UNLOADING FRACTURE——AN IMPORTANT TYPE OF FRACTURES IN THE JURASSIC TIGHT SANDSTONE, WESTERN SICHUAN BASIN

A number of comprehensive study data of cores,structures,inclusions,well testing performance and fracture pressure gradient have shown that there are several fracture types in the Jurassic tight sandstone reserviors of western Sichuan basin,such as local structures,subtraction (load relieving),stratification and corrosion etc.The leading origin type of fracture is the paleo subtractive one formed by uplifting subtraction during geohistorical process,which is the main percolation passage in the process of oil/gas concentration,reservoiring and production.