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Downhole logging

Excellent logging data quality, probably due to in-gauge borehole condition and relatively simple lithology consisting of sandstone, siltstone, shale, coal, and conglomerates, made clear identification of lithologic features based on the logging data characteristics. Because most lithology showed typical log response and was found to be intercalated by a number of marker layers (i.e., coals and cemented sandstones), correlation of the logging data to cores was also relatively easy and depth difference between these was 0.7 m or smaller.

Log characteristics suggested that the lithology of Unit I is most likely similar to that of Unit II, which consists of alternation of relatively thick layers of massive sandstones and siltstones (Fig. F10). Unit III was characterized by frequent coal layers in a few meters thickness of sandstone and siltstone alternation sequence. Based on correlations to visual core descriptions (VCDs), 7 coal layers, including the thickest two (7.3 and 3.5 m), were acquired in the cores among 13 layers that were >30 cm in thickness. Unit IV consists of thick (~200 m) massive shale in the upper half and alternation of sandstone and shale of a few meters thickness in the lower half, which also includes one thin coal layer (Core 337-C0020A-30R).

Resistivity borehole images suggested that sandstones in Unit II are massive and include granules, pebbles, and mollusks (i.e., Bivalvia), whereas those in Units III and IV consist of thin, a few centimeter thick sandstones or laminae of this scale, suggesting a change in the sedimentary environment at the Unit II/III boundary. By combining the logging data and core descriptions, sandstones that are supposed to be of high permeability were identified in Units II and III.

By using a formation-testing tool, in situ formation fluid samples were acquired from six permeable sandstones. The 35 “pretest” measurements prior to the fluid sampling indicated that formation pore pressure is hydrostatic or elevated by only a few percent of the hydrostatic value to depths of at least 2425 m wireline log matched depth below seafloor (WMSF) (i.e., the depth of the deepest reliable measurement for logging operations).

Borehole temperature was measured by two types of logging tools. The maximum temperature at the bottom of Hole C0020A was also estimated by examining the temperature build-up pattern during logging operations. Conclusively, the estimated temperature gradient is 24.0°C/km or slightly lower (Fig. F11).

Preliminary log-seismic integration was carried out based on the time-depth curve derived from vertical seismic profile (VSP) operations and synthetic seismogram calculation. The time-migrated seismic profile for Line ODSR03-BS (Taira and Curewitz, 2005) was converted with the time-depth relationship and compared with logging data. Strong reflectors are basically well correlated with the abrupt change on the logging curves.

Overview of logging results

The geophysical logging data set acquired during Expedition 337 showed characteristic log features corresponding to the lithology confirmed by the cores. The logging data set mainly used for lithology identification included gamma radiation (GR), spontaneous potential (SP), resistivity, density, and neutron porosity. Spectral gamma (uranium, potassium, and thorium) showed little change among the variation of rock types, and thus was not used. The responses of the sedimentary rocks, such as sandstone and shale, to the logging tools were typical, and therefore rock types could be distinguished without particular difficulty in the logging data. There were a number of anomalously high resistive layers, which are coal and mineral-cemented sandstones. These resistive layers can be a good marker to correlate the logging data to cores. Typical responses of major lithologies are summarized in Table T9.

Coal layers could be clearly identified, with very low gamma radiation, very high resistivity, exceptionally low density, and exceptionally high neutron porosity (see Table T10 for major coal layers). Mineral-cemented sandstones also showed a characteristic log response of very high resistivity, but those layers showed very high density and very low porosity, which is the opposite trend of coal layers.

Overall, the logging data showed that sandstones clearly responded to SP as low values, which is probably due to drilling fluid having higher salinity than formation water.

Data quality

Logging data quality is generally excellent, which is probably due to very good borehole condition without elongation or irregularity of the borehole wall. The lithology encountered may also contribute to log data quality, and the relatively simple rock type (e.g., sandstone, siltstone, shale, coal, and conglomerates) made clear identification of lithology based on the logging data characteristics. Because most lithology shows typical log response and is intercalated by a number of marker layers (coal and cemented sandstones), correlation of the logging data to cores is also straightforward.

However, there are some concerns in terms of the data quality. The Formation MicroImager borehole image quality (i.e., resistivity scanning on the borehole wall) might be significantly affected by the contact situation of the electrode pads and flaps. In the images acquired during Expedition 337, noise could be seen only in those taken by the flap edges. This may be due to weaker contact force of the flaps than that of the pads. When any material stuck to the pad and flap, those on the flaps might remain longer before being removed by friction to the borehole wall.

The top part of the logging data was affected by the 17½ inch hole and the 13⅜ inch casing pipe. Before the 10⅝ inch hole was drilled, the 17½ inch hole was drilled to 1263.0 m DSF and the 13⅜ inch casing shoe was installed at 1252.5 m DSF. Therefore, the quality of the logging data is reduced above this depth.

