DOWNHOLE LOGGING

Following completion of RCB coring operations in Hole 1258C, the hole was conditioned with a wiper trip and pumped with sepiolite mud. The drill pipe was initially set at 96 mbsf and pulled for all logging passes to 86 mbsf. The following two tool strings were run:

  1. The triple combo tool string with the Lamont-Doherty Earth Observatory (LDEO) Temperature/Acceleration/Pressure (TAP) tool and MGT tool and
  2. The FMS-sonic tool string (see "Downhole Logging" in the "Explanatory Notes" chapter for further tool specifications).

The wireline heave compensator (WHC) was used on all passes, with heave varying between 1.4 and 2.0 m throughout the operation. The first tool string run was the triple combo, which was lowered to the bottom of the hole at 488 mbsf logging depth (drillers depth = 485 mbsf). Two full logging-upward passes were made with the triple combo tool string. Following this run, control of the wireline passed to the downhole measurements laboratory (DHML) and the MGT was powered up and stabilized. Two full passes (the bottom of the hole was reached on both) were made with the MGT. The second tool string (FMS-sonic) was also successfully run to the bottom of the hole. Two full passes were made with the FMS-sonic tool string. Due to the reduced telemetry requirements of the Long Spacing Sonic (LSS) tool, it was possible to run this tool string at 548.6 m/hr (normal logging speed when running the FMS-sonic tool string with the Dipole Sonic Imager [DSI] = 274 m/hr). To check data quality, the first pass of the tool string was at 274 m/hr and the second at 548.6 m/hr. In summary, two tool strings were run during the logging operation, with six separate logging passes. All passes were from total depth into the pipe, providing a logged section of 402 m (Fig. F26). The wireline depth to seafloor was set at 3195 mbrf, determined from the step increase in gamma ray counts found at the sediment/seafloor interface, and recorded on the first pass of the triple combo tool string. The drillers depth to seafloor was 3203 mbrf. This discrepancy accounts for some of the differential between the drillers pipe and total depths (80 and 485 mbsf, respectively) and the wireline depths (86 and 488 mbsf, respectively).

Data Quality

Borehole diameter can affect the response of some tools, for example the Hostile Environment Litho-Density Tool (HLDT) and Accelerator Porosity Sonde (APS), and so the size and shape of the borehole is important for interpreting the quality of logging data. The caliper logs from the triple combo (one per pass) and the FMS-sonic (two per pass) tool strings provide information on the borehole size (Fig. F27). From the base of the pipe (86 mbsf) to 455 mbsf, the hole diameter is for the most part just beyond bit size, varying only in a few places (e.g., ~260 mbsf) but always less than the maximum extension of the FMS-sonic calipers (15.5 in; 39.4 cm). From 455 mbsf to total depth, the hole diameter varies significantly, sometimes extending beyond the FMS-sonic caliper maximum extension. Thus, FMS-sonic data in a few short sections of the borehole below 455 mbsf are unusable because of loss of wall contact. Caliper data from both the FMS-sonic (C1) and triple combo tool strings indicate hole enlargements at ~9.5-m spacings from 103 to 196 mbsf (Fig. F27). These data indicate borehole enlargement due to incomplete active heave compensation of the drill pipe, which is exacerbated during drill pipe addition and core barrel recovery and reload. Any time-series analysis on logging data through this interval that shows cycles corresponding to 9- to 10-m wavelengths should be treated with caution. The borehole conditions as indicated by the caliper logs were excellent.

Data from the triple combo tool string are good, with excellent repeatability and only minor depth mismatches between passes. Gamma ray data from the Hostile Environment Gamma Ray Sonde (HNGS), MGT, and Scintillation Gamma Ray Tool (SGT) are also well matched in depth and magnitude, providing easy depth shifting for the MGT and FMS-sonic logging runs. FMS-sonic data have been processed, and almost all of the 400-m logged section is good on both passes. This result indicates that when the tool string configuration allows (i.e., the FMS-sonic tool string is not running with the DSI), the FMS-sonic may be run at 548.6 m/hr without loss of data quality. Indeed, the image quality may be marginally better at the higher logging speed.

