Reservoir Characterization and Origin of Ultra-deep Tight Sandstones in Kuqa Depression of Tarim Basin, NW China

The Lower Cretaceous Bashijiqike Formation in the Kuqa Foreland Basin is tight gas sandstones deposited in a terrestrial setting. The sandstones include fine- to medium-grained feldspathic litharenite and debris arkosic sandstone, characterized by low compositional maturity, low cement content, and moderate textural maturity. Secondary pores created by dissolution and micro-fractures are well developed. The porosity is generally less than 6.0 %, and the permeability is less than 0.1 md. Due to the intense diagenesis and tectonism, the tight gas reservoirs are characterized by well-developed natural fractures, great reservoir heterogeneity, complex seepage system, and strong stress sensitivity. Compaction is the main factor for primary pore space reduction. The cementation of carbonate minerals, quartz, and clay minerals not only reduced the pore space of reservoir but also changed the pore structure, turning the intergranular tubular pore throat into laminar or sutural pore throat, which greatly reduce the reservoir permeability and porosity. Cementation is also a very important factor for the sandstone to be tight. The low porosity and permeability mainly results from sedimentation, digenesis and tectonic compression.