-
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%,
Following completion of APC and XCB coring operations in Hole 1257A, the hole was conditioned with a wiper trip and pumped with sepiolite mud. The drill pipe was initially set at 84 mbsf and pulled for all logging passes to 74 mbsf. The following three tool strings were run:
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 1.2 and 1.3 m throughout the operation. The first tool string run was the triple combo, which was successfully lowered to the bottom of the hole at 288 mbsf logging depth (drillers depth = 284.7 mbsf). Two full upward passes were made with the triple combo tool string. Following the trips, control of the wireline passed to the downhole measurements laboratory (DHML) and the MGT was powered up and stabilized in the pipe. Two full passes (the bottom of the hole was reached during both) were made with the MGT. The second tool string, the FMS-sonic, was successfully run to the bottom of the hole, although pump pressure was required for the tool to exit the pipe on both passes. Two full passes were made with the tool string. Because of the reduced telemetry requirements of the Long Spacing Sonic (LSS) tool, it is possible to run this tool string at 548.6 m/hr (normal logging speed when running the FMS with the Dipole Sonic Imager [DSI] is 274 m/hr). To check data quality, the first pass of the tool string was at 274 m/hr and the second was at 548.6 m/hr, which is the maximum for the FMS. The final tool string, the WST, was also successfully run to the bottom of the hole. Eight checkshot stations were acquired at a spacing of ~30 m, except for the top interval, which was shorter because of pipe proximity.
In summary, three tool strings were run during the logging operation, with seven separate logging passes. All passes were from total depth into pipe, providing a logged section of 210 m (Fig. F22). The wireline depth to seafloor was set at 2962 mbsl, 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 drillers depth to seafloor was 2951 mbsl. A discrepancy exists between the drillers pipe and total depths (70 and 284.7 mbsf, respectively) and the wireline depths (taken here as 74 mbsf for the pipe and 288 mbsf for total depth).
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. These logs are presented in Figure F23. From the pipe (74 mbsf) down to 110 mbsf, the hole diameter is variable but always less than the maximum extension of the FMS calipers (15.5 in). From 110 down to 170 mbsf, the borehole is elliptical, extending to just under the maximum extension width of the FMS caliper arms. The FMS caliper-2 data from both passes (FMS C2 in Fig. F23) indicates hole enlargements at 83, 92.6, 102.2, 111.8, 121.5, 131.1, 140.7, and 150.3 mbsf, which correspond to the bottom of Cores 207-1257A-10X to 17X. These data indicate borehole enlargement resulting from incomplete active heave compensation of the drill pipe, which is exacerbated during 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 suspect. At 170 mbsf, the borehole narrows and maintains a uniform diameter, punctuated only by a few minor washouts/breakouts. Below 245 mbsf, the hole narrows further to just beyond bit size. The borehole conditions, as indicated by the caliper logs, were very good. The WHC was run on all the passes, except for the WST tool, when it is not required.
Data from the triple combo tools 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 logging passes. FMS-sonic data are good for the logged sections, showing excellent repeatability between passes.
Core physical property data provide a rapid method for visualization of the core-log correlation (Fig. F24). The depth match between logging mbsf and core (drillers) mbsf is good. Elastic rebound and temperature changes following drilling and recovery of the core can affect the core index properties of density and porosity and, thus, velocity (Hamilton, 1976). A small offset of density and velocity values observed in core measurements vs. logging values may be related to this effect. No consistent pattern is observed in the resistivity data. Core-measured gamma ray values underestimate the in situ values. Overall, the patterns observed in the core physical properties are well matched to the logging data, suggesting postcruise core-log depth matching should be straightforward.
Four logging units were defined for Hole 1257A.
This unit is best defined in the porosity and velocity logs and, to a minor degree, in the density logs. Porosities are higher than the unit below and fluctuate rapidly (Figs. F24, F25). The velocity logs show a step increase of 250 m/s across the lower boundary of the unit (Figs. F24, F25). Density increases through the short section recorded by the HLDT. This logging unit corresponds to the lowest part of lithostratigraphic Subunit IIB (see "Lithostratigraphy") and the early Eocene (see "Biostratigraphy").
Overall, this unit is characterized by low-amplitude cyclic fluctuations in gamma ray, resistivity, porosity, density, and velocity (Fig. F25) and has been further subdivided into three subunits based on downhole gradient changes in physical properties.
