Riedel Shear Structures Reactivation may Induce Earthquakes Through Long-Term Steam Injection: A Case Study of a Heavy Oil Production Field in Northwestern China

Conventional heavy oil exploitation methods involve steam stimulation and flooding. An oil field in northwest China has been producing heavy oil via steam injection for several decades. The production area was seismically quiet until an increase in seismicity occurred several years ago. An array of 40 seismographs was deployed between July and October 2021 to monitor seismicity and resolve the possible causes of the recent seismicity. Using an end-to-end machine learning-based high-precision earthquake location workflow, we analyzed a microseismic sequence comprising 178 events that occurred in the study area. Numerical simulations incorporating Coulomb failure stress suggest that prolonged steam injection can reactivate faults and induce seismic events. Similarly, fluid diffusion through conduits may achieve the same effect. Analysis of the focal mechanism solutions of 21 strike-slip and thrust events with ML≥2 in conjunction with the background stress regime (SHmax azimuth = N15°W) revealed that the stress distribution is compatible with a left-lateral Riedel Shear Structure (RSS) model. Therefore, we can speculate that steam injection may induce earthquakes by reactivating pre-existing RSS fault structures. To conclude, the recent seismic events could have been induced by two possible mechanisms: (i) long-term steam injection may cause the static stress level on the faults beneath the reservoir to build up to critical levels, following which a slight stress disturbance can trigger an earthquake; (ii) fluid conduits may transport condensed water to basement faults, weakening the faults through fluid diffusion.