DOWNHOLE LOGGING

Following completion of RCB coring operations in Hole 1260B, the hole was conditioned with a wiper trip and pumped with sepiolite mud. The drill pipe was set at 90 mbsf for all passes except the first, when it was pulled to 80 mbsf. The following three tool strings were run:

The triple combo) with the Lamont-Doherty Earth Observatory Temperature/Acceleration/Pressure (TAP) tool and MGT;
The FMS-sonic tool string; and
The WST for a checkshot survey (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 1.3 and 2.0 m throughout the operation. The first tool string run was the triple combo, which was successfully lowered to the bottom of the hole at 515 mbsf logging depth (drillers depth = 509 mbsf). Two full logging-uphole passes were made with the triple combo, the first to seafloor. Following this run, control of the wireline passed to the downhole measurements laboratory and the MGT was powered up and stabilized. Two full passes (the bottom of the hole was reached on both passes) were made with the MGT. The second tool string, the FMS-sonic, was successfully run to the bottom of the hole. Two full passes were at maximum FMS recommended speed (548.6 m/hr). The WST was the final tool string. It was run successfully to the bottom of the hole and acquired a total of 14 checkshot stations at ~30-m spacing.

In summary, three tool strings were run during the logging operation, with seven separate logging passes. All passes were from total depth into the pipe, providing a logged section of 425 m (Fig. F27). The wireline depth to seafloor was set at 2553 mbrf, determined from the step increase in gamma ray counts found at the sediment/seafloor interface, recorded on the first pass of the triple combo. The official ODP depth to seafloor was 2560 mbrf. Thus, there is a 7-m offset between the core and logging depths. In all of the chapter diagrams, the core recovery and lithostratigraphic units have not been depth shifted, giving an apparent 7-m depth mismatch.

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), so the size and shape of the borehole are important when 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. F28). The borehole conditions as indicated by the caliper logs were excellent. From the pipe (90 mbsf) down to 450 mbsf, the hole diameter varies little, from 450 to 485 mbsf the hole widens out and then narrows again, and below 485 mbsf it remains constant, punctuated only by a few washouts/breakouts. Everywhere, the borehole diameter is less than the maximum extension of the FMS-sonic calipers (15.5 in; 39.4 cm). Because of excellent borehole conditions and successful heave compensation, the processed FMS data are good throughout the logged formation. Caliper data from both the FMS-sonic and triple combo tool strings indicate hole enlargements at ~9.5-m intervals from 100 to 250 mbsf (Fig. F28) and from 300 to 430 mbsf. 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 analyses on logging data from tools susceptible to borehole diameter effects that show cycles corresponding to 9- to 10-m or 5-m wavelengths (where short cores were taken) should be treated with caution.

Data from the triple combo tool string are good, with excellent repeatability and only minor (<1 m) 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, making depth matching the MGT and FMS-sonic logging runs to the first pass of the triple combo tool string straightforward.

Core physical property data provide a rapid method for visualization of the core-log correlation (Fig. F29). Drilling ahead with selective coring resulted in a lack of core data from Hole 1260B, which has been supplemented with core data from Hole 1260A. The core density and porosity data from the index property measurements are close to logging values. Small offsets may be related to unloading effects in the cores. Core velocities, measured directly on the core with the modified Hamilton Frame, are less than the logging values. This observation is interpreted to result from either sampling (removal from in situ conditions) and/or an overestimation of formation velocities by the Long Spacing Sonic (LSS) logging tool (see "Checkshot Survey and Synthetic Seismogram"). Core gamma ray values underestimate the logging values. The depth match between logging mbsf and core data are excellent when the core depths are corrected (7-m downhole shift).

Logging Stratigraphy

Four logging units have been defined for Hole 1260B.

Unit 1 (base of pipe [80 mbsf]–285 mbsf)

Unit 1 is characterized by a downhole increase in density (covarying with porosity) and velocity logs (Fig. F30). The porosity and gamma ray logs display cyclicity. This logging unit is further divided into two subunits.

