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doi:10.2204/iodp.proc.314315316.118.2009

Log characterization and lithologic interpretation

Log characterization and identification of logging units

Site C0006 logging units were characterized from visual inspection of gamma ray, resistivity, and caliper log responses (Fig. F1). Resistivity images were used to define finer scale characteristics within the units. Four primary logging units were defined based on the variability of log responses (Tables T6, T7).

Logging Unit I (0–197.8 m LSF) is characterized by a gradual increasing trend starting at 52 m LSF and variable gamma ray values from 20 to 70 gAPI. This logging unit is also characterized by high-amplitude fluctuation of ring (1–3 Ωm), shallow (0.5–2 Ωm), and deep (1–2.5 Ωm) button resistivity values. Ring and deep button resistivity values in this logging unit increase with depth from 0.5 to 2.0 Ωm. Caliper values show high-frequency and high-amplitude oscillation and the baseline of borehole diameter is consistently large (10–11 inches) over this logging unit (Fig. F1).

Logging Unit II (197.8–428.3 m LSF) is characterized by a gradual increasing trend of gamma ray baseline with depth from 60 to 80 gAPI. Four thick intervals of low gamma ray values (<30 gAPI) are observed at 216–223, 238–245, 298–305, and 330–335 m LSF (Fig. F1). Ring and bit resistivity logs exhibit a constant baseline trend between 1.8 and 2.0 Ωm, except for the interval 280–330 m LSF, which exhibits a variable trend from 1.5 to 2.5 Ωm. Ring and bit resistivity logs also exhibit four conductive intervals to 1.0 Ωm at the same intervals of low gamma ray values. Shallow and deep button resistivity logs show similar trends and four conductive intervals are sharply defined. This logging unit is also characterized by a decreasing trend of borehole diameter from 10 to 9 inches with distinct washouts as much as 14 inches in diameter corresponding to the low gamma ray intervals (Fig. F1).

Logging Unit III (428.3–711.5 m LSF) is defined as an interval of high-frequency and high-amplitude gamma ray fluctuations. This logging unit is divided into two subunits (Table T6). Logging Subunit IIIA (428.3–545.3 m LSF) is characterized by a gradual increasing trend in gamma ray baseline from 70 to 90 gAPI. Negative meter-scale spikes of gamma ray values to 50 gAPI are common within this subunit. All resistivity logs exhibit a near constant trend around 1.5 Ωm (Fig. F1).

Logging Subunit IIIB (545.3–711.5 m LSF) is also characterized by high-frequency and high-amplitude gamma ray log fluctuations. Gamma ray values are more variable (30–100 gAPI) than those found in logging Subunit IIIA (40–90 gAPI) and exhibit the highest values (up to 100 gAPI) in Hole C0006B. More than 20 spikes of low gamma ray values to 30 gAPI are observed in this subunit. Resistivity logs exhibit a variable trend between 1.0 and 2.5 Ωm, with increasing trends in resistivity observed over the intervals 594–652 and 656–710 m LSF. Borehole diameter shows nearly constant values (9 inches) at the upper part of this logging subunit and large variation from 9 to 14 inches below 652 m LSF. The base of logging Unit III is defined by a sharp decrease in gamma ray and resistivity values (Fig. F1).

Logging Unit IV (711.5 m LSF to TD) is characterized by the lowest gamma ray and resistivity values for this hole. The gamma ray log ranges between 20 and 50 gAPI except for the interval of high gamma ray values (up to 80 gAPI) at 712–730 m LSF. Resistivity logs exhibit a slight deceasing trend with depth from 1.0 to 0.5 Ωm. Resistivity logs show high-frequency oscillation over the interval 711.5–762 m LSF and decameter-scale cyclic variation over the lower part of this logging unit. Logging Unit IV is also characterized by high values and high-frequency oscillation of borehole diameter fluctuating between 10 and 14 inches (Fig. F1).

