Hole 1084A was logged with a full suite of sensors to continuously characterize the sedimentary changes and structures, to correlate and compare the lithostratigraphy with that for previous holes, and to provide data for core-log integration.
Hole 1084A was logged with four different tool strings. The first tool string (seismostratigraphy) included the NGT, DIT, and TLT sondes. The second tool string (lithoporosity) included the NGT, neutron porosity, gamma density, and TLT sondes. The third tool string (FMS, 1 pass) included the NGT, inclinometry, and FMS sondes. The fourth tool string (GHMT) included the NGT, magnetic susceptibility, and vertical component magnetometer sondes. The logs were run uphole from 605 mbsf (total depth) to pipe at 59 mbsf; the two first runs were logged to the seafloor. The natural gamma ray is the only parameter measurable through the pipe, but it should be interpreted only qualitatively in this interval. The pipe was set at 89 mbsf and pulled up to ~59 mbsf during logging for the first three runs and before logging for the fourth run (GHMT). The wireline logging heave compensator was started immediately upon entering the hole.
Hole 1084A is characterized by a regular hole size (~9- to 11-in diameter) with numerous small enlargements from the bottom to 170 mbsf (see caliper measurements; Fig. 37). Above this interval, the hole conditions are degraded and show a regular increase in the borehole size, with critical washout zones at the top of the logged interval (above 130 mbsf). The downhole measurements are affected by extreme enlargements at the top of the hole, and thus only the logging data in the 600- to 130-mbsf interval are of good quality.
The lithologic succession recovered from Hole 1084A is controlled mainly by changes in the nature and intensity of biogenic production vs. the type and amount of detrital input and is characterized by large changes in sediment composition and compaction, which should be reflected in the log physical properties measurements. The lithostratigraphic boundaries defined from core observation and smear-slide studies (see "Lithostratigraphy" section, this chapter) compare well with the main features observed in the downhole measurements.
Lithologic Unit IV is characterized by increasing magnetic susceptibility and decreasing uranium (U) content, with very steady density and acoustic velocity, which might be explained by changes of the ratio between lithic and organic components in the detrital input. The top of Unit IV is marked by an overall increase in resistivity, acoustic velocity, and density. The boundary between lithologic Units III and II at 498 mbsf is identified in the log data as a shift in all of the physical measurements (Fig. 37), which reflects the change in the nature and intensity of the biogenic production.
The top of lithologic Unit III is marked by a higher U content and small peaks of gamma-ray, resistivity, density, and magnetic susceptibility measurements. The top of the unit marks the FO of the organic-rich levels that were identified as black layers in the cores (see "Lithostratigraphy" section, this chapter). Above this depth, similar "black" layers with high gamma-ray, resistivity, acoustic velocity, and magnetic susceptibility dominate the downhole measurements signals, particularly between 270 and 170 mbsf. The distribution of the "black" layers is restricted to 410–150 mbsf, as shown by the detailed analysis of the FMS images (Fig. 38). The U content is highest between 270 and 170 mbsf (Fig. 37). Because of the presence of "black" layers, the lower part of lithologic Subunit IB is marked by a regular increase in gamma-ray, resistivity, and magnetic susceptibility, and the middle part shows higher amplitudes of the resistivity and the acoustic velocity (Fig. 37). The upper part of lithologic Subunit IB (between 150 and 130 mbsf) shows high resistivity, acoustic velocity, and density, but low U content. This pattern permits the identification of several "dolomitic" layers at this depth, confirmed by the mapping of the FMS images (Fig. 38). The velocity exhibits a regular decrease uphole, with a sharp change at 450 mbsf caused by compaction of the sediment.
Nineteen dolomitic layers and 114 "black" layers (or "organic-rich" layers) were tentatively identified in the logged interval (Fig. 37), both characterized by very high velocity, resistivity, and density, and by low gamma-ray intensity for the dolomites and high gamma-ray intensity for the "black" layers. Because of their high resistivity, the position and thickness of these layers can be identified on the FMS images (Fig. 38). Only one dolomitic layer was visually verified in cores recovered with the extended core barrel, and only 17 black layers were explicitly described in the cores (see "Lithostratigraphy" section, this chapter). These black layers were defined by being dark (e.g., 5y 2.5/1). They had a total reflectance between 20% and 25%. Numerous dark layers were not included in the visual core description, but are identified in Figure 38 based on the resistivity contrast of the FMS images.
The temperature tool measures borehole fluid temperature. The results suggest a downhole thermal gradient of 28.5°C/km, an estimate which is low because of the cooling effect of circulation during drilling.
The downhole measurements of the two neighboring holes are very similar, despite the higher sedimentation rate observed at Site 1084 in the NCB. Both general trends and details can be correlated and allow the establishment of a reliable depth-to-depth correlation, as shown by the gamma-ray intensity (Fig. 39). All the downhole physical properties parameters show the same range of variation at the two holes, despite the different sedimentation rates. The "black" layers are absent at Hole 1082A in the Walvis Basin.
The core MST and log measurements of natural gamma-ray intensity are very similar. Core data are recorded in counts per second (cps), whereas log data are presented in API (Oil Industry Standard) units. Detailed correlations between the core and log data sets (Fig. 40) promise to be reliable because of the high sedimentation rate at this site and the limited coring disturbance, except in the organic-rich interval (270–170 mbsf). The main trend observed in the gamma-ray log is not exhibited as clearly by the gamma-ray intensity measured on the cores, which is noisier. In Hole 1084A, log depth is close to core depth.