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Downhole logging

Logging-while-drilling operations

Hole U1378A 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 an 8½ inch bit and 6½ inch drill collars. For details, see “Downhole logging” in the “Methods” chapter (Expedition 334 Scientists, 2012).

During logging in Hole U1378A, the LWD tools were run without batteries, which did not reach Puntarenas before the R/V JOIDES Resolution sailed. The LWD tools were powered only by the mud turbine generator in the TeleScope. The main negative implications of running the LWD tools without batteries are that (1) no data are taken when the flow rate is less than the minimum required to power up the tools (~300 gal/min or 18.9 L/s) and (2) if the tools lose connection to the TeleScope their clock may be reset, which requires postcruise processing. The missing batteries, however, do not affect data collection during drilling, when pumps are on. The batteries were eventually delivered to the JOIDES Resolution by helicopter a day after operations started in Hole U1378A and were installed in the LWD tools used in Hole U1379A.

Hole U1378A was spudded at 537 m drilling depth below rig floor (DRF) at 0420 h on 18 March 2011 (all times are local Costa Rica time; UTC – 6 h). Drilling proceeded smoothly in the uppermost 300 m at total drilling rates of ~25 m/h (total drilling rates include time for connections). Drilling became more difficult at 300–450 mbsf, where rates dropped to an average of ~13 m/h. After the bit reached a depth of 455 mbsf at ~0500 h on 19 March, the drillers noted high standpipe pressures and backflow when making connections, indicating hole cleaning problems. Also, the top drive torque showed occasional large increases. The bit was lifted off the bottom, and the hole was repeatedly backreamed to improve circulation. These attempts continued for several hours. High standpipe pressures and top drive torques were consistently observed when the bit was deeper than ~850 m DRF (313 mbsf). After many attempts to return to the bottom of the hole, in the afternoon of 19 March the bit was still ~100 m above the total depth reached early in the morning. It was recognized that progress was too slow to significantly deepen the hole before the JOIDES Resolution would have to move to the next site, and at 1720 h it was decided to stop drilling in Hole U1378A, gaining a few hours for LWD at the next site.

Measurements recorded by the LWD tools were downloaded and processed without difficulties, except for the geoVISION resistivity image data. The Schlumberger logging engineers noted that the azimuthal orientation system of the geoVISION tool malfunctioned, and they sent the geoVISION data recorded in Hole U1378A to a Schlumberger LWD data processing center in Houston, Texas (USA). This attempt was unsuccessful, and no geoVISION resistivity image data are available for Hole U1378A. The geoVISION log measurements of resistivity and NGR, which do not rely on measuring tool orientation, were collected successfully.

Gas monitoring with logging-while-drilling measurements

As the first hole at Site U1378 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 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 caused by low-density gas entering the wellbore.

Figure F35 shows residual pressure that is the measured annular pressure minus the hydrostatic pressure (for a seawater density of 1025 kg/m3). From the seafloor to 340 mbsf, the observed annular pressure was very close to hydrostatic. The large downhole pressure fluctuations (100 psi and greater) observed below 340 mbsf are likely due to cuttings restricting flow in the borehole annulus, which required backreaming and hole cleaning in this interval and eventually led to abandoning the hole. These pressure fluctuations consisted of an increase followed by a decrease to the overall pressure trend as hole cleaning progressed. The pressure trend in this interval was higher than hydrostatic because of solid particles that increase the effective density of the borehole fluid. 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 F35 also shows the quality control logs for Hole U1378A. 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 F35 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 (10–12 inches; 25–30 cm) from the seafloor to 90 mbsf and at 290–310 and 340–370 mbsf. The density correction, which is also calculated from the difference between the short- and long-spaced density measurements, generally varies from 0 to 0.1 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 rate of penetration (ROP) was ~100 m/h between the seafloor and 30 mbsf, decreasing to ~40 m/h to 300 mbsf and 20–40 m/h below 300 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. The quality of the geoVISION resistivity image is best for ROPs of ~20 m/h, and while drilling Hole U1378A, 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 U1378A (e.g., Cook et al., 2009; Guerin et al., 2009). No geoVISION resistivity image data are available for Hole U1378A, however, and the possible effect of ROP on the resistivity image quality cannot be assessed.

The depths relative to seafloor were fixed for all LWD logs by identifying the step change in the gamma ray and density logs associated with the seafloor. For Hole U1378A, the logging pick for the seafloor was at 532 mbrf, ~5 m above the seafloor depth estimated by the drillers. The rig floor logging datum was at 9.8 mbsl.

Characterization of logging-while-drilling logs

Figure F36 is a summary of the LWD logs and images measured in Hole U1378A. 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 F36 also shows a 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 F36 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 F36 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 F36 also shows a high vertical resolution resistivity measured by the ring electrode in the geoVISION tool and two propagation resistivity curves that span the resolution of the resistivities measured by the arcVISION tool. These propagation 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 propagation resistivity curves in Figure F36 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, the resistivity in a sediment sequence is controlled by the 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 F36, with the exception of a few intervals where the logged densities show large negative peaks without corresponding drops in resistivity (e.g., 295–310 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. F35) and is discussed below, taking also into account core measurements.

