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doi:10.2204/iodp.proc.303306.105.2006

Downhole measurements

Logging operations

Downhole logging was performed in Hole U1305C after it had been drilled to a depth of 287.1 mbsf and displaced with sepiolite mud (see “Operations”). The pipe was initially set at 95.3 mbsf. Prior to logging, it was expected that two tool string configurations would be run, the triple combo with the Multi-Sensor Spectral Gamma Ray Tool (MGT) and the FMS-sonic (see “Downhole measurements” in the “Site U1302–U1308 methods” chapter). However, because of the problems detailed below, only the triple combo-MGT was deployed successfully. An impassable bridge was encountered at ~265.9 mbsf. A successful main pass was made from this depth to above the seafloor. In an attempt to save time, it was decided to run only a shortened repeat pass, and thus the tool string was lowered to ~204.2 mbsf. As the tool string approached the base of the pipe, the caliper arm failed to close, so logging was stopped. After many attempts to close the caliper, it was clear that it was failing to respond. Furthermore, it became clear that the tool could not be pulled, even partially, into the pipe and was probably trapped by the float valve. After ~3 h of pumping and altering the position of the pipe, the tool string eventually reentered the pipe and was pulled back to the surface. On the basis of the problems encountered downhole (most likely caused by the moderate to large heave) and an assessment of the state of the hole, it was decided to terminate logging operations. Inspection of the wireline revealed that it had indeed become trapped in the float valve. The caliper arm was missing and had most likely been lost in the open hole immediately prior to the first attempt to reenter the pipe. Details of the intervals logged and the position of the drill bit are shown in Figure F34. During logging operations, weather was poor and the sea state was moderate to rough with a typical heave of 2.5–3.5 m and peaks of >4 m. The wireline heave compensator was used throughout the logging operations in the open hole.

Data quality

The caliper data show that the diameter of the borehole ranged from ~13.6 to 18.0 inches (Fig. F35), resulting in data of variable quality. Reproducibility of data is high between passes (see gamma ray example in Fig. F35). The density and porosity tools require good borehole contact, and intervals with a large borehole diameter are characterized by high porosities and low densities (Fig. F36). Density and porosity data are also less reliable when the caliper is not open (i.e., above ~107 mbsf during the main pass).

Results

The downhole logging data suggest that the formation is fairly uniform in the open hole. As expected, the density and porosity data are generally inversely related to each other and show downhole trends of increasing density and decreasing porosity. Resistivity values are low, reflecting the generally moderate- to high-porosity sediments. Photoelectric effect factor (PEF) values range between 1.0 and 3.3 b/e^–, consistent with the clay-rich lithologies (see “Lithostratigraphy”). Extremely low PEF values (<1.8 b/e^–, the PEF value of pure quartz) may be the result of poor contact with the borehole wall or extremely porous intervals (seawater PEF = 0.807 b/e^–).

Gamma ray values increase slightly with depth, possibly as a result of increasing clay content (Fig. F37). The low uranium content of the formation results in very similar total spectral gamma ray (HSGR) and computed gamma ray headspace ([HCGR], summation of Th and K gamma rays only) values. The uranium data suggest that TOC values in the logged interval are consistently very low, as shown by discrete samples (see “Geochemistry”). Potassium and thorium display very similar trends downhole, suggesting that there are no major downhole changes in mineralogy.

Core-logging comparisons

All the downhole data sets display meter- to decimeter-scale variability that are most likely the result of subtle changes in lithology. A comparison of logging- and core-derived NGR and density records shows close agreement in downhole trends and patterns (Fig. F38). Measured density values are very similar in both core and logging data. Closer inspection of the gamma ray data suggests that 5 m scale patterns can be recognized in both the core and logging records (Fig. F38). Using the downhole logging records as a depth reference, it will be possible to map core measurements to equivalent logging depths using the software program Sagan to more precisely determine the amount of core expansion.

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