Characterization and evaluation of ultra-deep fracture-pore tight sandstone reservoirs: A case study of Cretaceous Bashijiqike Formation in Kelasu tectonic zone in Kuqa foreland basin, Tarim, NW China

Taking the Cretaceous Basijiqike Formation sandstone reservoirs in the Kuqa foreland basin as an example, this paper studies the characterization and evaluation methods of fracture-pore (fracture-primary pore and fracture-dissolution pore) ultra-deep reservoirs under the double effects of diagenetic compaction and tectonic compression. Buried over 6 000 m deep, the sandstone reservoirs of Basijiqike Formation are ultra-deep reservoirs, aimed at which, an ultra-deep fracture tight sandstone reservoir characterization technique is built, which integrates macroscopic microfacies-lithofacies identification, centimeter-micron scale fracture description, micron pore depiction and nano-throat characterization. The research indicates that the effective reservoir space consists of structural fractures, micron pores and nano-pore throats. The main radius of matrix pores is 2–100 μm, that of matrix throats is 10–500 nm and the main area of fracture opening degree is 100–300 μm. Effective reservoirs are mainly controlled by microfacies-lithofacies, tectonic compression and erosion. Relatively high-quality reservoir spaces consist of fractures, residual intergranular pores and dissolution pores, developed in weak compacted structure zone and stacked underwater distributary channel zone. Favourable reservoirs over 7 000 m deep can be in continuous band distribution, there still could be effective reservoirs in formations over 8 000 m deep.