Fracture development characteristics in the Upper Triassic Xujiahe Formation, western Sichuan depression (China)

Field joint surveys and electro-imaging logging were performed to study the characteristics of fracture development in the Upper Triassic Xujiahe Formation, which is located in the western Sichuan depression. Systematic joints with extensional characteristics have been identified; these likely formed as natural hydraulic fractures. There are four sets of fractures: nearly E-W trending fractures, nearly N-S trending fractures, NW-SE trending fractures, and NE-SW trending fractures; the nearly E-W trending fractures are the most widely developed. We analyze the timing of tectonic events of the Longmenshan orogenic belt and the western Sichuan foreland basin, terminal relationships, and filling characteristics of the joints. Based on this analysis, we proposed that some nearly E-W striking joints (after restored to the horizontal) formed during the Early Cretaceous (~100 Ma), although the Cenozoic (~60 Ma and onward) has been another important period for the formation of joints. This implies that horizontal, N-S wells could be drilled to reopen the sealed E-W striking joints and to gain access to the more permeable E-W striking joints.