Abstract:

The Cretaceous Bashijiqike formation in the Kelasu structural belt in the Kuqa depression of Tarim basin hosts a number of high- and steady-yield subsalt gas reservoirs in ultra-deep，high-temperature，and overpressure environment. For these subsalt tight sandstone reservoirs，the higher the porosity，the higher the salt content and the lower the apparent resistivity. The distribution of salt in the reservoirs not only significantly affects fluid identification but also has a noticeable impact on the reservoir physical properties. The distribution of salt in the subsalt reservoirs in the Bashijiqike formation were systematically analyzed based on the data of cores，cast thin sections，scanning electron microscopy，salt content，and conventional logs. According to the differences in salt content，resistivity，and salt source，three distribution patterns of salt in subsalt reservoirs were proposed: top source，lateral source and local sealing. For the top and lateral source patterns，the reservoir resistivity is only affected by salt content. In the reservoirs with the top source pattern，the salt content shows a vertical zonality，and the reservoir resistivity increases as the salt content decreases. In the reservoirs with the lateral source pattern，the salt content shows a lateral zonation，and the reservoir resistivity shows a trend of high to low and then to high value from the edge of structural belt towards its center. In the reservoirs with the local sealing pattern，the resistivity is influenced jointly by stress and salt content，and changes greatly because the distribution of salt content is sporadic. According to well logging responses，the reservoir is divided into intervals for each pattern. In an ideal top source pattern，the reservoir comprises a salt interval，a mudstone barrier，an interval strongly affected by saturated salt，an interval strongly affected by unsaturated salt，a transition interval affected by unsaturated salt，and a salt-unaffected interval from top to bottom. In an ideal lateral source pattern，there are several intervals affected by oversaturated salt. In an ideal local sealing pattern，the reservoir includes a salt interval，a mudstone barrier，a salt-unaffected interval with strongly compressed stress，a salt-stress hybrid affected interval，and a salt-stress unaffected interval.

Key words: Kuqa depression, deep stratum, Cretaceous, subsalt reservoir, high temperature and high pressure, salt content, logging response, salt distribution pattern