Core physical property data provide a rapid method for visualization of the core-log correlation (Fig. F28). Because of drilling ahead with selective coring, the lack of core data from Hole 1258C was supplemented by using core data from Hole 1258B. The core density values from the MST (see "Physical Properties" in the "Explanatory Notes" chapter) are lower than the in situ recorded logging values. Density and porosity data from the few index property measurements available for Hole 1258B are close to the logging values. Core resistivity data are suspect because of the RCB coring. However, core velocities (measured directly on the core with the modified Hamilton Frame) are close to the logging values. NGR values measured on the MST underestimate the logging natural gamma values. The depth match between logging mbsf and coring (drillers) mbsf is offset in Hole 1258C because of the 8-m depth difference in the estimated seafloor depths. Overall, the patterns observed in the core physical properties are matched to the logging data, although core-log depth matching will require some depth shifting of the core data.

Logging Stratigraphy

Four logging units have been defined for Hole 1258C.

Unit 1 (base of pipe [86 mbsf]–200 mbsf)

Logging Unit 1 is characterized by downhole increases in resistivity, density (covarying with porosity), and velocity logs (Fig. F29). The porosity and gamma ray logs display high-amplitude cyclicity. This logging unit is further divided into two subunits.

Subunit 1a (86–138.5 mbsf)

Resistivity, density, and velocity increase steadily downhole through this subunit (Fig. F29) and show only small-scale fluctuations. Porosity decreases downhole, as expected, but displays high-amplitude cyclicity. The high-resolution MGT gamma ray log also displays cyclicity through this subunit (Fig. F25). The increase in density is matched by an increase in carbonate content downhole (see "Organic Geochemistry").

Subunit 1b (138.5–200 mbsf)

Gamma ray, resistivity, density, and velocity decrease downhole to ~160 mbsf, where they begin to gradually increase again to the base of the subunit at 200 mbsf (Figs. F25, F29). Porosity covaries with density and displays high-amplitude cyclicity, also seen in the MGT log (Fig. F25). The change in density appears to relate to the change in clay content as revealed by the potassium contribution in the gamma ray spectrum (Fig. F25).

Unit 2 (200–398 mbsf)

The top of the unit is best seen as a peak in the gamma ray log and a step change in density. This change is closely related to the P/E boundary (see "Biostratigraphy") seen in cores from Holes 1258B and 1258C. Resistivity, density, and velocity all show increased amplitude fluctuations throughout this unit (Fig. F29), clearly distinguishing it from Unit 1 above. Porosity and gamma ray logs have well-developed cyclicity. This logging unit is further subdivided into three subunits.

Subunit 2a (200–268 mbsf)

Gamma ray counts increase downhole, as does the resistivity, density, and velocity (Figs. F25, F29). Porosity and gamma ray logs display well-developed cyclicity, which is matched in the FMS-sonic dynamic images (Fig. F30). The base of the subunit is marked by a spike and/or step shift seen in all the logs.

Subunit 2b (268–329 mbsf)

Density and velocity increase at the Subunit 2a/2b boundary, with a concomitant decrease in porosity. Gamma ray counts decrease across the boundary and then increase downhole. Between 317 mbsf and the base of the subunit, the photoelectric effect (PEF) log increases significantly, approaching and sometimes exceeding the calcite line (Fig. F29). The increase in the PEF log is not the result of increasing carbonate content (see "Organic Geochemistry") but is likely caused by the barite and pyrite concretions (see "Lithostratigraphy"). Even in relatively small amounts, these minerals will produce a significant deflection of the PEF log because of their high photoelectric absorption cross-section index (Pe) values. The Pe value for barite is 266.8 b/e and for pyrite is 16.97 b/e (Rider, 1996).

