Previous   |    Next

doi:10.2204/iodp.proc.314315316.113.2009

Log characterization and lithologic interpretation

Log characterization and identification of logging units

Hole C0001

Hole C0001C was drilled as the initial hole for LWD data; however, it was terminated at 78 m LSF because of poor hole conditions and inclination of the borehole (see “Operations”). Resistivity images are affected by the inclination of the borehole, and hence, the dynamic rbackesponse in particular exhibits conductive and resistive striping of the borehole wall. Resistivity peaks in the lower section (~50–70 m LSF) correspond to slight lows in the gamma ray log response and could represent higher contents of sand-sized particles (either terrigenous sand or volcanic ash). Also present in the image logs are small conductive patches that may be reduction spots. Distinct horizontal conductive-resistive-conductive bands (~1 m thick) typically correspond to a peak and dip in the button resistivity responses. These features may be artifacts, as there are no corresponding changes in the gamma ray or density log responses (Fig. F24). Overall, the characteristics of Hole C0001C are similar to those of Hole C0001D, Subunit IA (see next section).

Hole C0001D

We separated data from Hole C0001D into logging units on the basis of visual inspection of the gamma ray log and from resistivity log responses and images. The real-time density log was available to 500 m LSF. Based on the caliper log, however, only the upper 200 m LSF section was thought to be reliable, and it was not used for logging unit characterization. The overall log responses of both gamma ray and resistivity increase with depth. These vertical patterns may be a manifestation of compaction. The three primary logging units were defined based on contrasts in the log baselines and trends. The sonic traveltime log exhibited similar large-scale trends, where reliable (see “Data and log quality”). The three primary logging units were divided further into eight logging subunits (Table T6). Although the reasons for variation are not always obvious, statistical investigation of these units confirmed their distinctiveness (Fig. F25).

The real-time caliper log showed good borehole quality for the upper 200 m LSF before degrading slightly with some variability for the rest of the recorded section. The good-quality section falls within logging Unit I (0–198.9 m LSF). This interval exhibits fairly low and consistent values on both the gamma ray and resistivity logs. Near the base of logging Unit I, logging Subunit IB (190.5–198.9 m LSF) is characterized by a distinct series of strong negative and weak positive peaks in the gamma ray log. These responses could be due to interbeds of silt or sand-sized material with clay-rich sediment. A distinct increase in the gamma ray log baseline and a gradually increasing trend with depth define logging Unit II (198.9–529.1 m LSF). The resistivity values exhibit a small baseline increase but are similar to those in logging Unit I and remain fairly constant throughout the unit. Logging Unit II exhibits subtle internal variation and is divided into three subunits. Statistically, logging Subunits IIA (198.9–344.0 m LSF) and IA are similar, whereas logging Subunits IIB (344.0–434.7 m LSF) and IIC (434.7–529.1 m LSF) are similar in their resistivity profiles. However, they exhibit large enough changes in gamma ray and sonic profiles to justify statistical separation (Fig. F26).

The boundary between logging Units II and III (529.1–976 m LSF) coincides with a systematic change in the log characteristics (Fig. F27); all the curves below the boundary show strong increasing trends. In detail, however, the boundary is transitional and includes a highly fractured interval (see “Structural geology and geomechanics”). Logging Subunit IIIA (529.1–628.6 m LSF) displays large statistical variability in all the logs, and high-frequency fluctuations can be seen on the logs and images. Logging Subunits IIIB (628.6–904.9 m LSF) and IIIC (904.9–976 m LSF) are statistically well constrained but exhibit much higher ranges in gamma ray and resistivity log values and lower mean sonic traveltimes than other logging units and their subunits (Figs. F25, F26).

Log-based lithologic interpretation

Logging Unit I (slope sediments)

Logging Unit I consists of slope sediments that are well imaged by the seismic data (see “Log-seismic correlation”). Logging Subunit IA is characterized by nearly constant gamma ray values and gradually increasing resistivity. Small ranges of variation in log response indicate uniform lithology. Borehole images show that the sediments in logging Subunit IA are composed of alternating beds of relatively conductive and resistive layers a few decimeters to a few meters thick. Sedimentary structures cannot be recognized by borehole images because of the inadequate resolution of RAB images (see “Structural geology and geomechanics”).

