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doi:10.2204/iodp.proc.314315316.112.2009

Log characterization and lithologic interpretation

LWD measurements provide in situ petrophysical information on rocks and pore fluids while the hole is being drilled. These measurements are sensitive to changes in composition (changing curve magnitudes), textures, and structures (log shape, peak amplitude, and frequency, as well as information from image logs). Changes in the log response (values and/or frequency of the signal) are commonly associated with geological unit boundaries.

This section addresses the characterization of the LWD measurements and imaging tool response, focusing on zoning the well logs into logging units. Once representative petrophysical properties for the logging units were defined, they were incorporated in the log-based lithologic units. This process can be achieved by qualitative and quantitative methods.

Log characterization and identification of logging units

Qualitative analysis

The geometry of logging unit boundaries and bedding information was defined on the basis of borehole images and characterized from scalar LWD logs. Rock textures and structures were analyzed on borehole images, and vertical trends were analyzed on all the available logs. Composition and textural information was derived mainly from nuclear (spectral gamma, density, and PEF) and sonic logs. Data quality assessment was made by shipboard scientists through the examination of the potential effect of borehole diameter and conditions and drilling parameters on the logs prior to interpretation of the log response.

The first qualitative approach to unit definition was identification of the boundaries separating sections of different log responses and concomitant rock properties. For this type of analysis, natural- and induced-radioactivity logs, sonic logs, resistivity, and borehole images were the main input.

Integrated interpretation of all the available logs focused on the following items:

• Definition and characterization of logging units, subunits, and unit boundaries;
• Identification of compositional features within each unit; and
• Interpretation in terms of geological features (unit boundaries, transitions, sequences, and likely lithologic composition).

Quantitative analysis

As a consistency check of the qualitative interpretation and for quantitative log characterization, we applied statistical grouping. This involved investigating the percentile ranges and distribution of absolute values within the visually defined logging units.

Log-based geological/​lithologic interpretation

During Expedition 314, lithologies were interpreted based only on LWD logs without cores, unlike previous ODP/IODP expeditions. After log characterization and classification, logs were lithologically and geologically interpreted using a combination of log characteristics and borehole images for each site.

Compositionally influenced logs such as gamma ray and PEF logs were used to determine lithology from unit scale to bed scale. In particular, the identification of sand-rich intervals, clay-rich intervals, or alternating beds of sand and clay was a primary element of the interpretation. Sonic and all other logs were also used for lithology characterization. Borehole images provided useful information on mesoscopic features such as bedding, sedimentary structures, bed boundaries, unconformities, and faults. Additionally, lithologic information from ODP Legs 131, 190, and 196, all in the Nankai Trough off Shikoku Island, was an important aid in interpreting lithology.

These interpretations will be confirmed by correlation with core data from subsequent NanTroSEIZE project expeditions. A possible correlation to seismic units was also proposed for each site.

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