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Scientific objectives

The over arching objective of the LWD program was to provide borehole data that will be used in conjunction with cores to document the geology, physical properties, mechanical state, fluid content, and stress conditions at the drilling site locations. Specifically, we wanted to document the following:

  • Physical properties. We planned to record density, porosity, resistivity, and sonic velocity for each borehole. These logs provide key in situ information that, together with core-based sample data, will be used to quantify the mass and materials that make up the Nankai margin. Drilling targets for which physical properties are especially relevant include zones of anomalous compaction state, gas hydrate bottom-simulating reflectors (BSRs), and fault zones and adjacent wall rocks. In practice, we were able to obtain all of these types of logging data, but not all types from every site drilled. In particular, the lack of a neutron source during drilling of Sites C0004 and C0006 prevented us from acquiring porosity and density measurements and tool failure limited sonic data from Site C0006 to only the shallow portion of the hole.
  • Lithology. We also recorded natural gamma ray data. Together with the logs described above, these data were used to develop an integrated log-based lithologic characterization and logging unit definition.
  • Structural geology. Borehole imaging logs, principally azimuthal resistivity, permitted interpretation of bedding dip, fracture presence and orientation, and other rock fabric data, which will be useful in conjunction with core data to develop a complete structural description, including in situ orientation of structures.
  • In situ fluid pressure and stress. Borehole imaging logs were also used to detect borehole breakouts and drilling-induced tensile fractures (e.g., McNeill et al., 2004; Wu et al., 2007), which are useful for determining the orientation of the present-day horizontal principal stress axes. The APWD log is primarily a drilling parameter measurement, as it measures the fluid pressure in the open borehole near the bit; however, it can be an indicator of steep gradients in formation pressure or other anomalies. Sonic velocity is potentially another indirect indicator of stress and pore pressure conditions.
  • Core-log-seismic integration. Several of the logs provided key information for creating synthetic seismograms that tie 3-D seismic attributes to cores and borehole depth. The seismic while drilling (SWD), essentially a check shot–style VSP, is principal among these and provides borehole depth ties at similar wavelengths to that of the MCS data.
  • Drilling conditions. APWD, WOB, torque, drilling rate, and other parameters are primarily used to assess the drilling conditions. However, these measurements can provide useful information about formation strength, presence of faults, and other environmental variables relevant to the scientific objectives and also relevant for planning the well drilling and casing program for subsequent riser drilling.