Exploration of Seafloor Massive Sulfide deposits using natural gamma-ray logging: An application of through-the-bit logging

Abstract

During the cruise CK16-05 aboard the D/V Chikyu, we conducted natural gamma-ray logging with a through-the-bit logging system by using a memory-type geothermal exploration tool that has so-far been used only in terrestrial geothermal wells. This logging system was firstly tried during the cruise CK16-01 to reduce the cost and increase the efficiency of drilling surveys, but issues from vibration and collision impact within the drill hole were found. Based on these results, we re-examined and modified the logging tools and operation methods. As a result, we successfully obtained highly accurate and continuous data of natural gamma-ray intensity, temperature, and pressure within the drill holes at five sites in the Izena Hole, mid-Okinawa Trough, and accomplished significantly lower-cost logging at one order of magnitude lower than that of the Logging While Drilling (LWD) system. At three sites within the Izena Hole hydrothermal field, the gamma-ray intensity exhibited significant variations along with lithological changes composed of pumice, hemi-pelagic sediment, massive sulfide, silicified rock, and a K-rich altered clay layer. On the other hand, at the other two sites located ca. 330 and 670 m N-NNW from the hydrothermal sulfide mound, subseafloor strata were dominated by pumice with minor amounts of hemi-pelagic sediment and the gamma-ray intensity correlated well with the density and porosity of the core samples determined by the multi-sensor core logger (MSCL) and discrete sample measurements. Our results demonstrate that gamma-ray logging with the through-the-bit logging system is highly effective in identifying and characterizing the subseafloor lithology as well as the thickness of the massive sulfide layer, and can compensate for the often inevitable poor core recovery rate during coring operations at seafloor hydrothermal deposits. The versatility of this system could be further improved with additional sensor measurements (e.g., resistivity) in future drilling operations at seafloor hydrothermal fields.