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doi:10.2204/iodp.proc.341.106.2014

Downhole logging

Logging operations

Logging operations for Site U1420 began after completion of RCB operations in Hole U1420A, which was drilled to a total depth of 1020.8 m DSF at 1310 h on 20 July 2013. In preparation for logging, 50 bbl of high-viscosity mud was circulated to clean the hole. Because of high torque, further remediation was needed to prepare the hole for logging (see “Operations”). Another 75 bbl sweep was pumped, the pipe was raised to 981.2 m DSF, and the bit was released at 983.6 m DSF. The hole was then displaced with 450 bbl of barite-weighted mud (10.5 ppg), and the pipe was raised to 93.3 m DSF for logging.

Because of concerns regarding borehole stability based on poor core recovery and challenging coring conditions, only one logging tool string was deployed in Hole U1420A. This Sonic-induction tool string was designed to provide the highest priority measurements to meet science objectives with the lowest risk to logging tools. The tool string was composed of the Enhanced Digital Telemetry Cartridge, Hostile Environment Litho-Density Sonde (HLDS) without neutron source, Dipole Shear Sonic Imager (DSI), and Phasor Dual Induction–Spherically Focused Resistivity Tool, measuring total gamma ray, borehole diameter, sonic velocity, and resistivity (Fig. F26).

The Sonic-induction tool string was rigged up at 0630 h on 21 July. The tool string was found to be drawing too large a current and, after testing each of the tools, the DSI was determined to be at fault. The DSI was replaced with a back-up tool, and the tool string was lowered into the hole at 1020 h on 21 July. A downlog was recorded at a speed of 2800 ft/h. At 238 m WSF (~150 m below the pipe), the wireline heave compensator settings were tested in order to minimize downhole tool motion during logging. The downlog continued, reaching 288 m WSF, where the tool string was blocked and unable to progress deeper into the hole. From this depth, the first upward pass commenced at 2800 ft/h and ended just below the pipe depth. The tool string was run back into the hole, and a second upward pass began at 1419 h from 288 m WSF at 2800 ft/h. All logging tools were rigged down, logging operations were ended, and the rig floor was clear by 2335 h on 21 July. During logging operations for Hole U1420A, the heave was relatively low (~0.6 m, peak-to-peak average).

Data processing and quality assessment

The logging curves were depth-matched using the gamma ray measurement from the second pass of the sonic-induction tool string as a reference log, facilitating the generation of a unified depth scale. Logging data were then depth-shifted to the seafloor reference based on the step increase observed in gamma ray measurement from the downlog of the tool string, the only logging pass to record the seafloor in this hole. The seafloor was measured at 259 m wireline log depth below rig floor (WRF) and the resulting depth scale is wireline log matched depth below seafloor (WMSF).

Figure F27 shows a summary of the main logging data recorded in Hole U1420A. The caliper measurement shows that the borehole diameter exceeded 18 inches, the limit of the HLDS caliper arm, over several large depth intervals (Fig. F27). Borehole diameter was smaller (~15 inches) between ~140 and 200 m WMSF. Despite the large borehole size, the data seem to be of good quality, as measurements showed relatively consistent variability throughout the logged interval. A comparison of the two passes of the Sonic-induction tool string indicates reproducibility for all measurements (Fig. F28). In an interval where gamma ray drops dramatically (~130–140 m WMSF) and borehole diameter is >18 inches, there remains a coherent response in the resistivity data. The deep resistivity measurement (shown in red in Fig. F27) has an average radial depth of investigation of >1.2 m even at very low resistivity (0.1 Ωm), and the depth of investigation increases in higher resistivity formations. Thus resistivity is a reliable measurement even in wider boreholes.

The DSI recorded P&S monopole and lower and upper dipole modes in Hole U1420A, with standard (high) frequency for the monopole and upper dipole and low frequency for the lower dipole. The high coherence in sonic waveforms indicated by orange-red areas in the compressional velocity track (Fig. F27) suggests that the DSI was successful in capturing compressional arrivals through most of the logged interval, whereas there is low or no coherency in the shear wave arrivals. Higher velocities are shown in the intervals of smaller borehole diameter, which may be partially controlled by lithology, but also suggests that P-wave velocities recorded through wider borehole intervals may underestimate true formation velocity.

Logging stratigraphy

Logging Unit I

The logged interval in Hole U1420A was assigned to a single logging unit based on the minimal measurements recorded and the limited depth interval of the logging data in the context of the entire drilled depth. However, on the basis of distinctive changes in resistivity and velocity measurements, Logging Unit 1 has been divided into five subunits (Fig. F27).

