Operational strategy

To meet the scientific and engineering objectives of Expedition 338, primary operations in Hole C0002F were to drill from 860.3 to 2300 mbsf using LWD/measurement while drilling (MWD) and set 16 inch casing (Fig. F3). Wireline coring was to be conducted in Hole C0002F between 2300 and 2400 mbsf using a rotary core barrel (RCB) to obtain the highest quality and most complete core samples. Then, LWD/MWD, cuttings, and mud-gas analyses were to continue to 3600 mbsf where 13⅜ inch casing was to be set.

Following suspension of riser drilling operations, a contingency riserless operations plan was formulated that allowed coring at Sites C0002 (200–505, 902–940, and 1100.5–1120 mbsf), C0021 (0–194.5 mbsf), and C0022 (0–419.5 mbsf) as well as LWD at Sites C0012 (0–709 mbsf), C0018 (0–350 mbsf), C0021 (0–294 mbsf), and C0022 (0–420.5 mbsf) (Table T1).

Logging/Downhole measurements strategy in Hole C0002F

LWD/MWD tools provide the ability to monitor drilling parameters and conditions and to collect gamma ray and resistivity logs to define major lithologic changes in real time (MWD) as well as record high-resolution borehole and formation conditions (LWD). The MWD tool suite included annular pressure while drilling (APWD), downhole weight on bit (WOB), downhole torque, hole inclination, and gamma radiation. The Power-V tool was run during drilling to maintain hole inclination <3°. The geoVISION LWD tool was added to this suite to obtain azimuthal resistivity data and borehole resistivity images in order to further define stratigraphic boundaries and to characterize bedding, fractures, and any compressional borehole breakouts or drilling-induced tensile fractures (DITFs). In addition, the sonicVISION LWD tool was included to provide P-wave velocity data during drilling. A leak-off test (LOT) was conducted at 872.5 mbsf to help define the least horizontal principal stress.

Sampling and coring strategy at Site C0002

A total of 35 cores were cut from the 200–505 mbsf depth interval in Holes C0002K and C0002L (Table T1): 2 with the hydraulic piston coring system (HPCS), 4 with the extended punch coring system (EPCS), and 29 with the extended shoe coring system (ESCS). This interval includes the gas hydrate zone and the bottom-simulating reflector (BSR), which were not cored during Expedition 315 (Expedition 315 Scientists, 2009).

Nine RCB cores were cut from depth intervals 902–940 and 1100.5–1120 mbsf in Holes C0002H and C0002J (Table T1). This interval covers the lower part of lithologic Unit III (basal Kumano forearc basin) and the uppermost part of Unit IV (upper accretionary prism) according to the stratigraphy established during Expeditions 314 and 315 (Expedition 314 Scientists, 2009a; Expedition 315 Scientists, 2009).

During riser drilling in Hole C0002F (875.5–2005.5 mbsf), cuttings were collected every 5 m and mud gas was routinely sampled for geochemical analyses. Depth differences between LWD/MWD, mud gas, and cuttings data had to be carefully considered because of the mixing of cuttings caused by reaming while drilling (RWD). RWD technology allowed LWD/MWD analysis behind a 12¼ inch drill bit while simultaneously opening the hole above the LWD/MWD tool assembly with a 20 inch reamer bit.

Logging/Downhole measurements strategy in Hole C0012H

Logging operations in Hole C0012H used the same suite of LWD tools used for Hole C0002F LWD/MWD to 709 mbsf (Table T1). Logging data were collected from Shikoku Basin sediment overlying oceanic crust (~170 m crust penetration).

Logging/Downhole measurements strategy in Holes C0018B, C0021A, and C0022A

Logging with LWD/MWD tools in all three holes was conducted with the same suite of logging tools used in Holes C0002F and C0012H, with the exception of the sonicVISION tool (Table T1). Hole C0018B (0–350 mbsf) was designed to cover an interval cored during IODP Expedition 333 (Expedition 333 Scientists, 2012c). Holes C0021A (0–294 mbsf) and C0022A (0–420.5 mbsf) were logged to collect data from slope basin sediment.

Sampling and coring strategy in Holes C0022B and C0021B

A total of 55 cores were collected from Holes C0022B and C0021B (Table T1); however, only 41 cores from Hole C0022B were described on board the ship. Fourteen cores from Hole C0021B will be described and sampled in a shore-based sampling party at the Kochi Core Center. Both holes were cored using the HPCS (84.5 mbsf [or 8 cores in Hole C0022B] and 175.5 mbsf [or 12 cores in Hole C0021B]). After the formation consolidation state precluded further HPCS coring, EPCS coring was used for 3 cores and then ESCS was used for the final coring section in Hole C0022B (30 cores). In Hole C0021B, EPCS was used for the final 2 cores after the switch from HPCS.