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doi:10.2204/iodp.sp.315.2007

Drilling strategy

Continuous coring in the splay fault drilling at proposed Site NT2-03B (and its alternate proposed Site NT2-03C) are essential for characterizing the composition, structure, and architecture of the fault zones and wall rocks. Chemical composition, diagenetic processes, microstructures, and potential sealing/healing processes will be examined in the active splay fault zone and wall rocks. Physical property data, including porosity, electrical resistivity, and seismic velocity, are key parameters in assessing the mechanical and hydrologic behavior of fault and wall rocks. Whole-round and discrete core sampling are necessary for postexpedition studies such as permeability and consolidation experiments and mechanical (rock friction) tests. Pore fluid chemistry data will be important toward constraining the hydrologic behavior of the megasplay faults and the source of any chemically and/or isotopically distinct, deeply sourced fluids.

We will drill three holes during Expedition 315. Hole B is for coring only (Hole A is a LWD hole drilled during IODP Expedition 314). We will start drilling with the hydraulic piston coring system (HPCS) to refusal and continue with the extension shoe coring system (ESCS) to 600 mbsf. Hole C is for coring. We will drill out the upper 600 m and core with a rotary core barrel (RCB) to total depth (1000 mbsf) (Fig. F10). Expected lithologies are hemipelagite and slump deposits for the upper ~210 m section and well-consolidated and highly deformed sediments in the underlying accretionary prism section. We may also encounter fault rocks that are cataclastically deformed to various extents. Since we may encounter caving problems in these sections, we will use bentonite mud, if necessary. We will also prepare weighted kill-mud for potential overpressure beneath the upper slump sediments.

After coring and logging in Holes B and C, we will drill and case the upper 700 m of the future riser hole (Hole D). We will jet-in a 36 inch conductor pipe to 60 mbsf and drill a 26 inch hole to 700 m in riserless mode. Then we will set a 20 inch casing and place a wellhead and corrosion cap to suspend the well for the future riser drilling. Basically, no scientific operation is planned during Hole D drilling; however, if the coring in Hole B is unsuccessful, we will collect the core from this hole. The drilling schedule is shown in Figure F11 and Table T1.

Logging/downhole measurements strategy

Because Expedition 314 will conduct LWD operations at the same site, we will have no wireline logging program on our expedition as long as the previous expedition is successful. In the case that the Expedition 314 LWD operations are unsuccessful, there are two options. If the failure on the LWD expedition is due to other than hole conditions (e.g., mechanical problems, weather conditions, time limitations, etc.), we will run a full set of wireline logging with three runs on this expedition: triple combination (natural gamma radiation, density, neutron porosity, and electrical resistivity) in the first run; oriented electric resistivity imaging, natural gamma radiation, and sonic velocities on the second run; and vertical seismic profile (VSP) on the third run. On the other hand, if the reason for unsuccessful LWD operations is due to bad hole conditions, wireline logging is expected to be less successful. In past DSDP and ODP accretionary prism drilling, wireline logging was not completely successful; therefore, we will not run any wireline logging in case of poor hole conditions. The final decision regarding the wireline logging menu will be made immediately after LWD Expedition 314.

Estimating the geothermal gradient is one of the key issues for future deep riser drilling. Downhole measurement tools (third-generation advanced piston corer temperature [APCT-3] tool and Davis-Villinger Temperature Probe [DVTP]) will be used to measure formation temperature. The APCT tool is used every 30 m for HPCS coring intervals, and the DVTP is used every 50 m for ESCS coring intervals. Tool details can be found online at iodp.tamu.edu/tools/specs.html#dow.

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