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doi:10.2204/iodp.proc.339.109.2013

Downhole measurements

Logging operations

After the last core from Hole U1391C arrived on deck, the hole was prepared for logging. Following a wiper trip, the hole was flushed with sepiolite mud and displaced with 248 bbl of barite-weighted heavy mud (10.5 ppg). The pipe was set at 98.9 mbsf. Two tool strings were deployed in Hole U1391C, the triple combo and FMS-sonic (Fig. F34; see also “Downhole measurements” and Table T6 in the “Methods” chapter [Expedition 339 Scientists, 2013b] for tool acronym definitions).

The triple combo tool string started downhole at 0335 h on 15 January 2012. The Hostile Environment Natural Gamma Ray Sonde (HNGS), High-Resolution Laterolog Array (HRLA), and Hostile Environment Litho-Density Sonde (HLDS) were included. The Accelerator Porosity Sonde was omitted because it allowed the HNGS to be placed at the bottom of the tool string to record to the base of the hole. The tool string reached the base of the hole at 668 mbsf, and the main pass was logged up to seafloor. FMS-sonic tool string rig-up started at 0920 h, and the tool string also reached close to the bottom of the hole at 666 mbsf. The Pass 1 uplog extended to the seafloor, but bridged sections were noted from 317 to 324 and 330 to 334 mbsf. The hole was closing up, so Pass 2 was run up from 314 mbsf.

Maximum ship heave was generally 1 to 1.5 m maximum peak-to-peak, but reached 2 m at times. The tide was high during the triple combo run (+0.7 to +1.2 m) and rising during the FMS-sonic run (–0.8 to +0.7 m) (Fig. F35). The sediments were Pliocene–Pleistocene nannofossil muds and silty muds (see “Lithostratigraphy”). The wireline heave compensator was used to counter ship heave during logging.

Log data quality

In Hole U1391C, the borehole wall was very rugose, with many narrow washouts often exceeding 18 inches wide, reducing log data quality (Fig. F36). Some caves were found that were generally associated with low values in all the logs, for example, a large washout from 310 to 320 mbsf. Numerous 1–3 m thick narrow bridged sections with diameters <6–8 inches (bit size = 9.875 inches) were also found and were mainly concentrated in the middle section of the hole, from 245 to 390 mbsf. Of the all the logs, resistivity and sonic velocity were the least sensitive to variable hole diameter, although some reduction in values in the larger washouts is evident. The NGR log anticorrelates strongly with the caliper log. Density was highly affected, giving density values close to water density in washouts (Fig. F36). The photoelectric effect log gave anomalously high values below 390 mbsf, especially at washouts, because of the barite-weighted mud. FMS resistivity images were also dominated by poor contact with the borehole wall in the wide areas, although some intervals of good images are present.

Logging units

The baseline values of the Hole U1391C downhole logs change gradually downhole, with no major steps in the base levels. The entire logged interval was thus assigned to one logging unit (Fig. F36). At the scale of this unit, the NGR signal ranges on average from 30 to 65 gAPI, with peak values reaching 85 gAPI. The signal shows moderately high amplitude variability on a several-meter to submeter scale, and given the sedimentological context (see “Lithostratigraphy”), is primarily tracking clay content. This interpretation is supported by close correlation of the potassium and thorium logs (Fig. F37). Uranium generally contributes a relatively minor component to the total NGR signal and generally behaves as an independent constituent compared to potassium and thorium because it is not chemically combined in the main rock-forming minerals. The sonic velocity log increases downhole (Fig. F36), reflecting sediment compaction with depth, and generally co-varies with the NGR log. The logging unit is divided into two subunits, mostly on the basis of a relative absence of high NGR values below 562 mbsf and a small step down in the density, resistivity, and sonic velocity logs at this depth (Figs. F36, F37).

Logging Subunit 1A: base of drill pipe to 562 mbsf

Logging Subunit 1A is characterized by medium-amplitude alternations in bulk density, NGR (and its uranium, thorium, and potassium components), density, and sonic velocity values (Fig. F36). Several orders of cycles are observed in the NGR curves, varying from one to several tens of meters in thickness (Fig. F37). Potassium and thorium concentrations co-vary closely, suggesting that clay content controls these logs. The uranium concentrations correlate to potassium and thorium at some depths or vary independently at others. As expected from downhole compaction, the density and sonic velocity logs have an increasing downhole trend and are generally well correlated. These logs also correlate well with the resistivity curve at a medium scale (several tens of meters). Logging Subunit 1A at Site U1391 resembles logging Subunit 1A observed at Sites U1389 and U1390 (see “Downhole measurements” in the “Site U1389” chapter [Expedition 339 Scientists, 2013e]).

Logging Subunit 1B: 562–668 mbsf

Subunit 1B is distinguished from the subunit above by the presence of lower NGR values (Figs. F36, F37). In particular, two intervals with low NGR values are observed from 562 to 582 and 593 to 605 mbsf. These intervals suggest the presence of two several-meter thick sandy intervals that seem to correspond with intervals of low core recovery. A thin layer of high resistivity, high density (reaching 2.52 g/cm³), high velocity, and low NGR values is observed at 631 mbsf (Fig. F36). This high-resistivity horizon is caused by a well-consolidated dolomite mudstone identified in Core 339-U1391C-29R (see “Lithostratigraphy”).

Heat flow

Ten advanced piston corer temperature tool (APCT-3) downhole temperature measurements in Holes U1391A and U1391B ranged from 11.8°C at 32.0 mbsf to 13.3°C at 145.95 mbsf (Fig. F38; Table T18), giving a geothermal gradient of 14.2°C/km (Fig. F39). The measurements increase linearly with depth, and the trend line intersects the seafloor at 11.35°C. Some scatter and minor differences between the two holes is evident. These differences are probably caused by (1) uncertainty in fitting to the APCT-3 temperature equilibration curves to obtain the formation temperature and (2) lateral heterogeneity in heat flow leading to small temperature differences between the holes. Readings at shallower depths have more uncertainty because the APC barrel has more freedom of movement in the hole where the sediment is soft. Unlike at previous sites, the bottom water temperature was repeatably constrained from the APCT-3 temperature profiles (Fig. F38), both while the APC was held at the mudline (~11.6°C) and the minimum temperature in the profiles (~11.5°C), similar to, but a little higher than the estimate from the intersection of the linear temperature trend with the seafloor.

Thermal conductivity under in situ conditions was estimated from laboratory-determined thermal conductivity using the method of Hyndman et al. (1974) (see “Physical properties” in the “Methods” chapter [Expedition 339 Scientists, 2013b]). The calculated in situ values average 1.2% below the measured laboratory values. Thermal resistance was then calculated by integrating the inverse of in situ thermal conductivity over depth (Fig. F39). A heat flow of 17.5 mW/m² was obtained from the linear fit between temperature and thermal resistance (Pribnow et al., 2000). This value is very low compared to heat flow values for the nearby Marques de Pombal escarpment (Grevemeyer et al., 2009).

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