Finally, Figure F36 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 ~70:1 in the images of Figure F36.

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 U1378A are analyzed below.

Logging units

We define two logging units on the basis of the variation in logging properties displayed in Figure F36. Logging Unit 1 (0–82 mbsf) corresponds to a compacting sequence with a well-defined increase in density and resistivity and a corresponding porosity decrease with depth. The densities logged in this unit show some fluctuations over an increasing trend with depth, from 1.4 to 1.6 g/cm3. The resistivity log displays a similar increasing trend with depth from 0.5 Ωm at the seafloor to 1.0 Ωm at the base of the unit. Porosity starts at 70%–80% at the seafloor and decreases to 60% at the base of this unit. The NGR log also shows an increase with depth from the seafloor (~25 gAPI) to the base of the unit (~40 gAPI).

The top of logging Unit 2 (82–455 mbsf) is marked by a step increase of density and resistivity, which show only small increases with depth in this unit. Density increases from ~1.8 g/cm3 at the top to ~1.9 g/cm3 at the base and resistivity from just above 1 Ωm at the top to just below 2 Ωm at the base. Porosity shows a matching small decrease with depth, from 55% at the top of logging Unit 2 to 45% at the base. The NGR log shows nearly constant values (40 gAPI) at 82–190 mbsf, a positive excursion up to 70 gAPI at 190–196 mbsf, a gradual decrease with depth from 50 to 40 gAPI at 196–330 mbsf, and then a constant ~40 gAPI at 330–455 mbsf. The gamma ray peak at 190–196 mbsf is matched by a peak in the core measurements (see the next section) and corresponds to a sandy interval containing black organic matter particles in Core 334-U1378B-24X (see “Lithostratigraphy and petrology”).

Superimposed over the steady increase of density with depth, the logged values show large density fluctuations with sharp negative peaks at 295–310 and 355–370 mbsf caused by borehole enlargements (see the hole diameter logs in Fig. F35). 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 355–370 mbsf corresponds to fractured and brecciated zones observed in Cores 334-U1378B-44X through 46X (see “Structural geology”).

Borehole breakout analyses

Borehole breakout analyses were performed to assess the orientation of the present-day maximum horizontal stress within the borehole. Breakout features appear as two vertical dark bands in both density and radius images from Hole U1378A (Fig. F36). The borehole radius images were used to identify the breakout morphology. No obvious breakouts are identified from the seafloor to 110 mbsf. Breakouts occur discontinuously as two well-developed vertical bands of large borehole radius separated by 180° in the interval between 110 mbsf and the bottom of the radius image (438 mbsf). The widths of these breakouts are variable and generally range from 20° to 90°. Breakouts are typically developed in the northeast–southwest to east-northeast–west-southwest direction with a range of three sectors (22.5°–90°), indicating that the maximum horizontal stress at Site U1378 is oriented northwest–southeast to north-northwest–south-southeast. An exception is from 239 to 252 mbsf, where the breakouts are oriented northeast–southwest to north-northeast–south-southwest. Although breakouts are typically fragmented in segments of 0.5–5 m vertical length, continuous breakout intervals occur at 218–230, 241–252, 256–280, 395–406, and 410–433 mbsf (Fig. F37). Breakouts are wider and display disrupted structure at 294–311 and 334–375 mbsf, in coincidence with borehole enlargements indicated by the caliper logs (Fig. F35). The disrupted interval at 334–375 mbsf approximately corresponds to brecciated and fractured zones described in Cores 334-U1378-44X through 46X (see “Structural geology”). Detailed statistical analyses of breakout azimuths and widths will be conducted postexpedition.

Core-log integration

Figure F38 is a comparison of NGR, bulk density, and porosity measurements made by LWD in Hole U1378A and in core samples in Hole U1378B. 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 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 F38 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 F38. Most patterns in the log and core NGR records match closely, with only a few depth shifts likely caused by collection of data in different holes and by uncertainties in the depth measurement (e.g., the depth difference in the broad peak in NGR and logged values at 190–200 mbsf). This general agreement indicates a close correlation in the depths of the log and core records.

Figure F38 also compares IDRO bulk density 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 higher than the core densities and the corresponding logged porosities are unrealistically high (e.g., density lows <1.4 g/cm3 at 350–375 mbsf). These extremely low logged density values are likely caused by borehole enlargements.

An interesting interval in this bulk density comparison is between 110 and 200 mbsf, where core densities are systematically lower than logged densities. The differences are as much as 0.2 g/cm3, or ~11% of the bulk density. A contributing factor to this difference may be core expansion by elastic rebound (Moran, 1997), as many cores in this depth interval showed >100% recovery. 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.