Effectiveness and petroleum geological significance of tectonic fractures in the ultra-deep zone of the Kuqa foreland thrust belt: a case study of the Cretaceous Bashijiqike Formation in the Keshen gas field

The buried depth of the gas-producing reservoir in the Kuqa foreland thrust belt of the Tarim Basin exceeds 6000 m. The average matrix porosity of the reservoir is 5.5%, and the average matrix permeability is 0.128 x 10(-3) mu m(2). In order to reveal the characteristics and effectiveness of ultra-deep fractures and their effects on reservoir properties and natural gas production, outcrops, cores, thin section, image logs and production testing data are used to investigate the effectiveness of tectonic fractures in ultra-deep reservoirs in the Kuqa foreland thrust zone, and the corresponding geological significance for oil and gas exploration and development are discussed. Tectonic fractures in the thrust belt include EW-trending high-angle tensile fractures and NS-trending vertical shear fractures. The former has a relatively high filling rate, while the latter is mostly unfilled. Micro-fractures are usually grain-piercing-through cracks with width of 10-100 microns. In the planar view, the effective fractures are concentrated in the high part and wing zones of the long axis of the anticline, and along the vertical direction, they are mainly found in the tensile fracture zone above the neutral plane. The adjustment fracture zone has the strongest vertical extension abilities and high effectiveness, followed by the nearly EW longitudinal tensile fracture zone, and the netted fracture zone with multiple dip angles. The effectiveness of fracture is mainly controlled by fracture aperture and filling degrees. Effective fractures can increase reservoir permeability by 1-2 orders of magnitude. The higher part of the anticline is associated with high tectonic fracture permeability, which control enrichment and high production of natural gas. The netted vertical open fractures effectively communicate with pores and throats of the reservoir matrix, which forms an apparent-homogenous to medium-heterogeneous body that is seen with high production of natural gas sustained for a long term.