-
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 RCB coring operations in Hole 1261B, the hole was conditioned with a wiper trip and pumped with sepiolite mud. The drill pipe was set at 110 mbsf (logging depth) for all passes. The following three tool strings were run:
The wireline heave compensator (WHC) was used on all passes, with heave varying between 1.3 and 3.0 m throughout the operation. The first tool string run was the triple combo, which encountered a bridge at 210 mbsf. After working the tool for ~30 min, the triple combo passed through and reached the bottom of the hole without further trouble (665 mbsf). There was ~21 m of fill in the bottom of the hole. At the beginning of the first pass, the WHC began to stroke out because of a malfunction of one or both of the limiter switches. This problem was repaired, and during the first pass the WHC stroked out on one occasion (when heave reached ~3 m). Power was lost to the tool string at 325 mbsf and was reestablished at 316 mbsf. Pass 1 of the triple combo tool string was terminated at 314 mbsf in order to be used as the repeat section. The tool string was lowered, and pass 2 began. High heave again caused the WHC to stroke out on two occasions, between 488 and 470 and 434 and 410 mbsf. The WHC operated for the remainder of the pass (switched off at 141 mbsf), which continued up past the seafloor. A rise in the head tension on the upward pass indicated a tight spot at 210 mbsf but did not cause any difficulties. Following this run, control of the wireline passed to the downhole measurements laboratory and the MGT was powered up and stabilized. One full pass one short repeat pass from 172.5 mbsf into the pipe was made with the MGT. The second tool string, the FMS-sonic, was successfully run to the bottom of the hole, although a reduction in head tension was observed as the tool string passed through 210 mbsf. Two full passes were made with the tool string, logging at maximum FMS recommended speed (548.6 m/hr); the second was made to the seafloor. The WST also encountered the restriction at 210 mbsf but, due to its lower weight, was unable to pass through. Five checkshot stations were acquired between 210 mbsf and the base of the pipe (110 mbsf). In summary, three tool strings were run during the logging operation with seven separate logging passes. Two passes were conducted from total depth to above seafloor, one from total depth into the pipe, and three were reduced-length passes (Fig. F21). The wireline depth to seafloor was set at 1899 meters below rig floor (mbrf), determined from the step increase in gamma ray counts found at the sediment/seafloor interface recorded by the Scintillation Gamma Ray Tool (SGT) on the second pass of the FMS. The official ODP depth to seafloor was 1911.1 mbrf. Thus, there is a 12.1-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 12.1-m depth mismatch, but all core physical property data used in figures herein have been depth shifted.
Borehole diameter can affect the response of some tools (e.g., the Hostile Environment Litho-Density Tool [HLDT] and Accelerator Porosity Sonde) and so the size and shape of the borehole are important for interpreting the quality of logging data. The caliper logs from the triple combo (1 per pass) and the FMS-sonic (2 per pass) tool strings provide information on the borehole size (Fig. F22). From the pipe (110 mbsf) down to 150 mbsf, the hole diameter varies between bit size and 12 in. From 150 to 374 mbsf, the hole is in very poor condition, with rapid fluctuations beyond 18 in (45.76 cm), which is larger than the maximum extension of both the FMS-sonic (15.5 in; 39.4 cm) and triple combo (18 in; 45.76 cm) calipers. From 374 mbsf to total depth, the borehole is in excellent condition being at or just beyond bit size, except for a few short sections of washouts/breakouts. Because of the borehole conditions, all logs are good below 374 mbsf but above this depth FMS-sonic, density, and porosity have all been adversely affected. The repeat section for the triple combo tool string has only minor (<1 m) depth mismatches between passes, despite the problems with the WHC. Gamma ray data from the HNGS, MGT, and SGT are also well matched in depth and magnitude, making depth matching of the MGT and triple combo tool string logging runs to pass 2 of the FMS-sonic tool string straightforward.
