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doi:10.2204/iodp.proc.324.104.2010

Downhole Logging

Downhole logging data obtained from Hole U1347A included natural and spectral gamma ray, density, neutron porosity, photoelectric factor, and electrical resistivity measurements from three depths of investigation. Interpretations of gamma ray and electrical resistivity downhole logs were used to identify 15 logging units in Hole U1347A with 3 in the sediment sequences and 12 in the basaltic basement.

Operations

A wiper trip was completed throughout the open hole before the start of the wireline logging operations. The drill pipe was set at 130 m wireline matched depth below seafloor (WMSF), which is ~28 m above the sediment/basement interface. The hole was circulated with 83.5 bbl of 10.5 ppg barite mud. Downhole logging operations lasted 24 h beginning at 1955 h on 30 September 2009. Wireline logging operations consisted of two tool string deployments and testing of the wireline heave compensator (WHC). Logging operations in Hole U1347A took place in good sea conditions with ship heave of ~1 m. A planned third logging run with the Hostile Environment Natural Gamma Ray Sonde (HNGS)-General Purpose Inclinometry Tool (GPIT)-UBI tool string had to be abandoned because of communication problems with the tool string. Troubleshooting on the rig floor revealed a bad digital telemetry adapter (DTA-A) and communication problems between the GPIT and the UBI. The logging personnel were unable to resolve the latter in a timely manner and the deployment was aborted.

Tool string deployment

HNGS-APS-HLDS-GPIT-DITE

The wireline tool string deployment consisted of a 30.2 m long triple combo tool string that included a logging equipment cable head (LEH-QT), digital telemetry cartridge (DTC-H), HNGS, Hostile Environment Natural Gamma Ray Cartridge (HNGC), Litho-Density Sonde Cartridge (LDSC), Accelerated Porosity Sonde (APS), Hostile Environment Litho-Density Sonde (HLDS), DTA-A, GPIT, and the Digital Dual Induction Tool model E (DITE). Downhole logs were recorded in two passes: (1) a downlog from seafloor to 287 m WMSF and (2) an uplog from 314 to 128 m WMSF.

After the downlog was stopped and prior to starting Pass 1, 30 min was spent assessing downhole tool motion and optimizing the efficiency of the WHC for the water depth at Hole U1347A and heave conditions at the time of the logging operations. Once the best possible WHC parameters were chosen for the prevailing heave conditions, the tool string was lowered to 314 m WMSF to begin the first uplog.

HNGS-DSI-GPIT-FMS

The second wireline tool string deployment consisted of a 34.39 m long FMS-sonic tool string that included a LEH-QT, DTC-H, HNGS, DSI, DTA-A, GPIT, and FMS. Downhole logs were recorded in three passes: (1) a downlog from seafloor to 287 m WMSF, (2) an uplog from 314 to 115.5 m WMSF, and (3) a second uplog from 314.6 m WMSF to seafloor.

After completion of the downlog the first upward pass was started and run from 314 to 115.5 m WMSF. The tool string was then lowered to ~314 m WMSF to begin the second upward pass through seafloor.

Data processing

Logging data were recorded onboard the JOIDES Resolution by Schlumberger and archived in DLIS format. Data were sent by satellite transfer to Lamont-Doherty Earth Observatory–Borehole Research Group, processed there, and transferred back to the ship for distributing to the shipboard scientific party and archiving in the shipboard database. Processing and data quality notes are given below.

Depth shifting

In general, depth shifts are applied to logging data by selecting a reference (base) log (usually the total gamma ray log from the run with the greatest vertical extent and no sudden changes in cable speed) and aligning features in equivalent logs from other passes by eye. Logging data seafloor depth could not be determined by the step in gamma ray values because the gamma ray signal was affected by the presence of the FFF and drill collars. As a result, the seafloor depth given by the drillers (3461 m DRF) was used for depth shift (Table T12). The depth-shifted logs were then depth matched to the gamma ray log from the main pass of the triple combo tool string.

