Abstract:

The tight gas reservoir in the central Yuanba area of Sichuan basin is one of the major breakthrough in oil and gas survey in recent years. The fractures have important influence on the migration, enrichment and production capacity of the natural gas. Based on the outcrop, core, imaging log, inclusion analysis and acoustic emission experiment data, we studied the fracture characteristics and its genesis of structural fractures in this area. The low-angle shear and high-angle shear fractures lead to the tectonic origin. The fractures have the characteristic of middle scale, minor width(sealed), high thickness, lower density and less filled degree. The high-angle and vertical fractures have better validity. It shows that there are three fracture generations in central Yuanba area: the first stage fractures are formed in late-Yanshanian tectonic movement, NW—SE and NE—SE of plane "X" conjugate shear fractures and low section shear fractures are formed under the compression of SN tectonic stress. The maximum effective principal stress of the paleostructural stress is 18.8 MPa. The fractures in the second stage are formed in middle Himalaya tectonic movement, SN and EW of plane "X" conjugate shear fractures are formed under the compression of NE tectonic stress. The maximum effective principal stress of the paleostructural stress is 27.9 MPa. The third stage are formed in late Himalaya tectonic movement. The structural stress direction is still NE, which mainly forms NW angle section shear fractures and the maximum effective principal stress of the paleostructural stress is 38.6 MPa.

Key words: tight reservoir, fractures characteristics, formation stage, genetic pattern, Xujiahe Formation, central Yuanba area