Core physical property data provide a rapid method for visualization of the core-log correlation (Fig. F23). Because of drilling ahead with selective coring, there is a paucity of core data from Hole 1261B, so all of the core data presented are from Hole 1261A. Borehole diameter and wall rugosity effects are adequately displayed by the offset between the core and logging density and porosity values from 150 to 374 mbsf. MAD properties and logging data correlate very well for the remainder of the hole. Core gamma measurements slightly underestimate formation values, but the pattern match is good. Core velocities, measured directly on the core with the modified Hamilton Frame, are close to the logging values, except for the interval between 374 and 538 mbsf, where they are considerably less (100 m/s). The depth match between logging mbsf and core data is excellent when the core depths are corrected (12.1-m downhole shift).
Four logging units have been defined for Hole 1261B.
Unit 1 is characterized by high gamma ray counts and low resistivity and velocity values (Fig. F24). Borehole conditions throughout most of this unit are poor, but gamma ray, resistivity, and velocity tools are not greatly affected by these problems. The logging unit is further subdivided into two subunits.
The borehole diameter for most of this subunit is between bit size and 12 in (30.5 cm), and so all logs are expected to be of good quality. Gamma ray values are high at the top and decrease toward the bottom of the subunit (Figs. F24, F25). A submeter-scale cyclicity is apparent in the gamma ray logs. Resistivity, porosity, density, and velocity fluctuate moderately around a constant average (Fig. F24).
The borehole diameter throughout this subunit fluctuates greatly. The hole is enlarged to well beyond bit size in some places and is significantly under gauge in others. The under-gauge areas are interpreted to result from deformation (i.e., expansion) of the clay-rich layers (see "Lithostratigraphy"). Because of poor borehole conditions, density and porosity logs are not used in any descriptions or interpretations of this subunit. Gamma ray levels are high through the unit, reflecting high clay content. This trend is seen in the gamma ray spectrum as the computed gamma ray plots directly over the total gamma ray counts (Fig. F25). High-resolution data obtained with the MGT shows cyclicity down to the submeter scale (Fig. F26). Figure F26 also show that the under-gauge parts of the borehole equate to the clay-rich horizons. Correlation between core and logging data is excellent, given that the core data have only been depth shifted (–12.1 m) and not depth matched. This good core-log correlation also supports the interpretation that the gamma ray logs are valid in terms of the relative changes, if not absolute magnitudes. Velocities increase slowly downhole, suggesting a normal consolidation trend that is supported by the core bulk density data (Fig. F23) (see "Physical Properties"). Resistivity is low throughout the subunit. The base of this subunit correlates with the base of lithostratigraphic Subunit IB (see "Lithostratigraphy").
The top of Unit 2 is seen as a peak in the gamma ray log (Figs. F24, F25). Below the boundary, resistivity and velocity increase in magnitude (Figs. F24, F25). The borehole is still in poor condition, so no density or porosity data are used to define this unit. Between 330 and 340 mbsf, the borehole becomes uniform and density and porosity data are good (Fig. F27). The photoelectric effect (PEF) log indicates a shift toward more carbonate-rich sediment (Fig. F27) and cycles are observed in the FMS-sonic images. This interval is interpreted as a 10-m-thick raft of nannofossil chalk in a large debris flow. The equivalent section was seen in the core in Hole 1261A (Cores 207-1261A-16R and 17R) as clasts of nannofossil chalk up to 0.2 m (see "Lithostratigraphy" and "Paleomagnetism"). Gamma ray values first decrease and then increase before dropping off again at the base of the unit. Velocity values increase steadily downhole, but an increase in resistivity occurs only toward the base of the unit. The base of logging Unit 2 correlates with lithostratigraphic Subunit IC (see "Lithostratigraphy") and the middle Eocene–late Miocene unconformity. It is clearly observed in all of the logs, particularly in the borehole diameter data (Figs. F21, F24).
Logging Unit 3 is clearly visible in all logging data (Figs. F22, F24). Borehole conditions improved dramatically and are similar to those experienced in other holes logged throughout the leg (see "Downhole Logging" in the "Site 1257" chapter; "Downhole Logging" in the "Site 1258" chapter; and "Downhole Logging" in the "Site 1260" chapter). The top boundary is defined by a decrease in gamma ray and porosity, with a concomitant rise in resistivity, density, and velocity. The decrease in gamma ray (reduction in clay) is matched by a rise in the PEF log toward calcite, indicating an increase in the carbonate content (see "Organic Geochemistry") compared to logging Units 1 and 2. The unit is further subdivided into four subunits.
