Downhole Logging Summary
The primary objective of ODP Leg 202 was to document the history of climate and oceanographic changes and to investigate the role of such changes in biogeochemical systems in the southeast Pacific. A total of 11 sites were drilled, ranging in age from early Oligocene (~31.5 Ma) to Holocene, appropriate to reconstruct the evolution of upwelling, biota, biogeochemistry, sea-surface, and intermediate-water characteristics in the southeast and equatorial Pacific, as well as the closure of the Isthmus of Panama, the history of Andean uplift, and continental climate (Figure 1).
Figure 1: Map of Leg 202 Site Locations
Sites (1236, 1237, and 1241) targeted low sedimentation rates of <30 m/m.y. to obtain long sequences of climate change in the Neogene and, in some cases, the late Paleogene that are not subject to severe burial diagenesis. Other sites (1238 and 1239) targeted moderate sedimentation rates of 30-80 m/m.y. to assess orbital-scale climate oscillations at a resolution suitable for the tuning of timescales and examination of changing responses to orbital forcing during the late Neogene. Sediments that accumulated rapidly, at rates of 80-2000 m/m.y., were recovered from high southern latitudes (Sites 1232–1235) and near the equator (Sites 1240 and 1242) to assess equator-to-pole climate linkages at both millennial and orbital scales.
Downhole logging was conducted in the deepest hole at each of the 3 sites that penetrated beyond 390 mbsf (1238A, 1239A, and 1241B). Two tool string combinations were run at each site, the triple combo-MGT and the FMS-sonic. In all cases borehole conditions were good and all passes reached the base of the hole.
Pervasive meter-scale rhythmic variations were observed in borehole resistivity and core density logs at Sites 1238, 1239, and 1241. This variability is associated with fluctuations in the relative amounts of carbonate and biogenic opal in the sediments and was clearly resolved as banding in the FMS image logs (Figure 2, for examples).
Figure 2: Examples of the meter scale banding observed in FMS logs (dynamically normalized FMS logs from 1238A and 1239A shown here) associated with the carbonate/opal (nannofossil / diatom) cycles observed in the equatorial sites.
The meter scale cyclic variations in the downhole density logs at Sites 1238, 1239, and 1241, showed close agreement with core measurements (see example in Figure 3). Using the downhole log density as a depth reference, the recovered core sections were able to be mapped to equivalent log depths (ELD) using Sagan in order to identify more precisely the size and position of core breaks—particularly within the XCB section (Figure 3).
Figure 3: Comparison of gamma ray attenuation (GRA) bulk density data from core logging and downhole logging. Note the excellent correlation of variations in magnitude and depth at meter scale. Minor discrepancies at submeter scale may be the result of core disturbance or borehole irregularities.
Despite the close relationship between core and log densities, the lower resolution of the downhole log prevents core-log comparisons to the decimeter and centimeter scale. However, the relationship between resistivity and density in the downhole logs is strongly linear (r=0.96) throughout the logged sequence suggesting a common lithologic control on both properties (Figure 4).
Figure 4: Shown is the strong covariation between downhole density and resistivity in Hole 1241B.
This circumstance provided an opportunity for much higher resolution core-log comparison based on the FMS log which uses 64 micro electrodes to generate an electrical conductivity image of the borehole with resolution of ~1 cm. A comparison between the spherically focused resistivity log, with a resolution similar to that of the downhole density log, and the conductivity curve derived by averaging the 64 channels of FMS data, suggest that a significant amount of higher frequency variability is missed by the conventional resistivity log at this site (Figure 5).
Figure 5: Comparison between SFLU resistivity (blue) and the microconductivity curve derived from the averaging the 64 buttons of the FMS (red).
This higher frequency microconductivity variability is comparable with the GRAPE density records to the cm scale (Figure 6), suggesting the FMS data may allow cm scale core-log comparisons over large parts of the recovered sequences.
Figure 6: The Formation MicroScanner (FMS) image of the borehole at Site 1241 is shown to the right of the 64-button average (5-point smoothing) in red. The similarity of the smoothed FMS and the lower-resolution gamma ray attenuation (GRA) bulk density record (blue) in cores from Hole 1241B suggests that borehole logs will provide a relatively continuous proxy record of lithologic variability and will provide for detailed integration of depth scales between the borehole and core logs. Eld = equivalent logging depth.
Core and downhole log derived natural gamma radiation records also showed strong covariance in the three holes logged and were useful for confirming and improving the core-log depth mapping in Sagan (Figure 7).
Figure 7: Example of the comparison between natural gamma radiation data from core logging and downhole logging (1238A). Note that the data from two downhole tools (HSGR and MGT) and the MST-NGR correlated very well at meter scale throughout and at submeter scale over most intervals.
The high resolution 'Multi-sensor Gamma Tool' (MGT) was also run in each of the holes logged and provided much higher resolution natural gamma ray records than the conventional wireline tools (See example in Figure 8). The MGT consistently resolved decimeter-meter scale variations in natural gamma radiation which were either absent or heavily smoothed in the HNGS record.
Figure 8: Comparison between the Lamont (MGT) natural gamma log versus the Schlumberger HNGS record from Hole 1241B.
Ulysses S. Ninnemann, Logging Staff Scientist, Lamont-Doherty Earth Observatory, Columbia University, Borehole Research Group, Route 9W, Palisades NY 10964, USA