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doi:10.2204/iodp.proc.313.104.2010

Physical properties

The focus of this section is to provide a description of the petrophysical data sets collected on cores from Hole M0028A and their significant characteristics and variations, as well as concentrating in more detail on a number of interesting aspects. Some measurements display variation primarily associated with depositional sedimentary changes, whereas other measurements are controlled more by postdepositional factors such as the degree of cementation and the type of interstitial water. In addition, combined integration of the core and logging petrophysical data sets allows calibration of core data with in situ borehole properties and provides an assessment of the precise depth from which core was collected in the borehole.

Whole-core measurements were acquired on a multisensor core logger (MSCL): gamma ray attenuation density (here referred to as gamma density), P-wave velocity, noncontact electrical resistivity, and magnetic susceptibility. Thermal conductivity and natural gamma radiation (NGR) measurements were taken on whole cores prior to the commencement of the OSP (see "Physical properties" in the "Methods" chapter). Density, porosity, and P-wave velocity were measured and calculated from, on average, one discrete sample in every core section. Figure F35 provides an overview of the data. A synthesis of downhole and physical property data can be found at the end of "Downhole measurements" (Fig. F36).

Density and porosity

Gamma density and wet bulk density

Gamma density in Hole M0028A varies more than in Hole M0027A. Only values between 1.5 and 2.9 g/cm³ are considered below (Fig. F37). Wet bulk density calculated from wet and dry weight and dry volume of samples from cores varies between 1.65 and 2.57 g/cm³. Repeated measurements with resaturation of sediment before measurement indicate that the density of samples may be overestimated by up to 5%. Despite this uncertainty, there is a fair correspondence between gamma density and wet bulk density. Wet bulk density estimates tend to be lower than those in Hole M0027A (Fig. F38).

Density varies downhole with intervals of lower densities (~1.85 g/cm³) and higher densities (~2.1 g/cm³) (Fig. F37). These variations broadly correspond to lithology: respectively, clay- to silt-rich intervals and sand, glauconite, or cemented intervals. This correspondence to lithology is particularly clear when investigated in combination with porosity (see below).

Calculated porosity

Calculated porosity, here referred to as porosity, is calculated from the difference between wet and dry weights and the dry volume of selected sediment samples (see "Physical properties" in the "Methods" chapter). Some core liners had little drill fluid in them. As a result, sediments with high permeability may have been drained of water during storage and handling. Porosity may therefore be underestimated, particularly that of sands. Based on repeated measurements, we estimate that the porosity of sands may be underestimated by 1%–10% and perhaps up to 19%. Clays and silt with low permeability are not expected to have drained as much. Calculated porosity values vary from 20% to 61%.

Stratigraphically, porosity varies with intervals of low porosity (~40%) and high porosity (~50%–60%). This variation corresponds to lithology variation (Fig. F37). Clay- to silt-rich sediments in general have high porosities, and sand, glauconitic, or cemented sediments have low porosities.

The porosity of clay and silt intervals decreases with depth. The same tendency is observable for sand-rich intervals. Porosity reductions broadly follow the exponential decrease in porosity of sands, silts, and clays that have been observed from a number of passive margins and derived analytically (Sclater and Christi, 1980; Bahr et al., 2001).

Combined density and porosity as a stratigraphic tool

As in Hole M0027A, there is significant inverse variation between porosity and density of samples (Fig. F39). The different lithologies of Hole M0028A fall close to a mixing line between rock (zero porosity and density of 2.7 g/cm³) and saltwater (100% porosity and density of 1.024 g/cm³). Sand sediments and the majority of clay and silt samples plot close to the mixing line between rock and saltwater. Glauconite clearly plots above the mixing line with higher densities because mature glauconite has a density of 2.9 g/cm³ as opposed to ~2.65 g/cm³ for quartz. A significant portion of silt and clay samples plot below the line and have relatively reduced densities.

Downhole, there are significant differences between gamma density and density measured on samples. Gamma density corresponds to the main lithology variations (Fig. F40A, F40B, F40C). There is little separation between density and porosity curves. In this context, it is worth noting that at these shallow depths, porosities of clay and silt are still relatively higher than that of sands (cf. Bahr et al., 2001). As a result, the separation of the porosity and density curves is not fully expressed, as in deeply buried shale and sand successions (Rider, 2006).