GR, sonic, and VSP were also acquired in the 13⅜ inch casing pipe, and data quality was affected by the size of the borehole (17½ inch) and the situation of the casing cement that fills the gap between the casing pipe and the borehole. GR signals simply weakened because of less gamma ray penetration through the steel casing pipe and the increased distance from the tool to the borehole wall, which decreased the characteristics of the GR pattern. For the sonic measurements performed in the bottom 150 m part of the cased hole, most of the S-wave was not acquired, which may be due to the imperfect contact situation behind the casing pipe. P-wave quality was also significantly affected by that situation. VSP signals showed poor quality in the top 100 m interval of the cased hole, most likely due to the “ringing effect” to the shallow 20 inch casing pipe.

Unit descriptions based on wireline logging

Unit boundaries are based on sedimentological observations (shown in MSF), and then the exact depths were determined by log characteristics (shown in WMSF). The depth at the bottom of the hole is in drillers depth (DSF).

Unit I (647~1256.5 m MSF)

GR is the only tool available for the entire interval of this unit, and it shows a log response similar to that of Unit II. GR values are roughly the half of the Unit II GR values, which is likely due to the larger borehole size and effects of the casing pipe (see “Data quality”). For the bottom 150 m, Dipole Sonic Imager sonic data are also available, suggesting that this interval may be continuous from Unit II. Based on such limited information, the lithology of Unit I may be similar to that of Unit II.

Unit II (~1825.5 m WMSF)

Log characteristics of Unit II consist of alternation of relatively thick layers of massive sandstones and siltstones. Resistivity borehole images suggest that the sandstones in Unit II are massive and include granules, pebbles, and mollusks (i.e., Bivalvia).

The shallowest interval (~1429.1 m WMSF) of this unit consists of sandstone and siltstone of 60–70 m thickness. Several highly resistive layers (<1 m thickness each) are probably carbonate-cemented layers. Nuclear magnetic resonance (NMR) permeability and SP responses suggest that these sandstones are generally highly permeable (up to 4000 mD).

The middle part (~1617.4 m WMSF) of this unit is a thick (~170 m) sandy sequence, also highly permeable (a few hundreds to 1000 mD), and includes at least three coal layers (see Table T10) and frequent cemented resistive layers. This sandy sequence is characterized by a highly resistive zone (1429.1~1437.9 m WMSF) at the top, where a number of highly resistive layers of decimeter thickness each are concentrated. Around the base of this sequence, at least three thick conglomerate layers can be identified at 1592.5~1599.5, 1608.2~1611.2, and 1612.2~1617.4 m WMSF. Relatively low NMR values in these layers (100 mD or less) suggest poor sorting.

The bottom part (~1825.5 m WMSF) of this unit begins as a predominantly silty sequence and gradually changes to sandy layers with depth. Three permeable sandstones of a few meters thickness show 100–2000 mD at their best points. One of these sandstones forms the basal sandstone of this unit.

Unit III (~2055.0 m WMSF)

Unit III is characterized by frequent coal layers in alternation with a sandstone and siltstone sequence of a few meters thickness. Thirteen coal layers of >30 cm thickness were identified in this unit (see Table T10) as well as a number of thin coal layers. The sandstone layers are generally permeable (a few hundreds to 1000 mD), based on the NMR, laterolog resistivity, and SP logging data sets. There are also frequent intercalations of highly resistive cemented sandstone layers. As a whole, this unit forms the most colorful interval of this borehole. Resistivity borehole images show that the sandstones of this unit generally consist of a number of thin (centimeter thickness) sandstone layers or include laminae of this scale. Because such features are not commonly observed in the sandstones of Unit II, there may be a change in the sedimentary environment at the Unit II/III boundary.

This unit can be divided into shallow and deep parts at 1916.2 m WMSF, the top horizon of the thickest coal layer in this hole. The shallow part is silty and includes frequent sand layers of a few meters thickness and three coal layers of >30 cm thickness. In the deep part, lithology is more sandy with ten coal layers of >30 cm thickness, including the thickest two horizons (i.e., 7.3 and 3.5 m thick). Based on correlations to VCDs, seven coal layers of the deep part were acquired by coring (see Table T10 for the Unit III thick coal layers).

Unit IV (~2466 m DSF)

Unit IV consists of thick (~200 m) massive shale in the upper half and alternation of sandstone and shale of a few meters thickness in the lower half, which also includes one thin coal layer (Core 337-C0020A-30R; see Table T10). Resistive cemented layers are commonly identified. The sandstone layers are generally not as permeable (100 mD or less) as those above, based on the NMR, laterolog resistivity, and SP logging data sets.

