Calculation and evaluation of log-based physical properties in the inner accretionary prism, NanTroSEIZE Site C0002, Nankai Trough, Japan

Integrated Ocean Drilling Program (IODP) Expedition 338, the most recently completed drilling stage of the NanTroSEIZE project, targeted the Miocene inner accretionary prism off the coast of southwest Japan. NanTroSEIZE is a multi-stage project in which the main objective is to characterize, sample, and instrument the potentially seismogenic region of the Nankai Trough, an active subduction zone. Understanding the physical properties of the inner accretionary prism will aid in the characterization of the deformation that has taken place and the evolution of stress, fluid pressure, and strain over the deformational history of these sediments and rocks. This study focuses on the estimation of porosity and density from available logs to inform solid and fluid volume estimates at Site C0002 from the sea floor through the Kumano Basin into the accretionary prism. Gamma ray, resistivity, and sonic logs were acquired at Hole C0002F, to a total depth of 2005 mbsf into the inner accretionary prism. Because a density and neutron porosity tool could not be deployed, porosity and density must be estimated using a variety of largely empirical methods. In this study, we calculate estimated porosity and density from both the electrical resistivity and sonic (P-wave velocity) logs collected in Hole C0002F. However, the relationship of these physical properties to the available logs is not straightforward and can be affected by changes in fluid type, salinity, temperature, presence of fractures, and clay mineralogy. To evaluate and calibrate the relationships among these properties, we take advantage of the more extensive suite of LWD data recorded in Hole C0002A at the same drill site, including density and neutron porosity measurements. Data collected in both boreholes overlaps in the interval from 875 - 1400 mbsf in the lower Kumano Basin and across the basin-accretionary wedge boundary. Core-based physical properties are also available across this interval. Through comparison of density and porosity values in intervals where core and LWD data overlap, we calculate porosity and density values and evaluate their uncertainties, developing a best estimate given the specific lithology and pore fluid at this tectonic setting. We then propagate this calibrated estimate to the deeper portions of C0002F where core and LWD density and porosity measurements are unavailable, using the sonic and resistivity data alone.