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Downhole logging

Logging-while-drilling operations

Hole U1379A was drilled and logged with a Schlumberger LWD tool string that included (from top to bottom) the adnVISION 675 (density, neutron porosity, and ultrasonic caliper), the TeleScope 675 (measurement while drilling: power and data transmission and drilling parameters), the arcVISION 675 (propagation resistivity, gamma radiation, and annular pressure), and the geoVISION 675 (resistivity images and gamma radiation). The LWD tools were 6¾ inch diameter versions in a BHA with a 8½ inch bit and 6½ inch drill collars. For details, see “Downhole logging” in the “Methods” chapter (Expedition 334 Scientists, 2012).

Hole U1379A was spudded at 137 m drilling depth below rig floor (DRF) at 1000 h on 20 March 2011 (all times are local Costa Rica time, UTC – 6 h). Drilling started at high rates of penetration (ROPs) of 30–40 m/h in the first 6 h, slowing down to ~20 m/h afterward (these total drilling rates include time for pipe connections). Drilling proceeded smoothly until 624 mbsf at 1700 h on 21 March, when the driller noted high standpipe and downhole pressures and high torque on the top drive. Backreaming and mud sweeps were required to clean the hole before drilling could be resumed. These procedures were occasionally necessary while drilling the rest of the hole, but drilling progressed at overall ROP of ~8 m/h.

At 0300 h on 23 March, the real-time logs showed at 892 mbsf a sharp step change in resistivity from ~1 Ωm above to 2–3 Ωm below. This interface was interpreted as the boundary between slope sediments and basement. After logging this basement formation for ~75 m, drilling was stopped at 1100 h at a total depth of 966 mbsf, exceeding the original target depth of 950 mbsf.

The measurements recorded by the LWD tools were downloaded and processed successfully, except for the geoVISION data. The Schlumberger logging engineers noted that the geoVISION tool clock did not record time properly, and they sent the geoVISION data recorded in Hole U1379A to a Schlumberger LWD data processing center in Houston, Texas (USA), in an attempt to recover useful measurements. The data in the tool memory were found to be corrupted beyond repair, and data recovery was unsuccessful. No geoVISION data are available for Hole U1379A.

Gas monitoring with logging-while-drilling measurements

As the first hole at Site U1379 was drilled with LWD, the LWD data had to be monitored for safety to detect gas entering the wellbore. As explained in “Downhole logging” in the “Methods” chapter (Expedition 334 Scientists, 2012), the primary measurement we used in the gas monitoring was the annular pressure measured while drilling by the arcVISION tool and transmitted in real time to the surface. We looked for sustained decreases of >30–50 psi in the annular pressure, which could be due to low-density gas entering the wellbore.

Figure F41 shows a residual pressure that is the measured annular pressure minus the hydrostatic pressure (for a seawater density of 1025 kg/m3). The downward trend toward pressures higher than hydrostatic is due to solid particles that increase the effective density of the borehole fluid. The 50–100 psi downhole pressure fluctuations observed below 670 mbsf are likely due to cuttings restricting flow in the borehole annulus, which required backreaming and hole cleaning. These pressure fluctuations consisted of an increase followed by a decrease to the overall pressure trend as hole cleaning progressed. The only exception is a pressure decrease below 920 mbsf caused by an improvement in hole conditions in the basement rocks. No sustained pressure drops below the general trend that exceeded the 30–50 psi threshold set in the monitoring protocol and that could be due to gas entry were observed, and no drilling interruptions were necessary.

