Analysis of the Tectonic Stress Field of SE Sichuan and its Impact on the Preservation of Shale Gas in Lower Silurian Longmaxi Formation of the Dingshan Region,China

Tectonic stress is one of the most active deformation factors in the Earth’s crust. Therefore, determinations of its mode of action and form of conversion are critical for guiding research that evaluates preservation conditions in shale gas. Through a survey that targeted many structural features, such as vertical folds and conjugated joints, we investigated the revolution characteristics of the tectonic stress field, the mode of action and the sequence of tectonic stress in the Dingshan region of SE Sichuan by structural analysis methods. Meanwhile, based on a geologic analysis of tectonic stress mechanisms, the structural stress in the region using finite element numerical simulations is quantitatively analyzed. The results show that the region has experienced tectonic stress fields over four different periods and in four different orientations: NW–SE, nearly N–S, nearly E–W and NE-SW. The NW-SE force occurred earlier than that of the nearly N–S force, followed by nearly E–W compression, and finally N–E force. It is concluded that the structural framework of the study area was formed by the effects of the NE-SW compressional stress, which is still active today. The simulation demonstrated that overlapping parts of the intensive strain zone had a higher maximum principal stress, particularly in areas of energy concentration. As for mud and shale, which show excellent sealing capacity, the key to preserving shale gas is the accumulation of energy and a low degree of breakage. Considering the nip-out line of the Longmaxi Formation in the Dingshan region, the buried depth limit of shale-gas production (4500 m), and fault length, the preservation conditions of shale gas and identified favorable targets within the Longmaxi Formation is evaluated.