The top of this unit is clearly defined by a change in the resistivity, porosity, density, and velocity logs. High-amplitude variation in the data characterizes the unit. A shallow, increasing-downhole gradient in gamma ray, resistivity, density (covaries with porosity), and velocity is apparent (Fig. F26). This trend is interpreted to represent decreasing carbonate content (see "Organic Geochemistry" and "Lithostratigraphy") and increasing clay content downhole in the unit. The downhole increase in density is also indicated by the caliper logs (Fig. F23) as a narrowing of the hole diameter, presumably resulting from the increase in strength of the formation.
Logging Subunit 2b is characterized by lower-amplitude, shorter (depth)-period cycles in porosity, density, velocity, and resistivity but is best seen in the gamma ray logs. Increasing-downhole trends in density (covarying with porosity) resistivity, velocity (Fig. F26), and gamma ray (Fig. F25) values continue, but the gradient is slightly reduced. The caliper logs show the borehole widening downhole throughout this unit, suggesting that despite increasing density the sediment may be softer with depth. This trend may be explained by a small increase in clay content with depth combined with consolidation effects producing a density increase, but the higher clay content mechanically weakens the formation.
The top of this unit is defined by a peak, followed by a decline in the gamma ray and density (covarying with porosity) logs, and a velocity peak is observed just below the boundary (Figs. F25, F26). After the minimum is reached, values increase slowly toward the bottom of the unit, regaining the levels they had at the top. The logging Subunit 2b/3c boundary correlates with the lower Paleocene/lower Maastrichtian hiatus (see "Biostratigraphy") and lithostratigraphic Subunits IIIA and IIIB (see "Lithostratigraphy"). Despite the drop in density at the top of the unit, the caliper logs indicate the borehole narrowing downhole, suggesting increasing strength resulting from cementation with depth.
Logging Unit 3 is characterized by a large step increase in the gamma ray, resistivity, and porosity logs, with a concomitant decrease in the density and velocity logs. The gamma ray spectrum (Fig. F27) reveals the source of increased gamma ray emission to be potassium (clay) and uranium (organic matter). Large-amplitude fluctuations are observed in all the logs. These fluctuations are interpreted to result from the periodic presence of cemented layers, giving peaks in density (troughs in porosity), resistivity, and velocity, and are highlighted in the FMS resistivity images (Fig. F28). The photoelectric effect (PEF) log indicates that the cement in these layers is calcite (photoelectric absorption cross-section index [Pe] = 5.08 b/e– for calcite) (Fig. F28). 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, black shale (see "Lithostratigraphy" and "Organic Geochemistry"). The unit is further subdivided into two subunits based on the gamma ray logs (Fig. F27).
Logging Subunit 3a is characterized by three well-defined high-amplitude cycles in the gamma ray logs. The cyclic fluctuations are the result of synchronously varying potassium and uranium contributions (Fig. F27).
Uranium and potassium and, thus, total gamma ray counts decrease in logging Subunit 3b. Cycles in this subunit have a higher frequency (smaller depth range) and lower amplitude than cycles in logging Subunit 3a above. The base of the unit is defined by a sharp peak in total gamma ray counts that is dominated by uranium contribution, indicating a very organic-rich horizon (Fig. F27).
Logging Unit 4 is characterized by consistently high density (low porosity), resistivity, and velocity values recorded through the formation (Fig. F25), discounting the carbonate-cemented layers highlighted in logging Subunit 3a above. The unit has been subdivided into three subunits.
Density, resistivity, and velocity increase gradually in this unit, with only small-amplitude variations recorded. However, a spike in the PEF log, correlated with porosity and velocity peaks and a density trough, indicates a concentration of quartz at 223 mbsf. This horizon correlates with a calcareous sandstone layer identified in Holes 1257B and 1257C (see "Lithostratigraphy"). The PEF log indicates another calcareous sandstone layer at 237.5 mbsf (Fig. F25).
Three distinct peaks in resistivity and velocity combined with a step to higher gamma ray values characterize this subunit. The two lower peaks correlate with peaks in the density, porosity, and PEF logs and the FMS images and probably represent calcareous cemented sandstone layers. Total gamma ray counts are higher due to an increased contribution from both thorium and uranium.
Only short intervals of data from most tools are available for this subunit. Resistivity, density, and velocity values return to levels similar to logging Subunit 4a above. Gamma ray counts, however, remain similar to the levels in logging Subunit 4b, indicating the continuing presence of organic material (see "Organic Geochemistry"). Two peaks at the bottom of the resistivity log (Fig. F25) correspond to bright resistive layers in the FMS imagery and are interpreted to represent cemented layers, probably comprising calcareous-cemented sandstone similar to that found in logging Subunits 4a and 4b, described above.