In Subunit 3a, gamma ray counts are the lowest of the formation and fluctuate around a constant average. Density (porosity covarying) and velocity increase with depth and may follow a normal consolidation line, although the trend may also be due, at least in part, to increasing carbonate content as observed in the PEF log (Fig. F24). Resistivity increases downhole, punctuated by a step to lower values between 426 and 441 mbsf. A similar pattern is seen in the velocity log. Density and porosity data show only a single step change to lower and higher values, respectively, from which they gradually recover. Gamma ray and porosity logs display cyclicity to the submeter scale. The base of the subunit is marked by an increase in gamma ray, resistivity, and density.
Resistivity and velocity remain high at the top of the Subunit 3b for ~20 m and then decrease downhole to the bottom (Fig. F24). Density decreases steadily for the length of the subunit and appears to be related to a decrease in carbonate content, indicated by the PEF log (Fig. F24). The gamma ray log takes a step increase across the upper boundary and maintains a relatively constant average. The porosity log shows little change across the boundary and also fluctuates around a constant average. Both the gamma ray and porosity logs show cyclicity down to the submeter scale. The base of the subunit is defined as a peak in gamma ray and porosity curves and a trough in resistivity, density, and velocity logs and correlates with the P/E boundary (see "Biostratigraphy") and the lithostratigraphic Unit II/III boundary (see "Lithostratigraphy") (Fig. F24).
Resistivity and velocity magnitudes decrease sharply across the upper boundary of Subunit 3c and exhibit large-amplitude variations (Fig. F24). Density undergoes a smaller decrease but displays similar enhanced amplitude fluctuations (Fig. F24). The gamma ray and porosity log values conversely increase downhole, both displaying cyclicity down to the submeter scale. The base of the subunit is marked by a step increase in several of the logs, best seen in resistivity and velocity and only slightly less pronounced in density. Porosity and gamma ray values concomitantly show a step decrease.
Resistivity, density, and velocity values all decrease downhole from the upper boundary and have higher-amplitude fluctuations relative to the other Unit 3 subunits (Fig. F24). Gamma ray and porosity logs increase downhole, with the porosity log also displaying higher-amplitude fluctuations. The PEF log indicates increased carbonate content, and a number of peaks reach the calcite photoelectric absorption cross-section index (Pe), suggesting carbonate-rich layers. This apparent increase in carbonate is not supported by core lithostratigraphic descriptions, which indicates this subunit has an increased clay content. It is, however, supported by geochemical analysis that shows an increase in carbonate content to values just below 90 wt% (see "Organic Geochemistry"). The PEF-carbonate interpretation is supported by the gamma ray log, which indicates low clay content (Fig. F25). It is difficult at this stage, without detailed core-log depth matching, to determine if the top of the subunit correlates with the K/T unconformity (see "Biostratigraphy") or if the K/T unconformity is the trough in density, velocity, and resistivity (peak in gamma and porosity) seen 6 m below the upper subunit boundary. The base of the subunit is marked by a rapid increase in the gamma ray log and a smaller increase in the porosity log and correlates with the bottom of lithostratigraphic Subunit IIIB (now with a 10-m offset) (see "Lithostratigraphy").
Logging Unit 4 demonstrates a shift in the gamma ray and porosity logs to higher levels and the density and velocity logs to lower levels. All logs exhibit increased amplitude fluctuations, especially seen in resistivity (Figs. F24, F28). The unit is further subdivided into two subunits.
The sharp increase in gamma ray values at the upper boundary is maintained to the base of Subunit 4a, punctuated by a number of large amplitude fluctuations. The gamma ray spectrum (Fig. F25) shows that this increase is due mainly to an increase in uranium and, to a lesser extent, an increase in potassium content. The increase in uranium (bound to organic matter) is supported by core lithology descriptions (see "Lithostratigraphy"). Layers with high resistivity, density, velocity, and low gamma ray and porosity magnitudes are interpreted to be carbonate cemented, corroborated with peaks in the PEF log (Fig. F28). The upper boundary of the subunit correlates with the top of lithostratigraphic Unit IV (see "Lithostratigraphy"). The base of the subunit is marked by a step decrease in the gamma ray log.
