LOGGING PLANDifficulties in attempted wireline logging during previous legs at Nankai and other accretionary prisms clearly indicates that LWD will be required to acquire in situ physical properties in the Nankai accretionary prism. As noted above, Leg 196 LWD operations will be conducted at the beginning of the leg, prior to off-loading the tools at a midleg port call. We are planning to deploy the azimuthal density neutron (ADN) and resistivity-at-the-bit (RAB) tools (note that LWD sonic tools only operate in formations with Vp > 2 km/s and would likely not be useful in low-velocity formations [Vp < 1.8 km/s]). As demonstrated by the results from Legs 156, 170, and 171A, the information acquired from these LWD logs will also allow in situ pore pressures within the accretionary prism to be inferred.
The RAB tool is a unique LWD device which acquires full-bore, quantitative, azimuthal resistivity images of the formation while drilling. The RAB images detect resistivity, heterogeneity, and borehole structures such as fractures and stratigraphic contacts. The RAB tool also provides azimuthal gamma-ray data per quadrants. The ADN tool will provide borehole-compensated formation density, neutron porosity, and photoelectric factor measurements in four quadrants around the borehole. The ADN will also measure standoff from the tool to the borehole wall per quadrants using an ultrasonic caliper. A rate of penetration of 25 m/hr is required for each site to ensure good gamma-ray data quality and RAB image resolution. Drill-in casing operations down to 200 mbsf will be required at all proposed sites, so the upper 200 m may be logged in a separate penetration.
Based on the results from Leg 190, LWD operations at Sites 1173, 1174, and 808 are the highest priority. If additional time becomes available before the scheduled midleg port call, alternate sites include ENT-10A and ENT-2A.
Planned Sites | Table of Contents