Logging Subunit 1A (base of drill pipe to 129 m WMSF)

Logging Subunit 1A is characterized by very little net downhole variation in all data types (Fig. F27). Gamma ray data vary around an average of 36 gAPI. Resistivity and P-wave velocity are relatively constant with depth. Resistivity values range from 2.4 to 3.8 Ωm, and the mean P-wave velocity is ~1930 m/s. The borehole diameter for this subunit is ~18 inches.

Logging Subunit 1B (129–140 m WMSF)

Logging Subunit 1B is distinguished by abrupt decreases in gamma ray and resistivity measurements. The mean gamma ray value drops to 23 gAPI. There is a distinct separation between the shallow and medium resistivity curves and the deep resistivity curve (Fig. F27). The deep curve (mean = 2.1 Ωm) likely reflects formation resistivity, whereas the shallow and medium curves are mostly likely dominated by the resistivity of the borehole fluid, given the large borehole diameter. P-wave data coherence drops dramatically in this subunit (Fig. F27).

Logging Subunit 1C (140–172 m WMSF)

The Subunit IB/IC boundary is marked by a change in borehole diameter to values <18 inches and abrupt changes in all logging measurements (Fig. F27). Gamma ray values are elevated relative to those shallower in the hole, with a mean of 44 gAPI. Both resistivity and P-wave velocity increase with depth in this subunit, from ~3 to 6 Ωm and from ~1600 to 2200 m/s, respectively.

Logging Subunit 1D (172–190 m WMSF)

Logging Subunit 1D has a borehole diameter similar to that of Subunit 1C. Gamma ray values are slightly reduced relative to the shallower subunit (mean = 40 gAPI). Resistivity increases at the Subunit 1C/1D boundary (Fig. F27). The highest resistivity values measured in the logged interval occur within this subunit, with a maximum resistivity >8 Ωm. There is also an abrupt increase in P-wave velocity across the subunit boundary, with the highest velocity values recorded in the borehole (mean = 2375 m/s).

Logging Subunit 1E (190–282 m WMSF [base of logged interval])

Borehole diameter is <18 inches at the top of Subunit 1E but increases to >18 inches below ~204 m WMSF (Fig. F27). Gamma ray values decrease slightly with depth, with a mean value of 37 gAPI. Resistivity decreases from the top of the subunit to ~220 m WMSF and is relatively constant at deeper depths, ranging between 3.4 and 6.2 Ωm. P-wave velocity data show a similar trend, decreasing with depth to ~220 m WMSF and then varying locally around ~2050 m/s. There are correlated changes (peaks/troughs) in deep resistivity and P-wave velocity in this subunit. These local variations are most likely true responses to changing formation properties, which could be true increases/decreases in measured values or simply responses to changing borehole size that are also linked to lithology.

Resistivity

Electrical resistivity measured in Hole U1420A is distinctively different than that measured at Sites U1417 and U1418 (Fig. F29), although measurements at all three sites fall within the typical range of formation resistivity for clay/shale (2–10 Ωm; from Tittman, 1986). In Holes U1417E and U1418F, resistivity ranged from ~0.6 to 3.0 Ωm, whereas resistivity values in Hole U1420A are generally >3.0 Ωm, with the deepest resistivity curve showing values >8 Ωm in logging Subunit 1D (Figs. F27, F29).

Because of limited core recovery in the logged interval in Hole U1420A (<4%), it is difficult to speculate with confidence about the source of high resistivity. Formation resistivity is frequently controlled by the amount and distribution of water in a formation. When a formation is porous and contains saline water (e.g., resistivity of seawater at 24°C is 0.19 Ωm), its resistivity will be relatively low. High resistivity values may indicate a number of things: low porosity, a porous formation containing freshwater, or some degree of both. However, a formation’s resistivity depends on more than simply the amount of water it contains; this measurement is also responsive to lithology, compaction, overpressure, texture, and conductive minerals (e.g., Ellis and Singer, 2007). Rock materials themselves have high resistivity, but when a formation is also porous and all other factors are equal, the fresher the pore fluid, the greater the resistivity. The logged interval in Hole U1420A may contain both high-resistivity formation and relatively fresh pore fluid. The relatively high velocity values (~1700 to >2500 m/s) measured within this shallow logged interval (from ~92 to 282 m WMSF) support the idea that high resistivity is not simply due to the presence of freshwater in the formation. Although there is no significant core material to confirm, this interpretation is consistent with the recovery of pebbles and rocks in cores through the same depth interval (see “Lithostratigraphy”) and with reduced salinity measured in IW samples through the entire borehole (see “Geochemistry”).

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