The CK14-04/Expedition 907 was conducted at the active hydrothermal field on the Iheya-North Knoll at Okinawa trough Japan by D/V Chikyu in July 2014. High temperature hydrothermal fluid flow exceeding 300degC was expected at the active hydrothermal deposits from the previous surveys. To achieve successful survey and to protect LWD tools,
pre-survey dynamic temperature modeling was carried out,
continuous circulation system "Non Stop Driller", which makes constant drilling fluid circulation even during pipe connection, was used,
LWD tools were upgraded up to 175degC, was used, and
real-time annular temperature and pressure were monitored.
Flow rate was carefully controlled with the Non Stop Driller System after the pre-survey temperature modeling.
The six wells were drilled with LWD successfully. The water depths are 886m to 1132m. High quality natural gamma-ray and resistivity, annular temperature and annular pressure mounted on arcVISION and TeleScope throw a light to active hydrothermal deposits first ever. The maximum annular temperate anomaly at 84degC was observed. The combination of natural gamma-ray and resistivity logs, and time series of annular temperature and pressure allows us innovative interpretation for deep water hydrothermal deposits. These results suggest that a LWD survey can be a new method for investigating sea-floor hydrothermal deposits.
Core samples are taken at the five sites across the Iheya North Knoll including the sub sea-floor hydrothermal fluid reservoir in the IODP expedition 331 in 2010 by the deep sea drilling vessel (D/V) Chikyu (Takai et al., 2011, 2012). The results obtained from the drilling operations and cored samples indicated the possible existence of enormous subseafloor hydrothermal fluid reservoirs that were much more expansive that expected from the extension of seafloor hydrothermal activities. Probably at around the cap rock layers that sealed the hydrothermal fluid reservoirs, the subseafloor massive black-ores (Kuroko) were found (Takai et al., 2011; 2012). These findings suggested that the subseafloor hydrothermal fluid regimes of the Iheya North field, even of other mid Okinawa Trough hydrothermal systems, had great potentials to host subseafloor hydrothermal mineral deposits such as noble metals and base metals (Cu, Pb, Zn, etc) for future marine mineral resources of Japan. Using AUV exploration, two new hydrothermal systems in the Iheya North Knoll were found in 2013. The new hydrothermal fields named as Iheya North Natsu and Aki fields are located at 1.5 km south-southeast and 3.0 km south from the Iheya North Original field, respectively.