Logging data quality

Figure F41 also shows the quality control logs for Hole U1379A. A major control on LWD measurement quality is the borehole size, which is obtained by the adnVISION tool from the difference between the short- and long-spaced density measurements and from an ultrasonic traveltime measurement. Figure F41 shows the average borehole diameters estimated from these measurements. The most reliable measurement of borehole diameter is the ultrasonic caliper, which shows enlarged hole intervals (10–13 inches; 25–33 cm) from the seafloor to 120 mbsf and at 340–500 and 600–890 mbsf. The density correction, which is also calculated from the difference between the short- and long-spaced density measurements, generally varies from 0 to 0.2 g/cm3. This correction, however, did not result in accurate density values in some enlarged hole intervals, where the measured LWD densities were anomalously low (see the comparison to core data below).

The average instantaneous ROP was ~80 m/h near the seafloor, decreasing to 20–40 m/h below 200 mbsf. This ROP is the rate of penetration of the LWD tools while the hole is being drilled and does not include time for pipe connections. geoVISION resistivity image quality is best for ROPs ~20 m/h, and in drilling Hole U1379A some resolution in the shallowest interval was traded for drilling time. Previous experience shows that high-quality geoVISION resistivity images can be acquired at ROPs as high as those employed while drilling Hole U1379A (e.g., Cook et al., 2009; Guerin et al., 2009). No geoVISION data are available for Hole U1379A, however, and the possible effect of ROP on the resistivity image quality cannot be assessed.

Depths relative to seafloor were fixed for all of the LWD logs by identifying the step change in the gamma ray and density logs associated with the seafloor. For Hole U1397A, the logging pick for the seafloor was at 133 m DRF, ~4 m above the seafloor depth estimated by the drillers. The rig floor logging datum was located 9.8 m above sea level.

Characterization of logging-while-drilling logs

Figure F42 is a summary of the LWD logs and images measured in Hole U1379A. The two density curves are conventional bulk density (RHOB) and density estimated from the adnVISION azimuthal measurements to minimize the effect of sensor standoff (image-derived density [IDRO]). These two density measurements give very similar values. Figure F42 also shows density porosity (ϕ) computed from the IDRO bulk density ρb as

ϕ = (ρg – ρb)/(ρg – ρw),

where ρw is the water density and ρg the grain density. The density porosity curve in Figure F42 is calculated assuming a water density of 1.025 g/cm3 and a grain density of 2.65 g/cm3.

The comparison of density porosity and neutron porosity in Figure F42 shows that neutron porosity is always higher. As noted in “Downhole logging” in the “Methods” chapter (Expedition 334 Scientists, 2012), the likely reason for the higher porosity measured by the neutron log is the presence of clay minerals. The hydrogen in the clay mineral hydroxyls contributes to the slowing down of neutrons and increases the estimated porosity (Ellis, 1986). Density porosity is the more accurate measure of porosity in sediments containing appreciable amounts of clay.

Figure F42 also shows two propagation resistivity curves that span the resolution of the resistivities measured by the arcVISION tool. These resistivities are based on the attenuation and phase shifts of electromagnetic waves that travel through the formation (see “Downhole logging” in the “Methods” chapter [Expedition 334 Scientists, 2012] for details). The two resistivity curves in Figure F42 are the relatively low vertical resolution A40B, based on the attenuation measured at a transmitter–receiver separation of 40 inches (101.6 cm), and the high-resolution P16B, based on the phase shift measured at a transmitter–receiver separation of 16 inches (40.6 cm). The “B” in the acronym is for “blended,” because these curves include measurements at the two electromagnetic wave frequencies used by the tool (400 kHz and 2 MHz).

To a first approximation, resistivity in a sediment sequence is controlled by porosity. The sediment grains are effectively insulators, and the medium that conducts electricity is saline water in the pore network. This implies a close inverse relationship between resistivity and sediment porosity and a direct relationship between resistivity and bulk density. These relationships are evident in Figure F42, with the exception of a few intervals where the logged densities show large negative peaks without corresponding drops in resistivity (e.g., 600–670 mbsf). This difference could be due to a measured log density that is too low in enlarged hole intervals (see the hole diameter logs in Fig. F41) and is discussed below, also taking into account core measurements.