Evaluation of fluid sampling points

Formation fluid sampling points needed to be permeable sandstones. Lithology was identified using the logging data because lithologic characteristics can be distinguished clearly. The first list of 31 candidate zones in which sampling points were determined was made based on separation of the five laterolog resistivity measurements that suggested permeable layers. Then borehole resistivity images were used to define the exact depth of the zones. Based on borehole resistivity images, most sandstones consist of fine (centimeter thick) laminated sand layers that are frequently intercalated by electrically resistive layers. The resistive layers possibly have low permeability (e.g., cemented); therefore, those layers were removed from the potential sampling zones. NMR permeability was then examined to identify layers of possible free water. Most zones in the first list show high potential of free water with NMR permeability of 100–400 mD, and the zones were ranked according to the NMR permeability to form a second list. By comparing with VCDs, several zones of clean sandstone layers were replaced (or added as backups) to make the final list of potential fluid sampling points.

Prior to fluid sampling, pretests were conducted in 35 zones based on the final list to measure in situ mobility of the fluid in these zones (see “Downhole logging” in the “Methods” chapter [Expedition 337 Scientists, 2013b] for the definition of mobility). Because formation fluid viscosity may not significantly vary, mobility can be used to assess the sandstone permeability. Based on the mobility evaluations, actual formation fluid sampling zones were determined at six points as described in Table T11.

Formation pressure

By using the formation testing tool, formation pressure was measured as a part of the fluid sampling pretests. Pressure data at the 35 points indicated that formation pore pressure is hydrostatic or elevated by only a few percent of the hydrostatic value to depths of at least 2425 m WMSF (the depth of the deepest reliable measurement).

Temperature estimation

During logging operations, two types of logging tools (Environmental Measurement Sonde [EMS] and Modular Formation Dynamics Tester [MDT]) measured borehole fluid temperature in situ. The EMS measured the borehole mud temperature in detail, and the MDT recorded the temperature during the pretests and fluid sampling. We also used a method to estimate the bottom-hole static temperature by using the temperature measurements during the first three logging runs. The Horner plot (Dowdle and Cobb, 1975), a method commonly used for computing static formation temperature (Espinosa-Paredes et al., 2009), was applied and the corrected temperature was 63°C, indicating a maximum temperature gradient of 24.0°C/km. The Horner plot result was plotted together with wireline logging temperatures from EMS and the temperature measurements during the pretest (with the single probe) and during the fluid sampling (with Quicksilver probe) with the MDT (Fig. F11), and showed that the measured values were all lower than this corrected temperature gradient.

Log-seismic integration

We used the VSP to establish an accurate time-depth relationship. Detailed log-seismic correlation was achieved through the synthetic seismogram calculation and its comparison with the seismic profiles. Synthetic seismograms were calculated using P-wave velocity log and density log data (Fig. F12). The P-wave velocity log data were calibrated with the time-depth relationship derived from VSP data. We used a 30 Hz minimum phase ricker wavelet to generate synthetic seismograms. The seismic section was shifted down 10 ms to align the major reflectors with the synthetic seismograms. The synthetic seismogram matches well with the seismic profile. For example, strong reflections at ~2610–2650 and ~2780–2820 meters below sea level (mbsl) in the synthetic seismogram can be aligned to key reflectors at ~3180–3200 and ~3320–3350 ms two-way traveltime (TWT) on the seismic profile of Line ODSR03-BS, respectively. The synthetic seismogram and the seismic section also generally match at ~3100–3150 mbsl, corresponding to the coal layers. The uppermost part of this depth range in particular shows a good match; however, the reflection pattern in the lower part of this depth range shows some discrepancy between them. In the deeper interval of this borehole, the depths of the synthetic seismogram reflections are not exactly aligned to the seismic profile.

The log data in time domain, converted with the VSP time-depth relationship, can be directly compared with the time-migrated seismic section. Generally, the seismic reflectors are well correlated with the logging data. Figure F13 shows the gamma ray and resistivity logs in time domain laid on the seismic profile. The reflectors at ~3180, ~3320, and ~3580 ms TWT are correlated with the positive peaks in the resistivity log. The reflector at ~3440 ms TWT coincides with a slight positive shift in GR value. We correlated the lithologic units with the seismic profile (Fig. F14). The Unit I/II boundary can be correlated to a relatively strong, continuous reflector at ~3000 ms TWT on the seismic profile; however, the regional unit boundary was supposed to be an unconformity at ~2800 ms TWT. Unit II has two strong reflectors at ~3180 and ~3320 ms TWT, which are traceable in the regional scale. The top of Unit III might be correlated with a weak reflector at ~3490 ms TWT, but the major reflectors are located within Unit III. The bottom of Unit III is situated slightly below a series of relatively strong reflectors. Unit IV has several reflections; however, the lateral continuity of these reflectors is not well preserved in the regional range.