Previous   |    Next

doi:10.2204/iodp.proc.314315316.114.2009

Log characterization and lithologic interpretation

Log characterization and identification of logging units

Hole C0002A logging units were characterized from visual inspection of the gamma ray, resistivity, bulk density, and PEF log responses (Fig. F1). The resistivity images were used to define finer scale characteristics within the units. Four primary logging units were defined, as well as two zones of variation within logging Unit II (Table T5). Bulk density and P-wave velocity values increase slightly with depth, whereas neutron porosity decreases with depth, showing a clear compaction trend.

Logging Unit I (0–135.5 m LSF) exhibits a particularly homogeneous conductive character in the resistivity images. The ultrasonic caliper log indicates poor borehole quality between 43.3 and 135.5 m LSF. Gamma ray, resistivity, and density log values are relatively low throughout this uppermost section and change abruptly to higher values at 135.5 m LSF (Fig. F1). This shift is taken as the boundary between logging Units I and II.

At the upper boundary of logging Unit II (135.5–830.4 m LSF), the variability of the resistivity and gamma ray logs increases from the baseline values. Caliper values are good throughout most of logging Unit II, with the exception of 481.6–547.1 m LSF and the lowermost section (below ~700 m LSF). Bulk density values show intervals of larger and smaller variability; stronger negative spikes correlate well to higher caliper values, indicating washouts (Fig. F1). Two zones were identified within logging Unit II. Zone A (218.1–400.4 m LSF) is similar to the rest of logging Unit II in terms of gamma ray, PEF, and neutron porosity log responses. However, resistivity logs show >50 significant resistive spikes (2–50 Ωm). Zone B (481.6–547.1 m LSF) was defined as the highest sustained frequency and magnitude of log variations seen in logging Unit II. More than 50 conductive and low gamma ray layers occur within this zone.

Logging Unit III (830.4–935.6 m LSF) is characterized by relatively consistent responses in the gamma ray (75 gAPI), resistivity (1.4 Ωm), bulk density (2.0 g/cm³), sonic velocity (2500 m/s), and neutron porosity (0.5 PU) logs. The PEF values increase gradually over the upper few tens of meters and remain at a relatively elevated baseline (3.3 b/e^–) until decreasing at the base of the unit (Fig. F1).

In logging Unit IV (935.6–1400 m LSF) all of the log responses exhibit high variability. Caliper values are high throughout, and the density and sonic logs are correspondingly unreliable for significant portions. The neutron porosity and PEF curves reach their highest variability and lowest values within this unit. The gamma ray and resistivity logs have generally higher variability magnitudes than in any of the other logging units but do not exhibit as high or as many resistive peaks as in Zone A of logging Unit II (218.1–400.4 m LSF). Within logging Unit IV are several more homogeneous intervals that exhibit moderately high gamma ray, PEF, and resistivity log values.

Basic statistical analysis supports the broad logging units defined above. The fluctuation in the PEF curve and constraint of other parameters in logging Unit III indicate a difference in composition from the other units (Fig. F9). The variation in sonic log trend illustrates the unique characters of Zones A and B.

Log-based lithologic interpretation

Low resistivity associated with low gamma ray values and a higher average borehole diameter are common throughout the borehole. The difference between the shallow and deep resistivity log values for a given depth can be taken as an approximation of the degree of invasion of drilling fluid into the formation (Fig. F10). Intervals with significantly high resistivity difference (i.e., deep button resistivity is greater than shallow button resistivity) coincide with the gamma ray lows that are typically associated with sandier lithologies. Where borehole quality is good, high resistivity differences may indicate permeable layers which have been partially invaded. Therefore, lower (darker color) resistivity images alongside particular log responses throughout the borehole are indicative of higher sand content, with high conductivity potentially an effect of invasion.

A subtle change in bedding dip is exhibited at each of the logging unit boundaries, implying that they are angular unconformities (Fig. F11). Logging Unit IV is composed of steeply dipping beds with variable orientations. Bedding in logging Unit III exhibits a clear bimodal distribution between north and south dip directions. Logging Unit II consistently dips shallowly to the northwest except for the very top of the section, which begins to dip to the south. The bedding that can be seen in logging Unit I dips shallowly to the south.

Logging Unit I

Logging Unit I exhibits conductive resistivity images corresponding to sandy beds with significant washouts. Layering is also faintly visible. Accordingly, the lithology for logging Unit I is thought to be unconsolidated sandy/​silty mud. Low density and high caliper measurements, as well as sonic velocity measurements coinciding with the borehole fluid arrival, indicate that Unit I is the least consolidated of the units defined in this hole.

Logging Unit II

Below the logging Unit I/II boundary, resistivity images show more variation, with clearer bedding. Fine bedding in the images corresponds to high variation in the other logs. The high variability throughout the unit is attributed to the lithologic and compositional variation within turbidite sequences, which are characterized by higher amplitudes and frequencies of variation in the gamma ray, resistivity, and density values (Fig. F12A). When viewed at a finer scale, many of the gamma ray cycles show an abrupt drop from values indicative of mud below to those indicative of higher sand content above, followed by a gradual, fining-upward trend returning to muddier values. The sand beds identified in the gamma ray log also exhibit low P-wave velocity and resistivity and a slightly larger borehole diameter, implying that the beds are uncemented, soft, and permeable. Meter-scale cycles dominate and are locally expressed in the density profiles. Several zones of smaller scale cycles are interpreted as finely interbedded sand and mud (Fig. F12A). Some decameter-scale cycles are also observed, as well as slightly sandier intervals in the lower half of the unit (Fig. F12B). Overall, logging Unit II appears to consist of sandy/​silty turbidites and hemipelagic mud, with some variation in frequency of interbeds and composition.