Good hole conditions combined with low heave led to the acquisition of excellent logging data. 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.
The continuous data derived from the logs through the black shale interval (logging Unit 3) provides 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
om x om]/[(om x om) + ma(1 – om – fl)]) x 100%,
where
bk = density of the background sediment taken from the density log (1.88 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 (2.52 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 from core samples (see "Organic Geochemistry") in Figure F29. Despite the fact that the measured values are from Holes 1257A, 1257B, and 1257C and are not depth matched to the logging mbsf depths, the results are very satisfactory.
Both MGT and SGT passes and the second pass of the HNGS gamma ray tools all identified a major peak at 98.5 mbsf (Figs. F24, F25, F27). The other logs do not show evidence of a major change in physical properties. The gamma spectrum over the interval shows an increase in potassium, thorium, and uranium spectra, indicating contributions from both clay and organic matter (Fig. F30). The FMS image in Figure F30 suggests a higher-resistivity layer may be the source for the gamma ray spike. Core was only recovered across this interval in Hole 1257B. Given the poor core recovery and slumping activity associated with the top of lithostratigraphic Subunit IIIA at 85 mbsf (see "Lithostratigraphy"), it is possible that the gamma ray spike indicates slumping extending deeper into the section, bringing allochthonous material to the site from upslope.
During the description of logging Subunit 2b, mention was made of the counter-intuitive borehole widening as formation density increased. The gamma ray spectrum through this interval (Fig. F27) indicates that the increased gamma count results from a higher level of potassium. The step change in total gamma at the base of the unit and concomitant borehole narrowing is matched by a reduction in potassium counts. The uranium and thorium spectra show little change between logging Subunits 2b and 2c. One possible explanation for the formation softening is higher levels of illite (highest in potassium of the clays) in logging Subunit 2b, with a step change into logging Subunit 2c. Despite the decrease in density at the top of logging Subunit 2c, the borehole begins to narrow, suggesting perhaps better cementation associated with the hiatus between the lower Maastrichtian and lower Paleocene represented by the logging Subunit 2b/2c boundary.
The temperature record from the TAP tool must be interpreted with caution because the borehole has probably not reached thermal equilibrium following circulation of the drilling fluid. Nevertheless, abrupt temperature changes representing localized fluid flow into the borehole may be identified. The borehole temperature profile recorded on the first logging-downhole pass of the TAP tool (it is located on the bottom of the triple combo tool string) is shown in Figure F31. Pipe effects are clearly obvious to 90 mbsf. From 90 to 128 mbsf, there is a steep temperature gradient, indicating mixing between the cold pipe and warmer borehole fluids. What is interpreted to represent a more realistic formation borehole temperature begins at 138 mbsf and extends to the bottom of the hole. There is a clear perturbation (inflow of cooler fluid) in the temperature profile between 162 and 212 mbsf. This temperature perturbation is closely related to, and suggestive of, fluid flow through the black shale interval (Fig. F31). This observation provides direct supporting evidence for results from the pore water geochemistry studies (see "Inorganic Geochemistry").
A checkshot survey was conducted during logging operations in Hole 1257A, and eight stations were collected using a series of stacked shots at 30-m intervals up the borehole (Fig. F22). The checkshot survey provides a direct measurement of root mean square (rms) acoustic traveltime (Table T22) and, thus, formation velocity. Conversion of these rms values to interval velocities allows the checkshot data to calibrate the velocity log. For most sites, the logging velocity is higher than the measured interval velocity (Fig. F32), so a correction factor of –100 m/s was applied to the velocity log. Downhole impedance (velocity x density) was calculated, and the impedance contrast between successive layers gave the reflection coefficient series. An Ormsby wavelet was convolved with the reflection coefficient series to generate the synthetic seismograms (Fig. F33).
The synthetic seismogram accurately matches the seismic data, which allowed the regional B, B´, and C reflectors to be reinterpreted. Contrary to previous interpretations of the seismic data, Reflector C represents the base of the black shales, unconformably overlying the middle–upper Albian synrift sediments. Reflector B´ is the top of the black shales (173 mbsf), and Reflector B is the density and velocity step at 138 mbsf (the logging Subunit 2b/2c boundary), probably representing the lower Paleocene hiatus (see "Biostratigraphy") and the contact between lithostratigraphic Subunits IIIA and IIIB (see "Lithostratigraphy").