Subunit 2c (329–398 mbsf)

The top of this unit is defined by a sharp decrease in density and velocity with a covarying increase in porosity (Fig. F29). Density values continue to fall to ~344 mbsf and then rise back to levels seen at the top of the subunit, with porosities covarying. Gamma ray and velocity values increase gradually downhole throughout the subunit (Figs. F25, F29). In the middle and lower portions of the subunit, the gamma ray logs have a prominent cyclicity at a periodicity of ~10 m (Fig. F25). The Subunit 2b/2c boundary correlates with the lithostratigraphic Unit II/III boundary (see "Lithostratigraphy").

Unit 3 (398–455 mbsf)

Logging Unit 3 is characterized by a large step increase in the gamma ray and porosity logs, with a concomitant decrease in the density and velocity logs (Fig. F29). The gamma ray spectrum (Fig. F25) reveals the source of increased gamma ray emission to be from potassium (clay) and uranium (organic matter). The high organic matter content indicated by the uranium contribution to the gamma ray spectrum is corroborated by correlation of logging Unit 3 with lithostratigraphic Unit IV (laminated black shale) (see "Lithostratigraphy" and "Organic Geochemistry"). Large-amplitude fluctuations are observed in all the logs and are interpreted to result from the periodic occurrence of cemented layers giving peaks in density (troughs in porosity), resistivity, and velocity and are highlighted in Figure F31. The PEF log indicates that the layers are calcite cemented (calcite Pe = 5.08 b/e) (Fig. F31). Unit 3 is further subdivided into two subunits (Figs. F25, F29, F30).

Subunit 3a (398–428 mbsf)

Subunit 3a is characterized by lower total counts and lower-amplitude fluctuations in the gamma ray logs (Figs. F25, F31). A similar situation is observed in the porosity, density, and velocity logs (Figs. F29, F31).

Subunit 3b (428–455 mbsf)

Cyclic variations in gamma ray and density have markedly higher amplitude and average gamma ray, density, and PEF values than in Subunit 3a above (Figs. F25, F29, F30). The base of the unit is defined by a sharp decrease in all of the logs with porosity covarying (Figs. F29, F31).

Unit 4 (455–total depth [488 mbsf])

Logging Unit 4 is characterized by low resistivity (the lowest for the whole formation) and low gamma ray values (Fig. F25). Both density and velocity increase downhole from the pronounced drop that marks the top boundary of the unit (Fig. F24). This unit correlates with lithostratigraphic Unit V (see "Lithostratigraphy"). The borehole is widened beyond the HLDT caliper maximum in three locations (455.5–460, 461.5–463.5, and 471–472 mbsf). Density and porosity measurements are adversely affected by this hole enlargement, but the resistivity (especially intermediate and deep), velocity, and gamma ray logs should still be of good quality.

Discussion

Excellent hole conditions (mostly just beyond bit size) combined with good heave compensation led to the acquisition of high-quality logging data at Site 1258. For the most part, the logging units described above correlate well with the designated lithostratigraphic units (see "Lithostratigraphy"); however, a few further points of interest shall now be addressed.

Black Shale

The continuous data derived from the logs through the black shale interval (logging Unit 3) provide the opportunity for estimation of the TOC content in this unit. The result is only approximate because the shale porosity is assumed to equate that of the sediments above, and values for some densities (e.g., organic matter) that are not well constrained are also assumed. Following Rider (1996), the following three equations are used to calculate the TOC:

fl = (bkma)/(flma),
om = (bsbk)/(omma), and
TOC (wt%) = ([0.85 x om x om]/[(om x om) + ma(1 – omfl)]) x 100%,

where

bk = density of the background sediment from the density log (2.045 g/cm3).
bs = density of the black shale interval, taken from the density log.
om = density of the organic matter (assumed) (1.15 g/cm3).
ma = density of the matrix (grain density) averaged from five MAD measurements (2.44 g/cm3).
fl = density of seawater (1.05 g/cm3).
fl = water-filled porosity.
om = volume fraction of organic matter.

