Tectonic Stress at IODP Site C0002, Nankai, Indicated by Borehole Resistivity Images of Two Boreholes Drilled under Different Annulus Pressures

Abstract:

We constrain tectonic stresses at IODP Site C0002 in Nankai accretionary prism, SW Japan, using two boreholes drilled under different annular borehole pressures (APRS). The two vertical boreholes (C0002A and C0002F drilled in 2007 and 2012, respectively) are located at the southern margin of Kumano forearc, respectively drilled to depths of 1402 and 2006 mbsf. The two holes were drilled in different drilling modes: riserless drilling for C0002A and riser drilling for C0002F. Both holes were image-logged soon after drill bit penetration using the logging-while-drilling resistivity-at-the-bit tool, from which we detected borehole wall stress indicators (borehole breakouts and drilling-induced tensile fractures (DITFs)). We assume that there should be little difference in tectonic stress between the two drill sites (70 m apart) and between the two time frames (5 years apart). The resistivity images show that the patterns of borehole wall failures are dramatically contrast between the two boreholes, i.e., clear and continuous breakouts and no DITFs in C0002A, whereas intermittent breakouts and DITFs in C0002F, which is due to the difference in APRS between the two holes. Such different APRS and associated distinct borehole wall failure patterns enable us to constrain possible tectonic stress states that can produce observed borehole wall failures for given APRS conditions. Our analysis shows that while the stress states in the forearc sediments are predominantly in favor of normal faulting, those in the deeper accretionary prism are favorable for either strike-slip or reverse faulting although the differential stresses (between least and major horizontal principal stresses) are not significantly large. Throughout the drill depths, the borehole wall failures indicate that the maximum horizontal principal stress direction is NE-SW (perpendicular to subduction direction). However, a series of borehole wall failure zones at 1930-1980 mbsf, if they are breakouts, may indicate a stress rotation to NW-SE, which needs further investigation.