Differential deformation on two sides of Qiyueshan Fault along the eastern margin of Sichuan Basin, China, and its influence on shale gas preservation

Abstract

Structural deformation is one of the main factors controlling shale gas preservation in Silurian Longmaxi Formation in the Sichuan Basin. The exploration practices demonstrate that the shale gas preservation condition in the areas to the west of the Qiyueshan Fault (QYSF) (along the eastern margin of the Sichuan Basin) is better than that to the east. In this study, we synthesize seismic interpretation, apatite fission track (AFT), and triaxial rock mechanics experiment to determine the structural segmentation, deformation time, and shale fracturability of the QYSF belt. Seismic interpretation shows that the QYSF can be divided into three segments, among which, the central segment is transitional deformation zone, the burial depth and deformation structure difference of Silurian strata on the two sides of the fault are not variable obviously, and shale gas preservation conditions are good. AFT simulation shows that the initial deformation time of chevron syncline-thrust structure to the east of the QYSF was 160–120 Ma, while that of the chevron anticline-thrust structure deformation between the Qiyueshan and the Huayingshan was 100–80 Ma, and it gradually changes from the central to the north and south. Rock mechanics test reveals that under the same stress field, the difficulty of fracture is influenced the dip angle of the shale bedding plane. Shale is most likely to produce fracture when the dip angle of bedding plane 34°–37°, but horizontal formation is difficult to rupture. This proves that gentle anticline and syncline are more conducive to shale gas preservation. Therefore, shale gas is mainly enriched in the core of gentle anticline and synclinal. Combined with the analysis of typical wells, we believed that the two sides of the QYSF are different in deformation time, uplift amplitude, and later multi-directional stress reworking. These cause differences in the direction, scale, and density of fractures in the strata, thus affecting shale gas preservation on two sides of the QYSF.