The results are plotted along with values measured on core samples from Holes 1258B and 1258C (see "Organic Geochemistry") and are shown in Figure F32. Despite the fact that the measured values are from Holes 1258B and 1258C and are not depth matched to the logging mbsf depths, the results are very good. Logging Subunit 3a displays a ~2- to 3-m cyclicity pattern, which breaks down with the transition into Subunit 3b.

Photoelectric Effect

The PEF log can be used qualitatively to indicate lithology and changes in mineral composition. The calcite Pe (5.08 b/e) was used to indicate the presence of carbonate-cemented layers in parts of the formation (e.g., Figs. F29, F31). In the PEF log, a shift toward the calcite Pe in the carbonate-dominated sediments encountered during Leg 207 can be taken as an indication of increasing carbonate content. To demonstrate the accuracy of this approach, the PEF log from the first triple combo pass is plotted against depth along with measured CaCO3 percentages (see "Organic Geochemistry") from Holes 1258B and 1258C (Fig. F33), with the PEF log tracking the measured trends in CaCO3.

Gas Hydrates

Pore water chemistry indicated a freshening of the formation below 425 mbsf, which could be explained by the presence of gas hydrate (see "Inorganic Geochemistry"). The APS neutron porosity tool is essentially blind to gas hydrate and will record true formation porosities. If gas hydrate is present in pore spaces, the HLDT may record increased densities. Thus, if porosities derived from the density log are lower than neutron-measured porosities this difference may indicate the presence of gas hydrate. A porosity measurement can be obtained from the density log using the following equation:

= (mab)/(mafl),

where

= porosity and
b = bulk density.

The density-derived porosity is lower than the APS neutron porosity. However, this difference appears to be the case for the whole formation not just the zone of pore water freshening.

Gas hydrate–bearing sediments will exhibit high electrical resistivities compared to water-saturated units. Resistivity data from below 425 mbsf do not support the suggestion of a gas hydrate–bearing formation. There is no distinct change in the character of the resistivity logs downhole through logging Subunit 3b that cannot be explained by carbonate cementation. Logging Unit 4 shows a step decrease in formation resistivity, observed in both logging data (Fig. F29) and FMS imagery (Fig. F31).

Sonic velocities drop in logging Unit 4, except for a number of high-velocity peaks associated with cemented layers. If this unit was hydrate-saturated, higher average velocities would be expected. It is possible that there are small quantities of disseminated hydrate in the lower portions of the formation, perhaps concentrated in the higher-porosity layers in Unit 4. Following drilling disturbance, they would rapidly dissociate close to the borehole. However, the intermediate and deep resistivity logs would then show higher values than the shallow measurement. This response is not observed, so based on the downhole logging data it appears that there are no gas hydrates present in the formation.

Synthetic Seismograms

To compute a synthetic seismogram, formation density and velocity profiles are needed. Wireline logging provided high-quality density and velocity logs from the bottom of the hole up to pipe depth (86 mbsf). Density and velocity data for the remainder of the formation (above the pipe) were obtained from MAD (bulk density) and the Hamilton Frame (PWS3 velocity) measurements (see "Physical Properties"). Downhole impedance was calculated from velocity x density, and the impedance contrast between successive layers gave the reflection coefficient series (Fig. F34). An Ormsby wavelet was convolved with the reflection coefficient series to generate a synthetic seismogram (Fig. F35).

The synthetic seismogram matches the seismic data well, which allowed regional Reflectors B, B, and C to be correctly identified in the time domain (Fig. F35). Reflector C represents the base of the black shales. Reflector B is the top of the black shales, and Reflector B is the density and velocity step across the logging Subunit 2a/2b boundary and the lithostratigraphic Subunit IIB/IIC boundary (see "Lithostratigraphy").

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