Previous   |    Next

doi:10.2204/iodp.proc.308.103.2006

Physical properties

At Site U1319, laboratory measurements were performed to provide a downhole profile of physical properties at a reference site in Brazos-Trinity Basin IV. Fractures and voids that resulted from gas expansion, especially between 60 and 120 mbsf, degraded MST measurements. P-wave velocity logger (PWL) measurements were aborted after logging four cores unsuccessfully.

Density and porosity

Gas expansion and elastic recovery of clays were observed on the cores obtained at this site from 30 mbsf and deeper. This phenomenon reduces the bulk density from that in situ and, consequently, porosity is overestimated relative to in situ conditions. This error is assumed to be relatively small, and the data remain useful for interpretation and correlation. The downhole trends in gamma ray attenuation (GRA), measurement while drilling (MWD), and moisture and density (MAD) profiles correspond well (Fig. F34A). However, GRA densities are slightly higher in the uppermost 30 mbsf and slightly lower below 70 mbsf (the difference may be as much as 0.2 g/cm³). GRA density data also show considerably more scatter at all depths (Fig. F34A). Results from previous ODP legs typically exhibit higher MAD bulk densities compared to GRA densities. These differences have been attributed to differences in core diameter (e.g., Moore, Taira, Klaus, et al., 2001), but at this site the role of gas expansion cannot be disregarded. Compared to the MAD data, the MWD data underestimate density in the uppermost 30 mbsf, probably because of the larger borehole diameter in this section. MWD densities are slightly higher than MAD bulk densities below 70 mbsf. The lower MAD densities may be explained by expansion of the cores in the laboratory.

The following interpretation of density variations is based on MAD data. Bulk densities in lithostratigraphic Unit I (0–3.33 mbsf) increase rapidly with depth from 1.32 to 1.8 g/cm³ (Fig. F34A). Grain densities vary from 2.41 g/cm³ at the top of the unit to 2.74 g/cm³ at the bottom (Fig. F34B). The lowest values are probably due to the presence of organic matter that has a lower grain density relative to minerals. Porosities rapidly decrease from 81% at the top to ~60% at the bottom of this unit (Fig. F34C).

Low bulk densities in lithostratigraphic Unit II (3.33–17.25 mbsf) are correlated to low grain densities (e.g., 2–2.5 g/cm³) (Fig. F34). These grain densities were remeasured and the low values were confirmed. This change in grain density corresponds with lithostratigraphic Subunit IIB, a homogeneous black clay, rich in organic matter (see “Lithostratigraphy”). A bulk density peak at 1.6 g/cm³ in lithostratigraphic Subunit IIC correlates with the presence of silty laminae. Lithostratigraphic Subunit IID and Units III and IV show gradual increases in bulk density from 1.55 to 1.65 g/cm³ down to 26 mbsf, corresponding to additional silt laminae and thin beds. This increase in density is mirrored by a decrease in porosity from ~70% to 62% (Fig. F34C).

In Lithostratigraphic Unit V (29.5–31 mbsf) bulk densities measured by MAD, MST, and MWD all recorded sharp increases from ~1.59 g/cm³ at the top to ~1.75 g/cm³ at the base. There is fairly wide scatter in grain density measurements including a peak value of 2.78 g/cm³. Porosity declines rapidly from ~70% at the top to ~60% at the base, a 10% drop in only 1.5 m.

In lithostratigraphic Unit VI (31–156 mbsf), bulk densities increase from 1.57 to 1.99 g/cm³ (Fig. F34A). As density increases, porosity decreases from 65% to 44.5% within this unit (Fig. F34C), with a single major shift occurring at ~80 mbsf, where density is down to 0.1 cm³ lower than surrounding values. Grain densities range from 2.62 to 2.8 g/cm³, but the overall trend remains near constant around a mean value of 2.74 g/cm³ (Fig. F34B). Cores 308-U1319A-14X through 18X were cored using the XCB. MAD and GRA bulk densities are lower than MWD bulk densities in these cores (Fig. F34A). The difference between measurements shows that coring methodology may affect bulk density.

Noncontact resistivity

Noncontact resistivity (NCR) increases with depth in lithostratigraphic Units I, II, and III, with local resistivity peaks (Fig. F35A) that correlate to silty sediments (see “Lithostratigraphy”). In lithostratigraphic Units IV and V, NCR increases to 1.4 Ωm. This increase is followed by a slight decrease at ~40 mbsf near the top of lithostratigraphic Unit VI. NCR is constant at ~0.9 Ωm to TD. This pattern mirrors the porosity pattern derived from MAD data (Fig. F34C).

