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doi:10.2204/iodp.proc.314315316.115.2009

Log characterization and lithologic interpretation

Log characterization and identification of logging units

In Hole C0003A only real-time data were available because of the loss of the LWD drill collars. Data are available to a maximum depth of 532 m LSF, including gamma ray; ring, bit, and deep resistivity; neutron porosity; and density log responses. Because of the poor quality of the sonic log (see “Data and log quality”), it was not used to define the logging units. The top ~55 m LSF of data for all the logs was discarded because of poor sampling. Three logging units (Table T3; Fig. F1) were defined based on the variability of the available log responses. Statistical analysis supported the differences among these units (Fig. F7).

Logging Unit I (55–76.6 m LSF) is characterized by a moderately high gamma ray baseline (50–55 gAPI) with high variability across this baseline. All three resistivity responses are similar and remain fairly constant (~0.9 Ωm). The density and neutron responses are moderately high (~1.6 g/cm³ and 60 porosity units, respectively), with minor fluctuations. The caliper indicates that the borehole quality is poor (9.5–11.5 inches), and this may explain some of the variation observed in the density and neutron log responses.

The base of logging Unit I (76.6 m LSF) is a prominent boundary characterized by a sharp drop in gamma ray, resistivity, and density baseline values and an increase in neutron baseline values. These log responses correspond to a sharp decrease in borehole quality. Logging Unit II (76.6–151.5 m LSF) is characterized by very low gamma ray values (~30 gAPI) with meter-scale variations. The resistivity responses are consistently low (0.65 Ωm), with sporadic positive peaks. The measured density is low (~1.2 g/cm³) and highly variable, which correlates with the continuously poor borehole quality. The base of logging Unit II is characterized by a sharp increase in all the log responses, accompanied by a return to good borehole conditions.

Logging Unit III (151.5–509 m LSF) exhibits a broad increasing trend in the gamma ray log (average = 77 gAPI), whereas the resistivity profiles (average = 1.65 Ωm), density (average = 1.9 g/cm³), and neutron (55–65 porosity units) log responses remain fairly constant. Subtle internal variation is exhibited throughout logging Unit III. The gamma ray log exhibits high-frequency (meter scale) and high-amplitude variations. The resistivity and density logs exhibit relatively constant values with local negative peaks, usually corresponding to washouts indicated by the caliper and low gamma ray values.

Log-based lithologic interpretation

Logging Unit I (slope sediments)

Within the 21.6 m section of logging Unit I, moderate gamma ray, low resistivity, and low density log values suggest a lithology of mud with interbeds of sandy and silty sediments. Logging Unit I corresponds to the base of a sequence of slope sediments seen in the seismic section (see “Log-seismic correlation”).

Logging Unit II (thrust sheet)

Logging Unit II is defined by a washed out zone, characterized by low gamma ray and resistivity values. Shipboard correction of the gamma ray log (applying LWD GR hole size correction [GR-4 Schlumberger chartbook, Schlumberger 1995]) was performed to investigate the possibility of borehole effects causing the low gamma ray measurements. The gamma ray values corrected for actual borehole diameter using the real-time caliper are still low (average = 32 gAPI). This range of low gamma ray values is typical for coarse sediments, most likely sandy, uncemented (washed out and low density), and porous (low resistivity formation).

Logging Unit III (thrust sheet)

When compared to logging Units I and II, logging Unit III displays overall higher gamma ray values increasing with depth, increased density, and higher resistivity values. The most likely lithologic interpretation is clay-rich sediment showing no significant compaction trend with depth.

Six washed out intervals (Table T4; Fig. F8) identified within this unit are considered as potential deformation-related zones and display distinctive log responses. Within these intervals, changes in density and resistivity values might be caused by changes in physical properties along zones of deformation, as recognized in the seismic data (see “Structural geology and geomechanics” and Fig. F9). One of these intervals (415.7–450.5 m LSF) is characterized by very poor quality caliper and corresponding low density and high neutron porosity values. The resistivity and gamma ray logs exhibit very little change across this zone. This area of washout is coincident with a fault zone interpreted in the seismic profiles (see Fig. F9). Internal subtle changes in gamma ray trends are correlatable with washouts.

A comparison of gamma ray values between Expedition 314 sites and Ocean Drilling Program (ODP) Site 808 is shown in Figure F10. Gamma ray values in logging Unit II are exceptionally low. The values in this unit are also lower than logging Unit 1 at Site 808, which is composed of sand and silt turbidites (Mikada, Becker, Moore, Klaus, et al., 2002).

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