doi:10.2204/iodp.proc.324.106.2010
Downhole Logging
Downhole logging data obtained in Hole U1349A included natural and spectral gamma ray, density, 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 U1349A, with 1 in the section covered by the bottom-hole assembly (BHA), 5 in the sedimentary sequences in the open hole interval, and 13 in the basaltic basement section.
Operations
A wiper trip was completed throughout the open hole, and the RCB bit was released at the bottom of the hole using the mechanical bit release before the start of wireline logging operations. The hole was displaced using 56 bbl of barite mud, and the drill pipe was set at a depth of 118.9 m wireline matched depth below seafloor (WMSF). Logging operations in Hole U1349A consisted of two tool string deployments and took place in good sea conditions with ship heave fluctuating between 0.5 and 1.5 m. Downhole logging operations began at 2020 h on 11 October 2009 and were concluded at 0950 h on 12 October after the tools from the second tool string were rigged down.
Tool string deployment
HNGS-HLDS-DITE
The wireline tool string deployment consisted of a 22 m long modified triple combo tool string that included a logging equipment cable head (LEH-QT), digital telemetry cartridge (DTC-H), Hostile Environment Natural Gamma Ray Sonde (HNGS), Hostile Environment Natural Gamma Ray Cartridge (HNGC), Hostile Environment Litho-Density Sonde Cartridge, Hostile Environment Litho-Density Sonde (HLDS), and Digital Dual Induction Tool model E (DITE). During the initial descent, the short spacing sensor in the HLDS began giving high voltage readings. The tool had tested within normal parameters during the initial check on the rig floor. After several checks, the logging operation continued without the short spacing measurements. The tool string was lowered to a depth of 3079 m DRF at ~1160 m/h. Downhole logs were recorded in a downlog from seafloor to 248 m WMSF at 550 m/h, in uplog Pass 1 from 250 to 103.5 m WMSF at 275 m/h, and in repeat uplog Pass 2 from 250 m WMSF to seafloor at 550 m/h.
HNGS-GPIT-FMS
The second wireline tool string deployment consisted of a 34.39 m long FMS-sonic tool string that included an LEH-QT, DTC-H, HNGS, HNGC, digital telemetry adapter, General Purpose Inclinometry tool (GPIT), and FMS. Downhole logs were recorded in a downlog from seafloor to 222 m WMSF at 550 m/h and in uplog Pass 1 from 250 to 148 m WMSF. A second uplog Pass 2 was recorded from 250 m WMSF to seafloor at 550 m/h.
Data processing
Logging data were recorded onboard by Schlumberger and archived in digital log information standard (DLIS) format. Data were sent by satellite transfer to the Lamont-Doherty Earth Observatory Borehole Research Group, processed there, and transferred back to the ship for archival in the shipboard database. Processing and data quality notes are given below.
Depth shifting
In general, depth shifts that 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 features in equivalent logs from other passes are aligned by eye. The downhole logs were first shifted to the seafloor based on the logger's seafloor depth of 3133 m wireline depth below rig floor (Table T11). This depth differs 5 m from the drillers bottom-felt depth. The depth-shifted logs were then depth matched to those of HNGS-HLDS-DITE tool string Pass 2.
Data quality
The quality of wireline logging data was 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. In addition, the photoelectric effect measurements are strongly affected by the use of heavier mud.
A wide (>30.5 cm) and/or irregular borehole affects most recordings, particularly those like the HLDS that require eccentralization and a good contact with the borehole wall. The density log roughly correlates with the resistivity logs, but it is largely affected by hole conditions. Hole diameter measurements were recorded with the hydraulic caliper on the HLDS tool (LCAL) and show a very irregular borehole. Good repeatability was observed between Pass 1 and Pass 2, particularly for measurements of electrical resistivity, gamma radiation, and density.
The FMS images are generally of good quality below 220 m WMSF as a result of the relatively good hole condition (hole size < 35.6 cm) and of intermediate quality above 220 m WMSF because of the larger borehole size, ranging from 26 to 43 cm (Fig. F66). The irregular and possibly elliptical shape of the borehole occasionally prevented 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 ~ 1.2 m or less. The wireline heave compensator was used during the entire logging operation.
Preliminary results
Electrical resistivity measurements
Three electrical resistivity curves were obtained with the DITE. The spherically focused resistivity (SFLU), 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, respectively) and different vertical resolutions (76, 152, and 213 cm, respectively). Downhole electrical resistivity measurements covered 129.4 m of the open hole sedimentary and basement lithostratigraphic sequences drilled in Hole U1349A (Fig. F66). The DITE was the only tool that reached the bottom of the logged interval in Hole U1349A because it was the bottommost tool in the logging tool string (see Fig. F19 in the "Methods" chapter).
