Method of Evaluating Vertical Sealing of Faults in Terms of the Internal Structure of Fault Zones

There is an internal structural discrepancy between plastic faults and fragile faults: a fragile fault zone consists of a crushed zone and an induced fracture zone characterized by fault rock and associated fractures; a plastic fault zone consists of several big fractures filling the fault gouge without an induced fracture zone. Associated fractures inside a crushed zone, a fault rock zone without adhesive power and induced fracture zones may all allow petroleum migration. So a fragile faultis vertically sealed only when all three migration pathways are sealed. When the associated fractures are sealed, the plastic fault is also sealed. Based on the seal mechanism, the sealing conditions of the three migration pathways are analyzed in this paper: the seal on a fault rock zone without adhesive power is dependant on the content of fault mud; the seal of an associated fracture inside a crushed zone is dependant on the relationship between the section pressure and fragile strength of the fault gouge; the seal quality of an induced fracture zone is controlled by the quantity of later rock-forming fill. This paper presents a method forevaluating the vertical seal of disparate faults using the section pressure, content and plastic strength of fault gouge and the quantity of later rock-forming fill. For example, the vertical seal of the F₁ fault of Kela 2 structure in Kuqa depression was evaluated according to this method. The results indicate that vertical seal on the F₁ fault had the following properties: section ① and section ③ show fragility, but section① is not sealed, as a result of an induced fracture that was not filled; section ③ is sealed; section ② is a plastic fault with a good vertical seal. This is one of the key reasons that natural gas gathered on a large scale and formed a field in Kela 2 stucture.