Characteristics and formation stages of natural fractures in Lower Silurian Longmaxi Formation in Tiangongtang area of Sichuan Basin

Abstract

The Lower Silurian Longmaxi Formation on the southwestern margin of the Sichuan Basin holds significant shale gas reserves. The characteristics and formation stages of fractures play a crucial role in shale gas accumulation and productivity. Focusing on the Longmaxi Formation in the Tiangongtang area, this study employed core analysis, FMI (Formation Micro-Imager) logging, rock acoustic emission experiments, carbon-oxygen isotope analysis of fracture fillings, fluid inclusion homogenization temperature testing, and burial-thermal evolution history analysis to investigate the development characteristics and formation periods of natural fractures in the shale. The results indicate that the natural fractures in the Longmaxi Formation in the study area are characterized by the coexistence of tectonic vertical and low-angle fractures. The core fractures exhibit high development density, short extension, and high filling degree. Comparison of imaging log fracture dip angles, core fracture cross-cutting relationships, fracture filling fluid inclusion tests, and rock acoustic emission experiments suggested that the fractures in the Longmaxi Formation in the Tiangongtang area were associated with three tectonic events. Combined with burial-thermal evolution history analysis, the formation periods were confirmed as follows: the first stage involved NW-oriented and NNE-oriented planar shear fractures, and NEE-oriented cross-sectional shear fractures formed during the mid-late Yanshanian period (130-62 Ma) with tectonic stress orientation near SN (345°±5°) and inclusion homogenization temperatures of 185-206 ℃; the second stage involved NE-oriented and NW-oriented planar shear fractures, and NNW-trending cross-sectional shear fractures formed during the late Yanshanian to mid-Himalayan period (62-34 Ma) with tectonic stress orientation near EW (80°±5°) and inclusion homogenization temperatures of 165-184 ℃; the third stage involved near SN-oriented and NEE-oriented planar shear fractures, and NE-oriented cross-sectional shear fractures formed from the mid-Himalayan period to present (34 Ma to present) with tectonic stress orientation near NW (315°±5°) and inclusion homogenization temperatures of 125-162 ℃. Based on the geomechanical background, a three-stage tectonic fracture evolution model for the Longmaxi Formation in the Tiangongtang area was established.

Keywords

• shale fracture
• fracture characteristic
• tectonic stage
• longmaxi formation
• lower silurian
• sichuan basin