Logging Tools

The downhole logs were recorded by Halliburton-Sperry Drilling Services using the LWD/MWD (Logging-While-Drilling/Measurement-While-Drilling) system, which employs logging instruments that are part of the drill string itself. The advantages of this technique include being able to log in formations that would not provide a stable hole for wireline logging (e.g. the upper section of sedimentary formations) and logging a hole immediately during drilling, so that it is in good condition and largely free of wash-outs. The data is recorded in the tool’s memory and downloaded when the drill string returns to the rig floor. A small subset of the data is transmitted in real time up to the ship during drilling.

The following services were employed in Hole C0002N:

DGR / EWR-P4 / PWD (gamma ray, resistivity, annulus fluid temperature and pressure)

The rate of penetration ranged from 5 to about 50 m/hr but most of the time was around 40 m/hr.

Processing

Depth shift: The original logs have been depth shifted to the sea floor (- 1967.5 m) by JAMSTEC personnel. The two runs were spliced at 2008.6 m LRF (41.1 m LSF).

Environmental corrections: The gamma ray and resistivity tools have been corrected for borehole size and type of drilling mud. However, both the gamma ray and resistivity values acquired at hole C0002N are lower than those obtained in the adjacent hole C0002F. The most likely reason is that the borehole diameter was larger in hole C0002N (17 inches) and the environmental corrections applied were not sufficient to compensate beyond the specified standard range for the tools.

Quality Control

During the processing, quality control of the data is performed by inter-comparison of all logging data to ensure that reasonable values are returned for expected lithology types and features on the logs reflect true formation characteristics and are not artifacts. The overall quality of the logging data is good.

In hole C0002N the logging data were acquired in two runs separated by a WOW (waiting-on-weather) event. Overall the quality of the logs is acceptable, but some factors must be taken into account when interpreting the log data. The exposure time tipically taken by the electromagnetic resistivity tool wave to reach the formation after it is drilled may vary from 30 minutes to 2 hours but in this hole was occasionally as long as 50 hours, when incidents such as WOW or extended mud circulation occurred. Such long exposures resulted in degradation of the borehole conditions (cavings and invasion of the formation by the drilling mud), which in turn affected the quality of the logs. Despite the mud and borehole size corrections applied to both gamma ray and resistivity logs, the absolute values of these logs are not to be trusted, especially in the following intervals:

1205-1221, 2022-2038 m LSF (due to mud infiltration into the formation related to extended circulation) and 1662-1678, 1992-2008 m LSF (due to WOW events). The resistivity logs at these depths are noisy and very low compared to the response above and below these intervals.

In general, it is recommended to use the two deepest attenuation and phase resistivity measurements, which are less affected by mud invasion and therefore are more likely to represent the true resistivity of the formation.

Additional and more detailed information about the drilling and logging operations and the quality of the measurements performed can be found in the Operations and Logging sections of the expedition report, Proceedings of the Integrated Drilling Program, Expedition 348.

For questions about the online database, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia

For questions about the procesing of the resistivity images, please contact:

Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu

For questions about the logs, please contact:

Yoshinori Sanada

E-mail: sanada@jamstec.go.jp