Controls of fault-bend fold on natural fractures: Insight from discrete element simulation and outcrops in the southern margin of the Junggar Basin, Western China

Abstract

The fault-bend fold, a typical structure in foreland thrust belts, is suitable for hydrocarbon accumulation. Natural fractures are ubiquitously developed and usually interlaced to form a complex network in fault-bend folds, which can play a fundamental role in hydrocarbon migration and enrichment. This study utilized outcrops and the discrete element method (DEM) to simulate and analyze the characteristics, distribution patterns, and development mechanisms of natural fractures in the fault-bend fold. The study anticline, located in the southern margin of the Junggar Basin, is a well exposed fault-bend fold. Statistical results of dips and dip direction of fracture in different structural positions show that the fold-related fractures strike NWW-SEE parallel to the axis of the study fold. The fractures include layer-parallel shortening related fractures (LPSF), interlayer slipping related fractures (ILSF), active hinge parallel shearing related fractures (AHSF), and curvature-related fractures (CF). The LPSF and ILSF are strata-bound fractures, whereas the CF and AHSF are not strictly confined in layers. Furthermore, results of the DEM and outcrop measurements demonstrate that LPSF are formed at the embryonic stage. ILSF and AHSF are formed in the early stage, and the mid-late stage is the main formation period of CF. By integrating strata deformation history with fracture types and development characteristics, a development pattern of fractures within the fault-bend fold has been established, and five deformation panels are developed (panels I–V). From panels I to V, the fracture area density increases consecutively, and panel I (unfolded) is limited to regional fractures. Next, LPSF and ILSF are more highly developed in panel V (forelimb) than in panels IV (strongly folded layers of the backlimb), III (core), and II (weakly folded layers of the backlimb). Moreover, CF are predominantly found in panel III, and AHSF are mostly developed in panel IV. Overall, this study investigated the characteristics of fractures in deformation panels of the fault-bend fold and thus provides guidance for locating favorable reservoirs in foreland thrust belts.