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doi:10.2204/iodp.proc.303306.114.2006

Downhole measurements

Logging operations

After a successful CORK deployment in Hole U1315A on the Vøring Plateau, downhole logging operations were carried out in a nearby legacy hole from ODP Leg 104, Hole 642E. Hole 642E had been drilled in 1985 to a total depth of 1229 mbsf (~40% recovery) with the upper 370 m cased into basalt basement. The entire drilled sequence consists of Eocene-age interbedded lava flows and volcaniclastics with estimates of up to 135 lava flows. The main operational goal was to obtain a good downhole temperature profile at this site because one could not be obtained at the new CORK site. In order to not disturb the hole, no hole preparation (i.e., wiper trip) or mud was circulated through the hole prior to logging. A secondary objective was the relogging of a legacy site to both evaluate hole conditions after 20 y and to use a new generation of downhole tools, particularly the FMS imaging and sonic tools. The drill pipe was set just ~15 mbsf (1304 mbrf) prior to logging. During logging operations, the sea state was very calm with a typical heave of 1 m or less (see “Operations”). The Lamont-Doherty Earth Observatory-Borehole Research Group (LDEO-BRG) wireline heave compensator (WHC) was used throughout the logging operations in the open hole.

The plan was to use two tool string configurations, the triple combo tool string with an additional General Purpose Inclinometer Tool and the FMS-sonic tool string (see “Downhole measurements” in the “Sites U1312–U1315 methods” chapter). The TAP tool was deployed with the triple combo tool string and we logged down slowly, stopping every 5–10 m over the upper 100 m, and then logged continuously at 1800 ft/h to total depth of 588 mbsf. While collecting the downhole temperature data, we also logged down with the triple combo tool string. At 588 mbsf, we reached an impassable obstruction and stopped the downhole logging. We then logged the hole up into casing to a depth of 335 mbsf. Details of the intervals logged are shown in Figure F4.

After the triple combo tool string, the FMS-sonic tool string was also deployed to ~580 mbsf after again reaching the same hole obstruction as before. The second pass of the FMS-sonic tool string was only able to reach a total depth of ~440 mbsf before reaching an obstruction. So, a shortened second run was made from that depth into casing until 310 mbsf. Upon finishing the main logging operations, testing of new software for the Schlumberger WHC system was carried out to continue performance improvements made earlier during Expedition 306. After rigging down all logging tools, we lowered the empty instrument housings (with new O-rings) of the MGT that had leaked previously at Site U1313 to the seafloor (1277 mbsf) for testing. The housings remained dry at pressures equivalent to 2500 psi (four times more than when they had leaked at the previous site). Both the MGT and the housings appear to be sound with possibly a faulty O-ring responsible for the earlier failure.

Data quality

The temperature data collected by the TAP tool yielded detailed results based on ~3 h of continuous down logging (3600 measurements/h). The temperature profile results are discussed in detail below. Initial examination of the logging data from the triple combo tool string showed good quality data for several logs including natural gamma radiation, density, resistivity, and porosity. The centimeter-scale results from the FMS resistivity imaging were quite good and should be very useful in delineating flow boundaries, fracture densities, lithology, and the relationship to physical properties. The sonic waveform data appear to be of good quality, but analysis of waveforms will be performed postcruise onshore and will not be discussed further here.

The hole conditions, despite short intervals of obstructions, were generally fairly good. The intervals with obstructions could be clearly linked with high-porosity, low-resistivity zones in volcaniclastic layers scattered through the sequence. This is supported by the caliper data, which show that the diameter of the borehole (drill bit size = 9.75 inches) ranged over just a few inches from ~10 to 15 inches over the entire interval (Fig. F5). The density and porosity tools require good borehole contact and are held against the borehole wall by an eccentralizer that is only effective in the open borehole below the drill pipe and casing. Density and porosity data are also less reliable when the caliper has been closed before the tool string enters the base of the casing (i.e., above ~370 mbsf) (Fig. F5). Resistivity data from the formation are quite consistent between shallow (spherically focused resistivity), intermediate (medium induction phasor-processed resistivity), and deep (deep induction phasor-processed resistivity), indicating that the tool is getting reliable results.

