Open shear fractures at depth in anticlines: insights from the Kuqa foreland thrust belt, Tarim Basin

Abstract

Shear fractures that close under compressive stress regime were described by previous workers. Here, we show the existence of hinge-parallel shear-opening mode fractures at a certain depth within an anticline structure, through core observations, Formation Micro Imager (FMI) log interpretations and production data. Studied reservoir is a tight sandstone body of Cretaceous age, which is emplaced in KS2 Anticline, with depth ranging between 6000 and 8100 m. We explore the mechanism of hinge-parallel shear fractures by stereographic technique and backtilting, as well as show field evidences for opening displacements. These shear fractures mainly consist of vertical tension set, moderate-dipping shear set, steeply dipping shear set and sub-horizontal shear set. Crosscutting relationships and surface textures suggest that although the moderate-dipping shear set is older and maybe sealed by cements, it may have been opened by local stress due to outer-arc extension through folding. Calcite bridges with rhombohedral structures may support fracture walls helping them to remain open in case of compressive stress. Our work documents shear fracture openings in the anticline at depth, and fluid flow in subsurface reservoirs.