Data quality

The quality of wireline logging data were assessed by evaluating whether logged values are reasonable for the lithologies encountered and by checking consistency between different passes of the same tool. Gamma ray logs recorded through the BHA should be used only qualitatively because of the attenuation of the incoming signal. The thick-walled BHA attenuates the signal more than the thinner walled drill pipe.

A wide (>30.5 cm) and/or irregular borehole affects most recordings, particularly those like the HLDS that require eccentralization and good contact with the borehole wall. The density log roughly correlates with the resistivity logs, but it is largely affected by hole conditions. The hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL) and shows a very irregular borehole with intervals exceeding the maximum caliper aperture. Good repeatability was observed between Pass 1 and 2, particularly for measurements of electrical resistivity, gamma ray, and density.

Bulk density (HLDS) data were recorded with a sampling rate of 2.54 cm, in addition to the standard sampling rate of 15.24 cm. The enhanced bulk density curve is the result of a Schlumberger enhanced processing technique performed on the MAXIS system onboard the JOIDES Resolution. In normal processing, short-spacing data are smoothed to match long-spacing data; in the enhanced processing this is reversed. In a situation where there is good contact between the HLDS pad and the borehole wall (low-density correction) the results are improved because the short spacing has better vertical resolution.

The FMS images are generally of good quality below 155 m WMSF as a result of the relatively good hole condition (hole size < 35.6 cm) and of intermediate quality above 155 m WMSF because of the large borehole size where it ranges from 35.6 to 48.3 cm. The irregular and possibly elliptical shape of the borehole occasionally prevents some FMS pads from being in direct contact with the formation, resulting in poor resolution or dark images. Hence, the FMS images (and the high-resolution resistivity logs) should be used with caution in this depth interval.

The sea state was relatively calm with a peak to peak heave of ~1.0 m or less. The WHC was used during the entire logging operation.

Preliminary results

Electrical resistivity measurements

Three electrical resistivity curves were obtained with the DITE. The spherically focused log (SFL), medium induction phasor-processed resistivity (IMPH), and deep induction phasor-processed resistivity (IDPH) profiles represent different depths of investigation into the formation (64, 76, and 152 cm) and different vertical resolutions (76, 152, and 213 cm). Downhole open hole electrical resistivity measurements covered 26.8 m of the bottommost sedimentary sequences and the uppermost 159.8 m the basement lithostratigraphic units (Fig. F61). The DITE was the only tool that reached the bottom of the logged interval in Hole U1347A because it was the bottommost tool in the logging tool string (see Fig. F19 in the "Methods" chapter).

In the bottommost sedimentary sequences IMPH values range from 1.1 to 2.0 Ωm, IDPH values range from 1.1 to 2.3 Ωm, and SFLU values range from 0.85 to 2.0 Ωm. The interbedded layers of sediment within the basaltic basement have IMPH, IDPH, and SFLU values between 1.9 and 4.0 Ωm. In the basaltic basement units IMPH measurements range from 6.2 to 123.8 Ωm, IDPH measurements range from 7.9 to 186.5 Ωm, and SFLU measurements range from 3.8 to 1211 Ωm (Fig. F61).

Gamma ray measurements

Standard, computed, and individual spectral contributions from ⁴⁰K, ²³⁸U, and ²³²Th were part of the gamma ray measurements obtained in Hole U1347A with the HNGS. The total gamma ray measurements through the BHA show two anomalous peaks between 12.8 and 18.8 m WMSF and between 78.5 and 97.5 m WMSF (Fig. F61).

Downhole open hole gamma ray measurements covered 26.8 m of the bottommost sedimentary sequences and 160.6 m of the basement lithostratigraphic units. Total gamma ray measurements in the bottommost sediments of Hole U1347A are moderately variable, ranging from 11.0 to 40.7 gAPI with a mean of 23.4 gAPI. Potassium values are also relatively high with values between 0.46 and 1.45 wt% with a mean of 0.82 wt% (Fig. F62). Uranium values are mostly between 0.33 and 1.7 ppm with a mean of 2.84 ppm. In contrast, thorium values are relatively low, ranging from 0.01 to 0.55 ppm with a mean of 0.15 ppm.