Subunit 1a (80–199 mbsf)

Density (porosity covarying) and velocity increase steadily downhole through this subunit (Fig. F30) and show only small-scale fluctuations. Porosity decreases downhole, as expected, and displays high-amplitude cyclicity. The high-resolution MGT gamma ray log also displays cyclicity through this subunit. The increase in density is manifest in the borehole caliper logs as a gradual narrowing of the borehole. The density gradient likely results from normal consolidation as there is no concomitant increase in carbonate content (see "Organic Geochemistry") or clay content (see the gamma ray spectrum on Fig. F31). The base of this subunit is marked by a short-lived peak in resistivity, density, and velocity (Fig. F30). The FMS images indicate a 13.5-m-thick region of higher resistivity, containing 15 distinct high-resistance bands varying from 1 to ~20 cm thick (Fig. F32). The photoelectric effect (PEF) log indicates a peak in carbonate levels through this interval but with increased amplitude variation between high and low carbonate levels. Core was only recovered from the topmost part of this interval and indicates chert nodules (see "Lithostratigraphy"). Based on the core and logging descriptions, this interval appears to be a carbonate-cemented zone with layers of chert or carbonate and chert nodules. The logging Subunit 1a/1b boundary is equivalent to the change from lithostratigraphic Subunits IIB/IIC.

Subunit 1b (199–285 mbsf)

Density (porosity covarying) and velocity continue to increase downhole to the bottom of the unit (Fig. F30). Gamma ray counts increase across the top of Subunit 1b and continue rising to ~240 mbsf, when they begin to fall (Fig. F31). Resistivity stays low in the top part of the unit but begins to increase from 230 mbsf to the bottom of the subunit (Fig. F30). Porosity and gamma ray logs display cyclicity down to the submeter scale. The change in density appears to follow a normal consolidation line, showing little correlation with changes in carbonate (see "Organic Geochemistry") or clay content.

Unit 2 (285–397 mbsf)

The top of Unit 2 is best seen as a peak in the gamma ray log and a step in the porosity log (Fig. F30). This change is close to the P/E boundary (see "Biostratigraphy") and the change from lithostratigraphic Units II to III (see "Lithostratigraphy"). Resistivity, density, and velocity values all show increased amplitude fluctuations throughout this unit (Fig. F30), clearly distinguishing it from Unit 1. Porosity and gamma ray logs have well-developed cyclicity. The logging unit is further subdivided into two subunits.

Subunit 2a (285–328 mbsf)

Gamma ray counts, resistivity, density (covarying porosity), and velocity values all gradually increase downhole in Subunit 2a (Figs. F30, F31). Porosity and gamma ray logs display well-developed cyclicity, which is also observed in both FMS dynamic and static images (Fig. F33). The variability of the logs in this subunit presumably reflects concomitant fluctuations in depositional processes (see "Lithostratigraphy"). Density continues to increase along what appears to be a normal consolidation line (Fig. F30). The base of the subunit is marked by a peak–trough couplet seen in all the logs. The subunit lower boundary correlates with the lithostratigraphic Subunit IIIA/IIIB boundary (see "Lithostratigraphy").

Subunit 2b (328–397 mbsf)

Resistivity, density, and velocity values increase rapidly from the top boundary of Subunit 2b, with a concomitant decrease in porosity and gamma ray counts. There is a marked peak in porosity, a trough in resistivity, density, and velocity, and a low resistivity region in the FMS imagery at 339 mbsf (Fig. F34), correlating with the location of the K/T boundary. Below the K/T boundary, resistivity decreases gradually downhole. Density and velocity fluctuate about a relatively constant average, punctuated by a small increase between 370 and 378 mbsf, before dropping off rapidly in the lowest 12 m of the subunit (Fig. F30). Porosity and gamma ray values increase continuously downhole, still displaying cyclicity down to the submeter scale (Figs. F30, F31). Resistivity, density, and velocity all show increased amplitude fluctuations compared with Subunit 2a above. The increase in resistivity, density, and velocity are linked to a higher carbonate content than in the subunit above (see "Organic Geochemistry"), which is also indicated by the PEF log (Fig. F30). The base of this subunit correlates with the base of lithostratigraphic Subunit IIIC (see "Lithostratigraphy").

Unit 3 (397–485 mbsf)

Logging Unit 3 is characterized by a large increase in gamma ray and porosity log values, with a concomitant decrease in the density and velocity (Fig. F30). The gamma ray spectrum (Fig. F31) reveals the source of increased gamma ray emission to be dominantly uranium (organic matter), with some contribution from potassium (clay). The high organic content indicated by the uranium contribution to the gamma ray spectrum is corroborated by correlation of logging Unit 3 with lithostratigraphic Unit IV (calcareous claystone with organic matter) (see "Lithostratigraphy" and "Organic Geochemistry"). Large-amplitude fluctuations are observed in all the logs and are interpreted to result from periodic cemented layers, giving peaks in density (troughs in porosity), resistivity, and velocity, and are highlighted in Figure F35. The PEF log indicates that the layers are calcite cemented (calcite photoelectron absorption cross-section index [Pe] = 5.08 b/e^–) (Fig. F33). Unit 3 is further subdivided into two subunits (Figs. F30, F31, F35).