In the 24 m of sedimentary sequence in open hole the resistivity measurements are consistent with minimal sediment variability. IMPH and IDPH average 1.5 Ωm, whereas SFLU averages 1.2 Ωm. In the basaltic basement units, resistivity shows large variability with a general trend of increasing values downhole, with the exception of logging Unit XIIIb (stratigraphic Unit V) where resistivity values tend to be more uniform (Fig. F66).
Gamma ray measurements
Standard, computed, and individual spectral contributions from 40K, 238U, and 232Th were part of the gamma ray measurements obtained in Hole U1349A with the HNGS. The total gamma ray measurements through the BHA show one significant anomaly, a peak between 74.1 and 82.9 m WMSF (Fig. F66).
Downhole gamma ray measurements in the open hole covered 45.2 m of the sedimentary sequences and 82.9 m of the basement lithostratigraphic units. Total gamma ray measurements in the sediments of Hole U1349A show little variability in logging Units Is and IIs (see "Lithostratigraphic correlations"), ranging from 7.9 to 19.6 gAPI, with a mean of 11.7 gAPI. Logging Units IIIs–Vs show an increase in total gamma radiation at the top of Unit IIIs. Potassium (<1 wt%), U (<1 ppm), and Th (<1 ppm) values are all low and show little variability in Units Is and IIs (Fig. F67). In logging Units IIIs–Vs, K and U have higher values, with a large peak in U in logging Unit IIIs.
Total gamma ray measurements in the basaltic basement units are considerably higher than those obtained in logging Units Is and IIs, with values ranging between 11.5 and 116.5 gAPI (Fig. F66).
Potassium values are relatively high in the basaltic basement and logging Unit IIIs compared to the shallower sediment section, with values between 0.6 and 3.0 wt% (Fig. F67). Uranium values are mostly between 0.0 and 0.7 ppm (Fig. F67). Uranium is the largest contributor to the peak in total gamma radiation between 143.5 and 148.6 m WMSF, with a high value of 7.8 ppm. Thorium ranges from 0.0 to 2.0 ppm, with a mean of 0.5 ppm (Fig. F67).
Density measurements
Density values range from 1.5 to 2.8 g/cm3 over the sediment section of the open hole (Fig. F68). In the basaltic basement section, density values are between 1.7 and 2.9 g/cm3. A comparison between discrete physical property samples and downhole density log shows that discrete sample data (MAD) are consistent with the downhole data (Fig. F68).
Magnetic field measurements
Measurements of total magnetic moment, magnetic inclination, magnetic intensity, and hole deviation were obtained with the GPIT (Fig. F69). The mean magnetic inclination and total magnetic moment from 148 to 250 m WMSF are ~50° and 0.47 Oe, respectively. The magnetic intensity is 0.37 Oe on the z-axis and varies between –0.3 and 0.30 Oe on the x- and y-axes. The mean hole deviation using Pass 1 is 1.1°.
Formation MicroScanner images
FMS images were obtained for the open hole interval between 160 and 250 m WMSF. The diameter of the hole recorded by the FMS calipers varies between 25 and 43 cm. High-quality FMS images were obtained in sections of the hole with a diameter <35.6 cm. FMS images from the basaltic basement section show sections with subhorizontal contacts, moderately dipping contacts, and vesicular or brecciated textures (Figs. F70, F71).
Lithostratigraphic correlations
Preliminary interpretation of the downhole log data divided Hole U1349A into 19 logging units within 3 main sections: the section covered by the BHA, the sedimentary sequences in open hole, and the basaltic basement sequences (Figs. F66, F67, F68). 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 contain sedimentary sequences were also interpreted on the basis of the gamma ray fluctuations, whereas the basement sequence was characterized using both the gamma ray and resistivity logs.
One logging unit was qualitatively identified in the section covered by the pipe (Fig. F66):
Logging Unit Ip (74.1–82.9 m WMSF) is distinguished by an increase in total gamma ray measurements. Spectral gamma data indicates large contributions from U, Th, and K (Figs. F66, F67).
Five logging units were identified in the sedimentary sequence in open hole below the BHA based on gamma ray downhole logs (Fig. F66):
Logging Unit Is (119.7–127.2 m WMSF) is characterized by gradual increases in resistivity, density, and gamma ray measurements. Gamma radiation decreases at the base of this unit. Spectral gamma ray measurements indicate a peak in potassium to ~1 wt%.
Logging Unit IIs (127.2–143.1 m WMSF) is defined by a decrease in total gamma ray measurements at the top of the unit. Resistivity values show a gradual increase throughout the unit (Fig. F67). Density data increase sharply in the middle of the unit (Fig. F68), corresponding to the boundary between nannofossil ooze and sand-silt-claystones. Spectral gamma data indicate that uranium, potassium, and thorium are low throughout the unit.