Total gamma ray measurements in the basaltic basement are low with basement values between 5.31and 10.86 gAPI. The sediment layers interbedded within the basement show higher values between 6.89 and 24.0 gAPI, highlighting the locations of the sediment layers (Fig. F61).

Potassium values are relatively low in the basaltic basement with values between 0.072 and 0.35 wt% (Fig. F62). The sediment interbeds, by contrast, show higher potassium values between 0.45 and 0.85 wt%. Uranium values are mostly between 0.0 and 2.45 ppm with a gradual increase from 250 m WMSF to the base of the hole (Fig. F62). The sediment interbeds within basement have low uranium values of <0.5 ppm. Thorium values range from 0.01 to 1.32 ppm with a mean of 0.64 ppm (Fig. F62). A significant gamma ray anomaly was recorded while in-pipe during FMS Pass 2, but it was not recorded during the deployment of the HNGS-APS-HLDS-GPIT-DITE tool string.

Density

Density values range from 1.2 to 1.8 g/cm³ over the lowermost sediment section in Hole U1347A (Fig. F63). In the basement section, density values are between 1.8 and 3.1 g/cm³. Sediment interbedded within basement has values between 1.5 and 1.9 g/cm³. A comparison between discrete physical property samples and the downhole density log shows that discrete sample data (MAD) are consistent with the downhole data (Fig. F63).

Sonic velocity measurements

Downhole velocity data were obtained for the open hole interval between 127.16 and 295 m WMSF (Fig. F63). In the sediment section average velocity is ~1.7 km/s. In the basement section velocities range between 3.6 and 6.8 km/s with an increasing trend with depth. A comparison with discrete sample measurements of P-wave velocity shows that the core data are consistently faster than the downhole measurements.

Neutron porosity measurements

Overall, downhole neutron porosity measurements show good agreement with physical property measurements obtained from core samples (Fig. F63). Most high-porosity values in the upper 250 m correspond to sections where the borehole was enlarged and therefore the log returns artificially high measurements. The bottommost section of Hole U1347A, where flows are massive, shows porosities slightly below 20% and good correlation with discrete physical property measurements.

Magnetic field measurements

Measurements of total magnetic moment, magnetic inclination, and magnetic intensity were obtained with the GPIT (Fig. F64). The mean magnetic inclination and total magnetic moment from 131.8 to 313.9 m WMSF are 44° and 0.43 Oe, respectively. The magnetic intensity is 0.29 Oe on the z-axis and varies between –0.46 to 0.43 Oe on the x- and y-axes.

Formation MicroScanner images

FMS images were obtained for the open hole interval between 134 and 315 m WMSF. The diameter of hole measured by the FMS calipers varied between 10.2 and 38.1 cm. High-quality FMS images were obtained in sections of the hole with a diameter <35.6 cm. FMS images from the basement section showed sections with high fracture density, well-defined pillow structures, and massive flow units in the basaltic sequences (Fig. F65). Large numbers of fractures and veins are clearly visible, which should allow reorientation of core pieces postcruise.

Lithostratigraphic correlations

Preliminary interpretation of the downhole log data divided Hole U1347A into 15 logging units within three main sections, the section covered by the BHA, the sedimentary sequences in open hole, and the basaltic basement (Figs. F61, F62, F63, F65). Logging units in the section covered by the BHA were interpreted on the basis of the gamma ray downhole logs, and only intervals that showed significant anomalies were characterized as logging units. Logging units within the open hole section that contained sedimentary sequences were also interpreted on the basis of the gamma ray fluctuations, whereas the basaltic basement was characterized using the resistivity logs.

