Observed stress state for the IODP Site C0002 and implication to the stress field of the Nankai Trough subduction zone

Abstract

Conducting a stress state analysis is one of major challenges for evaluating earth crust and formation conditions. Stress magnitude is especially essential to comprehend stress heterogeneity, significant structural anisotropy, and/or pre-existing fracture systems. Stress state is influenced by rock strength, structural properties, and near-field principal stresses, which impact borehole integrity during drilling. Factors that we can measure through Logging While Drilling (LWD) and Measure while Drilling (MWD) to better understand borehole conditions, include mud weight, overburden weight and velocity. The LWD resistivity images in Holes C0002A and C0002F of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) indicate that most of the drillings in stable environments. However, the bottom hole assembly became stuck around 3 km depth in Hole C0002P and did not reach the mega-thrust fault. We evaluated the stress state for the Site C0002 by the geomechanical model. With the breakout width and rock strength estimation, we constrain the stress profiles in the drilled depth. Our analysis inferred that the instability of Hole C0002P had been caused by an overpressure drilling fluid state and low horizontal principal stress. Furthermore, we developed an optimally oriented plane (OOP) model to exam the stress state in the Site C0002 from Nankai accretionary prism to the Nankai Trough subduction zone. The normal faulting stress regime modeled from OOP is consistent with the geomechanical model in Site C0002. The horizontal principal stresses magnitude appears to less than the overburden weight above this subduction zone.