Finally, Figure F42 shows two images of bulk density and borehole radius measured by the adnVISION tool. The image display is highly compressed in the vertical direction. For a 10 inch (25.4 cm) diameter borehole, the unwrapped borehole images are ~80 cm wide and the vertical scale is compressed by a factor of ~150:1 in the images of Figure F42.

These images are obtained by azimuthal measurements in 16 sectors and thus are sampled at a relatively coarse interval of 22.5°. Because of their limited angular resolution, these images typically do not resolve fine-scale sedimentary or structural features. On the other hand, the radius image clearly displays vertical bands of large borehole radius, which are typically interpreted as borehole breakouts caused by differences in the principal horizontal stresses (e.g., Chang et al., 2010). Borehole breakouts in Hole U1379A are analyzed below.

Logging units

We define four logging units on the basis of the variation in logging properties displayed in Figure F42. Logging Unit 1 (0–492 mbsf) corresponds to a compacting sequence where the density and resistivity progressively increase and density porosity decreases with depth, reaching nearly constant values of ~1.9 g/cm3, 1 Ωm, and 45% porosity at the base of the unit. The NGR log shows an initial increase from the seafloor (~30 gAPI) to 220 mbsf (~50 gAPI) and then shows minor fluctuations around a constant value to the base of the unit, except for a sharp peak in NGR at 476 mbsf corresponding to volcanic ash layers in Core 334-U1379A-58X (see “Lithostratigraphy and petrology”).

The top of logging Unit 2 (492–600 mbsf) is marked by a small density and resistivity step increase that remains constant within this unit at ~2 g/cm3 and 1.3 Ωm. The average density porosity is ~40% and the NGR log shows constant values of ~50 gAPI. Logging Unit 3 (600–892 mbsf) does not show an appreciable change in resistivity from the unit above, but it is clearly distinct because it contains many borehole enlargements, clearly shown by the borehole diameter logs in Figure F41 and the borehole radius image in Figure F42. Although density follows an increasing trend with depth, the logged values show large fluctuations with sharp negative peaks caused by borehole enlargements. These enlargements are likely to correspond to intervals containing unconsolidated sands or fractured intervals, which are prone to washout during drilling. For example, a negative excursion in the density log at 640–650 mbsf corresponds to fractured and brecciated zones in Cores 334-U1379C-76X and 77X (see “Structural geology” and “Core-log integration”). The cores also have higher sand content in logging Unit 3 compared to the sediment above (see “Lithostratigraphy and petrology”).

Logging Unit 4 (892–955 mbsf) corresponds to the basement rocks of the sedimentary sequence, inferred by the seismic reflection profile and confirmed by coring in Hole U1379C. The top of logging Unit 4 is clearly identified by a sharp shift in baseline NGR, photoelectric factor, density, and resistivity logs. Compared to the slope sediments above, this unit shows a markedly higher average density and resistivity (2.3 g/cm3 and 2.5 Ωm) and lower density porosity (~20%). These properties indicate a well-lithified sedimentary rock. In this unit, the resistivity logs show local peaks that reach 5–6 Ωm, suggesting lithologic variation such as blocks of more lithified rocks in the basement. Differences in NGR and photoelectric factor with the sediments above suggest a different mineral composition.

Borehole breakout analyses

Borehole breakout analyses were performed to assess the orientation of the maximum horizontal stress direction within the borehole. Although these density and radius images are sampled only in 16 azimuthal sectors (every 22.5°), obvious breakout features appear as two vertical dark bands in both images from Hole U1379A (Fig. F42). As borehole radius images are constructed by direct measurement of borehole diameter, these images were used to estimate borehole morphology. No obvious borehole breakouts are identified in the interval from the seafloor to 292 mbsf and in the basement, and breakouts occur discontinuously in the interval between 292 and 885 mbsf as two vertical bands separated by 180°. The widths of the breakout bands are variable and typically range from 20° to 90°. The breakout bands are wider and show a complicated morphology at 335–445 and 600–682 mbsf because of a large-diameter, irregular borehole. In contrast, distinct narrow breakouts are developed at 445–600 and 720–860 mbsf, where the borehole is less enlarged (Fig. F43). The average azimuth of the breakouts is roughly north–south to north-northwest–south-southeast with a range of three sectors (315°–22.5°), indicating that the maximum horizontal stress is oriented east–west to east-northeast–west-southwest. Detailed statistical analyses of breakout azimuths and widths will be conducted postexpedition.