Figure F12 illustrates ring resistivity and gamma ray distributions for the logging units and subunits. The gamma ray log exhibits a trend of gradual increase from logging Unit I to Subunit IIIB, with extremely low values for logging Unit IV. Ring resistivity shows a broad range of values in logging Unit I and a gradual decreasing trend from logging Units II to IV. Figure F13 shows a cross-plot of gamma ray values versus ring resistivity. Logging Unit III is characterized by high gamma ray values. Logging Unit IV is characterized by low gamma ray and resistivity values.

Lithologic interpretation

Log responses in conjunction with resistivity-at-the-bit (RAB) images in Hole C0006B show lithologic characteristics and detailed sedimentary/structural features (Figs. F1, F14, F15).

Logging Unit I

Logging Unit I is characterized by an increasing baseline trend and oscillation of gamma ray and resistivity values. Resistivity logs and RAB images demonstrate clear decameter- to centimeter-scale alternating bedding. The caliper log shows significant washouts in logging Unit I resulting from the unconsolidated state. These log signatures suggest that the lithology of logging Unit I consists of unconsolidated and uncemented interbeds of sand and mud. Based on the interpretation of borehole images, the general trend of the bedding plane is westward dipping, which is obviously different than trends in lower logging units (Fig. F14).

Logging Unit II

Logging Unit II is characterized by higher values of gamma ray baseline with several prominent, ~5 m thick layers with low gamma ray values. This log character is interpreted as mudstone with major, thick sand layers. Fining-upward sequences in these sand layers are clearly seen in the RAB images as a transition from dark (conductive) at the base of the sequence changing gradually to light (more resistive) toward the sequence top. A similar trend is shown on gamma ray and resistivity logs (Fig. F15). The upper and lower pairs of sand layers are presumably stratigraphic repetition. However, these sand layers and the surrounding formations are not correlated with each other in a simple way because of the complexity of faulting (see “Structural geology and geomechanics” and “Log-seismic correlation”). Bedding dip and orientation patterns within this unit show variability (Fig. F14) that is likely the result of disruption of the original bedding caused by complex deformation.

Logging Unit III

Logging Unit III is characterized by a high gamma ray baseline with frequent thin low gamma ray layers. These log responses are interpreted as alternating beds of mudstone and sand. Thickness and frequency of coarse layers are quite different from those of logging Unit II. The gamma ray log also suggests that the sand/mud ratio in logging Subunit IIIB (sand-dominant facies) is larger than that formed in logging Subunit IIIA (mud-dominant facies) (Fig. F1).

Based on borehole image interpretation, a number of alternating bedding features are recognized but specific textures or sedimentary features are not clearly imaged. A general trend of bedding is northward and southward dipping for logging Subunit IIIA and northward to northwestward dipping for logging Subunit IIIB (Fig. F14). Logging Subunit IIIA is relatively homogeneous and conductive. Logging Subunit IIIB is characterized by three decameter-scale resistive (light) intervals changing to conductive (dark) sediments with depth. These trends are consistent with gamma ray trends and are likely related to textural and compositional changes.

Logging Unit IV

Very low gamma ray values (lowest in Hole C0006B) suggest that the lithology of logging Unit IV is sand dominated. Borehole diameter is as large as that of logging Unit I, possibly caused by an unconsolidated/​uncemented state. Based on borehole image interpretation, sedimentary structures and deformation features are not very clear because of the possible massive character of the formation and also washouts. A general trend of the bedding planes is northward dipping throughout the unit. This bedding structure is slightly different from that of logging Unit III (Fig. F14). All log responses suggest that the lithology of logging Unit IV is uncemented massive sand.

Based on the seismic section (see “Log-seismic correlation”), logging Unit IV is correlated to possible underthrust sediments. Although the lithologic contrast demonstrated by the abrupt decrease of gamma ray and resistivity logs is large between logging Units III and IV, deformation structures between the two logging units are poorly identified (see “Structural geology and geomechanics”) and the seismic and RAB structural interpretation places the likely main thrust fault ~50 m higher at 657 m LSF within logging Subunit IIIB.

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