Results

The downhole logging data suggest that the formation is made up of two main lithologies that can be delineated in almost every downhole log. The individual basalt flows have lower density at the top (~2.0–2.2 g/cm³) and higher density at the bottom (2.6–2.9 g/cm³). The pattern in resistivity appears to mimic the density with higher resistivities (20–200 Ωm), corresponding to higher densities. The peaks in gamma ray data correspond with low-density and high-porosity volcaniclastic intervals and relatively low resistivity values (<20 Ωm). The total spectral gamma ray (HSGR) data show that these higher gamma ray intervals are characterized by up to 1.5 ppm U and Th and up to 1 wt% K, probably driven by higher clay contents.

As is usual, the density and porosity logs are generally inversely related to each other and show a cyclical pattern of increasing density across lava flows, as noted above, and decreasing porosity (50%–10%). Intervals with very high porosities (>80%) correspond to the volcaniclastic layers. Photoelectric effect factor values increase from 3.0 b/e^– (clay-rich) in the high porosity-low density-low resistivity intervals to as high 5.5 b/e^– in the fine-grained basalts, consistent with the lithologies. Gamma ray values from the HSGR log vary between 3 and 35 gAPI throughout the entire 220 m interval (Fig. F6).

Log(new)-log(old) comparisons

An important part of revisiting this ODP legacy site is an evaluation of hole conditions after 20 y. Caliper data provide a quick measure of changes in hole shape that may yield important information related to weathering downhole. The rotary bit size used for coring this site was 9.75 inches. The original caliper log is compared against two calipers from the FMS tool (Fig. F7). As can be seen, the original caliper (density tool) was not very reliable in showing a much larger than bit size hole for almost the entire length of the cored interval. What might be useful information from the old caliper data is the lack of any zones showing intervals much larger than 12 inches, as in the new caliper logs. Most of the intervals with hole sizes larger than 12 inches in the new caliper logs correspond to high-porosity, low-resistivity zones.

A comparison of porosity logs shows a very good correlation downhole. The overall variability of porosity is much larger (10%–95%) than the original measurements (15%–70%) (Fig. F7) and is attributed to a more sensitive porosity sonde. In combination with detailed FMS resistivity measurements and imaging and sonic data, it should be possible to obtain reliable permeability estimates. Understanding the permeability should allow better understanding of fluid flow and temperature gradients observed in the borehole.

Measured total gamma ray data from the old and new logs in Hole 642E are generally close overall; however, the original Th portion of the spectral gamma ray logging data showed typically more variable and lower Th in the formation relative to U and K. The new logs show Th and U values almost equal and more consistent, if not higher than Th, across the interval we examined (Fig. F7). This may be attributable to how the energy windows were processed or improvements in the natural gamma tool. Density logs (not shown) from both studies also appear to be reliable between the two data sets, with most values ranging between 2 and 3 g/cm³.

Whereas the basic physical property logs from the triple combo tool string confirmed the previous results in Hole 642E, most new information will come from the FMS-sonic tool string, which was not available (FMS) or has been significantly upgraded (sonic). FMS imaging of the hole yielded good measurements and will allow easy correlation to existing core data and filling in the gaps (~60% of the formation). Examples from the volcaniclastic and fine-grained basalt intervals are shown in Figures F8 and F9, respectively. The fine-scale (centimeter) resistivity data will allow high-resolution studies of fracture density of basalts and porosity within the sequence. Combined with new shear wave data from the sonic tool, it should be possible to construct more reliable permeability estimates as well as revised synthetic seismograms that may yield better depth-velocity correlations.

Temperature log in Hole 642E

A temperature log (Fig. F10) was obtained in Hole 642E using the LDEO-BRG TAP tool. This tool logs at a rate of 1 Hz, has a precision of 5 mK, and has an accuracy of 1 K. The temperature was logged on the downhole run. The TAP tool was held off of the seafloor for a few minutes and indicated a bottom water temperature of ~0.2°C. The top 10 m of the borehole has a very steep thermal gradient (~2500°C/km). Below this section, the borehole has a relatively low gradient of ~22°C/km. The borehole is cased to a depth 370 mbsf. At a depth of ~500 mbsf, a positive temperature excursion may indicate in-flow. This excursion may correlate with a high-permeability zone indicated in the other logs. The temperature log as a whole indicates significant fluid discharge that may be as much as tens of meters per year.

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