Natural fracture opening preservation and reactivation in deep sandstones of the Kuqa foreland thrust belt, Tarim Basin

Abstract In deep to ultra-deep sandstones, natural open fracture patterns strongly influence the directions, magnitudes, and heterogeneities of fluid flow. We present an analysis of the natural fracture patterns, critically-stress state, and productivity of 35 wells in the Keshen and Dabei gas fields of the Kuqa foreland thrust belt. Our results show that reservoir performance is governed by the critical stress that acts on existing fractures to enhance their permeability. There is a good correspondence between the critically-stressed fractures and the natural open fractures in cores. These open fractures have patterns of barren cement, quartz bridges, and slickensides. Barren cement and quartz bridges indicate the preservation of permeable fractures, while the slickensides indicate shear reactivation. Barren fractures developed due to extension at the late stage of structural activities. Quartz bridge can prop up fracture walls to preserve porosity and permeability when compression. Shear reactivations consist of strike-slip and dip-slip, which broke the gypsum and calcite seal. Shear-reactivated fractures could reflect stress state transition from thrust regime to strike-slip regime during rapid burial. Combined with the analysis of fracture morphologies and kinematics, we indicate that open fractures development may relate to folding and fault reactivation. Besides, our case indicates that the E-W open fractures in the Keshen field and NE-SW open fractures in the Dabei field are not consistent with the direction of modern-day maximum horizontal compressive stress (σHmax). We conclude that the quartz bridge and shear reactivation govern the persistence of open fractures in tight and ultra-deep sandstones, even if the open fractures are perpendicular to σHmax.