Improvement of reservoir quality of ultra-deep tight sandstones by tectonism and fluid: A case study of Keshen gas field in Tarim Basin, western China

Abstract

The Keshen gas field is one of the most important natural gas supply sources in the Tarim basin, western China. The main gas producing interval there is the Lower Cretaceous Bashijiqike Formation(K1bs), it is an ultra-deep tight reservoir whose buried depth exceeds 6000 m, and it shows a low matrix porosity (<10%) and extremely low matrix permeability (<0.1mD). However, this reservoir can supply extremely high and stable gas production due to improvement of reservoir quality by tectonism and fluid.Based on tectonic evolution analysis by plenty of lab data of core plugs or thin sections, the enhancement models of tectonism and fluid are built, evidence suggests both tectonism and fluid improve the reservoir quality greatly. Tectonic evolution produces lots of natural fractures in 3stages which promote the ultra-deep tight reservoir permeability 10–200 times, then, testing results of new boreholes without fracturing show reservoir permeability underground can reach 20 mD. Furthermore, fluid dissolution increases reservoir porosity 1–2 times, the main dissolved mineral is feldspar, all fluid dissolution came from the unconformity controlling the dissolution distance. Tectonism and fluid interact on each other: Tectonism controls fracture distribution and types of diagenic fluid in reservoir, but fluids influence fracture cements and dissolution. Both tectonic folding and the fluid flow control the sweet point reservoir located in upper 150 m formation.

Keywords

• Reservoir quality
• Fluid interaction
• Tectonism
• Diagenesis
• Deep gas