The gamma ray spectrum indicates that the decrease in gamma ray value is due mostly to a reduction in uranium and, to a lesser degree, in potassium (Fig. F25). Uranium contribution increases again toward the base of the subunit. Resistivity, porosity, density, and velocity logs fluctuate at higher amplitudes than in logging Subunit 4a (Fig. F28). Layers with high resistivity, density, and velocity values are distinct in the FMS imagery and are interpreted as carbonate-cemented layers. At 614 mbsf, the PEF log extends beyond the calcite line and correlates with a coarse-grained glauconite-rich layer seen in Sections 207-1261A-45R-2 and 207-1261B-9R-4 (Fig. F28) (see "Lithostratigraphy"). The base of the subunit extends to the bottom of the logged interval. Lithostratigraphic Unit V is not represented in the logging stratigraphy because 21 m of fill limited the penetration of logging tools to 665 mbsf, which is at the lithostratigraphic Unit IV/V boundary (when depth shifted 12.1 m to the logging seafloor depth).
High heave conditions initially caused problems, but data from those runs have been depth matched to pass 2 of the FMS-sonic tool string when the WHC operated throughout, thus removing any problems related to increased heave. Poor hole conditions in the upper part of the borehole (above 376 mbsf) limited the use of some logging data in this region. The main focus was the black shale interval, which was adequately recorded and is readily interpreted with the logging data. For the most part, the logging units described above correlate well with the designated lithostratigraphic units (see "Lithostratigraphy") once the core depths have been shifted down 12.1 m to account for the logging and drillers seafloor depth mismatch.
The continuous data derived from the logs through the black shale interval (logging Unit 4) 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
om x om]/[(om x om) + ma(1– om – fl)]) x 100%,
where
bk = density of the background sediment from the density log (2.043 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 (2.483 g/cm3).
fl = density of seawater (1.05 g/cm3).
fl = water-filled porosity.
om = volume fraction of organic matter.
Calculated TOC results are plotted along with values measured on core samples from Holes 1261A and 1261B (see "Organic Geochemistry") and are shown in Figure F29. The core depths have been depth shifted but are not depth matched to the logs.
A checkshot survey was conducted during logging operations in Hole 1261B, but a bridge encountered at 210 mbsf limited the depth of penetration of the WST tool string (see the first paragraph of "Downhole Logging"). Five stations were collected, using a series of stacked shots at 20-m intervals up the borehole (Fig. F21). The checkshot survey provides a direct measurement of acoustic traveltime (Table T21) and, thus, formation velocity. Conversion of these traveltimes to interval velocities allows the checkshot data to calibrate the velocity log, which has proved particularly useful at Site 1261 given the poor borehole conditions in the upper part of the hole and the lack of physical property measurements (see "Physical Properties"). To compute a synthetic seismogram, formation density, and velocity profiles are needed. Wireline logging density and velocity logs were used from the bottom of the hole up to 248 mbsf. Interval velocities calculated from the checkshot survey were used for the upper part. Downhole impedance was calculated from velocity x density, and the impedance contrast between successive layers gave the reflection coefficient series (Fig. F30). An idealized Ormsby wavelet was convolved with the reflection coefficient series to generate the synthetic seismogram (Fig. F31).
The synthetic seismogram matches allowed lithostratigraphic units and seismic stratigraphy to be correlated. Reflector B´ ties to the top of the black shale sequence (574 mbsf), and Reflector B matches the density and velocity step (538 mbsf) at the Subunit 3c/3d boundary, equivalent to the lithostratigraphic Subunit IIIA/IIIB boundary (see "Lithostratigraphy"). Reflector A correlates with the logging Unit 2/Subunit 3a boundary, the middle Eocene–late Miocene unconformity (see "Biostratigraphy"), and is equivalent to the lithostratigraphic Subunit IC/Unit II boundary (see "Lithostratigraphy").
