the release of gas from cores also induced poor contact. Different contact conditions between the core and core liner caused by the difference in the actual core diameter that was observed in the X-ray CT images might also lead to a loss in quality for velocity data. Varying quantities of drilling fluid seeping out and filling the core liner as well as varying core diameters are other potential causes of noise in the P-wave data.

Magnetic susceptibility, NGR, and electrical resistivity of LDC (Core 337-C0020A-8L) measured by the MSCL-S doesn’t match with those of RCB measured by the MSCL-W. The difference of the NGR values between the two procedures can be caused mainly by the difference in core diameter. However, the causes of discrepancy between the other measurements, GRA, magnetic susceptibility, electrical resistivity and P-wave velocity, are not clear. The sensitivity of the sensors and different core disturbances mediated by mud/core treatment (larger core is probably associated with less disturbance) may be the cause.

Moisture and density measurements

Moisture and density (MAD) were analyzed on every discrete sample from all recovered cores from Units II–IV. The four categorized cuttings samples (i.e., >>4, >4, 1–4, and <1.0 mm) from Units I–IV were also subjected to MAD analyses. The lithology of discrete samples for MAD analysis was also categorized in four types (sand/sandstone, silt/siltstone/shale, coal, and carbonate-cemented sandstone and siltstone). In discrete core samples, cementation was verified by stereomicroscopic observation and the addition of hydrochloric acid. No lithologic categorization of cuttings samples was made because cuttings are a mixture of various rocks.

Results

Porosity profiles categorized by core/cuttings size are shown in Figure F16A. Cuttings samples have generally higher porosity than core samples. Among the four categories of the sieved cuttings, larger size fractions show generally lower porosity. Despite such variations in cuttings size fraction, porosity in each category shows a gradual decrease with depth.

Figure F16B shows porosity profiles categorized by lithology. For discrete core sample porosity, sandstone in Unit II mainly ranges 30%–50%, whereas siltstone plots mainly within 34%–42%. In Unit III, siltstone porosity shows a drop to 20%–30%. Sandstone shows a similar minimum porosity of ~2% but ranges up to a maximum of almost 50%. Coal porosity (gray inverted triangle in figure) ranges 24%–38%. Carbonate-cemented sandstone and siltstone show very low porosities of 2%–14%.

In Unit IV, sandstone and siltstone porosities generally range 26%–32% and decrease slightly to 23%–27% in Core 337-C0020A-32R. Carbonate-cemented sandstone is less porous than other lithologies in Unit IV, with values of 14%–17%. A porosity shift in siltstone was observed at the Unit III/IV boundary.

Figure F17A shows bulk density profiles of core samples and cuttings. In general, cuttings samples show lower bulk density than core samples. Among the four categories, the larger size fraction shows the highest bulk density. Core samples in Unit II range mainly between 1.9 and 2.3 g/cm³ with some deviations. In Unit III, core sample bulk density is scattered widely from 1.2 to 2.7 g/cm³, whereas values in Unit IV are clustered tightly between 2.1 and 2.3 g/cm³. In the bulk density scatter plots in Unit III, some outliers with high values of 2.5–2.7 g/cm³ were identified as carbonate-cemented sandstone and siltstone (Fig. F17B). On the other hand, coal samples have low density values around 1.2–1.4 g/cm³. In Unit IV, sandstone and siltstone density narrowly range between 2.2 and 2.3 g/cm³. Carbonate-cemented sandstone and siltstone have generally higher bulk densities than other lithologies, with values between 2.4 and 2.6 g/cm³. The relationship of porosity to depth for siltstone decreases in Unit IV (Fig. F16B), where only a very slight increase in bulk density is recorded (Fig. F17B).

Grain density of core and cuttings samples gradually increases with depth from ~2.5 g/cm³ at 700 meters below seafloor (mbsf) to ~2.7 g/cm³ at 2400 mbsf (Fig. F18A). Differences in grain density among core samples and sieved cuttings samples are not obvious, although cuttings samples seem to have slightly lower values than core samples and smaller cuttings have more variable grain density values. Grain density values in Unit III cover a particularly wide range (1.3–2.7 g/cm³). Among different lithologic rock types, coal has a very low grain density of 1.3–1.6 g/cm³ (Fig. F18B).

Discussion

Cuttings samples show higher porosity, lower bulk density, and lower grain density than core samples. These differences are considered to be caused by excess moisture on cuttings surfaces because the ratio of surface area to bulk volume increases with decrease in the fraction. Additional fractures and damage might be formed in cuttings samples during the transfer from depth to the surface, and the damage produces extra pores and increases porosity. Lower grain density in cuttings samples is also influenced by the deposition of NaCl (solid density = 2.14 g/cm³) on the surface by drying. The smaller size fraction likely contains excess water that may affect wet mass; therefore, the larger size fraction produces more reliable MAD data. During the treatment of discrete cores, multiple fracturing likely develops parallel to the bedding. This might cause overestimation of porosity and underestimation of bulk and grain density.