A number of packages of gradual overall uphole increasing density and decreasing porosity can be delineated based on the average "log motif" of gamma density and sample-based density and porosity (Fig. F40A, F40B, F40C). The packages are, in general, separated by abrupt density decreases and porosity increases. These packages correspond to large-scale coarsening-upward successions ending in abrupt fining (horizontal arrows; Fig. F40A, F40B, F40C) (see "Paleontology"). Sometimes the top of the packages and the following abrupt density decrease is masked by large fluctuations in density and porosity caused by glauconite and cemented sections (Fig. F40C). Significant boundaries based on the averaged log motif are identified at ~254, 323, 340, 388, 495, 610, and 636 mbsf. (see Table T12 for all key physical property boundaries). A boundary is also present at 553 or 538 mbsf but is difficult to pick because high glauconite content and cementation mask the log motif. A significant proportion of the identified boundaries correspond to flooding surfaces or other important sequence stratigraphic surfaces, as determined by sedimentology and stratigraphic correlators (see "Lithostratigraphy" and "Stratigraphic correlation"). Intervals with abundant phytoclasts show reduced densities.

P-wave velocity

In most of Hole M0028A, velocity measurements on whole cores do not always correlate well with density measurements (Fig. F36). There is considerable scatter and some offsets in the data are due to the variable liner fill and saturation of cores. No downhole sonic log was obtained in Hole M0028A, so this data cannot be used for comparison with the core measurements.

Discrete samples were measured for velocity from 220 mbsf, where coring began, to the base of the hole. Between 220 and 320 mbsf (Fig. F41), velocities are <2000 m/s and are relatively constant, with a slight increase between 290 and 320 mbsf (Subunits IIA–IIC). These trends are matched by MSCL data. At ~323 mbsf (Subunits IIC into IID), velocities observed in both discrete samples and the whole core despite some scatter increase in some cemented sandstone horizons and glauconitic sandstone. Between ~272 and 282 mbsf, MSCL velocities increase in an interval of cemented sandstones, but velocities from discrete samples remain similar to the units above; this is thought to be due to sampling the less indurated parts of sections in this interval. In the sandier units below, only a few discrete samples could be measured. Here, MSCL velocities are low but increase again (with scatter) between 285 and 293 mbsf where sandstone is present.

Throughout Units II and III there is, on average, a slight increase in velocity with depth. At the Subunit IIIA/IIIB boundary, both MSCL velocity and density increase significantly in a carbonate-cemented medium-grained sandstone. In Subunit IIID (513 mbsf), both MSCL velocity and density increase as glauconite appears in the sands. Discrete velocities do not increase significantly in this unit. In Unit V (Section 313-M0028A-129R-2), a sandstone bed is identified by an increase in velocity (also clearly picked out by a change in density, the acoustic image, and conductivity). In the lower part of Subunit VB (~581–599 mbsf), some higher velocities display some correlation with MSCL conductivity, and discrete velocities increase again from ~610 mbsf in the tan-colored silty clays of Unit VI.

Magnetic susceptibility

Magnetic susceptibility was measured at 1 cm intervals and illustrates changes in lithology associated with variations in the nature of magnetic minerals and/or their sizes. Core measurements and log measurements correlate extremely well, although absolute values are different. From 223 to 350 mbsf (Unit II and the top of Unit III), the magnetic susceptibility signal fluctuates from high values to intervals of low values. High-value intervals are described below. From 350 mbsf downhole, the magnetic susceptibility signal is very flat to 516 mbsf, where it increases drastically. This high-value interval extends over 34 m to the base of the glauconitic-rich/-poor sand transition at 550 mbsf. Slight variations then occur in the rest of Unit V to ~610 mbsf. Below this depth, Unit VI is characterized by a high magnetic susceptibility signal with alternating high and medium values.

From the seven main intervals of higher magnetic susceptibility observed from 220 mbsf to the base of Hole M0028A, four occur within Unit II through Subunit IIIA, followed by generally low magnetic susceptibility until Subunit IIID, where glauconite reappears in the sediments. The first interval of high magnetic susceptibility occurs in Subunit IIA, with a maximum at ~235 mbsf. In this interval, there are some glauconitic sand, glauconitized burrows, and also some sulfide (possibly the magnetic sulfide mineral, greigite). This unit may be similar to the clays of Unit II in Hole M0027A. Around 275 mbsf and again at 325 mbsf (Subunits IIIB and IIID), increased magnetic susceptibility is clearly related to increased glauconite in the sediments. A smaller peak is evident in Subunit IIIA, again due to glauconite. At ~515 mbsf, magnetic susceptibility is again high and the sediment glauconite rich. Susceptibilities decrease slightly below the glauconite layer. A large broad peak in susceptibility around 615–660 mbsf (Unit VI) is not related to glauconite. Figure F42 shows the relationship between K, K/Th ratios, and magnetic susceptibility, along with the glauconite content estimated by the sedimentologists.

