Operations Summary During Riserless Drilling to >7700 mbsl in the Japan Trench for IODP Expedition 343 & 343T JFAST and Discussion of the Relationship Between Drilling Parameters and Rock Damage

Abstract

During IODP Expedition 343: The Japan Trench Fast Drilling Project (JFAST), five boreholes were drilled from the D/V Chikyu in >6800 m water depth. Three of these crossed the main fault target. A logging-while-drilling (LWD) hole that penetrated to 850.5 meters below seafloor (mbsf) (total depth [TD] = 7740 meters below sea level [mbsl]) was documented using a suite of LWD tools. From an adjacent partially cored hole drilled to 844.5 mbsf (TD = 7734 mbsl) 21 cores were acquired that spanned the two main fault targets. During the follow-up expedition 343T a third borehole was drilled to 854.8 mbsf (TD = 7752.3 mbsl) and a simple temperature observatory was deployed in the wellhead. The drilling operation, which lasted 88 days, was very technically challenging. Notably, the drill string had to be withdrawn a number of times due to high seas, and technical issues. In certain intervals, rather than core we recovered loose, subrounded fine gravel clasts of the two major lithologies penetrated to those depths (silt and mudstone). Particle shape and size of these clasts was analysed. Results demonstrate (1) particle shape variations apparent visually are not easily quantified, (2) there are distinct variations in particle size distributions. We discuss whether these relate to variations in drilling parameters.