Two logging units were qualitatively identified in the section covered by the BHA (Fig. F61):

• Logging Unit Ip shows a significant increase in total gamma ray measurements between 12.75 and 18.20 m WMSF. Spectral gamma data show a large contribution from thorium and uranium (Figs. F61, F62).

• Logging Unit IIp (80.2–97.5 m WMSF) shows an increase in gamma ray values with three distinct peaks (Fig. F61). Spectral gamma data show that the peak has a significant contribution from uranium (Fig. F62).

Only one logging unit was identified in the sedimentary sequence in open hole below the BHA based on gamma ray downhole logs (Fig. F61):

• Logging Unit Is (127.78–154.93 m WMSF) has gamma ray values between 11 and 41 gAPI.

The basement sequence below 154.93 m WMSF is divided into 12 logging units using the downhole resistivity and natural gamma logs (Fig. F61):

• Logging Unit Ib (154.93–167.9 m WMSF) resistivity shows a steady increase from the sediment/basement interface through the unit with maximum values of 60 Ωm. Caliper data show the borehole is larger than the bit size in the unit with a maximum diameter of 35.6 cm. Density averages 2.7 g/cm³.

• Logging Unit IIb (167.9–173.2 m WMSF) shows lower resistivity values of ~6.7 Ωm. Caliper data show the borehole has a consistent diameter of 27.3 cm within the unit. Total gamma shows a small peak up to 7.78 gAPI with spectral gamma showing that potassium makes the largest contribution to the peak (0.26 wt%).

• Logging Unit IIIb (173.2–183.7 m WMSF) has resistivity values of ~20 Ωm with a peak of ~75 Ωm. Caliper data show the borehole in the unit has a diameter of 27.3 cm. Natural gamma ray measurements are low in the unit and density has a mean value of 2.6 g/cm³.

• Logging Unit IVb (183.7–191.6 m WMSF) is characterized by low resistivity values of ~1.9 Ωm. Caliper data show a diameter of 32 cm. Potassium is high with a peak at 0.69 wt%.

• Logging Unit Vb (191.6–196.9 m WMSF) resistivity values range between 12.5 and 61.5 Ωm. Natural gamma and potassium measurements are low. Borehole diameter is ~32 cm.

• Logging Unit VIb (196.87–206.75 m WMSF) has resistivity values averaging ~2.9 Ωm. Borehole diameter ranges between 45.7 and 29.7 cm. Potassium peaks at 0.87 wt%.

• Logging Unit VIIb (206.75–237.07 m WMSF) resistivity values range between 10.6 and 175 Ωm. Natural gamma and potassium measurements are low. Borehole diameter averages 26.7 cm.

• Logging Unit VIIIb (237.07–242.30 m WMSF) resistivity values average 1.9 Ωm. Potassium peaks at 0.77 wt%. Borehole diameter is wide in this unit with a maximum of 44.5 cm.

• Logging Unit IXb (242.30–256.79 m WMSF) resistivity values show a peak of 1000 Ωm. Potassium ranges between 0.1 and 0.35 wt%. Borehole diameter is wide in this unit with a maximum of 42.4 cm.

• Logging Unit Xb (256.79–258.76 m WMSF) resistivity values average 5.75 Ωm. Potassium peaks at 0.86 wt%. Borehole diameter averages 29.7 cm.

• Logging Unit XIb (258.76–290.12 m WMSF) resistivity values range between 18.2 and 246 Ωm. Potassium and natural gamma measurements are low and show a decreasing trend through the unit. Borehole diameter averages 22.7 cm.

• Logging Unit XIIb (290.12–315.0 m WMSF) resistivity values are considerably higher than the upper logging units, ranging between 42.3 and 1153 Ωm. Borehole diameter is consistent throughout the unit with a diameter close to bit size of 25.9 cm.

Within the basement several highly altered sediment layers were recovered. These layers correspond with logging Units IIb, IVb, VIb, VIIIb, and Xb, which show high natural gamma values. Elevated potassium concentrations in the sediment layers may be indicative of alteration of the sediment.

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