Two zones within logging Unit II demonstrate differences potentially related to postdepositional properties and thus are classified differently. Zone A resistivity logs show >50 significant resistive spikes (2–50 Ωm), ranging from 20 to 140 cm in thickness and fewer conductive spikes than the rest of logging Unit II (Fig. F12A). More than half of these resistive peaks occur between 350 and 400 m LSF and are generally >75 cm thick (Fig. F13A). Low gamma ray values, high velocity, and better borehole diameter are associated with these occurrences.

Both resistivity and sonic baseline values increase slightly throughout Zone A and decrease sharply at the base. Interpretation of the seismic data indicates a negative polarity BSR near 400 m seismic depth below seafloor (SSF). The interval above the BSR probably contains gas hydrates, which yield high resistivity and velocity values but have low density and little effect on the gamma ray log. Upon closer examination (Fig. F13A), resistivity peaks are usually found at the base of asymmetric gamma ray log lows, which have been interpreted as fining-upward sequences with gradational tops. This relation indicates a host bed grain-size control on porosity and gas hydrate formation.

More than 50 possible sand layers occur within Zone B, with thicknesses ranging from 20 cm to 1 m (Fig. F13B). The sand beds are thickest between 500 and 525 m LSF, whereas the thinnest beds are observed near the top and bottom of the zone. Above and below Zone B are mud-rich zones, which yield less variation in resistivity log responses and a lower magnitude of separation between the deep and shallow resistivity values. This implies that the strata in Zone B, particularly the central portion (500–525 m LSF), are more permeable and form a possible conduit for gas and liquid. Increased magnitude of variation in the density log through this zone, slightly depressed sonic log values, and the presence of a negative polarity bedding reflection in the seismic profiles (Fig. F14) suggest the presence of abundant gas.

Logging Unit III

Logging Unit III displays distinctly different log responses from the logging units above and below (Fig. F15). Low variation from gamma ray and resistivity baselines, which are near the high end of logging Unit II values, indicates a uniform lithology within logging Unit III, similar in some respects to the mud-rich end-member of the Unit II variation. Values of PEF near 3.5 b/e^– are particularly high, in contrast to 3 b/e^– in the section above. This difference suggests a compositional change to a very low sand content, as 3.5 b/e^– is close to pure shale or mudstone values (Rider, 1996). Resistivity images show very fine layering and a decrease in breakout occurrences. The most likely lithology is hemipelagic mud.

Several conductive peaks in the resistivity logs, corresponding to low gamma ray, PEF, and density log values, are found in the middle portion of logging Unit III and are interpreted as thin sand or silt beds. These beds are at depths (~890 m LSF) consistent with an unconformity seen in the seismic profile (top of lower sediments I in Fig. F14). Distribution of dips within this unit shows that above this unconformity the dips are dominantly southward, whereas below the dips are dominantly to the north (Fig. F11). This differs somewhat from the seismic interpretation, which shows the upper part of logging Unit III (seismic Unit Kumano 11) dipping to the north. However, the surface of this unconformity may be uneven, and the southward dip may be a local phenomenon. Above the unconformity, the PEF curve decreases upward to the top of the unit; however, other logging parameters show no difference from the lower portion of the logging unit. Classification of a lithologic boundary at this unconformity is therefore not warranted.

Logging Unit IV

Logging Unit IV corresponds to the accretionary prism inferred in the seismic interpretation (Fig. F14). The boundary between logging Units III and IV is sharp and marked by a negative spike in each log except for the neutron porosity. All of the logs except for resistivity and PEF return to values similar to logging Unit II and III baselines within 40 m of the logging Unit III/IV boundary. Observed bedding dips are steeper and more variable in logging Unit IV than in the logging units above. Intervals which exhibit more variation, particularly in the resistivity, and intervals which appear more homogeneous are both observed within Unit IV. Some of the intervals of high variability are similar to Unit II, with some fining-upward cycles and meter to decameter scales of variation. High caliper values (9.5–14 inches) are believed to indicate higher concentration of uncemented sand both within these cycles and overall. These zones are interpreted as strongly deformed sandy/​silty turbidite sequences.

The more homogeneous intervals of logging Unit IV are distinctive on the image logs, where they appear mottled (Fig. F16). These more homogeneous intervals are characterized by relatively constant, high gamma ray and density values and significantly less variability in resistivity logs. Associated high resistivity could be attributed to compaction, which would make the formation more stable and is consistent with good borehole quality. The homogeneous appearance indicates higher clay-size content, although the mottled appearance on the resistivity images lends itself to several interpretations. Given the deformed nature of the rest of Unit IV, it is possible that the mottled sections are the dismembered remnants of clay-rich horizons. Incorporation of volcanic ash or volcaniclastic inputs are also possible with this interpretation. An alternative hypothesis is that logging Unit IV may be composed predominantly of deformed interbeds of sand and mudstone with local mass flow deposits. The mottled sections would correspond to more homogeneous intervals.

Top of page   |    Previous   |    Next