Assuming that discrete samples from cores produce more reliable MAD data, we infer that porosity gradually decreases from 60% to 25% with depth. However, carbonate-cemented sediments strongly deviate from the consolidation curve. Cement minerals possibly have a higher density than major minerals (e.g., quartz and feldspar) in the same samples because grain density in several cores is higher than in noncemented sections. These low-porosity cemented rocks are located in units with coal layers. Porosity and bulk density shifts were observed in siltstone at the Unit III/IV boundary. The apparent shift may be caused by undercompaction of sediment because of the generation and maintenance of high pore pressure in Unit IV. Several mechanisms are involved in high pore pressurization in sedimentary basins (Osborne and Swarbrick, 1997), and one of the plausible processes is that the effective sealing of low-porosity layers developed in Unit III prevented fluid flow from depth.

Thermal conductivity

Results

Thermal conductivity was determined on samples from recovered cores in Units II–IV. For the measurements, full-space configuration using a needle probe (standard VLQ probe) was applied to semiconsolidated sediments at two points in the whole cores (337-C0020A-1R and 2R). For other cores, a half-space line source probe (mini-HLQ probe) was applied.

The collected thermal conductivity data gradually increase with greater depth from a range of 1.0–1.8 to 1.5–2.1 W/(m·K) except for Unit III, which shows a wider variation of 0.4–3.5 W/(m·K) (Fig. F19). Carbonate-cemented sandstone and siltstone, which were identified by stereomicroscopic observation and hydrochloric acid reaction, have higher values than others (red triangles in figure). The highest thermal conductivity value was 3.473 W/(m·K) in highly carbonate-cemented, hard sandstone in Sample 337-C0020A-22R-2, 77 cm. Thermal conductivity in coal shows lower values in the range of 0.4–1.4 W/(m·K) (solid inverse triangles in figure).

Discussion

The major trend in thermal conductivity is obtained from sandstone and siltstone data, and the increasing thermal conductivity may indicate increasing compaction with depth. The scattered thermal conductivity values in Unit III are caused by more diverse lithologies, which include coal and carbonate-cemented sandstone in addition to sandstone and siltstone. Very low thermal conductivity values observed in coal are lower than that of saline water at room temperature (0.62 W/[m·K] at T = 22°C) (Beardsmore and Cull, 2001). The increase in thermal conductivity with depth is mainly caused by the reduction of porosity in sedimentary rocks because thermal conductivity of porous media is described as the average for pore fluid and lithic material thermal conductivity. High thermal conductivity of carbonate-cemented rock is explained by carbonate materials of high thermal conductivity (>4 W/[m·K]) and less fluid of low thermal conductivity. The sandstone is mostly composed of quartz particles, which have generally higher thermal conductivity (>7 W/[m·K]) than other major rock-forming minerals (Beardsmore and Cull, 2001); therefore, the thermal conductivity variation of sandstone as well as the low thermal conductivity of coal is affected by the quartz contents.

P-wave velocity

The P-wave velocity measured in Unit I cuttings, which were measured after preparing the large fraction cuttings cubic samples, ranged from 1.67 to 1.90 km/s (Fig. F20). Velocity in this unit averaged 1.81 km/s and decreased with depth. In Unit II, discrete core sample velocity ranged from 1.51 to >3.00 km/s for sandstone and siltstone. A very high velocity of 5.6 km/s was observed in carbonate-cemented sandstone at 1753 m CSF-B. Siltstone velocity was on average higher than in sandstone and cuttings. Siltstone velocity decreased with depth. Coal from Unit III shows velocity values of ~2.2 km/s. Velocities of sandstone and shale range from 1.6 to 1.8 km/s, although several shale samples show velocity values similar to those of coal. Carbonate-cemented sedimentary rocks show high velocity (4.4–5.6 km/s). Velocity of sandstone and siltstone in Unit IV is scattered. Sandstone velocity ranges from 1.51 to 2.12 km/s, and siltstone velocity ranges from 1.57 to 2.38 km/s. On average, sandstone and siltstone velocities in Unit IV are relatively higher than in Unit III. Carbonate-cemented sandstone velocity ranges from 2.5 to 4 km/s, which is lower than in Unit III.

