Characteristics and controlling factors of multi-scale fractures in reservoir: A Jurassic-Cretaceous case from the southern margin of Junggar Basin

Fractures are widely developed in the Jurassic-Cretaceous reservoirs in the southern margin of the Junggar Basin. Based on the observation of the outcrop sections, cores and thin sections, combined with the use of logging data, computed tomography (CT), confocal microscope and scanning electron microscope (SEM), the study analyzed the classification, distribution and origin of the fractures in the reservoir at different scales. The fractures in research areas can be divided into three levels, which are level-1 macro fracture, level-2 mini fracture and level-3 micro fracture. Based on the length and origin of fractures, macro fracture and micro fracture respectively can be further divided into two sub-levels. The results indicate that the development degree of macro fractures decreases from the Tianshan orogen to the inner basin. The density of macro fractures reduces from 1. 6 fractures per meter in the first row of the structural belt to 0. 8 fracture per meter in the third row of the structural belt. Mini fractures are mostly developed in the sandstone reservoir in the structural belt area, whereas micro fractures are mostly developed in the conglomerate reservoir in the first row of structural belt and in the sandstone reservoir in the northern basin-slope area. The development of fractures in Jurassic-Cretaceous reservoir is controlled by the tectonic stress, lithology and the thickness of the reservoirs. Fractures are more easily to develop where the tectonic stress is stronger. Fractures are better developed in medium-fine sandstone than siltstone, coarse sandstone and conglomerate, especially in the reservoir whose bedding thickness is thinner. Fractures are more easily developed in the reservoir which has more felsic minerals and carbonate cement. © 2022, Editorial Board of Journal of CUMT. All right reserved.