Natural Fracture Patterns in Fault-Related Folds in the Tight Sandstone Reservoirs, the Southern Margin of Junggar Basin, Northwestern China

Abstract

The spatial distribution and development of natural fractures in foreland thrust belts are extremely related to the structural styles and deformation strength of stratum. The southern margin of Junggar Basin in northwestern China, bounded by the northern Tianshan orogen, has experienced four orogenies since the Permian, as indicated by the characteristics of Mesozoic–Cenozoic fault-related folds. Most hydrocarbon reserves in this area are stored in Jurassic and Cretaceous tight sandstone reservoirs, and natural fracture systems have a significant impact on the performance of such reservoirs. This paper focuses on the potential control that fault-related folds may exert on the fracture patterns using the data of outcrops, cores, image logging, 3-D seismic and others. The research indicates that the characteristics of natural fractures show a clear relationship to the fault-related folds in both the outcrop and subsurface. In high curvature regions of folds, such as cores and forelimbs, fracture intensities are high and fractures are short. In low curvature regions fractures have variable intensities and are longer. The dominant orientations of fractures vary in different regions of fold. Fractures usually intersect the bedding planes at high angles, and some inclined fractures with a large scale are developed near the faults and the core of folds. The development degree of fractures decreases exponentially with increasing distance to the fault plane, and there is a fracture belt near the fault. These variations are also relate to strain history of fault-related folds. Structures with longer deformation histories exhibit consistent fracture attributes, despite present day low curvature. This is in contrast to younger structures with similar curvature but shorter deformation histories. We suggest in high strain regions fracturing is influenced by structural styles, whereas in low strain regions lithology, layer thickness, sedimentary facies and others become more important in influencing fracturing. Therefore, according to the structural positions, curvature and strain characteristics, we divide the fault-related folds into six fracture domains. In the same fracture domain, the developmental characteristics of fractures have a strong similarity. The results of this study can help us to better understand the distribution of natural fractures in fault-related folds and reduce the uncertainty of fracture prediction in such reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018