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doi:10.2204/iodp.proc.347.109.2015

Downhole measurements

Logging operations

Hole M0065A

Hole M0065A was drilled to 73.9 m drilling depth below seafloor (DSF). In preparation for logging, the hole was circulated with seawater and the drill string was pulled back in the hole to 14 m wireline log depth below seafloor (WSF). Logging operations started in Hole M0065A with rigging up the Weatherford logging setup.

For downhole logging in Hole M0065A, two tool strings were deployed:

• gamma ray tool (MCG)/array induction tool (MAI) tool string, measuring natural gamma ray and electrical resistivity, and
• MCG/spectral gamma ray tool (SGS) tool string, measuring total gamma ray and spectral gamma ray.

The MCG/MAI tool string was lowered and downlogged to 41 m WSF. The hole was then uplogged to the seafloor. The tools provided continuous and good quality log data. The wireline depth to the seafloor was determined from the step increase in gamma ray values.

After this, the MCG/SGS tool string was lowered and reached only ~16 m WSF while downlogging.

The decision was taken to run in the hole with the drill pipe again to ~45 m DSF and do a wiper-trip of the hole. After this, the tool string with total gamma ray and spectral gamma ray was deployed again but only reached ~20 m WSF. After this unsuccessful attempt, logging operations were abandoned.

Hole M0065C

Hole M0065C was drilled to 47.9 m DSF. In preparation for logging, the hole was circulated with seawater and the drill string was pulled back in the hole to 13.2 m WSF. Logging operations started in Hole M0065C with rigging up the Weatherford logging setup.

For downhole logging in HoleM0065C, two tool strings were deployed:

• MCG/SGS/sonic sonde (MSS) tool string, measuring natural gamma ray, spectral gamma ray, and sonic velocity, and
• MCG/microimager (CMI) tool string, measuring total gamma ray and high-resolution electrical images.

The MCG/SGS/MSS tool string was lowered and downlogged to 40 m WSF. The hole was then uplogged to the seafloor. The tools provided continuous and good quality log data. The wireline depth to the seafloor was determined from the step increase in gamma ray values.

The MCG/CMI tool string was lowered to 40 m WSF with the calipers closed. The hole was then uplogged with open calipers at high resolution.

Hole M0065A logging units

Hole M0065A is divided into five units on the basis of the logs (Fig. F25). The uplog was used as the reference to establish the wireline log depth below seafloor depth scale.

Logging Unit 1: base of drill pipe to 20 m WSF

Logging Unit 1 is characterized by high values in NGR between 100 and 120 gAPI indicating high clay content. Resistivity is constantly increasing through this unit as a normal compaction trend.

Logging Unit 2: from 20 to 26 m WSF

After a sudden drop of NGR at 20 m WSF, natural gamma ray values slightly increase again with depth showing some large fluctuations at 23 m WSF. Resistivity is constantly increasing. The sudden drop in NGR can be explained by the clay/silt intraclasts as described in “Lithostratigraphy” for Subunit IIIa.

Logging Unit 3: from 26 to 34.1 m WSF

After a negative excursion at 26 m WSF and large fluctuations between 26 and 28 m WSF, NGR shows constantly high values through this logging unit. Resistivity increases with depth at a constant rate. Again these fluctuations can be due to the presence of clay/silt intraclasts.

Logging Unit 4: from 34.1 to 37.1 m WSF

NGR is characterized by large changes in this logging unit. Between 35 and 36 m WSF, NGR values are very high after a minimum of 60 gAPI at 34.1 m WSF. NGR shows a sudden drop again at 37.1 m WSF. Resistivity shows some oscillation in this logging unit but still generally increases with depth.

Logging Unit 5: from 37.1 to 41 m WSF

Logging Unit 5 is characterized by an increase in NGR after the natural gamma ray values dropped to 60 gAPI at 37.1 m WSF. Resistivity values are slightly decreasing and show invasion of bore fluid into the formation.

Hole M0065C logging units

Hole M0065C is divided into two units on the basis of the logs (Fig. F26). The uplog was used as the reference to establish the wireline log depth below seafloor depth scale.

Logging Unit 1: base of drill pipe to 18 m WSF

Logging Unit 1 is characterized by a constantly decreasing natural gamma ray. The sonic log stays constant in this logging unit. This decrease in NGR could correspond to the diminished organic content in lithostratigraphic Unit II (see “Lithostratigraphy”).

Logging Unit 2: 18–40 m WSF

In logging Unit 2, NGR values are constant with some fluctuations, possibly caused by the clay/silt intraclasts (lithostratigraphic Subunit IIIa; see “Lithostratigraphy”). There are no major changes in the sonic log. The calipers show that borehole conditions were very poor in this last logging unit.

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