Changes in gamma ray values in logging Subunit IA include four decametric cycles at intervals 0–54, 54–100, 100–156, and 156–191 m LSF (Fig. F28). Gamma ray values gradually increase with depth in the upper three intervals, and several meter-scale cycles are also observed within the interval. Most of these cycles are characterized by decreasing gamma ray values uphole. The lowermost interval (156–191 m LSF) is characterized by higher frequency submeter-scale gamma ray fluctuations. The four main cycles may reflect abrupt changes in the sedimentary sequence, including the formation of subtle angular discontinuities in the seismic section (Fig. F29), but they are not necessarily a result of changes in sediment composition or texture.

Log-core correlation at Ocean Drilling Program (ODP) Site 808 off Shikoku Island (Mikada, Becker, Moore, Klaus, et al., 2002) provides some constraints on our interpretation of lithology for logging Unit I in Hole C0001D. Gamma ray values in Hole C0001D, logging Subunit IA, range from 56 to 64 gAPI (Fig. F28), similar to those of Site 808 logging Subunits 2a and 2c (58–66 gAPI) and different from the values of 38–55 gAPI seen in Site 808 logging Unit 1 (Table T7). Therefore, the lithology in logging Subunit IA could be silty sediments and hemipelagic mud.

Alternating beds of conductive and resistive sediment characterize logging Subunit IB. A negative peak on the gamma ray log to ~20 gAPI occurs in the lower part of this subunit, which is consistent with sandy sediments. The layers evidently dip to the north and gradually steepen with depth. No significant structural discontinuities can be observed at the top and bottom of this subunit; therefore, we interpret the base of logging Subunit IB as an unconformity between slope sediments and the accretionary prism (see “Structural geology and geomechanics”).

Logging Units II and III (accretionary prism sediments)

Logging Unit II is divided into three subunits. In logging Subunit IIA a general increase in gamma ray and resistivity suggests a gradual decrease in porosity with depth. The gamma ray log also indicates possible meter-scale cycles. In the lower part of logging Subunits IIB and IIC, the gamma ray and other logs exhibit at least five different lower frequency, sharp-based, decameter-scale sequences. Sequences of both decreasing and increasing gamma ray values can be identified (Fig. F28). The uppermost section of logging Subunit IIB does not show the same clear pattern, but a sharp change in dips within that section may indicate structural overprint on original depositional features. Higher frequency variations inside these subunits outline minor sequences of gently increasing gamma ray values.

Changing trends in bedding dips within logging Unit II are probably related to structural deformation (possible tilting and rotation) given the abrupt changes of dip and direction without noticeable changes in composition (Fig. F29). Interpretation of the resistivity images suggest the presence of tilted blocks with varying dip angles and azimuths. Dominant bedding dips identified on the images are roughly south-dipping and changing to other orientations at tilted and/or deformed intervals.

There is very little evidence to constrain interpretations of lithology within logging Unit II. However, if we compare the gamma ray values to those for Site 808 logging Subunit 4a, they are similar (Table T7). Also noteworthy at Site 808 was the presence of breakouts in hemipelagic mud intervals. In Hole C0001D breakouts are confined to logging Units I and II. This favors hemipelagic mud as the likely lithology for logging Unit II.

Interpretation of the “transitional section” from logging Unit II to Unit III (Subunit IIIA) is subject to uncertainties indicated by drilling-related borehole damage and also by the effect of fractures intersecting the borehole wall, as clearly seen on the resistivity images (Fig. F27). The increased gamma ray values at these points mask and overprint the signature of the possible original sedimentary features on the logs. The base of logging Subunit IIC is also strongly affected by fracturing.

Logging Unit III displays the most distinctive features of the entire section drilled in Hole C0001D: increased resistivity as shown on the static processed resistivity images, overall increased gamma ray values, increased resistivity, and sonic traveltime average values that drop at the depths where the hole was intersecting faults and fractures (650, 800, 835, and 860 m LSF). The images show no evidence of intense deformation below 885 m LSF.

Logging Subunit IIIC and the lowermost part of logging Subunit IIIB exhibit more homogeneous base values in comparison to the overlying section. This suggests a difference in sedimentary and depositional features between the upper subunits of logging Unit III and the lower subunits. However, there is currently insufficient evidence to determine the lithology of Unit III.

Top of page   |    Previous   |    Next