Magnetic susceptibility

Magnetic susceptibility increases from ~17 × 10^–5 to ~30 × 10^–5 with depth in lithostratigraphic Unit I (Fig. F35B). Lithostratigraphic Unit II has relatively constant values (~30 × 10^–5 SI) with peaks superimposed. Two of these peaks correlate with silt layers (see “Lithostratigraphy”). Mudstones in lithostratigraphic Units III, IV, and V show variable magnetic susceptibilities centered at ~20 × 10^–5 SI. In lithostratigraphic Unit IV, peaks correlate with silt and sand layers.

At the top of lithostratigraphic Unit VI (31–156 mbsf), magnetic susceptibility sharply increases and then remains relatively constant, with mean values centered at ~60 × 10^–5 SI (Fig. F35B). This unit contains wide scatter, especially in the lower part of the section below 80 mbsf, where the lithology log shows alternation between reddish brown and greenish gray clay layers with interspersed pyritic dark gray clay layers. The magnetic susceptibility lows seem to be associated with the reddish brown clay layers.

Thermal conductivity

Thermal conductivity increases with depth (Fig. F36A). Measured thermal conductivities at 68 mbsf and below 117 mbsf are not considered in the analysis because these values are almost equal to the thermal conductivity of water at 25°C (0.58 W/[m·K]), which is considered too low in view of the sediment densities and porosities. Most likely the probe was not in full contact with the sediment and thus did not accurately measure thermal conductivity of fully saturated sediments. The resolution of measurements is low and data are relatively scattered, oscillating between 0.75 and 1.24 W/(m·K) (Fig. F36A).

P-wave velocity

Only velocity measurements above 30 mbsf were reasonable. Below this depth, voids within the core due to gas expansion compromised the measurements. Velocity measurements in the upper 15 m of cores, measured both longitudinally (PWS1) and perpendicular to the core axis (PWS2), are close to the velocity of water, reflecting high porosity (Fig. F36B). P-wave velocities increase with depth, and a small peak corresponds to the more silty material of lithostratigraphic Subunit IIA (see “Lithostratigraphy”).

The x-axis (PWS3) measurements are lower than the y- (PWS2) or z- (PWS1) axis measurements (Fig. F36). These low values likely result from bad picks of the first arrival wavelet.

Shear strength

Undrained shear strengths increase with depth (Fig. F37A). The trend is approximately linear in the uppermost 80 mbsf, with a maximum value of 40 kPa and then suddenly increases and displays more erratic behavior, with values ranging from as high as 90 to as low as 40 kPa. Variations in peak shear strength increased when the coring method changed from APC to XCB, implying that the XCB coring process strengthens core material relative to the APC coring process. Peak shear strength measurements by both automated vane shear [AVS] and pocket penetrometer are quite similar (Fig. F37B). The sedimentary sequence below 80 mbsf appears to have low undrained shear strength in several intervals, possibly linked to the presence of weaker layers. The irregular undrained shear strength pattern observed in lithostratigraphic Unit VI is probably associated with a change in lithology in the lowest part of this unit (see “Lithostratigraphy”). The lower part of lithostratigraphic Unit VI alternates between greenish gray, reddish brown, and pyritic dark gray clay layers. Low sensitivity shows that most of the tested layers within the drilled section are nonsensitive (Fig. F37C). The high sensitivity observed at 147 mbsf is due to low residual shear strength.

The ratio between the measured undrained shear strengths and the vertical hydrostatic effective stresses derived from the sediment bulk densities and unit weights ranges between 0.025 and 0.1 (Fig. F38A). The increase in variation from ~115 mbsf downhole is probably due to disturbance by XCB coring. These ratios are relatively low, especially when compared to the values observed in the uppermost sediment column for conventional piston cores.

Summary

Bulk densities generally increase linearly with depth from 1.3 to 2.0 g/cm³. Grain densities show relatively constant values (~2.7 g/cm³), except in the upper part of the sediment column where the presence of sediment with high organic matter content causes low densities. NCR and magnetic susceptibility also match variations in porosity and grain size.

Porosity measurements at this site show an approximately exponential trend with depth, with values decreasing from 80% to 45%. NCR measurements mirror porosities derived from MAD measurements. Silt-sand layers interbedded with clay are recognized in the NCR profile.

In lithostratigraphic Unit II, peak values of magnetic susceptibility are correlated to black clays containing pyritic minerals. In clay layers of lithostratigraphic Units III–VI, peak values correspond to silt and sand layers, indicating a change in mineral composition between these layers and the embedding clay.

Thermal conductivity values at Site U1319 are relatively low and the general trend is an increase with depth. Thermal conductivities increase from 0.75 to 1.24 W/(m·K) between 0 and 117 mbsf.

At Site U1319 undrained shear strengths tend to increase almost linearly with depth from 2 to ~80 kPa, as expected. Below 80 mbsf the sedimentary sequence is characterized by alternating clay layers of different colors, and undrained shear strength values show a more scattered pattern.

Top of page   |    Previous   |    Next