Downhole measurements were made after completion of APC/XCB coring in Hole 1167A and while drilling Hole 1167B. During coring, operational problems were experienced with the drill pipe sticking and the lockable flapper valve jamming with sediment. Hole conditioning involving a wiper trip and circulation of 195 barrels of sepiolite mud was therefore undertaken prior to logging. Despite this precaution, however, similar problems hampered wireline operations in Hole 1167A.
The triple combo tool, which measures resistivity, density, porosity, and natural gamma, was the only tool string run in Hole 1167A (Fig. F40; Table T12). While exiting the pipe during the initial run into the hole, the triple combo got stuck with only 10 m of the tool string protruding out of the base of the pipe. It was likely that the lockable flapper valve had not latched open properly when the go-devil was pumped downhole. The problem was resolved by rigging up the circulator and increasing the pump pressure until the tool string eventually came free. After these initial difficulties, the triple combo tool string was lowered to 151 mbsf, where its passage downhole was halted by an obstruction. A conglomerate interval was observed in the cores at this depth, and the drillers had noted sticking just below this depth while pulling pipe for both the wiper trip and while waiting for an iceberg to pass.
The hole was logged from 151 mbsf up to the base of pipe at 85 mbsf, covering an interval of 66 m, without extending the HLDS caliper arm. The depth to the seafloor was determined to be 1649 mbrf from the decrease in gamma-ray values at the sediment/water interface (driller's mudline depth = 1651.3 mbrf) (Table T12).
High tension was recorded at the head of the tool when the tool string entered the pipe, even with continued pumping. Because of time constraints, poor hole conditions, problems encountered with the lockable flapper valve, and the high probability of our encountering further difficulties, we decided to switch to LWD.
LWD operations were carried out in Hole 1167B, offset by 50 m from Hole 1167A. The aims of LWD/MWD at this hole were twofold: to record spectral gamma and resistivity logs with the compensated dual resistivity (CDR) LWD tool and to record weight on bit and other drilling parameters for the engineering experiment on the efficacy of the passive heave compensation system. The spudding of Hole 1167B was delayed for 4 hr by a storm; spudding with LWD tools required low heave because the tools were narrower and hence weaker than the drill collars above them. Low pump rates were used for the top 17 m of the hole, so looser sediment was not washed away to leave a wide hole that would have degraded resistivity and natural gamma readings. However, measurement-while-drilling (MWD) communications required a higher pump rate of 70 spm (350 gal/min), so below 17 mbsf, the pump rate was increased and the MWD tool began communication. The data transmission rate was 3 bits/s, and the frequency of the mud pulse was 12 Hz; no problems were experienced with MWD data transmission. The hole was drilled to 261.1 mbsf at an average rate of 22 m/hr.
For the engineering experiment, data were recorded under a range of operating conditions from low weight on bit in softer formations to 25 KlbF (1000 lb force) in harder formations in the deeper parts of the hole. The amplitude of the heave varied considerably during the course of drilling.
Total gamma-ray emission and resistivity data from the CDR were downloaded from the tool when it came back to the surface. The data were then processed by the Anadrill engineer to move the data from evenly spaced time intervals to a depth scale. Correlation with the overlapping interval of wireline logs was mostly good (Fig. F41). Differences can be attributed to washouts in the wireline-logged hole that caused intervals of anomalously low natural gamma, resistivity, density, and porosity logs. LWD measurements are made only minutes after the hole is drilled, so the borehole is likely to be in good condition for logging. Although the total natural gamma data from the CDR were good, the logs derived from the natural gamma spectrum (potassium, thorium, and uranium) required further processing.
Unit 1 is a clay-rich unit, corresponding to lithostratigraphic Unit I. It is characterized by slightly higher gamma-ray values and lower resistivity values relative to the underlying unit. Resistivity values are low at the surface because the sediments are soft and less lithified; however, a relatively rapid increase in resistivity with depth is observed as a result of compaction.
A general compaction trend of increasing resistivity values is observed with depth. Clay-rich beds can be detected as drops in the resistivity values (Fig. F42). The gamma-ray log highlights the presence of a more radioactive interval at 62-90 mbsf, coincident with a red-colored interval seen at 65-84 mbsf. The lower radioactivity levels observed between 90 and 120 mbsf and from 215 to 255 mbsf may correspond to a decrease in the proportion of granitic clasts or a change from a clay-rich to a sandier matrix.
This subunit is characterized by relatively low gamma-ray values and increasing resistivity with depth. There are thin intervals with low resistivity and low gamma-ray values at 18, 35, and 41 mbsf. A clay bed was observed in Core 188-1167A-5H, and since clay beds generally have a lower resistivity than diamict, we are inclined to interpret the resistivity lows as clay beds within the diamict. However, clay layers might not be expected to have the observed lower gamma-ray values because they contain radioactive potassium and thorium. On the other hand, it is quite likely that K-feldspars and heavy minerals contribute significantly to the natural gamma signal at this site, so that an increase in clay minerals would have a minor effect on the logs. Possible alternative lithologies for the low-resistivity intervals are clay with microfossils, silty or sandy clay, or perhaps gravels.
This subunit has higher gamma-ray values than the subunits above and below; it contains no major drops in resistivity. Red-colored beds are observed in the core from this interval.
This subunit is similar to Subunit 2a, apart from having generally higher resistivities attributed to compaction with depth. It contains several drops in resistivity; those at 213 and 215 mbsf were observed in Core 188-1167A-25X as clay beds. In contrast to Subunit 2a, these resistivity lows are accompanied by natural gamma highs (Fig. F42). A major interval of resistivity lows (116-127 mbsf) is found in a zone of zero core recovery. Conglomerate and sand beds were observed in Cores 188-1167A-19X and 22X respectively; they are tentatively correlated with small positive peaks in the resistivity log at 153 and 181 mbsf. However, the conglomerate bed might also be the cause of the large low in resistivity values at 148 mbsf if the material between the clasts is poorly compacted (which is conceivable since the conglomerate is clast supported).
This subunit is defined by a drop in the gamma-ray values relative to Subunit 2c, above. This is likely to be related to the decrease in granitic clasts and the increase in sandstone clasts observed in the cores around this depth; granites are typically more radioactive than sandstones. Resistivities in the upper part of Subunit 2d are higher than in Subunit 2c; there is one drop in resistivity at 235 mbsf.
We interpret the thin intervals of low-resistivity values as clay beds, given the known low resistivity of clay compared to diamict and the absence of other major lithologies from the cores in the logged interval. The position of the clay beds appears to be related to the "sawtooth" trends observed in the core magnetic susceptibility record (Fig. F42). Clay beds are found at the step in susceptibility values at 116-127 mbsf and 190-126 mbsf, but the relationship is not so clear cut in the upper 100 m of the hole.
The Lamont-Doherty Earth Observatory temperature-acceleration-pressure (TAP) tool recorded the temperature of the fluid in Hole 1067A as the triple combo tool string was run (Fig. F43). The measurements underestimate the formation temperature, as the fluid temperature does not have time to equilibrate to the formation temperature. A temperature of 4.5°C was recorded at 151 mbsf. The downgoing and upgoing curves have an offset, owing to the borehole still reequilibrating during acquisition.