Fractographic features on joints in KTB drill cores as indicators of the contemporary stress orientation

Abstract

Fractographic features on joints in cores of the superdeep KTB drilling were used to estimate the orientation of the contemporary maximum horizontal stress (SH). The unique opportunity to investigate cores from a depth down to approximately 8085 m reveals information on in situ stress orientation with increasing depth. We recognized on the cores the main normal stress axes which are reflected by common fractographic pattern on coring-induced fractures, present in all sections. The analysis is based on the symmetry of joint characteristics: 3D (out-of-plane) shape of the joint and surface features of joint planes. The symmetry axes of these joint features are related to the orientation of principal stress, which permits the determination of the orientation of the contemporary maximum and minimum horizontal stress axes (SH≠Sh), if the core orientation is determined by other methods. For that purpose, the reorientated cores of the pilot borehole (VB) could be used down to 4000 m. The main borehole (HB) cores are generally not reoriented by several reasons. Some investigations were done by members of the KTB laboratory to determine the supposedly northward orientations of HB core sections by comparison with the foliation and of drilling-induced fractures at the borehole wall (using FMI/Formation MicroImager and FMS/Formation MicroScanner log data) and within the core. Independently, the maximum horizontal in situ stress orientation down to 7800 m was determined from drilling-induced vertical joints, and the strike and dip of the foliation from borehole measurements down to ca. 8600 m, carried out by the staff members at the KTB. Additionally, rock fragments from below 9050 m represent a palaeo-tectonic joint system developed at a depth of ca. 9 km.