Subunit 3a (397–445.5 mbsf)

Subunit 3a is characterized by lower total counts and lower-amplitude fluctuations in the gamma ray logs (Figs. F31, F35). The converse is observed especially in the resistivity, density, and velocity logs (Figs. F30, F35).

Subunit 3b (445.5–485 mbsf)

The top of the subunit is marked by a large increase in the amplitude of gamma ray fluctuations and in the overall counts, which subsequently fall continuously to the base of the unit (Figs. F30, F32, F35). Average resistivity and velocity values drop marginally (Figs. F30, F35), whereas density shows a small increase downhole as does the PEF log (Figs. F30, F35), indicating an increased carbonate content that is corroborated by geochemical analyses (see "Organic Geochemistry").

Unit 4 (485 mbsf–total depth [515 mbsf])

The top of Unit 4 is defined by a sharp increase in resistivity, density (porosity covarying), and velocity (Figs. F30, F35). Resistivity, density, and velocity then decrease (as does the PEF log), indicating decreasing carbonate content. The FMS images highlight 10 prominent high-resistivity cemented layers, the thickest being 0.4 m. Gamma ray counts are still high, reflecting the high clay content (see "Lithostratigraphy"). This unit correlates with lithostratigraphic Unit V (see "Lithostratigraphy").

Discussion

Excellent hole conditions (mostly just beyond bit size) combined with good heave compensation led to the acquisition of high-quality logging data. For the most part, the logging units described above correlate well with the designated lithostratigraphic units (see "Lithostratigraphy").

Total Organic Carbon

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 to 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 = (_bk – _ma)/(_fl – _ma),
_om = (_bs – _bk)/(_om– _ma), and
TOC (wt%) = [(0.85 x _om x _om)/(_om x _om ) + _ma (1 – _om – _fl)] x 100%,

where

_bk = density of the background sediment from the density log (1.938 g/cm³).
_bs = density of the black shale interval 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 (2.441 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 1260A and 1260B (see "Organic Geochemistry") and are shown in Figure F36. The core depths have been depth shifted but are not depth matched to the logs. The results are good in terms of the magnitudes of TOC.

Checkshot Survey and Synthetic Seismograms

A checkshot survey was conducted during logging operations in Hole 1260B, and 14 stations were collected, using a series of stacked shots at 30-m intervals up the borehole (Fig. F27). The checkshot survey provides a direct measure of acoustic traveltime (Table T22) and thus formation velocity. Conversion of these traveltimes to interval velocities allows checkshot data to calibrate the velocity log. For most checkshot locations, the logging velocity is higher than the measured interval velocity by ~100 m/s. To compute a synthetic seismogram, formation density and velocity profiles are needed. Wireline logging provided density and velocity logs from the bottom of the hole up to pipe depth (90 mbsf). The wireline velocity data were corrected by –100 m/s. Density and velocity data for the remainder of the formation, above the pipe, were obtained from MAD property (bulk density) and Hamilton Frame (PWS3 velocity) measurements (see "Physical Properties"). Core velocities were also corrected for in situ temperature and pressure. Downhole impedance was calculated from velocity x density, and the impedance contrast between successive layers gave the reflection coefficient series (Fig. F37). An idealized Ormsby wavelet was convolved with the reflection coefficient series to generate the synthetic seismogram (Fig. F38). The synthetic seismogram matches accurately to the seismic data, which allowed Reflectors A, B, B´, and C to be accurately identified. Reflector C represents the base of the black shales, unconformably overlying the middle–late Albian synrift sediments. Reflector B´ ties to the top of the black shale sequence (397 mbsf), and Relector B matches the density and velocity step at 330 mbsf just below the logging Subunit 2a/2b boundary, equivalent to lithostratigraphic Subunits IIIA and IIIB (see "Lithostratigraphy"). Reflector A correlates with the middle–upper Eocene hiatus (see "Biostratigraphy"), equivalent to the lithostratigraphic Subunit IIA/IIB boundary (see "Lithostratigraphy").