There is a linear correlation between density and magnetic susceptibility in several intervals (Fig. F43). Glauconite was identified in all of these intervals. Linear correlations probably represent a mixing line between sediment rich in mature glauconite with high density (2.9 g/cm³) on the one hand and high magnetic susceptibility (~120 SI) and with siliciclastic sediment with a lower density (2.5 g/cm³) and very low magnetic susceptibility (a few SI) on the other. Glauconite's influence on density is also seen in the density–porosity crosscorrelation (above and in Hole M0027A).

Electrical resistivity

Resistivity is affected by a number of variables, including lithology, pore fluid and salinity, and core liner saturation. In general, there appears to be a decrease in the amount of drill fluid saturation of the core liner down a recovered core. As a result, the last section of every core has very high resistivity. Therefore, all Section 3s (generally short sections) were removed from the analysis below. Most very short sections have low core liner fluid saturation. High-pass filtering attempts to remove this effect (Figs. F37, F40).

Resistivity is highly variable from low values of ~0.3 Ωm to high values of 6 Ωm. High resistivity is encountered at a number of levels that are cemented. Low resistivity (~0.7 Ωm) is registered between 220 and 414 mbsf, with brief intervals of higher resistivity. From 415 mbsf downhole, background resistivity gradually increases from ~1.2 to 1.6 Ωm. This interval also has frequent brief increases in resistivity. The overall variability is greater below 415 mbsf than above.

Overall, the correspondence between resistivity measured on cores and the chlorinity of formation waters or lithology is not as clear as in Hole M0027A (see "Downhole measurements").

Digital linescans and color reflectance

All cores from Hole M0028A were measured using an X-ray fluorescence (XRF) digital linescan system. Data were recorded at a resolution of 0.068 mm as both images and as red-green-blue (RGB) values down the core centerline. All suitable cores were also scanned for color reflectance with the Minolta logger. Color reflectance data may reflect changes in such factors as glauconite or iron content by analysis of a specific wavelength or a*/b* ratios. The scatter in the data set brought into question the suitability of the core section for this measurement (see "Physical properties" in the "Methods" chapter). Efforts were made to compare the changes of color reflectance as measured by the Minolta logger with other measurements and observation, but results still need improvements.

Thermal conductivity

Thermal conductivity measurements are scattered down to 300 mbsf, with lower values around 1.4 W/(m·K). Scattering does not allow easy interpretation of the variation. Below 300 mbsf, thermal conductivity to a first order is related to the chlorinity of interstitial waters. From 315 to 420 mbsf, high thermal conductivity fits with high electrical conductivity and salty interstitial waters (Fig. F41). Thermal conductivity decreases to 1.4 W/(m·K) at 460 mbsf, following the decrease in chlorinity of the interstitial waters. Electrical conductivity does not follow this trend. Below 520 mbsf, high thermal conductivity values again fit with the variation of both chlorinity and electrical conductivity. The low spatial resolution of thermal conductivity precludes identification of detailed chlorinity spikes. Comparison with downhole conductivity measurements and chlorinity is discussed further in "Downhole measurements."

Natural gamma radiation and core-log correlation

NGR was measured on all cores, primarily for core-log correlation. Data collected are equivalent to the total gamma ray (TGR) downhole log, although absolute values are different. Gamma ray trends are described in "Downhole measurements." In general, correlation between the core and log data sets is excellent, with only minor (<1 m and often <0.4) adjustments required in some intervals toward the top of the cored section (see "Stratigraphic correlation"). Correlations show a most straightforward match where there are both good core recovery and distinctive peaks in downhole and core/MSCL data sets.

The correlation between core depths and logging data was verified using other petrophysical data sets, like magnetic susceptibility.

Petrophysical surfaces and intervals

Important surfaces and intervals can be identified based on pronounced or sharp changes in trends in one or more of the petrophysical measurements. Many of the abrupt shifts correspond to surfaces or intervals with trend changes in wireline logs. A preliminary summary of the most important surfaces or intervals are given in Table T12. Some surfaces and intervals correspond to important surfaces, as determined from sedimentology and/or seismic data (see "Lithostratigraphy" and "Stratigraphic correlation").

A synthesis of petrophysical data and downhole logs and derived quantities of each lithostratigraphic unit are presented at the end of "Downhole measurements."

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