Logging Unit IIIs (143.1–148.5 m WMSF) is defined by a sharp increase in resistivity, with maximum values of ~33 Ωm. Gamma ray measurements increase to 117 gAPI (Fig. F66). Spectral gamma ray data indicate that uranium makes the largest contribution to this peak, with a value of 7.7 ppm.
Logging Unit IVs (148.5–155.2 m WMSF) decreases sharply in resistivity and total gamma radiation. Spectral gamma ray measurements indicate that thorium, uranium, and potassium are low (Fig. F67). Density is ~2.0 g/cm3 in this unit (Fig. F68).
Logging Unit Vs (155.2–165.1 m WMSF) has low resistivity values and high total gamma ray values throughout the unit. Gamma ray values are mostly controlled by uranium readings, which peak at 3.0 ppm (Fig. F67). Density values steadily increase through the unit to 164.7 m WMSF, where they increase sharply.
The basement sequence below 165.1 m WMSF is divided into 13 logging units using the downhole resistivity and natural gamma logs (Fig. F66):
Logging Unit Ib (165.1–170.1 m WMSF) increases sharply in resistivity values (Fig. F66). Total gamma radiation decreases in this unit. Density values are high, averaging ~2.6 g/cm3.
Logging Unit IIb (170.1–176.9 m WMSF) is characterized by a decrease in resistivity and gamma ray values (Fig. F66). Potassium increases, whereas U and Th have low values. Density decreases through the unit but sharply increases near the base of the unit (Fig. F68).
Logging Unit IIIb (176.9–179.1 m WMSF) has high resistivity values and a small peak in gamma radiation (Fig. F66). Spectral gamma data indicate that the peak is due to a contribution from K, with low Th and U (Fig. F67). Bulk density is high throughout the interval (Fig. F68).
Logging Unit IVb (179.1–185.8 m WMSF) is defined by a sharp decrease in resistivity values at the top of the unit. Gamma radiation increases at the top of the unit and then decreases in the lower part of the unit (Fig. F66). Density data decrease sharply at the top of the unit and gradually increase until reaching 185.3 m WMSF, where a sharp increase occurs (Fig. F68).
Logging Unit Vb (185.8–188.6 m WMSF) increases sharply in resistivity values, whereas gamma ray values decrease. Spectral gamma data indicate that potassium, thorium, and uranium are low (Fig. F67). Density values decrease sharply through the unit (Fig. F68).
Logging Unit VIb (188.6–190.4 m WMSF). Resistivity values decrease sharply at the top of the unit. Gamma ray data show a peak that has a large contribution from potassium (Figs. F66, F67). Thorium and uranium are low. Density values within the unit are high.
Logging Unit VIIb (190.4–192.6 m WMSF) is characterized by high resistivity values, whereas total gamma ray values are low. Density decreases sharply and values are low throughout the unit.
Logging Unit VIIIb (192.6–197.8 m WMSF) is defined by sharply decreasing resistivity values at the top of the unit. Total gamma ray increases sharply at the top of the unit and then stays consistently high throughout the unit (Fig. F66). Potassium values are high throughout the unit. Density values are high and show an increasing trend throughout the unit (Fig. F68).
Logging Unit IXb (197.8–206.8 m WMSF) is defined by a sharp increase in resistivity increases sharply at the top of the unit that steadily increases to higher values. Total gamma radiation steadily decreases throughout the unit (Fig. F66). Thorium and U values in the unit are low, whereas K shows a similar trend to the total gamma ray curve, indicating that the total gamma ray is mostly controlled by the contribution from potassium. Density values increase throughout the unit.
Logging Unit Xb (206.8–210.8 m WMSF) is characterized by increasing resistivity values, particularly the SFLU values that peak at ~1200 Ωm. Total gamma radiation is low. Density values are high but show a decreasing trend in the unit.
Logging Unit XIb (210.8–217.9 m WMSF) is characterized by SFLU values that decrease at the top of the unit but otherwise remain fairly constant. Total gamma ray values show an increasing trend though the unit, whereas density values show a decreasing trend with increasing depth.
Logging Unit XIIb (217.9–223.5 m WMSF) is characterized by increasing resistivity values at the top of the unit. Total gamma radiation decreases slightly in the unit, whereas density values sharply decrease to very low values at the base of the unit.
Logging Unit XIIIb (223.5–248.7 m WMSF) is defined by decreasing resistivity values at the top of the unit that remain steady to the base of the section. Total gamma ray data only cover the upper 11 m of the unit, showing a decreasing trend to the base of the unit. Density averages ~2.4 g/cm3 and exhibits some cyclicity.