Subsurface SHMAX determined from a borehole image log, onshore southern East Coast Basin, New Zealand

Understanding the in-situ stress field in the subsurface provides insights on the tectonic processes and seismic hazards of a basin. This paper presents observations and interpretations of present day maximum horizontal stress (SHmax) orientation from well Ranui-2 located in the onshore southern East Coast Basin. Fractures identified in the Ranui-2 image log have been divided into distinct subsets based on fracture orientation. These subsets correspond in a general sense, to changes in lithology in the well, which are associated with changes in formations. Analysis of the image log shows the present day SHmax in the borehole has two primary stress directions, W–E and NW–SE, and most of the fractures strike NE–SW. The change in SHmax occurs at a depth of circa. 1130 m measured depth (918 m true vertical depth subsea), below the depth at which the borehole changes from vertical to ≥10° deviation. When these data are compared to the regional stress field obtained using other stress indicators such as earthquake focal mechanisms, the results are consistent with each other. This study shows that shallow subsurface indications of present-day SHmax in southeastern North Island reflect SHmax orientations from deeper within the subsurface.