The stress heterogeneity in the axis of Nankai Trough observed in the scientific drillings Borehole C0004B and C0010A, NanTroSEIZE

Site C0004 is the first expedition of NanTroSEIZE to understand the geological characters and stress states in the shallow mega-splay fault near the Nankai Trough axis. In NanTroSEZIE Exp. 319, well C0010A was drilled to compare with well C0004B in physical properties, fault zone architecture, and the variations along-strike of Nankai Trough. The character of the mega-spray fault diverges remarkably between C0010A and C0004B in the seismic reflection data. The high resolutions of LWD resistivity images are collected to 550 meter below sea floor (mbsf) in each borehole. The stress state in site C0010A and C0004B can be modified by the borehole conditions, logging data and the physical properties. This geo-mechanical model used the stress polygon to analyze the reprocessing resistivity images for picking the breakout and tensile fracture azimuth and width in well C0010A and C0004B. Several faults were identified by the rotated borehole breakout orientation. The fractures distributions also supported the stress anomaly in the vicinity of the faults. However, the magnitudes of the horizontal stresses in C0010A vary in the different units comparing to the C0004B due to the different behavior on the logging data. The difference of two sites indicated that the local structure near the Nankai Trough would be the factor to influence the stress state in the boreholes. Normal fault stress regime was determined in both boreholes. The low stress level in the shallow portion near the Nakai Trough was obtained in these scientific drilling projects.