Characteristics and main controlling factors of fractures in gas reservoir of Xujiahe Formation, Dayi Structure

The gas reservoir of the Xujiahe Formation in the Dayi Structure is a fractured reservoir. The generation degree of fracture is the key factor for natural gas enrichment and high productivity. According to the core and logging data from 14 wells, combined with structural and sedimentary facies analyses as well as ancient tectonic stress field simulation, it has been concluded that 3 genetic types of fractures exist in the study area, including structural fracture, stratification fracture and abnormal pressure blowdown fracture. Structural and stratification fractures are relatively more effective; however, they are usually in smaller amount, and extend for short distance and narrow width. They are filled for higher degree and have stronger heterogeneity. Small formation stress and weak deformation are the primary causes for the poor development of structural fractures. Induced feather fractures during the Himalayan period control the formation and distribution of structural fractures. Parallel beddings and inclined beddings which develop in the distributary channels of delta plains and delta fronts are the main bedding types for stratification fracture formation. The genetic types of fractures, filling characteristics, later denudation and current ground stress field control the effectiveness of fractures.