Electrical impedance

A frequency of 25 kHz was selected for the impedance measurements to minimize the imaginary part of the impedance, resulting in a phase angle of <3°. Resistivity of the paper filter on both the top and bottom sides of specimens is <1 Ωm, which has a smaller effect on the resistivity data. Electrical resistivity of cuttings ranges from 0.40 to 0.86 Ωm (Fig. F21A). On average, resistivity has values of ~0.64 Ωm and increases with depth. In Unit II, the resistivities of both sandstone and siltstone increase with depth in a similar trend. In Unit III, resistivity increased in the order of sandstone, siltstone, coal, and carbonate-cemented rocks. Several sandstones show lower resistivity than in Unit II, although several silt and shale samples have higher resistivity than in Unit II. Coal resistivity ranges from 9.5 to 30 Ωm, and carbonate-cemented rocks have values of ~60 Ωm, with the highest values reaching nearly 100 Ωm (i.e., more than two orders of magnitude larger than sandstone). Unit IV resistivity slightly increases with depth, and the resistivity at 2465 m CSF-B is ~3.3 Ωm in shale. Carbonate-cemented rock resistivity is higher than that of noncemented rocks, although the resistivity is one order of magnitude lower than that observed in Unit IV.

The relationship between formation factor and depth (Fig. F21B), estimated from the resistivity data, shows a similar trend with electrical resistivity. This is because the pore water temperature in cubic samples ranged only slightly from 23° to 25°C among samples. The resistivity shift between Units III and IV was observed to a small degree for siltstone, with a trend resembling the porosity shift (Fig. F16B).

MSCL versus core sample

Generally speaking, data from the MSCL show lower values than those data from discrete samples. GRA density by MSCL is lower than the bulk density measured with pycnometer for discrete samples (see Figs. F15, F17B for bulk density, Fig. F20 for P-wave velocity, and Fig. F21 for resistivity). The consistently lower values from the MSCL, despite the differences in measurement principles from discrete sample analysis, resulted from incomplete filling of core samples in core liner. The void space within core liner, and the smaller volume of sample material than assumed in the theory, would result in the smaller values in the measurement results. In particular, poor contacts of the core material with the core liner hampers accurate measurement of P-wave velocity. A secondary cause might be fractures and drilling-induced disturbances (filled with drilling mud) in the WRC material used for MSCL measurement, as discrete samples were selectively collected from intact parts of working halves.

On the other hand, general variations with depth matched with each other between MSCL and discrete sample analysis. We therefore use MSCL data only for physical properties interpretation on a broader scale.

Vitrinite reflectance analysis

Vitrinite reflectance (R_o) was measured in five samples from coal formations (in Sections 337-C0020A-15R-3, 19R-7, and 30R-2), small coal fragments from cuttings (Sample 337-C0020A-97-SMW), and a sandstone layer (Section 2R-2). Figure F22A shows the R_o variation with depth. R_o at 1211 m CSF-B had the lowest value of 0.24%, and the highest value of 0.35% was observed at a coal formation (Section 15R-3). The deepest coal in Hole C0020A was 0.29%. The depth trend for R_o is not clear.

R_o values are very low through the horizon, suggesting coal maturity is quite low at this site. The range of coal porosity in Hole C0020A is in good agreement with R_o values (Rodrigues and Lemos de Sousa, 2002). Low-grade maturation makes it difficult to measure the reflectance properly because the color of low-maturity vitrinite (in such case, huminite) is very heterogeneous and depends on the origin of the huminite. In addition, observable spots (1.6 µm) for R_o analysis are limited, and approximately half of the area measured for R_o almost approached the limitation of field of view. For this reason, R_o values in some samples are inaccurate (see the R_o histogram in Fig. F22B). Microstructures in coal fragments become more uniform with depth, which results in easier identification of vitrinite. Therefore, R_o values are more accurate for the deeper horizon. In low-maturity coal, it is necessary to separate pure vitrinite tips from other minerals for accurate vitrinite measurement. Collecting smaller fractions is one of the ways to address these problems, as we treated the fraction >150 µm for R_o measurement.

Anelastic strain recovery analysis

Diameter measurement results show that the average RCB core diameter ranged from 57.3 to 58.4 mm. In most of the samples, periodicity of the diameter was observed during core rotation at a constant rotation speed, and maximum (d_max) and minimum (d_min) diameter were identified (Fig. F23A). Deviation of the diameter between d_max and d_min normalized by d_min is shown in Figure F23B. S_Hmax – S_Hmin increased with depth. Even though the Young’s modulus of each sample is not investigated, the trend does not change because the Young’s modulus of sedimentary rocks can increase with depth via sediment consolidation. Therefore, the core diameter measurement results imply that tectonic horizontal stress is more effective at depth in Hole C0020A.

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