Core-log integration

Figure F44 is a comparison of NGR, bulk density, and porosity measurements made by LWD in Hole U1379A and in core samples in Hole U1379C. This comparison is useful to correlate depths in the LWD logs and depths of core samples and to integrate information from log and core measurements.

NGR log measurements are calibrated to a degree API (gAPI) scale by comparison to a standard artificial formation built to simulate about twice the radioactivity of a typical shale and conventionally set to 200 gAPI (Ellis and Singer, 2007). The NGR measurement made on whole-core sections on the R/V JOIDES Resolution is in counts per second (for a detailed description of the NGR apparatus, see Vasiliev et al., 2010). The comparison of log and core NGR measurements in Figure F44 shows that their curves overlap if 1 cps equals ~2 gAPI. Occasionally, low NGR values are measured at the end of core sections. To avoid these end effects, measurements from the uppermost and lowermost 10 cm of each section have been excluded from the NGR data plotted in Figure F44. Most patterns in the log and core NGR records match closely, with only a few exceptions (e.g., the high NGR values at 270–315 mbsf or the sharp peak in the downhole log at 476 mbsf). This general agreement indicates a close correlation in the depths of the log and core records.

Figure F44 also compares IDRO logs to densities measured on whole-core sections by GRA and on discrete core samples by MAD analysis. The bulk density values are generally comparable, with the exception of several intervals where the log densities are clearly lower than the core densities (e.g., from the seafloor to 20 mbsf or at 600–670 mbsf). Possible reasons for these differences are discussed in the next section.

An interesting interval in this bulk density comparison is between 110 and 500 mbsf, where the core densities are systematically lower than the logged densities. The difference is ~0.05 g/cm3, or ~3% of the bulk density. Several cores in this depth interval showed >100% recovery, and the small decrease in density between core and log may be due to core expansion by elastic rebound (Moran, 1997). The MAD porosities are density porosities calculated with the measured grain densities in each sample. As the MAD densities in this interval are slightly lower than the log densities, the MAD porosities are slightly higher than the porosities computed from the density log.

Figure F45 shows a detailed comparison of log and core data in an interval near the top of logging Unit 3 where there are large fluctuations in borehole size. The purpose of this comparison is to distinguish low logged density values that are due to an enlarged borehole from log measurements that are representative of formation properties. As noted earlier, the electrical resistivity is mostly controlled by water content and hence porosity, and the logged electrical resistivities are not very sensitive to borehole size. Therefore, a negative peak in logged density that corresponds to a borehole enlargement but is not matched by a low electrical resistivity suggests that the measured density is affected by borehole size. A few of these density lows caused by borehole size are indicated in Figure F45. Also, the porosities of ~80% corresponding to these density lows are unrealistically high.

Intervals do exist, however, where low logged densities correspond to low resistivities (e.g., 616–619 and 642.5–644.5 mbsf; shaded intervals in Fig. F45). The low density logged in these intervals is likely to be representative of the actual formation density. This conclusion is supported by MAD measurements at 616–619 mbsf, which also show a density decrease. The fractured and brecciated zone observed in Cores 334-U1379C-76X and 77X corresponds to an interval where MAD data do not match the low logged densities (642.5–644.5 mbsf). The low logged density and resistivity values in this interval may reflect in situ fracture porosity, which is not measured in the MAD samples.