Formation Mechanism of High-Obliquity Faults in Platform Areas of Chinese Western Basins and Their Control on Petroleum Migration and Occurrence

High-obliquity faults (superior to 45°) are very commonly encountered in the platform areas in Chinese western basins. Geomechanical analysis demonstrates that high-obliquity faults are produced by the compression-torsion stress field existing in Chinese western basins, and they may also be related to the stratum's brittle deformation. Static sealing pressure, which can be calculated from Poisson's coefficient, burial depth, obliquity degree of the fault, and maximum and minimum principal stresses, is used as an index of the sealing capacity or openness degree of a fault for petroleum migration. The calculation of static sealing pressure indicates that the openness degree of a fault increases with the fault's obliquity degree. The floating force decomposition illustrates that the decomposed force along the fault surface also increases with the fault's obliquity degree. Therefore, vertical, cross-formational petroleum migration via a high-obliquity fault is easier than via a low-obliquity fault. This conclusion is applied to interpret the control of high-obliquity faults on petroleum migration and occurrence in the Tarim and Junggar basins of western China.Conclusionsare: (1) Regional high-obliquity faults crossing different dynamic fluid systems control the petroleum distribution in different systems, especially as petroleum migration pass-ways for the middle-upper systems, as they are essential for pool formation in the latter; (2) Inside each dynamic fluid system, petroleum migrating along high-obliquity faults is most likely tofill the newest layer cut by the faults. The conclusions obtained have clear implications for petroleum exploration and these conclusions can be extended to other basins where high-obliquity faults exist.