Stress estimation in a 3 km-deep geothermal borehole: A snapshot of stress state in Southern Cathaysia block, China

Abstract

The present-day stress state at depth in Southern Cathaysia block remains poorly understood. We retrieved twelve high-quality granite core samples in a 3 km deep borehole of the Huangshadong geothermal field in South China and conducted core-based stress measurements using the anelastic strain recovery (ASR) method to estimate the full stress tensors. The measured stress data of hydraulic fracturing in an adjacent, 600 m-deep geothermal borehole was also collected. The results show that the maximum principal stress σ1 is sub-horizontal from 1566 m to 3008 m in depth. The maximum and minimum horizontal principal stresses, SH and Sh, and the vertical principal stress, Sv, generally follow SH > Sv ≈ Sh in their magnitudes, thus indicating a thrust/strike-slip faulting transitional stress regime in the study area. Our stress orientations by ASR method based on paleomagnetic analysis and the impression test of hydraulic fracturing further suggest that SH is approximately oriented N72° ± 5°W, and this is consistent with both the regional stress field and the observed motion pattern of the Tonghu strike-slip fault in the study area. Interestingly, we found that the difference between the magnitudes of SH and Sh is small, indicating a low level of shear stress in the shallow crust. Also, because of the presence of the lithological changes and fracture zones, there are some snapshots of localized heterogeneous stress states in the granite geothermal reservoir. The present stress state has an impact on the occurrence and exposure of hot springs in the Huangshadong geothermal field. We indicated that the fracture zones with weak stress accumulation are potential transport channels of hot water. The stress field of the Southern Cathaysia block is mainly constrained by the southeast push of the Tibetan Plateau and the northwest subduction of the Philippine Sea plate.